How do I follow an ILS glideslope on the ADI in Falcon BMS? I'm currently flying the ILS landing mission in Falcon BMS, I'm practicing landing using the instruments (head down) and although I'm mostly OK with the LOC, I struggle to maintain a 2.5 degree glideslope.

In the training doc for Falcon BMS there is the following information:

  ADI gives an angular reference to the position of the glideslope. Each
  dot on the left side of the ADI represents an angle of 2.5°

But I don't know how to achieve the above. I watched some YouTube videos but I didn't see anyone aiming for those dots on the ADI (yet they managed to land perfectly).

I don't want to chase the glide path bar on the ADI, so how do I hold a proper glideslope using the instruments?

The trick to flying a raw-data ILS is to find the pitch attitude that gives you the descent rate that achieves the target glide slope as far out as possible, then hold the attitude as you monitor the ADI, making small attitude changes and waiting for the result when corrections are required.

So try this procedure:

As the ADI comes alive in level flight, gradually lower the nose as the indicator approaches the middle index to an attitude that is giving about 6-700 fpm down The required descent rate depends on ground speed - a shallow glide slope like that going 130-140 kt will be somewhere around that descent rate; finding it by trial and error is easy enough (a normal 3 degree glideslope, which you are probably really referring to, will need about 750-800 fpm going that speed).  Note the pitch attitude in degrees and hold the attitude.
Monitor the ADI.  You want it to be very slowly creeping toward the target. If it stops above the target index or starts rising, you are descending a bit too fast.  Raise the nose 1 degree on the pitch index, hold the new attitude and wait. Vice versa if the opposite problem.  If it keeps moving the wrong way, change the pitch another degree and wait.
When the ADI is on the target, pitch a degree to stop it from moving, and wait.
At some point with these little tweaks you will find yourself on slope with the ADI not moving.  Note the pitch attitude and concentrate on holding that pitch attitude.
If the ADI starts to drift up or down, same deal, pitch up or down one degree and wait. If that's not enough, pitch another degree and wait.  And so on.  The key thing is to keep the corrections small and be patient.

As you get farther down the ILS the sensitivity goes up and the deviations become harder to track so it becomes critical to get sorted out as far out as possible and have the pitch attitude nailed as you get close in.  If you have the pitch angle and hold it the slope will generally take care of itself unless you encounter a change in wind that affects your ground speed.

The other thing is trim trim trim.  When you are on-target and on-speed, trim to hands off. Now only slight pressures are required to make corrections, and the "default" or relaxed, control input is the nominal correct one.


What is the tail strike pitch angle of the ATR-72? What is the maximum pitch attitude an ATR-72 can be during takeoff or landing, without having a tail strike?

The FCOM says "tail strike may occur if pitch attitude exceed 8° during the flare
depending upon vertical speed at touch down" (Ctrl + F "tail", matches 31 and 32).


How does engine noise change during descent / approach? When on the ground beneath a STAR transition I hear commercial jets making a dramatic change in sound. I assumed it was the sound of high-bypass engines being pulled back at stepdown fixes, but when a friend recently asked me what the sound was, I wanted to make sure I was providing the correct answer. I am familiar with the sound made by the Fuel Over Pressure Protector (FOPP) cavity on A380s but this is a different, much lower pitched and shorter duration sound. Any help?

It is important to notion that an aircraft does not provide a uniform sound but features several sources of noise. The noise generated by the individual sources has separate characteristics and is transmitted into different directions.
For example, the sound of the engine front fan is sent into the direction of flight, creating a cone. The sound of exhaust jet and low-pressure turbine is transmitted backwards. Passive sound sources sent according to their positioning on the aircraft. Underside structures transmit downwards.
When you experience an aircraft transition overhead, the sound created by the aircraft may be steady but you experience the sound of changing sources.


How does my Casio watch know the altitude in flight? Wrist watches usually use the barometric pressure sensors they have to calculate the altitude. I have also confirmed this from the manual of my watch, which states that:

  The watch displays altitude values based on air pressure readings taken by a built-in pressure sensor.

Fine that's understood and makes sense and has always worked for me wherever I go.

Question

But then how does it know altitude during flight? The cabin of all commercial airliners are pressurized and hence the air pressure is not changing inside the cabin with the change in altitude (as happens for descent or ascent on ground). I know that even the aircraft itself uses pressure sensors for many calculations but those sensors are mostly usually on the Pitot tube and they sense the real outside pressure and not inside an pressurized cabin

I would also like to mention that yes the altitude reading it takes during a flight is not always close to what the real value is but still it does track the altitude difference when pressure inside the cabin remains same, how so?

Please note

That I didn't include the exact details of my watch because neither am I promoting any special model or a brand nor am I asking specifics of how a watch works, focus is on air pressure inside the cabin, not on watch. If needed I can provide the model and manual reference links.

The pressure in the cabin does not remain the same. It decreases, but much more slowly, in such a manner that when the aircraft is at 37000 feet the pressure in the cockpit is the equivalent of 7000 or 8000 feet. Keeping the cabin pressure at sea level would pose incredible stress on the fuselage, due to huge difference in pressure between the inside and the outside.


What are Forward flight, Straight flight, Level flight, and Cruise flight in a helicopter? Pretty straightforward: what is the difference between forward flight, straight flight, level flight, and cruise flight in helicopters?

Forward flight is how the helicopter flies in most situations. It flies similar to an airplane in this situation, with pitching up and down to increase and decrease airspeed. This is controlled by use of forward and aft cyclic.

Straight flight is where the tail rotor is set so that it opposes the turning tendency of the main rotor. This essentially prevents the helicopter from yawing while in flight. This is controlled by using the anti-torque pedals.

Level flight is simply when the helicopter is flying at a constant altitude. This does not mean, however that its nose is pointed ahead, or that it is flying forward, merely that it is not gaining or losing altitude.

I'm not sure exactly what cruise flight is. When a helicopter is "cruising", it most likely is maintaining altitude and flying straight ahead, but I don't think there is a separate term for it.


Who manages air traffic during Red Bull air races? I'll preface this by saying I'm not a pilot so I don't know much about Air Traffic Control.

How does Air Traffic Control work for Red Bull air races? Does:

(1) each plane taking off need to contact the local airport's ATC and let them know what they plan on doing/request permission for it, or

(2) do the event organizers schedule a block of time and airspace with ATC and take care of managing that block by themselves?

The latter seems feasible from my (non-pilot) perspective:

there's only one plane at a time on course (easy to manage)
it's a pretty well defined area that they'll be staying within.

It would help the event by:

perhaps allowing some of the long-range communication systems to be removed from the airplanes (reducing weight - useful for racing)
give the pilots fewer things to think about prior to their run on the course
transferring the control of take-offs and landings to the organizers makes it easier for them to stay on schedule

I think there would be advantages for them to do it the second way, and I think it would be technically possible to safely do it that way. However, it's not what I think is "standard procedure" and I wonder if they strayed from standards for their events' organization.

So are they doing it the first or second way? If they're doing it the first way, why? It seems inefficient, could they do it the second way (legality/regulation -wise)? Did I miss something that would make method (2) more trouble than it's worth? Or maybe they're doing it some other way I didn't think about?

This is a fairly broad question since the races move around the world the the regulations vary heavily depending on where its occurring but ill answer for the FAA understanding that most countries treat this kind of stuff similarly. The Air race its self has its own control tower of sorts that talks directly to the pilots to clear them onto and off of the course (this seems to be #2 in your question) but local ATC will be involved for departures.

Depending on where they intend on flying they may need to get a special flight permit, these are applied for in advance with the FSDO. I dont know if this is something the teams need to do or if this is something organizers handle. If the country has such regulations a TFR may be issued, for example this one from the french race (thanks to @mins for digging it up!). This keeps traffic out of the way so the organizers don't to worry about a Cessna 150 putting through the course @90Kts trying to "watch the race".

The planes typically depart from a local field close to the course. Here in the US they need to follow the same procedures as anyone else, if its controlled that means ground and tower coms if its uncontrolled thats CTAF announcements.

Once in the air they will head to the race course. Since these planes are effectively VFR radio com's is not mandatory outside of controlled airspace. The tower may simply transition them outside of the terminal area to the course frequency when appropriate.

According to this walk through the aircraft has one radio for taking to "race control" who clear them through the course. This behind the scenes video will walk you through how race control works and what they do.

No communication occurs during the race its self, you can see in this video the pilot is mostly concentrating on breathing and flying.

Once off the course they will either loiter around if they need to run again or head back to the airport, heading back is the same as any flight, fly on over and call up the tower to request landing.

This podcast is also a pretty great overview on the race and what it takes to be a pilot.


How can one calculate the maximum instantaneous bank angle a plane can do in a certain time? I know that there is a limitation to do an instantaneous banking angle a plane can do. These limitations are due to the airplane (infrastructure - aerodynamics) and limitation on the pilot (due to his ability to withstand a certain maneuvering calculated in g), so how can one calculate an accurate or approximate value for the maximum instantaneous bank angle taking into account the (military and civilian) planes infrastructure and also taking into account the pilot (with or without) a g-suit.

Also i want to know how long can a pilot withstand that instantaneous bank angle?

Thanks in advance.

This question has been asked and answered in over 20 various forms, a few are....

How much g-force is experienced in a 45° turn?

What is the relation between airspeed and rate of turn?

Best procedure to Turn around in Canyon, Turn radius as a function of velocity

How to calculate angular velocity and radius of a turn?


Does the TIS-B "hockey-puck" contain VFR mode-C targets with unconfirmed altitude? When receiving TIS-B traffic via ADS-B In, I'm curious what mode-C targets are included.

Is it only mode-C traffic with confirmed altitude in contact with ATC services (presumably with a transponder code)?

Or does it include traffic with unconfirmed altitude that's probably squawking 1200? Does it leave off altitude in that case? How would it know if traffic is within +/- 3,500' of your aicraft to give you your "hockey-puck" of traffic?

The AIM covers most of the operations and limitations of TIS-B on page 4-5-18

  TIS−B is the broadcast of ATC derived traffic information to ADS−B
  equipped (1090ES or UAT) aircraft from ground radio stations. The
  source of this traffic information is derived from ground−based air
  traffic surveillance sensors. TIS−B service will be available
  throughout the NAS where there are both adequate surveillance coverage
  from ground sensors and adequate broadcast coverage from ADS−B ground
  radio stations. The quality level of traffic information provided by
  TIS−B is dependent upon the number and type of ground sensors
  available as TIS−B sources and the timeliness of the reported data.
  (See FIG 4−5−8 and FIG 4−5−9.)

