If the efficiency of a turbofan engine is 35%, where does the rest of the fuel energy go? There have been a couple of questions on this site about the efficiency and propulsive power of turbine engines (here, here, here).

In a high bypass turbofan engine, what are the losses in the different stages? Where does the unutilised energy go, and what exactly is left over to make the aeroplane fly?

There are a couple of sources of loss throughout the process, as indicated in the figure from an old paper format uni book. I've had to translate the labelling, open to suggestions there. The percentages are valid for a high bypass turbofan manufactured in the late 80s.

The total energy input starts with the fuel flow: chemical energy per second.
Combustion converts the chemical energy into a heat flow pretty successfully, with about 1% lost in incomplete combustion. This takes place in the combustion chamber.
The turbine extracts mechanical energy from the heat flow, and uses a portion of it to power the compressor. The thermodynamic efficiency of the Carnot cycle determines the resulting power fraction. I've labelled the resulting net mechanical power as Gas Power which sounds a bit dicey. This Gas Power can be converted into shaft power and/or into jet power, depending on the type of turbine engine.

Note that the thermodynamic efficiency depends on the inflow speed of the air into the combustion chamber: it is decelerated and compressed in the intake, allowing for higher pressure ratios which result in higher efficiencies.
We now need to utilise the Gas Power to raise the kinetic energy of the medium that is used for propulsion. This increase in kinetic energy (from the reference frame of the aeroplane) is labeled Propulsive Power.

Aero engines can be divided into two major groups:

a. Transformation of available Gas Power into shaft power, delivering mechanical energy which can drive a propeller or rotor, which then increases the kinetic energy of a mass flow of surrounding air - a turboshaft.
b. Direct transformation of available Gas Power into kinetic energy by expansion in a nozzle - a turbojet.

The principal difference between the two is that in a. the mass flow  delivering the propulsion is larger than the mass flow through the turbine, while in b. the mass flow through the turbine is equal to the propulsion mass flow. Since thrust T = $\dot{m} \cdot \Delta V$, this means that at a given thrust and entry velocity, the exhaust velocity of a. must be lower than of b. Note that both the turboprop and the turbofan are a mix of a. and b. since a portion of the turbine mass flow is expanded for jet propulsion.

Converting Gas Power into Propulsion Power cannot take place isentropically (loss free): in a turbofan about 5% is lost into heat flow, less in a turboprop.
The propulsive power is the power transferred to the medium used for propulsion (air or combustion gas). The Propelling Power is the power transferred to the aeroplane. There are efficiency losses in this process as well: the medium exits the propulsion assembly at a higher velocity than the airspeed of the aeroplane, and therefore has a certain absolute velocity (relative to earth). The corresponding kinetic energy flow must be considered as a loss. Of course, the outflow velocity must be greater than the airspeed in order to generate thrust.

This power transformation is therefore also associated with an efficiency factor, $\eta$ propulsion. It costs power to drive those fan blades, which have induced drag and profile drag just like a wing does.

When using these definitions, we can see that an aeroplane on the runway, just before take-off, with the brakes applied and the throttle fully open, has:

Maximum propulsive power since the airflow has maximum $\Delta V$. As computed in this answer.
Zero propelling power, since none of the propulsive power has been transferred to the aeroplane yet. All propulsive power is transformed into kinetic energy of the gas flow.


What is a rubber engine? I hear it a lot in engine design , I think that belongs to some phase about we can change the engine parameters according to requirements but I am not sure about it.

A rubber engine is not to be confused with a rubber motor (a rubber band which is twisted in order to store energy which can be released when the rubber band untwists. This type of engine is good for short energy bursts in small model airplanes).

A rubber engine is an engine deck (tables of engine data) which can be scaled according to your needs. In a way, you stretch one parameter of the data like a rubber band, and all other parameters change accordingly. In most cases, you adjust thrust to your needs and the scaled engine deck will produce mass, size and fuel consumption for the most likely actual engine with the given thrust. This is very helpful in the initial stages of engine design when you search for the smallest possible airframe to fulfil given mission requirements.


How many total airframe cycles should a B737-400 be put through before it becomes too costly to maintain? I know maintenance can extend the lifespan of an aircraft but when looking at large aircraft such as the 737-300/400 is there a standard amount to go by?

According to this chart from the FAA the B737-400 variant was designed for 75,000 cycles. You can find more general info in this question as well as this discussion and this one. However there are lots of other factors that may make a 737 too costly to maintain long before it reaches the cycle life time.