According to the AIM all transponder equipped (A/C/S) aircraft are included

  Only transponder−equipped targets (i.e., Mode A/C or Mode S transponders) are transmitted through the ATC ground system
  architecture. Current radar siting may result in limited radar
  surveillance coverage at lower 10/12/17 AIM Surveillance Systems
  4−5−19 altitudes near some airports, with subsequently limited TIS−B
  service volume coverage. If there is no radar coverage in a given
  area, then there will be no TIS−B coverage in that area.

section d. TIS-B Limitations offers some more info

The system is not perfect and unconfirmed altitudes may display as shadows

  (b) The ADS−B−to−radar association process within the ground system
  may at times have difficulty correlating an ADS−B report with
  corresponding radar returns from the same aircraft. When this happens
  the pilot may see duplicate traffic symbols (i.e., “TIS−B shadows”) on
  the cockpit display.

More on transponder requirements

  (d) The TIS−B system only uplinks data pertaining to
  transponder−equipped aircraft. Aircraft without a transponder will not
  be displayed as TIS−B traffic.

So pilot A in his Cherokee with a mode S transponder will show up but pilot B in his cub with no electronics and only a hand held who may be on radar will not show up.

This technical order covers the topic important phrase

  TIS-B is the ADS-B In function that provides for the in-cockpit
  display of other aircraft (without ADS-B) in the vicinity that are
  being seen via FAA secondary radar. General observations can be made
  during the flight to verify that known radar targets are appearing in
  the correct relative position on the ADS-B display(s) in the flight
  inspection aircraft. Of course, if there are no such
  targets-of-opportunity in the area during the flight, the opportunity
  to check TIS-B in this way will not be possible.

So if you are coming up on secondary radar you will be included in the ADS-B/TIS-B broadcast. ATC receives 1200 codes with altitude (encoder provided) so they will show up, and in my recent experience flying in the northeast most VFR traffic does show up. Aircraft with no transponder will not show up (radar target only). Im not sure what happens in the case of no altitude reporting.


What is this stripe on the windshield of the Embraer E-Jet? I noticed that there is a stripe along the upper edge of both forward windshields in the Embraer E-Jets:

(cropped from Wikimedia)
I tried to find a better picture, but couldn't find anything closer. This YouTube video clearly shows the stripe several times:

What is this stripe used for? The only thing I can think of is window heating, but these elements usually look quite different, as can be here: What is this pattern on the A350 front window?

It's a heater bus bar. In fact, the Q/A about the sensor loops that you linked has a great example of the same if you scroll to the bottom of the best answer.


How does afterburner efficiency vary with thrust? Afterburners are much less efficient than turbine engines at converting fuel into thrust, so AFAIK no jet will engage afterburner unless it is already at full throttle on the turbine.
But what if an afterburner were used at less than full throttle?  How, if at all, is the efficiency of an afterburner related to the thrust of the engine feeding it?

Normally, what is throttled back is the afterburner. The principle of reheating an already hot gas is so inefficient that partial wet thrust is achieved by running the engine at full power and using less than full afterburner.
The efficiency of the afterburner is proportional to the ratio of gas temperatures (peak to entry temperature), but at partial jet thrust the pressure level and volume flow in the jet exhaust is also diminished, so the available thrust increase from the afterburner will also decline sharply. To produce the same thrust, it will be much more efficient to run the jet at full speed and to throttle the afterburner.
The Tornado attack aircraft, designed for nap-of-the-earth flight, was so inefficient at higher altitudes that during the Bosnian war of the mid-Nineties it had to run one engine in partial afterburner in order to keep up with the tanker aircraft. For inexperienced crews which used most of their allotted time for coupling with the tanker, the tank stop could result in a negative fuel gain because so much was needed to keep up with the tanker.


Why would a copilot's window be shaded/covered? I'm sitting in a small regional airport in Vietnam and I've noticed something odd about the aircraft I've seen arriving and departing. All of these aircraft appear to have the copilot-side window covered or shaded. (The windows are covered from the inside with what looks like the reflective covers you put against your car windshield if you park in the sun to keep the inside of your car from heating up so badly.)

All of these aircraft are ATR 72's, and they're all flown by the same airline. The one I just observed depart had the copilot's side window and cockpit window covered at the time they taxiied to the runway. The one I just saw land had just the side window covered when it taxiied off of the runway and over to the "gate." I can't speak to whether the window was still covered when the plane actually took off since I didn't have a clear view from the airport of the actual runway.

(What I'm calling "copilot-side" is the left-hand side if you're standing in front of and looking towards the tail.)

What is the purpose of keeping some of the cockpit windows covered? Is it a regular practice? (Is it safe? It seems like a safety hazard to cut your visibility in half while taxiing around an airport, even if this place is just an airstrip.)

Aircraft flight decks can become very warm at times and pilots will always try their best to avoid letting the sun add any additional heat.

Most aircraft have sun visors or sun screens as part of the flight deck equipment. These are usually a clip-on design or pull-shade design. Both types are translucent and can be left in place for all phases of flight.

Many pilots will also bring their own temporary sun shades to use in the flight deck. Theses can be translucent or opaque. Many pilots even use their charts as temporary make-shift shades while parked at a gate in the blaring sun.

Since you described them as reflective covers, I suspect what you saw may have been some kind of temporary shade supplied by the company. They would not be used on the forward windscreen so they would be safe to use during taxiing.

Here are some examples of a pull shade being used in an ATR 72.

Here is an example of what looks like a reflective shade.


What is the drag induced by a high bypass jet engine on an airliner? What is the drag induced by a turbo bypass jet engine in a typical most modern airliner like the 777, the Jumbo Jet (Boeing 747-8) and the Airbus A380?

Given that the jet is travelling faster than the replacement of the air displaced by the engine suction, do parts in close proximity to this area where suction is largest cause less drag to the aircraft because of this micro second vaccum period occuring at all times when the aircraft is in cruise mid air?
I refer to turbo bypass engines because they are the ones most used because of fuel efficiency, noise, etc., in airliners, I do not ask about bypass ratio. I ask about the displaced air outside the engine, and the overall effect on drag caused by the engine as a physical component attached to the plane colliding with air.

Does the fact that it is sucking a very large amount of air around it reduce this drag and by how much compared to if it were not sucking any air at all?

What you are asking about is inlet performance.

Instead of sucking air, think of it as eating air, that's a better more correct analogy (like Pac-Man eating dots). As the plane flies through the air, it may literally bite off more than it can chew. Causing spillage. The extra air stuck in the inlet tries (but fails) to go out. Creating drag.

But this drag for the subsonic airliners you ask about is offset by the suction at the inlet due to the inlet's design.

See here for how they cancel each other: How does inlet spillage drag produce a lip suction effect?

Related: Does the air resistance of a jet engine air intake count towards the jet's total air resistance?


What are the procedures for flight checking a glideslope and localizer? What are the FAA and ICAO requirements and procedures for flight checks and calibration of glideslope and localizer transmitters? I'm interested in both the engineering and piloting aspects.

It's all laid out in Order 8200.1D United States Standard Flight Inspection Manual.

That and other documentation, including the inspection reports can be found from the FAA website under Flight Inspection Information.


If a circling airplane (AC-130) fires a bullet how does the plane's trajectory alter the shell's flight path? Background: An AC-130 is a military aircraft that has a cannon that fires out the side of its fuselage.

If an AC-130 (or similar aircraft) is traveling in straight, level flight at a constant rate of speed calculating gun lead is straightforward.  For example if the plane is traveling forward at 100 meters per second, the target is 1000 meters to the side and its cannon shell travels at 1000 meters per second simply fire 1 second or 100 meters early to score a hit.

However what if the AC-130 is traveling in a circular orbit at 100 meters per second around its target instead of beside the target?  I'm not sure how to calculate gun lead in that circumstance.

Hopefully the image I attached below will help this make sense.  Thanks so much in advance for any input!

The bullet leaves the barrel with the combined velocity of barrel muzzle velocity plus the instantaneous aircraft true air speed velocity (actually, the instantaneous true air speed velocity of the tip of the muzzle). That puts the velocity vector of the bullet ahead of the line of the barrel (ahead of the 90 degree wing line). The fact that the barrel is rotating in inertial space is irrelevant. Whether the aircraft is straight and level, turning towards the tank, or away from it, if, at the moment the bullet leaves the muzzle, it is traveling in inertial space towards the tank, (and sufficiently above it for gravity drop) it will hit it.

And since the bullet's velocity leaving the barrel is a simple vector sum of the muzzle velocity, and the aircraft's instantaneous velocity at that instant. The velocity vector of the aircraft before, (or after), the bullet leaves the barrel is of no relevance.

So if the aircraft is moving due North at 100m/s and the muzzle velocity is 1000m/s due East relative to the aircraft, the bullet is actually traveling
SQR(100^2 + 1000^2) = 1004.98 ~1005m/s in a direction slightly North of due east by an amount equal to the arctangent of 100/1000 or ArcTan(0.1) = 6.345 degrees
So the bullet is traveling 1004 m/s on a heading of 90-6.34 degrees = 83.66 degrees true.
So, to hit the tank, the pilot has to fly so that the gun barrel is pointed behind the tank by 6.34 degrees (and above it by a sufficient amount to account for gravity drop)


Does SDAC display data for both the PFD, ND and the ECAM display? On the Airbus A320 the System Data Acquisition Concentrators (SDAC) aquire system data, process it and sends system page data to the three Display Management Computers (DMC). The DMC process the data and generate an image.

Is this for the PFD and ND only? Or does the DMC also generate the image for the upper and lower ECAM too?

EIS, source

If the lower ECAM, supplied by DMC 2, fails then the E/WD displays which has priority take its place; and DMC 1 normally supplies the Engine/Warning Display (E/WD).

Does it mean DMC 1 has a channel or a way to display on the lower ECAM as it just did?
Likewise, does DMC 2, which normally supplies the lower ECAM, have a channel or some sort of way to display on the upper ECAM?

The DMC can process simultaneously 3 pages among them only one system page. DMC 1 and DMC2 are linked to both central displays.

Therefore the display page arrangement is done by the DMCs not by the SDACs, nevertheless lot of data computed by the SDACs are displayed on the ECAMS and on the PFD and on the ND:

On the ECAMs for instance all system pages and AMBER alarms via the FWCs
On the ND and on the PFD flags and degraded conditions, such as the cross « X » when data is NOT computed for the relevant parameter.

Of course my answer is not exhaustif but sufficient to justify that the SDACs are concerned by data displayed on all EIS displays.


Why can the aircraft not turn on pitot heaters itself? There have already been some posts about pitot heaters: when to turn them on/off, why not leave them on etc.  I don't want to go over that.

I am an engineer, so I'd like to ask:  if the flight management avionics detect an out-of-bounds disagreement from the pitot transducers, and the heaters are off, why not turn them on, in addition to whatever other warnings etc. they already perform?

An ECAM message about turning on the heaters could be made and, if the pilot explicitly wants the heaters off for some reason, s/he can instruct the computer or pull a breaker, so overriding the software.

There's much simpler and safer logic. Most modern Western transport aircraft automatically switch the pitot heaters on when in flight mode or at least one engine is running. Pilots can manually force pitot heat on, but there's no direct option to switch them off. Normal procedure is to leave it in auto.

Boeings since the 757/767 are automatic (exception: 737), and Airbus has had it automatic since at least the A300-600. ERJ is auto, not sure about the CRJ200, but the 700 is auto.

Pitot heat indication systems are already required by certification standards. 14 CFR 25.1326:

  If a flight instrument pitot heating system is installed, an indication system must be provided to indicate to the flight crew when that pitot heating system is not operating. The indication system must comply with the following requirements:

  (a) The indication provided must incorporate an amber light that is in clear view of a flight crewmember.

  (b) The indication provided must be designed to alert the flight crew if either of the following conditions exist:

    (1) The pitot heating system is switched “off”.

    (2) The pitot heating system is switched “on” and any pitot tube heating element is inoperative.

EASA CS 25.1326 is basically identical.

The Antonov 148 is certified to EASA CS-25, so it must have had such a warning system.


Where can I get a history of METARs? Is there a place where past METARs are logged?

Suppose I want all the METAR for a given airport from October 1st to October 15th 2015. Where can I find them?

If it matters, I'm looking in particular for METARs from French airports.

OgiMet Professional information about meteorological conditions in the world is such a site that offers 30-day history for METARs/SPECIs and TAFs reports.
The coverage is said to be worldwide. Here is a glance at the query interface:

The site states these conditions of use for the reports:

About all the codified met. reports (SYNOPS, METAR ...), the copyright
are from every country or source institution. To use them,
read WMO resolution 40.

(essentially it says it's free for use.)
The site accepts requests for up to 31 days at once.
Example: METARs for month of September 2020 for Orly / LFPO:


Is possible to stick two '2-blade' propellers to get multi-blade propeller? Does propeller blades' airflow interfere with each other blades and decrease overall thrust? Should I consider high pitch propeller for this purpose?
My purpose: I'm looking for a 20-inch four-blade propeller for a tail-sitter Drone. But commercially I don't find anything like that. So started looking at making a  4-blade/6-blade propeller with 2-blade propellers. Thrust density is my first priority compared to efficiency.

Sure, this is not only possible, but even has some advantages over single-plane propellers. The catch is you ideally want the two to spin in opposite directions to cancel out torque and vibration.
You're building a contra-rotating propeller:

Pictured is a Sun Flightcraft contra-rotating gearbox kit for Rotax 503 and Rotax 582 engines.
Although more efficient (on the order of 10%) than single-plane propellers, they are also much more complicated to design. The gearbox will be heavier and harder to maintain. And they are also often much louder, although the Rotax kit pictured above is claimed to be quieter.
They're not just for small engines. Here's a Tu-95 strategic bomber:

On helicopters, the same idea is called coaxial rotors:

"Co-rotating" propellers are also possible, but far less common, as the key advantages (efficiency and cancellation of torque and vibration) are reduced or lost. I could only find some academic papers on it. Here is another.


Is it possible to build a flight-capable aircraft powered by compressed gas? I pray this isn't a physics SE question.

A star engine can be considered "gas operated". So what if we power a star engine with unleashed, formerly compressed gas instead of... actual gas/aero-kerosene/etc...? So instead of fuel tank (oh the temptation to pun is so strong here) you use gas tank?

Note that since nothing has to BURN, we can use lighter-than-metal material, as engines usually play a big part when it comes to weight of aircraft.

Can you?

And if that's a YES, can you estimate the flight time?

In aircraft design, the main problem is energy density of the energy source. Even with the phenomenally high energy density of kerosene fuels (>42 MJ/kg), the fuel still represents a sizable chunk of the weight of an aircraft. For example, fueling an Airbus A330 will double its weight!
According to Wikipedia, compressed air has a practical energy density of about 0.1MJ/kg. There is no way you can offset the enormous amount of compressed air required by downsizing your engines. You would go from 110.000 kg of fuel to over 400 million kg of compressed air.
A more relatable example would be a Cessna 172. It can carry 212 litres of fuel, or about 150kg. On top of that, it can carry about 190kg of useful payload. If we instead use compressed gas, we would need over 60.000kg of compressed gas to get the same range.


Considering a piston engined aircraft, what resistance pressure would be generated within its engine bay against its cooling intake? Consider a piston engine with all its coolers and components within an aircraft engine bay. To cool this engine, cooling intakes are positioned facing the direction of flight. Now, the condition at the inlet surface need not reflect the flight conditions exactly, one reason for this would be since the resistance pressure from within the engine bay would influence the conditions at the inlet itself.

The question is specific to this resistance pressure as a first approximate at least. I know there are a variaty of other reasons as to why the conditions at the inlet surface would change (e.g pressure distribution at location of intake position, inlet type, etc...) but those aren't a concern here yet.

A good answer to this based on actual data, or statistical data or even experience would be greatly appreciated. Sources as to where one may find the answers to this would also be much welcomed. Thanks in advance!

3 main points come to mind:

The space above the cylinders is a pressure plenum so that the air pressure and velocity flowing past the cylinders is the same everywhere.
The space below the cylinders is a low pressure zone and to achieve that the exhaust opening should be larger than the inlet opening(s) to get the highest differential at low speed, but can be reduced as ram pressure increases (hence a cowl flap).  The pressure balance has to take into account air bled off the high pressure plenum to pass through coolers or other services that need ram air.
The baffling that controls the flow between the high pressure plenum above to the low pressure exhaust zone below past the cylinders has be tight to the cylinders, actually touching the fins, so that the cooling air is forced between the fins.  Cooling air that passes beyond the edges of fins does no good.

As for details and numbers, you should probably obtain this book on piston cowling design.


What are the forces present in a coordinated turn? During straight and level flight, coordinated flight is assumed when there is no net lateral force (no slip or no skid). But this concept totally breaks down when it comes to turning, in a co-ordinated or unco-ordinated turn; there will always be a net lateral force due to centripetal force provided by the horizontal component of the lift force. The centrifugal force (inertial force) found in the books, in an attempt to explain it doesn't make sense to me as the centrifugal force is an imaginary force. So how is co-ordinated flight achieved in term of the forces?
Thanks in advance.

To get a deeper understanding beyond the superficial, it helps to break down the forces and moments present on an aircraft that may affect its rigid-body motion:

Aerodynamic forces: These are the forces and moments exerted by the airflow on the aircraft

Ground forces: These are the forces and moments exerted by the ground on the aircraft, transmitted through tyres and landing gear. Not applicable when it's flying.

Propulsion: Forces and moments due to direct thrust. For simplification, let's assume that thrust acts inline with the forward axis.

Gravitational forces: Gravity pulls the aircraft towards the ground. It's rather special because everything, be the aircraft structures, you, me, or the accelerometers, get pulled at the same rate ($g$)1. This is distinctly different than the other two types of forces, which are only present when airflow affects exposed areas, or when contact with the ground has been made.

Inertial forces: These are fictitious forces and moments that are required to maintain non-uniform motion. This includes your centripetal force. Inertial forces must always be equal to the sum of all the aforementioned external forces.

First of all, let's consider any turn a steady-state maneuver (thereby ignoring the transients like rolling in and rolling out), which means that the vector sum of all the external forces, including gravity, must sum to the inertial forces. As you've correctly pointed out, the sum of aerodynamic forces + gravitational forces must be equal to the centripetal force, which in the turning plane is provided by lift, side force and gravity. This must hold true for any steady-state turn, whether coordinated or not.

There are two ways to define a coordinated turn. With all engines operating, they are approximately equivalent:

A zero-sideslip turn
A ball-centered turn: we are going to use this definition

What ball-centered means is that there are no aerodynamic forces acting laterally on the airplane: lift must provide all the centripetal force required. Ball centered provides the best average feel for the occupants, since the forces are directly inline with the floor, and there's no side force causing sway. Since everything feels gravity at the same rate, occupants or the ball cannot detect gravity.
For illustration:

For the more math oriented, in inertial frame, Newton's second law is stated as:
$$\vec{F_{i}}+m\vec{g_i}=m\frac{d\vec{V_i}}{dt} \tag{1}$$
The entire right hand side is considered to be inertial forces. However, if the measurements occur in a rotating frame (on an airplane, for example), then we need to express everything into the body frame. The left-hand side is easy:
$$\vec{F_{b}} = C_{bi}\vec{F_{i}} \tag{2}$$
$$\vec{g_b} = C_{bi}\vec{g_i} \tag{3}$$
where $C_{bi}$ is the rotation matrix transforming a vector from inertial frame to body frame.
The right-hand side requires some adjustments, because the Euler angles of the body themselves are changing:
$$\frac{d\vec{V_b}}{dt}=\frac{d(C_{bi}\vec{V_i})}{dt}$$
Apply chain rule, and we have:
$$\frac{d\vec{V_b}}{dt}=\frac{dC_{bi}}{dt}\vec{V_i}+C_{bi}\frac{d\vec{V_i}}{dt} \tag{4}$$
It can be mathematically shown the following identity:
$$\frac{dC_{bi}}{dt}\vec{V_i} = -\vec{\omega_b} \times \vec{V_b} \tag{5}$$
Substitute (5) into (4), and we get:
$$C_{bi}\frac{d\vec{V_i}}{dt} = \frac{d\vec{V_b}}{dt} + \vec{\omega_b} \times \vec{V_b}$$
Finally, if we multiply both sides of (1) by $C_{bi}$ and simplify with (2), (3) and (5), we get:
$$\vec{F_{b}}+m\vec{g_b}=m \left( \frac{d\vec{V_b}}{dt}+\vec{\omega_b} \times \vec{V_b} \right) \tag{6}$$
(6) is the Newton's second law in a rotating frame. The entire right-hand side are still inertial forces (same as (1)), except now we also have the cross-product, which produces the centripetal inertial forces:
$$\vec{\omega_b} \times \vec{V_b} = \begin{bmatrix}p \\ q \\ r\end{bmatrix} \times \begin{bmatrix}u \\ v \\ w\end{bmatrix} = \begin{bmatrix} qw - rv \\ ru - pw \\ pv - qu \end{bmatrix} \tag{7}$$
You will readily identify terms like $ru$, which is very close to the familiar $a_c=\omega V$ for a restricted 2D centripetal motion (whence $p=0$ and $q=0$).
One final note, the right-hand side is fictitious in that they are not real forces! They are a result of the kinematic motion itself, and must require the left-hand side (which are the real forces), to sustain.