Just to give an idea of flight cycles to time comparisons. Lets say a 737 flys an average of a 2 hour flight and 30 minutes to turn the plane around. 75,000 * 2.5 = 187,500. There are 8760 hours in a year so if the plane were to fly continuous 2 hour legs with 30 minutes to turn the plane around it would take 21.4 years to hit the 75,000 cycle life of the fuselage. However we all know that planes need service and most airlines dont have 100% airframe utilization all the time so it will take well over 21.4 years to hit that cycle limit.


What causes the sudden spool-up sound from an F-16 when enabling afterburner? What causes the sudden spool-up sound from an F-16 when enabling afterburner?

I've included a video to make it clear what sound I'm referring to:

at 1:16, I assume the pilot engages afterburner (again), and there is a sudden spool-up sound associated with this.

Am I even correct in assuming this is what's happening? I don't understand what this sound is though, it doesn't seem to appear consistently. Throughout the video, it appears the pilot engages afterburner and turns it off multiple times, but the sound only appears once.

Bonus question: What is the purpose of running the engine like this, and how is the F-16 attached to the ground?

My first guess based on the sound repeating when the thrust is idled near the end of the video is that's the sound of the actuators of the variable geometry nozzle.

Googling that actually auto-completed it to Great F-16 Actuator Sound (video).

Bonus answers to the bonus questions:

It's a ground test of the engine, making sure it's working fine with no leaks.
The F-16 has a tail-hook like carrier-borne planes, if you look closely a cable is attached to that hook, which itself is attached to the ground. The F-16, F-15, and other land-based fighters have it in case of an emergency, they'd land on a runway with a braking wire across it.


What is the purpose of the inlet doors of the Pratt & Whitney JT3D? I have a question about the 707 Engine Inlet Doors, according to Wikipedia the Pratt & Whitney JT3D inlet doors purpose is to provide additional air. Is the thrust higher on more air intake? And why modern engines don't have it anymore?

The intake speed at the compressor is between Mach 0.4 and Mach 0.5 and changes little with airspeed. That means that at low speed the intake has to suck in air from a wide capture area, (even from behind the inlet lip at low speed) while at high speed only the part that flows into the center of the intake is digested, while surrounding air will spill over the intake lip and flow around the nacelle.

The surface that separates the flow going into the engine and that which does not is called the stagnation stream line or surface. This surface is way aft of the inlet lip at low speed, and the pressure near it is the highest (stagnation pressure). The challenge for the inlet designer is to design the lip with a rounded shape so that the flow going into the engine is uniform. It won't be if the lip is as shaped in the sketch. The high pressure on the outside and the low pressure inside the lip open the doors to admit a small percentage of the air to the engine inlet at low speed. As airplane speed increases, the stagnation point (or line) shifts to the lip itself and the pressure equalizes on the doors, closing them.

The fairly sharp intake lip of the JT3D will capture air rather inefficiently at low speeds (the topmost case in the sketch above), which could even lead to flow separation right after the intake lip, reducing mass flow and compressor efficiency. By adding the suction-activated doors, the intake can capture more air and keep the flow attached.

More recent intake designs have thicker intake lips, so the flow stays attached even at very low airspeed.


What is the safe separation for landing a Cessna-172 behind a commercial airline jet? When is it safe to land a Cessna-172 after a commercial airline jet has landed?

It depends on the size of the preceding aircraft. ICAO PANS-ATM-Doc 4444 Air Traffic Management has some values.

5.8.2 · Arriving aircraft
5.8.2.1 Except as provided for in 5.8.l.l a) and b), the following separation minima shall be applied.
5.8.2.l.l The following minima shall be applied to aircraft landing behind a HEAVY or a MEDIUM aircraft:
a) MEDIUM aircraft behind HEAVY aircraft- 2 minutes;
b) LIGHT aircraft behind a HEAVY or MEDIUM aircraft- 3 minutes.

Note that in case of A380, it is more- 4 minutes.
In case of distances, the light aircraft are to be landed 5nm after a medium aircraft, 6nm after a heavy and 8nm after A380.
Note: The categorisation of aircraft is done as below:

4.9.1 Wake turbulence categories of aircraft
4.9.1.1 Wake turbulence separation minima shall be based on a grouping of aircraft types into three categories according to the maximum certificated take-off mass as follows:
a) HEAVY (H)- all aircraft types of 136 000 kg or more;
b) MEDIUM (M)- aircraft types less than 136 000 kg but more than 7 000 kg; and
c) LIGHT (L)- aircraft types of 7 000 kg or less.