1: Technically, this is only true locally, because Earth is a sphere and does not exert a uniform field at different altitudes. But at the range of altitudes that airplanes will be flying, this is a rather good approximation.


How does the rotor blade material affect aircraft performance? Helicopter rotor blades in newer helicopter designs are made from composite materials. This results in improved performance. Is this simply because they are lighter or is there another reason behind that?

For example, for the new CH-53K the blades are made from titanium fiberglass composite. The use composite rotor blades results in better "hot+high performance".

How can that be? Isn't performance simply weight/lift ratio?

The Hot and High conditions refer to the worst atmospherical conditions for the performance of an aero-engine in general. This is due to two phenomena:

HOT: When the outside temperature is high, on one hand, the pressure decreases (due to the decrease of density of the air) and thus the mass flow rate and thus the power decreases (the power is proportional to the mass flow rate). On the other hand, the ratio TIT/Ta (Turbine Inlet Temperature of the atmospheric temperature) will also decrease. This will lead to a reduction of P and to a reduction of the effective efficiency (the power and the effective efficiency are proportional to this ratio), the result is also an increase off the specific fuel consumption (SFC).
HIGH: At high altitude, the pressure decreases and thus the density and the mass flow rate. The power will then decrease (as above for the HIGH condition) and the SFC will increase.

What are your options then ? Well, you can put a more powerful engine to compensate for these conditions but you will most probably increase the weight and the fuel consumption a lot (and the cost for sure). You can use the same engine but at a higher regime, but again, you will consume a lot which is not ideal. But there is another solution: you can make your helicopter or aircraft lighter. This whey, you need less power. And that's what brings composites in general; an important gain of weight.

Hope it helps !


Why is there a drift value between the IRS position and the FMGC position present even after performing a full IRS alignment on the A320? I have noticed this many times across my company's fleet of A320's that is even after a full IRS alignment has been completed many a times there is still a drift value present between the FMGC position and the IRS position. Could any help explain as to how this happens ?

TLDR: The FMGC position is a mix of GPS and IRS (called GPIRS), but the IRS position is aligned to the airport reference coordinate, not to the GPS position.

The difference you observe is between the FM position and the MIX IRS position. This difference is called the bias.

Position Computation
Each FMGC computes its own aircraft position (called the "FM position") from a MIX IRS position (see below), and a computed radio position or GPS position.
The FMGS selects the most accurate position, considering the estimated accuracy and integrity of each positioning equipment.
GPS/INERTIAL is the basic navigation mode provided GPS data is valid and successfully tested. Otherwise, navaids plus inertial or inertial only are used.

(Airbus A320 FCOM - Auto Flight - Flight Management)
The FM position will move towards the GPS position in flight, if GPS is available (otherwise to the radio position):

FM Position
At flight initialization, each FMGC displays an FM position that is a mixed IRS/GPS position (GPIRS).

At takeoff, the FM position is updated to the runway threshold position, as stored in the database, possibly corrected by the takeoff shift entered on the PERF TO page.
In flight, the FM position approaches the radio position, or the GPS position, at a rate that depends upon the aircraft altitude.

Note: The FM position update at takeoff is inhibited when GPS PRIMARY is active.

Bias
Each FMGC computes a vector from its MIX IRS position to the radio or GPIRS position. This vector is called the "bias". Each FMGC continuously updates its bias, if a radio position, or a GPIRS position is available.

(Airbus A320 FCOM - Auto Flight - Flight Management)
The MIX IRS position is determined from all three IRSs:

MIX IRS Position
Each FMGC receives a position from each of the three IRSs, and computes a mean-weighted average called the "MIX IRS" position.

(Airbus A320 FCOM - Auto Flight - Flight Management)
I think your confusion comes from the fact that you think the IRS is aligned with the GPS position and therefore MIX IRS and GPIRS should agree directly after alignment. This is however not true:

Initialization
The F-PLN origin airport coordinates are extracted from the FMS database. These
coordinates appear on the MCDU INITA page, and are normally used for initialization.
They are the airport reference coordinates.
If a high navigation performance is desired, (i.e. for long-range flights without GPS
and without radio navigation updates, or if low RNP operation is expected), the crew
should adjust the airport reference coordinates to the gate coordinates, provided that
this data is published or available on board.

(Airbus A320 FCTM - Normal Operations - Pre Start - ADIRS Initialization)
As you can see the IRS is aligned against whatever is in the MCDU INITA page (airport coordinate by default), not GPS. Therefore, the IRS position can deviate from the FMGC position (GPIRS), even right after alignment.
This deviation will remain in flight, only the FM position will move towards GPS and the bias vector will describe the difference.


How is thrust of engines on the two sides of an airplane kept exactly equal to prevent yawing? I assume that the thrust of engines on the two sides must be kept exactly equal otherwise the plane would start yawing. What mechanism is used to prevent this from happening?

It isn't. The rudder is used.

Yes, if there is asymmetry in thrust, the plane will yaw. And there is asymmetry in thrust. The engine thrust decreases slightly with wear and the engines are often worn out differently. There are effects like P-factor that shift the thrust axis depending on speed and attitude. And of course an engine may fail.

So thrust asymmetry happens and needs to be compensated. And it is compensated by generating some sideways force with the rudder. Either by pilot pushing the pedal, or by applying trim.

As @mins mentions, there are also effects that can cause asymmetry of drag, e.g. icing, and there are situations where the aircraft intentionally needs to fly somewhat sideways, e.g. cross-wind take-off and landing, so rudder is needed for many other things too. And when it's there, it can compensate thrust asymmetry (often, though, the engine failure dictates the minimum size of the vertical stabilizer and rudder).

Note, that P-factor occurs in single-engine propeller aircraft too, so they also have some thrust asymmetry.


How to find coordinates of the geometric center of a NACA 4-digit airfoil? For a project that my team and I are working on, we are using Autodesk Inventor Professional 2018 (student license) for 3D modelling. We are working on NACA airfoils.

I need to create an airfoil within an airfoil so that the thickness distribution is uniform at around (10/12/15)mm.

To do this modelling, I've used two methods, neither of which is satisfactory. Please advise if using geometric center of airfoil is better than aerodynamic center. And how to find it. Please suggest any other method to get the uniform thickness distribution.

Use a NACA 6415, c=600mm and NACA 6412, c=580mm. Superimpose their aerodynamic centres.

The issue with this method is that thickness is more at Trailing Edge (TE) than at Leading Edge (LE).

Use a NACA 6415, c=600mm and scale it down to 0.85 with respect to the aerodynamic centre.

Issue with this is that the thickness variation is too high.

Cross posted at researchgate

I guess you wish to create a halo.

You can use Airfoil Tools' NACA 4 digit generator ( http://airfoiltools.com/airfoil/naca4digit ) to generate your NACA shape. It also features a plotter were you can set chord and halo thickness.
Positive values for internal halo and negative for external.
Internal halos are interesting to build wing ribbons.

Airfoil Tools generated NACA 6415

NACA 6415 600mm chord with 15mm halo (JPG)

Airfoil Tool's NACA 4 Digit generator


Why do some towers reference departures by compass direction, others by leg of pattern? Got my Private about 6 months ago, and I've always wondered - let's say I request a VFR departure to the South, with the active runway being, say, 2. Some towers will give my departure clearance in reference to the requested compass direction, i.e.
    cleared for takeoff runway 2, right turn to heading 180 approved
whereas other towers will reference it to a pattern leg, such as cleared for takeoff runway 2, right downwind departure approved. Similarly, I've noticed pilots based at airports that use the latter convention will follow that convention when making their departure requests. Is there any rhyme or reason behind which convention a tower follows?

It depends on local custom.

Some airports have multiple runways, so controllers will use compass points instead of pattern legs, and this habit will carry over when they are assigned to a single-runway airport.

Other airports have noise abatement procedures which have specific names. Below are the departures for San Carlos airport, and pilots are expected to use a named departure, such as "Belmon Slough Departure" instead of "right-crosswind departure," and tower will readback the named departure.


Is it allowed to exit the runway by opposites High Speed Taxiway? Is it allowed to exit the runway on a High Speed Taxiway to the opposite side? (see red line in image below). Assume the situation allows it, like airplane limits, airport congestion, weather, etc.

It's not really a regulation question, at least not in the way you think. There's 2 possibilities:

A controlled airfield: in an airfield which has air traffic control all movements require a clearance, if the tower clears an airplane to use a taxiway then that is all that airplane is allowed to do unless there's an emergency of some kind. It would be unusual for ATC to direct an airplane to use a taxiway which has such a tight turn, but there's no reason they wouldn't if it the situation called for it
An uncontrolled airfield: in this situation there's nobody in a tower telling pilots what to do, all movements are at pilots' discretion. Pilots can use any taxiway in any way they please as long as it doesn't put them in potential conflict with another airplane


What are the durations of strokes and events in a 4-stroke engine? I'm learning about engines, and I understand the basics of how a 4-stroke engine works, but I've seen lots of true or false questions such as "The power stroke is shorter than the intake stroke" or "The power stroke is longer than the power event". What's the difference between strokes and events, and where can I see more about how long each one takes?

The duration of the stroke is simply the time the piston takes to run from the bottom to the top of the cylinder or vice versa, half a rotation.  So if the engine is running 2000 rpm, or 33.33 revolutions per second, or 16.66 strokes per second, the stroke duration is .06 seconds, half of one full revolution.

That's the physical mechanical stroke.  However the "events" overlap the strokes.  Ignition on an aircraft piston engine starts normally around 24 degrees before Top Dead Center, so the power "event" actually starts before the compression stroke is completed.  Same with exhaust; the exhaust valve may open slightly before the power stroke is complete (piston not quite at bottom dead center) and the intake valve may open just before exhaust stroke is complete.

This is all done to take advantage of inertial effects of the mass of the air fuel charge.  There is a lot of overlap of all these events with the mechanical stroke and is a function of valve and ignition timing, so for a given engine you need to look at a diagram of the timing points for ignition and valve opening/closing for that particular engine.


Are the items in newbie "kits" actually used during or after training? I want to start training for my Private certificate but I do not want to waste my money.  I'm already purchasing an online course for ground school.  Generally speaking, are the following used during or after training?  And, if so, would a $150 US price tag be worth it?

Electronic E6B Flight Computer
FAA Aviation Weather Text Book
FAA Airplane Flying Handbook
Private Pilot Maneuvers Guide
Pilot's Handbook of Aeronautical Knowledge
Pilot's Log Book
FAR/AIM Fuel Tester
Flight Gear HP Crosswind Bag
Sectional Plotter
VFR/IFR Flight Plan Forms

Thanks, in advance.  I'd rather spend my money on flight time than nonsensical items.

Well going by Sportys own price lists

Flight Gear HP Crosswind bag - 50
PHAK - 16
AFH - 16
AWTB - 16
FAR/AIM - 16
Fuel tester - 11
E6B - 40
Logbook - 16
Plotter - 16
Private Pilot Maneuvers Guide - 16

TOTAL: 213

If the list price is 150, that represents a savings of 63.

Of all that equipment, save for the maneuvers guide, you will need the rest of it to complete your training knowledge test and checkride.  The E6B and the chart plotter are a little anachronistic these days but can still be put to good use for time or fuel sensitive flights.   You will make good use of the flight bag for the rest of your flying career.  All in all it’s a pretty good kit.

One thing they left out and I highly advise you buy is a flashlight and spare batteries.  It’s a $30 item and it could save your life if you ever have an electrical failure at night.  I would recommend the Smith and Wesson Captain’s Flashlight for this.  Avoid tactical flashlights with high output white lights; these are counterproductive when flying and will wreck your night vision.  Another good option are the head mounted lights popular with cavers and climber.  Select one with white/red output and, again, not too bright.

Another useful item is a headset.  High end ANR headsets cost over 1000 USD but ther are economical and good quality one in the 200-300 USD range.  I used the David Clark H10 series headset during my primary flight training and still have the headset handy for passengers.

I would recommend spending the money on the following apps

Either ForeFlight or Garmin Pilot as an EFB.  Best thing since sliced bread for charts, maps, weather products, flight planning, filing, etc.  much lighter and a hell of a lot cheaper than paper products for this.

ASAs FAR/AIM - Heck of a lot lighter than a paper FAR/AIM, can search regulations faster and it automatically updates annually, instead of having to buy a new paper one every year.


Why is it said that the Tupolev Tu-144 had superior aerodynamics to the Concorde? A quote from this article:  "The Concorde had better range, braking and engine control but the Tupolev had vastly superior aerodynamics."

What is the basis, if any, for this statement on the aerodynamics of the two aircraft?

Unfortunately, there is no hard data available (at least in open literature) to back up these claims.

One for the claim may be due to the fact that the Tu-144 was designed to fly higher than Concorde as its primary flight path was over land rather than water. Because of this, the L/D ratio of the Tu-144 was higher than that of Concorde (7 vs 8 in Tu-144D). Also, the Tu-144 was bigger and was meant to carry more passengers.

Photo Courtesy: globalsecurity.org

The Tupolev site offers an interesting explanation for this claim. Basically, they say that despite the engines and other subsystems being inefficient, the Tu-144 performed similar to the Concorde due to superior aerodynamics of the aircraft. The performance of the Tu-144's RD-36 engines was later brought up to that of Concorde's Olympus 593, at least in fuel consumption.

To quote,

  It should be added that the engines were less perfect (by specific parameters the engines of TU-144 a/c were brought close to “Olympus” only in their last modification). All these negative initial data were compensated in the course of development of the project by high aerodynamic cleanness of TU-144 which was achieved by making the design more complicated and by lowering maintainability of the aircraft.

But without any hard data, it is not possible to compare the two aircraft directly.


Why would there be airflow from the back of a TAT probe? During walk-around checks, I occasionally find airflow coming out from the back of the TAT probes on the Airbus A320. I wonder if it's air to cool down the TATs as they are heated during flight for anti-ice purposes.

I had a look at the description and operation of the TAT probes in the AMM but I couldn't find any information about that airflow and where it comes from.

This is a so called aspirated TAT sensor, optional on the Airbus. Air is being drawn through aspirated TAT sensors to ensure that they correctly indicate the temperature even when the aircraft is not moving.

Normal TAT sensors develop a bias when the aircraft is static.

This video gives a nice explanation.


How does EMAS reduce damage in case of an aircraft overrun? I saw a report about Mike Pence's plane overrun today on Twitter link: Mike Pence plane overrun photo on Twitter.

As can be seen there is huge cracks and breaking of the runway but the plane was not damaged. According to twitter this is due to Engineered Materials Arresting System (EMAS). The FAA is adopting a plan to expand the use of this method for arresting overruns on runways that do not have enough land for a runway safety area (RSA).

I am interested to know how this system works. Is it similar to gravel runaway truck arrestors on the steep downhill roads? Do we have any specifications and test data?

There is a link in the FAA tweet you link to, you should probably read it.

  EMASMAX® is the latest, most durable version of ESCO’s EMAS, developed with and technically accepted by the FAA. EMASMAX arrestor beds are composed of blocks of lightweight, crushable cellular cement material designed to safely stop airplanes that overshoot runways.
  - FAA Fact Sheet – Engineered Material Arresting System (EMAS)

So yes it's like gravel runoff areas, where the crushable material slows down the aircraft by providing resistance through the landing gear. The difference being this is a bit more engineered so you don't get bits flying up into the engines etc. like gravel would.

As for specs, the same link claims it can stop an aircraft going 80 mph but doesn't state a size of aircraft.

  A standard EMAS installation can stop an aircraft from overrunning the runway at approximately 80 miles per hour.
  - FAA Fact Sheet – Engineered Material Arresting System (EMAS)


Can climbing above RVSM airspace cause temporary loss of separation with an aircraft in RVSM airspace? Consider Aircraft A cruising at FL400 and Aircraft B cruising at FL410. Both in close proximity, heading in the same direction. Ignore altitude for direction of flight issue. When aircraft B requests to climb to FL430, would allowing him to do that cause a loss of separation?
I'm not sure if my understanding is correct. In the beginning, both aircraft are in RVSM airspace, have 1000ft vertical separation and are therefor correctly separated. When Aircraft B climbs above FL410, they enter non-RVSM airspace, requiring 2000ft. vertical separation and therefor loosing separation. When Aircraft B is between FL410 and FL420, there is less than 2000ft of separation between them (Aircraft B being in non-RVSM airspace).
It seems really counterintuitive that increasing vertical separation between two aircraft can cause a loss of separation.
I'd love to know the answer to this question for both ICAO and FAA land with 4444 and 7110.65 citations if anyone knows them.
Thanks!

I think the way rules regarding RVSM are worded in ICAO documents leaves some room for interpretation. When RVSM was implemented, it was probably not very common for aircraft to be able to climb above FL400, so this specific problem was probably not considered when writing the rules. But I would say, when in doubt, use the more conservative interpretaion, which would be that: Yes, this would be a loss of separation.
It sounds weird that increasing the distance between two aircraft can cause a loss of separation, but RVSM airspace is not actually the only place where this can happen. I can think of a few other examples:

Two aircraft flying on parallel tracks with exactly 3 NM between. They are flying parallel to an airspace border. On one side of the border, the required separation is 3 NM, on the other side, the required separation is 5 NM (this is quite a common setup, actually). Imagine one of them is flying almost directly on the airspace border. If this aircraft now turns slightly away from the other one, increasing the distance between the two, it crosses the border into the airspace where 5 NM of separation is required. That's a loss of separation.

Losing lateral separation when no other separation exist. For example, imagine a visual reporting point 1.5 NM from the runway. In the local rules, it is defined that VFR traffic orbiting overhead this reporting point are separated from aircraft taking off and landing. When an aircraft takes off whild a VFR flight is orbiting over this point, that's all well and good. By definition, they are separation. But if the VFR flight now leaves the reporting point, separation may be lost - even if the VFR flight takes a heading directly away from the other aircraft, so the physical distance is increasing. As the VFR flight leveas the reporting point, the rule about lateral separation no longer applies, so if the flights are closer than the prescribed radar minima, that's a loss of separation.

Two aircraft taking off after each other. Initially, the tower controller can visually see both aircraft and ensure separation. When the controller can see both aircraft, normal minima do not apply - it is up to the controller to visually make sure there is no risk of collision. Now imagine the first aircraft climbs into a cloud, so the controller can no longer see it. Even if the higher aircraft is climbing faster than the lower one, so physical separation is increasing, if the required vertical separation does not exist when the controller loses visual contact with the first aircraft, that's a loss of separation.

Two aircraft flying on opposite, parallel tracks. The required radar separation is 3 NM for aircraft flying less than 250 knots, and 5 NM for aircraft flying faster than that. Both flying at 250 knots, they pass each other with exactly 3 NM in between, and are now flying away from each other. One aircraft now accelerates to 251 knots, while there is still less than 5 NM between them. Although they are rapidly moving away from each other, that would be a loss of separation.