Is takeoff permitted from an aerodrome inside a CTR when it's outside the tower's operating hours? I am departing from an airfield which is within the CTR of a nearby airport but outside tower operating hours. The chart doesn't specify this situation.
According to ICAO rules,
Am I allowed to take off or not?
EDIT: this is not an EASA or FAA specific question. Rather international, ICAO rules, but if there are different conventions it's also good to know.

In the United States, if you are VFR at an airport in Class G or E airspace, you may take off without needing clearance or permission from ATC.
Normally Class D airspace reverts to E or G outside of tower operating hours, therefore no communication with tower is required or even possible.


How to calculate Blade angle? If we know the helix angle is 3angle, forward speed is 150mph, and rotational speed of propeller is 1500rpm

How can i calculate the blade angle of propeller ?

In case you want to know at what angle does the blade 'see' the incoming wind, you first choose an station of the prop, say at 70% of the prop radius, measured from the axis. If the prop has a radius of 1,2 meters, that station is at a distance 1,2 x 0,7 = 0,84 meters from the axis. Let's imagine, to start with, that the plane is stationary, and the prop turns at 1500 rpm. That's 25 revs per second. In every revolution, our station moves a distance of 2 x π x 0,84 = 5,27 meters. Thus, the chosen station sees the wind coming parallel to the plane of rotation, at an speed of 5,27 x 25 = 131,75 m/s.
But if the plane is moving forward at 150 mph, that's a frontal airspeed of 67,04 m/s. That additional, frontal wind is 'seen' by the station as coming from 90º to the plane of rotation. If you now draw the triangle of velocities, you have one side measuring 131,75 and the other 67,04. The angle (with the plane of rotation) of the total relative wind obtained by (vectorially) adding the two winds is arc tan 67,04/131,75 = 27º
Hope that helps...


What are the differences between ACARS and CPDLC? What are the differences between Aircraft Communication, Addressing and Reporting System (ACARS), and Controller Pilot Data Link Communication (CPDLC)?

Can anyone please let me know in short, what is the difference between ACARS and CPDLC? So far, I have learned that both use data link communication. A ground station is necessary for ACARS and I am not sure weather ground station in necessary for CPDLC or not. ACARS can works on VHF and HF frequency also on SATCOM. Does CPDLC also works on these or it requires different system to work on? CPDLC gives the messages on a digital display. Does ACARS also provide information on a digital display or it is like fax machine where information come out on a paper from the machine.

I become confuse while figure out the differences between ACARS and CPDLC. Can anyone help me out?

CPDLC
CPDLC is used for communication to/from ATC facilities. As the name suggests, it's a controller—pilot link. An aircraft with CPDLC has one of the following panels/interfaces:
787 example:

The pilot can accept, cancel, or reject an instruction.

Uploaded instructions are automatically fed into the different MCP windows, for example UL (uplink) 250 for the speed. And a transfer (XFR) button to activate it. The RL in the heading window is the turn direction.

For route changes, those are fed into the CDU. There's also a text reply option via the CDU (multi function display on the 787). The message also appears on the aux display (outboard side of the display unit for the PFD):

The pilots can also send requests.

A330 example:
(wikimedia.org) The Datalink Control and Display Unit (DCDU) on an Airbus A330, the pilot interface for CPDLC messages.
Similar system but without integration into the autopilot control panel.
ACARS
ACARS is accessed via the control display unit, the same unit for controlling the FMC. Messages can be printed. It's mainly for company communication (between an airline and its pilots). The company uses the ACARS for Aeronautical Operational Control.

Sending wind aloft data, expected delays, METAR, load sheets, etc.
Receiving position reports, systems status, etc.

Also, messages can be relayed to/from ATC via the company during communication difficulties, for receiving delivery clearances, submitting oceanic position reports, etc.

Summary:

CPDLC is an ATC tool, and ACARS is a company tool but it can be used to relay messages to/from ATC.
CPDLC coverage is not global yet, it depends on the FIR to have it.
CPDLC utilizes VHF Data Link (VDL) and satcom. ACARS uses VHF, VDL, HF, HFDL, and satcom.