These are all real examples that, although they shouldn't happen, could result in a loss of separation between two aircraft that were initially separated, while distance between them actually increases. I think the key point here is the fact that in each example, the rules regarding separation minima change, which is what causes the loss. A rule that allowed the aircraft to be relatively close together, for one reason or another, no longer applies, and so we have to fall back to another rule, which may require larger minima.
While this may not specifically answer your question regarding RVSM, I think the exact same logic can be applied. In your example, initially, one rule applies: RVSM, which allows 1000 ft vertical separation. As aircraft B leaves RVSM airspace, the RVSM rule no longer applies, so we have to fall back to the general rules for vertical separation, which says we need 2000 ft. Thus, your example would indeed be a loss of separation.


Why are magnetic headings used for designating runways? Runways are numbered based on compass heading. Why?

Does this help pilots in locating the runway, even in today's GPS age? Or are there some other technical reasons behind this?

There are two parts to the question:

Why number runways based on their direction?

The runways have to be identified somehow and the identification has to be directional. So the, rounded, heading offers itself and it has advantage that:

The pilot can verify they are approaching the intended runway by looking at the compass, which

works at night, in bad visibility or early on the approach when the numbers on the ground are not visible yet and
serves as cross-check for verifying the instruments are set correctly during instrument approaches.

Since the pilot needs to know the runway heading for navigation and for checking the wind anyway, it makes it easier to remember.

Why use magnetic heading instead of true?

In the early days of aviation, magnetic compass was the only tool available for determining direction. It is simple and reliable and well tested in naval navigation. So magnetic headings became the obvious standard.

Later gyrocompass was introduced to overcome the limitation of magnetic compass that it is biased in turns and requires some time to stabilize. However gyrocompass precesses, so it has to be periodically corrected, which was, and is, still done by reference to the magnetic compass.

On the other hand GPS is still a rather recent technology. It is only precise enough to determine heading (or rather tracking) for last 15 years. It requires a complex electronic device that can fail in a myriad of ways and it is affected by many external factors including the state of ionosphere and the selective availability not being turned on again.

And not all aircraft have GPS anyway. So if you wanted to switch the ones that use GPS to true headings, there would still be some that would need to use the magnetic and it would be a mess. On the other hand when you have GPS, you have a computer already and it can contain and use the magnetic declination tables to give you magnetic heading as well.


What radio frequencies are used for intercom in formation flying? What radio frequencies are used for air-to-air intercom in formation flying? I assume there is a difference between military and civil aviation (e.g a group of GA aircrafts flying as a formation).

I found a couple of civil UNICOM frequencies here (e.g. 122.8 MHz, 122.975 MHz, ...). 122.750 MHz (Aviation Air to Air Communications) seems to be what I am looking for (correct?). But what if there is more than one team?

I also wonder if in military or professional formations (aerobatic formations) something outside the Very High Frequency (VHF) Aviation Band is used (to provide better quality, e.g. digital communication).

In this article I can see lot of military frequencies, but not telling me what to use for military formation flying.

For the US, according to section 4-1-11 of the AIM (Designated UNICOM/MULTICOM Frequencies) for private fixed-wing flights you should use 122.75:

  Air-to-air communication (private fixed wing aircraft): 122.750

  Air-to-air communications (general aviation helicopters): 123.025

  Aviation instruction, Glider, Hot Air Balloon (not to be used for
  advisory service): 123.300, 123.500

I have no idea what the military does and their tactical comms might even be encrypted anyway, but they frequently use UHF instead of VHF, at least in the US.

Nor do I know what to do if you have two formation flights at the same time, but I guess that in reality it's either an air show with some form of semi-official control and perhaps even NOTAMs, or it's something private and in that case people make their own arrangements, e.g. picking some frequency that doesn't overlap with any nearby assigned frequencies. Using 123.45 is apparently common, but definitely not official.

Non-VHF communications for private flights would be regulated by the FCC, not the FAA, and personally I'm not sure if the benefits would be worth the extra equipment and the risk of confusion, but then again I know nothing about formation flying.

There's some useful information from the FAA in this policy order clarifying the use of 121.5 and 123.45. It says that 123.45 is for operational use in oceanic regions, and within the US it's reserved for "non-government flight test operations". There's also an FAA document on requesting temporary frequencies for airshows etc. but it's air to ground only.


What is the time before an alarm is given after a fire sensor perceives a fire? I've got a very specific question:

When a fire sensor perceives a fire, does it wait 20 or 60 seconds before giving the alarm?

The fire sensor's time to give the alarm is customizable depending upon the fire safety alarm being used. Two widely used types of continuous-loop systems are the thermistor type detectors, such as the Kidde and the Fenwal systems, and the pneumatic pressure detector, such as the Lingberg system. (Lindberg system is also known as Systron-Donner and, more recently, Meggitt Safety Systems.). A typical delay of 10 Seconds is quite common. Here is the Source for your further reading


When are ATC frequencies preset in the radios? I've noticed that sometimes pilots tend to present the next ATC frequency in the standby radio, and when required to switch frequencies, they can just push the frequency transfer switch to switch the preset frequency to the active radio.
When do pilots preset frequencies? I.e. tower or ground frequencies are known through the briefing material (i.e. Jeppesen charts), so they could be preselected to accelerate operations. Are there any rules as to when preselect the next frequency, is it also done during cruise flight or only in the terminal area?

I'm not aware of any rules, hopefully others can contribute, but you can usually predict the next frequency with a high rate of precision.

For example (exact stations will differ depending on airport and airspace type) a typical IFR sequence of stations.

Clearance delivery
Ground
Tower
Departures
Sector/area
...
Sector/area
Approach
Tower
Ground

The initial tuning will put delivery in live and ground in standby.  Then as you say, just press the toggle.  When the next frequency gets dialled in is not, as far as I know, governed in anyway and is probably not even in SOPs since it is just about using the equipment more efficiently.

Say delivery has given you a clearance.  They will probably follow up with something like "call ground 129.7" for push and start".  When you are ready for push and start, toggle to ground then put the tower into the standby frequency and so on.

Frequency changes are certainly made in cruise, when passing from one ACC sector to the next. These frequencies are published in the national AIP's and visible on en-route charts.  For a sector handoff, I'd probably dial the next frequency in when the current sector instructs me to contact xyz.

It's really not a big deal if you have to contact someone unexpected.  Modern boxes with push button tuning are almost as quick to tune manually as to toggle.


Which is more accurate for height measurement: GPS or barometer? I am not a pilot and have come to this site to ask a question about the measurement of height.  This is as a result of the comments made as a result of a question in the Physics Stack Exchange forum "Does barometer improve gps accuracy?"

I feel that there will be more of you on this site who have real time experience with flying and so can answer my question which is:

Which is more accurate for height measurement: GPS or barometer?

I have noted that those engaged in geocaching often have GPS units with built-in barometers to improve height accuracy.

One of my friends who is a pilot responded as follows:

  A light aircraft relies on a barometer that is set to the standard
  1013 millibars when in cruise at height (or the regional QNH when
  lower) but they set to the local reading for the airfield (QFE) when
  planning their decent. Commercial aircraft also have ground proximity
  radar but they also have a standard set of old fashioned flight
  instruments in case the electronics fail. My old aircraft GPS did
  indeed read out height as well but you were told never to rely on it!

For surveying glacier movement, or a mountain range's growth/decay, GPS is more accurate.

For airplanes: refresh rate, reliability, and simplicity matter more. When it comes down to accuracy, ground augmented GPS provides ILS CAT I accuracy using LPV. So alongside refresh rate, and accuracy, comes the question of application. For airplanes in cruise, where separation matters, reliable devices are best.

GPS signals can be jammed. As can be seen in the news lately in California and Egypt. And GPS signal can be lost due to ionospheric interference. So, any equipment that doesn't rely on external sources is safer.


What kind of information is shown on airport surface traffic surveillance displays? What kind of information is displayed by the ground surveillance radar scope on Advanced Surface Movement Guidance and Control Systems (A-SMGCS) or similar systems like ASDE-X?

Advanced Surface Movement Guidance and Control System (A-SMGCS) provides controllers (eventually pilots and  vehicle  drivers)  with  situational
awareness on the movement area and  normally  consists of three type of sensors. Surface movement radar, MLAT and ADS-B.

The kind of information is displayed to the controller is dependent on what are the sensors used and the capability of aircraft and vehicles on the ground.

If the A-SMGCS is equipped with Surface Movement Radar (SMR) it can display the moving vehicles and aircraft on the maneuvering area. (except in areas where tracking is inhibited ) to the controller. But manual correlation is required to identify the target.
If ASMGCS is ADS-B capable, controllers display will show Aircraft or vehicle  Identification (ACID), Essential Flight Plan details like destination etc and type of aircraft also in addition to the exact location of the aircrat or vehicle. For this to work, Aircraft/vehicle should be ADS-B capable.
using  MLAT (Multilateration ) ASMGCS system displays postion and ACID and other FPL details to the controller if the aircraft SSR transponder is in operating mode. System correlate the transponder code with the flightplan.

Depending upon the level of ASMGCS operation , it can also display the taxiway route, SID, Runway assigned etc to the controller.

If the target is obtained from ADS-B or MLAT then it is called a cooperative sensor target. It will be displayed in a different symbol.


Does the B787 (Dreamliner) operate with higher cabin pressure? The Boeing 787 Dreamliner's fuselage is almost completely made of composite carbon fibre material, which is not susceptible to metal fatigue.

The main reason why the cabin pressure in a pressurized aircraft is kept as low as possible is to reduce the expansion and shrinking of the fuselage due to changes in pressure differences, reducing metal fatigue in the long run.

Does the 787 use higher cabin pressures than other commercial aircraft? Boeing touted this as one of the revolutionary new features of the aircraft back in 2006, but does it actually use higher cabin pressure now that the plane is in use by airlines?

According to this document by Boeing:

Altitude: How High Is Just Right?
Today's airplanes are pressurized to a typical cabin altitude of 6,500
to 7,000 feet (1,981 – 2,133 m), with a maximum certification altitude
of 8,000 feet (2,438 m). Because the advanced composite materials that
make up the 787’s fuselage do not fatigue, the 787 can be pressurized
more, which allows for lower cabin altitude levels.
Studies at Oklahoma State University explored the effect of altitude
on passengers to determine optimum levels. After testing at various
altitudes, it became clear that lowering the cabin altitude to 6,000
feet (1,830 m) provided meaningful improvements. Lowering the cabin
altitude further, however, provided almost no additional benefit.
Based on that knowledge, Boeing designed the 787 to be pressurized to
a maximum cabin altitude of 6,000 feet.

So if we assume a constant altitude of 43,000 (the 787's service ceiling1), we would get a maximum differential of:

6,000 ft Cabin Altitude = 9.06 psi
8,000 ft Cabin Altitude = 8.11 psi

This would indeed be a higher cabin pressure than what Boeing considers "typical" at maximum altitudes.

1 It is probably actually an even bigger difference since the ceiling of a lot of airplanes isn't this high so the max differential would be even lower.


My kid wants to be a pilot My 7 yo is really keen on taking up aviation as a career.
I have seen multiple times where kids at the age of 15-16 get licenses to fly. How does that happen?
Looking for some advise on how to start this journey for my little man?

The following information is based on US FAA regulations and training programs.
In the US, there is no age limit (minimum or maximum) you must be in order to fly at the controls of an aircraft. There are age limits specified in Title 14 of the Code of Federal Regulations Part 61. Here are some of the particulars:

14 or older to solo a ballon or glider.
16 or older to get a balloon or glider private pilot certificate.
16 or older to solo a powered aircraft.
17 or older to get a powered aircraft private pilot certificate.
18 or older to get a commercial pilot certificate.

In the meantime, you can get your child into one of several youth aviation programs like Civil Air Patrol, or the Young Eagles.


How do I find out what happened on this flight? On 2017-01-03 DAL58 from Boston to London apparently returned to New York after 90 minutes of flight.

This seems quite irregular to me, and I had expected to find details about this published somewhere. I tried Google, and I searched at avherald.com for the date and also for B763, but didn't find anything.

Was this not sufficiently irregular to warrant some kind of public information about what happened on this flight? Or if such information is published, where could I find it? Or is such information only available to certain people?

Issues in flight are more common than we think: engine shutting down in flight, fumes in cabin, ... Aircraft are huge pieces of engineering with many parts and sensors that can fail. Aircraft have many redundancies which most of the time allows for a safe landing.

I tried to look on the avherald, a reliable and extensive source of aircraft reports, incidents and accidents, mainly fed by its owner and its users but the flight does not appear there. However you can appreciate the number of incidents occurring every day and this is a small part of them.


How do propeller-driven, fixed-wing UAVs counter negative torque from engine rotation? What cancels the rolling moment produced from the rotation of the engine?

In exactly the same way as any conventional, single-prop aircraft does

Which is to say, by using stabilizer and aileron/rudder trim, and the natural resistance that wings/stabilizers give.

The wings/stabilizers will naturally resist rolling forces to some extent, particularly at flight speed. At lower speeds (eg when taking off or landing) the control surfaces can counter the force quite easily - a little opposite rudder and aileron is all that's required.


Why do Vx and Vy coalesce at an airplane's absolute ceiling? I'm struggling to figure out why that is the case. I'm trying to determine why the best angle of climb speed equals to best rate of climb speed at the absolute ceiling on airplanes.

I think the answer to your question lies in the fact that approaching the absolute ceiling, the terms "climb rate" and "angle of climb", in regards to an airspeed, begin to lose their meanings.

As a plane approaches the absolute ceiling, the climb rate will be zero or close to zero, so the best angle of climb approaches 0 degrees above the horizontal plane as well making your "best-rate" speed also be your "best angle" speed; both the rate of climb and thus the angle of climb being, effectively, zero.

Any speed which yields less than the best rate of climb (if you want to call it that) of zero at the absolute ceiling would be causing a descent, and thus a negative angle of climb as well making any speed other than the best rate of climb speed also not be the best angle of climb speed, which is why that is the altitude at which best-rate and best-angle speeds coalesce.


What is the business purpose of scheduling two separate flights on the very same route with 5-10 minutes separation? As in question. I have noticed, for example, that Berlin-Tegel has TXL-CDG route served by both AirBerlin and AirFrance with exactly five minutes separation. First plane takes off at 09:50 and second one at 09:55. I have also noticed that the same rule goes for longer routes, i.e. LHR-SYD goes with around 20-40 minutes separation, served by both Qantas and British Airways.

Can someone explain me a business or economic reason for doing so?

Why these two planes are not scheduled with a bigger separation? I.e. 2-3 hours or even half of day in case of long routes? Wouldn't that assure bigger chance or getting more customers? If route interest is big enough that two planes, near each other, needs to be scheduled, then why simply one airline don't use a larger airplane and the other one schedules its own flight 2-3 hours later or so to capture "other part of day" travellers?

If you look at just the schedules it says nothing. If you look at the prices you can see that Air Berlin was pricing the flight significantly lower than Air France. As was stated above, it was competition.

Unfortunately, things did not end well for Air Berlin. The airline declared bankruptcy in August 2017, and was unable to obtain further funds from its primary shareholder, Etihad. The company closed its doors in October of the same year.


What are airline "flag" and "supplemental" operations? I constantly see regulations which refer to "flag" and "supplemental" operations.  For example:

  14 CFR 121 — OPERATING REQUIREMENTS: DOMESTIC, FLAG, AND SUPPLEMENTAL OPERATIONS

  Subpart Q—FLIGHT TIME LIMITATIONS AND REST REQUIREMENTS: DOMESTIC OPERATIONS

  Subpart R—FLIGHT TIME LIMITATIONS: FLAG OPERATIONS

  Subpart S—FLIGHT TIME LIMITATIONS: SUPPLEMENTAL OPERATIONS

What exactly are they and how are they different from "normal" Part 121 operations?  I can't seem to find them defined in Part 121 or 1.1 (definitions).

Part 121 certificated carriers are given specific permission within their Operations Specifications (OpSpecs) to conduct one or more of those three types of operations -- Domestic, Flag, and Supplemental.

The OP asked: how are these different than "normal" Part 121 operations. The answer is: they're not. A "normal" 121 operation must be operated under one of those three types, with slightly different rule requirements for each.

Each is defined in Part 110.2. (the previous reference to 119.3 is out of date) The gist of it is as Shawn said: US-to-US location is a domestic operation, US-to-nonUS location is a flag operation, and all-cargo or "large aircraft charters" are a supplemental operation. These are not exact, and there are exceptions and legalese within the 110.2 page-long definitions for each one. Please read the FAR for the complete definition.

Each individual flight needs to be released and operated according to the rules of the operation type that is applicable to that specific flight.

Airline themselves are not "domestic" or "flag" or "supplemental" (per definition), but it is common to call an airline a "domestic airline" who only has OpsSpecs approval to conduct domestic operations. But that is not a legal term.  Most (I suspect all?) of the majors are OpsSpec'd to conduct all three types of operations.

Let's take an example of Alaska Airlines. An aircraft is scheduled for (important) and operated on a flight from Seattle to LAX. This flight meets the definition of a domestic operation, so all of the "domestic" rules would apply to this flight. That same aircraft is then operated on a flight to Mexico City. This would be a "flag" operation, and so some slightly different rules would apply to how this flight is dispatched, released, and operated. (alternate airport requirements, fuel requirements, and how much crew rest was required prior to the flight, etc. are slightly different)

Finally, the Washington State Huskys football team charters an Alaska Airlines aircraft to transport them to the Rose Bowl. The flight from Seattle to Los Angeles would be operated under "supplemental" rules. The previous paragraph's example domestic flight might have used the same exact routing, the same exact crew, and the same exact aircraft. But the rules for this specific flight were slightly different, due to the different type of operation being conducted.

The air carrier, "Alaska Airlines", is not a "Domestic Airline", nor a "Flag Airline", nor a "Supplemental Airline"... rather, the company's certificate grants them permission to conduct all three types of operation.


Can I simply use multiple turbojet engines to fly supersonic? the jet engine FJ33 produced by Williams International produces around 6 kN thrust (maximum, at sea level).
Imagine: to fly supersonic, I need for my aircraft 9 kN of continious thrust.
I am wondering now: is it possible to use just two FJ33 jet engines, so that I would just double the maximum thrust ?
What are the restrictions to this?
My thoughts:
If I fly supersonic with a jet engine which is designed actually for subsonic flight, there can be many aerodynamic effects like stall and strong rise in drag at the gondola, for which the engine is not designed: So I would need a diffusor to bring the supersonic flow down to an optimal flow velocity for the FJ33.
Is this correct? Do you know some other restrictions?

is it possible to use just two FJ33 jet engines, so that I would just double the maximum thrust ?

Yes, you will double the maximum thrust on the ground and at rest. You will also double thrust at altitude and flight speed, but that thrust will be much lower than your sea level static thrust. Below is a thrust diagram over speed and for several altitudes for a small turbofan engine with a bypass ratio of 3. It is not the Williams, for which I don't have this kind of diagram, but very similar.

The kink in the low level thrust line is from internal engine pressure limits. At low altitude and high speed the pressure inside the compressor and burners must be limited so they will not be overstressed.
Below 10,000 ft it is not legal to fly faster than 250 kts and supersonic flight is illegal over a lot of areas. In order to achieve legal supersonic flight you need to be high and over unpopulated land. The higher the better, but as you can see from the diagram, thrust drops with altitude. For turbofans it also drops with airspeed. Now it would be interesting to know at what altitude and speed those 9 kN of thrust are required - maybe that number is for a lower altitude and the two FJ33 are indeed sufficient. At the moment that question is unanswerable with the information given.

So I would need a diffusor to bring the supersonic flow down to an optimal flow velocity for the FJ33.

Yes, but that is simpler than it seems. Every engine intake will slow down and pre-compress the incoming air stream at high subsonic flight speed simply because the engine will not be able to swallow all the air that could flow through the intake at high speed. Instead, some air will spill over the intake lips so only the core of the streamtube that flows toward the intake will be ingested. At supersonic speed the intake will do this precompression by means of a shock, the strength of which is again influenced by the swallowing capacity of the engine. Still, the intake cross section needs to widen between the intake face and the engine face and must be carefully shaped to avoid flow separation or speed variations at the engine face (that is actually the tricky part) or you will not be able to achieve the full performance of the engine. A simple pitot intake should be sufficient but it will need some length to do its work properly. Therefore, it is advisable to not put the engine in gondolas (B-58 style, also called nacelles) but to integrate them into the fuselage.
Thrust is the difference between intake an exit impulse, so you need high exit speed in order to have useable thrust at supersonic speed. While a simple convergent nozzle can be used to achieve supersonic flight, the best results are obtained with a convergent-divergent nozzle. Due to the high temperature of the exit flow, even a subsonic exhaust stream will bring you to supersonic speed, but only just. Given the relatively high bypass ratio of the FJ33, if supersonic flight is achieved, the maximum Mach number in level flight will not be much above 1 (if the drag characteristics of the airframe allow even this).


What does a pilot mean when he tells ATC that he "has the numbers"? What does "have the numbers" mean when talking to ATC?

This is from the FAA's Pilot/Controller Glossary:

  HAVE NUMBERS − Used by pilots to inform ATC that they have received runway, wind, and altimeter information only.

The AIM has some more information in 4-1-13 and 4-1-18, which comes down to "having 'the numbers' is not the same as having the ATIS". The equivalent controller information is in the ATC Orders 3-9-1, 3-10-1 and 7-1-3. For example:

  3−10−1. LANDING INFORMATION

  Provide current landing information, as appropriate, to arriving
  aircraft. Landing information contained in the ATIS broadcast may be
  omitted if the pilot states the appropriate ATIS code. Runway, wind,
  and altimeter may be omitted if a pilot uses the phrase “have
  numbers.” Issue landing information by including the following:

  NOTE−
  Pilot use of “have numbers” does not indicate receipt of the ATIS broadcast
  [...]

In most cases, pilots will just tell ATC that they have the ATIS anyway, as Ron described in his answer. But, there are some towered airports that have no ATIS, only AWOS, e.g. Columbus, Indiana (KBAK). In that case, telling the tower controller that you have the numbers is the only thing you can actually do (other than not tell them anything and let them give you the information, of course).


What exactly is a hot air pressure regulating valve, and what is its purpose? Someone else has asked this question but the answer just says that it regulates the pressure of hot air, and I don't understand what that means. Does "hot air pressure" just mean "hot air"?

So I've been learning about an Airbus A320 and its air conditioning system and I'm wondering what's the point of the hot air pressure regulating valve? Why can't bleed air, which is hot compressed air, be used as hot air? Send some of the hot compressed air to the packs and some hot compressed air to the trim air valves to heat the air from the mixing unit.

Please explain what is the purpose of the hot air pressure regulating valve and even what it is. What's even the purpose if you have bleed air? Please don't tell me that the hot air pressure regulating valve regulates the pressure of hot air, I already know that. I just want to know why it is generating hot air pressure, why not just hot air? Or is hot air pressure the same thing as hot air?

The hot air pressure regulating valve does not generate hot air. The hot air is generated from the engine bleed air.

But that air does not go straight from the engines into the cabin, it would be too hot. If you want to adjust the temperature, you need a way of limiting the amount of hot air going into the cabin. That is what the regulating valve does (in conjunction with a couple of other components).


Why did water come into USAirways Flight 1549 if an airplane is sealed and pressurized? Airplanes are supposed to be air-tight and pressurized.
However, when USAirways Flight 1549 landed on the Hudson river; water started to come into the airplane.

How is this possible if the airplane is sealed? Was there a crack in the fuselage?

Two reasons:

Airplanes are not completely, perfectly hermetically sealed. They are pressurized using positive air pressure from the engine bleed air and packs. If there is a failure in the system, the aircraft internal pressure will return to the outside ambient pressure. Air will escape the aircraft or enter the aircraft if the outside ambient pressure is greater. Water will react the same way. Especially when you have the combined greater pressure of the water and the atmosphere above it.

Combine this with the structural damage caused by the dynamic pressure of the aircraft impacting the water, the aircraft will have ways of water entering the aircraft through burst seams. The structural and hydrodynamic load alone, created by the rapid deceleration of the aircraft will be enough to tear apart the airframe, open sealed doors and bulkheads, and blow out windows.


Why should autofeather only be used for take off on the Dash-8 Q400? The Bombardier Dash-8 Q400 FCOM says (emphasis mine):

  The autofeather system provides automatically initiated propeller feathering following an engine failure during take-off, and power uptrim of the operating engine (Figure 12.22-5). Autofeather is selected on for take off only, using the AUTOFEATHER switchlight on the engine instrument panel.

(source: smartcockpit.com)

It only says autofeather should be used for take off only, but fails to explain why. I would assume autofeather would also be useful when landing. The uptrim (automatic power increase for remaining engine) might not, but it is independent anyway:

  Uptrim is triggered (regardless of autofeather selection) [...]

Why should autofeather not be selected during landing?

It's because the critical case that autofeather caters to is engine failure right after V1, on takeoff, where the normal identify/feather drill uses up precious time.  An engine failure on a missed approach or go-around starts well above the low energy state of an engine failure after V1 and there is much more time and energy margin to do the normal feathering procedure.

The system is quite sensitive and there are risks associated with having the autofeather system armed and "ready to fire" you might say.  Mainly, having it put a prop in feather when you don't want it to.  Or even BOTH props in feather.  That risk outweighs the benefit of having it armed outside of the take-off case.

It's a bit like having a policy of taking a firearm off SAFE only when there is a critical need to able to fire instantly when clearing a house.


Is it normal for instantaneous GLOC to occur at 5 G's? Is it normal for a trainee to have an instantaneous G-force induced loss of consciousness (GLOC) at 4-5 G's in a light military trainer aircraft like Socata TB 30 Epsilon, after doing a manoeuvre sequence?

That sounds really unlikely to be a G-LOC.  That usually happens at much higher loadings after 3-4 seconds of straining.  The counscious and cognitive centers of the brain take a few seconds to shut down once the systolic and diastolic pressures hit zero in the brain.  It may be an indication of another underlying medical issue.


Could the V-22 Osprey be used as a close air support platform? I saw the US was looking for a new close air support plane. Why do they not use the V-22 Osprey? It is able to travel slowly and could carry more weapons than an A-10, for example.

Can it be? Sure.

Let's take a look at what it would be replacing.

The A-10 was designed from the ground up to be a platform for moving the GAU-8 into range of the target.

It has good low speed flight characteristics.
The engines are mounted high and to the rear to protect them from FOD (both debris kicked up by the GAU-8 and hostile weapons fire).
The cockpit is within a 540Kg titanium armored tub to protect the pilot and critical flight systems.
It has wing hard-points for carrying additional munitions should the GAU-8 not be sufficient for the current mission.

Why won't the Osprey be adapted for CAS?

Because it travels slowly, it's susceptible not only to MANPADS and other traditional AA systems, but simple small arms fire becomes a threat.

For example, the Douglas A-1 Skyraider (which the A-10 replaced) - a "propeller-driven design was also relatively slow and vulnerable to ground fire. The U.S. Air Force and Marine Corps lost 266 A-1s in action in Vietnam, largely from small arms fire." (Source, emphasis mine)

Those huge props (38' diameter, which is smaller than the optimal 43' (Source)) are a very vulnerable target - damage to them may make the aircraft unflyable.
Hits to the engine rotation system that may not cripple the aircraft may cause it to have to land with the props in the vertical plane instead of horizontal. They're designed for this without destroying the rest of the aircraft, but that means thorough inspections and expensive prop replacement prior to its next flight.
It could have an armored cockpit surround retrofitted, but it's not designed for it.
It could have hard-points mounted, but they would need to be integrated with this swing-wing mechanism which would significantly increase the design/build cost and complexity

The swing-wing (used for stowing the plane on a carrier) could be eliminated, but that would defeat one of the main uses of the aircraft.

It's highly likely that the replacement for the A-10 (current thought trends toward the F-35, but that's contentious) will be more similar to the A-10 itself. That kind of role has some unique requirements that can be fulfilled by a more multipurpose aircraft, but not as well.


Do USAF fighter jets go supersonic over land? Does the US Air Force operate supersonic fighter jets over land?

I heard that the FAA banned supersonic flight over America due to the shockwaves creating loud booms and breaking windows on the ground, therefore the Concorde only operated over the Atlantic Ocean.

However, is the US Air Force allowed to operate supersonically over land? How about in training missions, or drills, or a real event? Did they go supersonic during 9/11?

Generally speaking, FAA does not allow supersonic flights over land except in special restricted military flight corridors. These are called High Altitude Supersonic Corridor (HASSC) in the US. For example, the one near Edwards AFB is:

NASA also mentioned about supersonic flights:

  The F-18s flew in Edwards' High Altitude Supersonic Corridor at 32,000
  to 40,000 feet for the supersonic runs.

However, I read that FAA's jurisdiction is only until 60,000 feet, hence above this altitude, there are no speed limits:

  According to the FAA regulations the controlled airspace extends up to
  60,000 feet. Anything flying above may fly at “unlimited speeds.”

During 9/11 attacks, US Air Force had its aircraft fly supersonically over mainland.


Do fan blades use bell shaped lift distribution curves? Seeing the organic curvature of some newer fans in turbofan engines, I'm curious if they use bell shaped lift distribution curves in their design, or something else all together.

The bell shaped lift distribution has been used on the Horten flying wings in order to reduce lift at the outer, rearward parts of the wing for stability and better controllability.

The fan blade of a jet engine has no need for a tailored reduction in lift towards the tips; instead it tries to maximize the thrust from the available area. The wavy shape is the result of a compromise which tries to sweep the outer, supersonic part of the blade aft and at the same time to minimize the torsion resulting from that sweep. Therefore, the blade is first swept forward and then aft.

RR Trent fan (picture source)


What is this hole at the front of the engine pylon on a Boeing 707? Images of the Boeing 707 show a hole at the front of the engine pylon just above the engine.

Can someone please tell me what is the purpose?

Also why do images show the hole absent on the left outer engine pylon but present on engines 2, 3 and 4?

ref: wikimedia commons

This is because those engines have turbocompressors which are used to supply cabin air.

The engine bleed air drives the turbine of the turbocompressor, which in turn would drive a compressor which would pull in air from the atmosphere through the front hole, compress it, and send it down to the Environmental Control System for use as cabin air. The discharge outlet for this bleed air is the long teardrop shaped hole in the side.

The tubocompressor can be seen in the image below.

Norm King at work on the turbo-compressor on Number 3 engine on 17 August 2006. Photo: Karen Glass; image from 707.adastron.com