How do I keep a C172 straight on the centerline during landing/rollout? During rollout upon touching down, I'd often find myself veering off the runway centreline to the left. Every time I input rudder, I have this feeling that I am getting pulled out of the turn (while that is normal just like turning round a corner at high speeds in a car), and that the aircraft might just flip over. But you see I know my limitations of the car, and that it won't flip unless I yank the steering wheel.

My instructors tell me to just simply keep my feet active, but it's the fear of too much input that will cause me to lose directional control of my aircraft.

Upon touching with my main wheels, to maintain centreline, should I just really apply rudder without fear, even though I feel, and visually believe, that I am going to flip over?

What correct techniques should one apply to keep the aircraft straight during the rollout?

My guess here would be that you are not truly tracking the centerline of the runway on final or on roundout.  There’s an easy way to solve that when flying final:  if the runway centerline appears pure vertical from your perspective in the airplane while stationary in your field of view, you are tracking the centerline.  If it has a slight angle to it or is translating, then you’re not totally on centerline.

The differences can be subtle, but can lead to flying a serpentine track toward the runway on final, during roundout, and during ground roll.  It can also result in a touchdown with a slight drift to it which indicates poor piloting technique.

To counter this, first, remember to get the final approach stabilized ie on centerline, on glidepath, and on airspeed as soon as possible after rolling out from the base to final turn.  Remember the more corrections you need to make to do this closer to the runway threshold, the more difficult the approach becomes and the more likely errors in technique will grow unchecked.  Once on a stable final approach, observe the runway centerline and judge if it is a vertical, straight line.  If so, you’re tracking straight down the centerline.  If not, use shallow, coordinated turns to align the airplane so.  This technique will also work when flying a crabbed final in crosswind conditions.

As you roundout, remember you are in slow flight at an increased AoA, so the propeller will tend to pull the airplane’s nose to the left.  Use additional right rudder to prevent this and track straight.

After touchdown, use quick precise feetwork for rudder inputs - think “happy feet happy feet happy feet” - to track centerline.  Also, if doing a crosswind landing, DON’T RELAX AILERON PRESSURE AFTER TOUCHDOWN or the airplane will tend to veer off centerline rapidly.  Remember, fly the airplane all the way through the landing, including the complete rollout.


Can VOR indication be reliable if one of its antennas breaks? What will be the reliability of the VOR indication inside the cockpit if one of the two outside antenna is broken ?

Can we trust it to make a VOR approach?

What will be the consequences assuming that we are in a general aviation aircraft like cessna 150 / 172 / 182 .

Picture of antenna:

(Image Source: http://www.simmer.fr/)

No, it should not be used.

Twin antennae, as you have shown, are balanced and fed into a balun (BALanced to UNbalanced) to feed the single transmission line to the receiver.   Together, they provide the correct impedance for the aerial system.  With an antenna broken, it is likely that the impedance of the circuit is wrong and therefore, the input to the receiver is, at best, suspect.

[GUESSWORK]

It's impossible to predict the indicated behaviour without knowing precise details of the aerial fit but a reasonable guess would be that the standing wave ratio in the feed will be altered and therefore, a phase error will be introduced.  The magnitude of the error will vary in proportion to the sine of the actual phase angle so my guess would be that the bearing will be accurate when due South or North of the VOR then increasingly inaccurate as you move round with an maximum error at 90 or 270.

[EDIT]

Early in the morning when I first wrote this but I think I am wrong about being accurate due North or due South.  It will be accurate somewhere on the compass and at 180 degrees opposite to that, with increasing inaccuracy as you move towards 90 degrees away from that somewhere.  Only calibration would tell you where the somewhere is.


Why do military jets sometimes have elevators in a depressed position when parked? Why do they have elevators in such a position as if the joystick is pushed forward, even though there's nobody inside the cockpit?

It generally means that the hydraulic actuator (power control unit) driving the surface has an "idle" facility that allows fluid to move internally between the two sides of the actuator piston, or just circulate in the pressure/return lines, and when unpressurized it acts more or less like a hydraulic damper even though the input spool valve is at its "null" (no command) position.

The result is that when unpressurized it's free to move (although damped) and if gravity wants to pull the surface in one direction it'll do so.  As soon as hydraulic pressure is applied, it'll snap back to whatever position is being called for by the input system.

Sometimes you have PCUs that are "non-idling" and when unpressurized they are hydraulically locked which is only broken when there is a control input to the controlling spool valve.  Such surfaces stay where they are even when the hydraulic pressure has bled off, unless you move the controls to break the hydraulic lock.

On the CRJs, the elevator PCUs are idling, and the elevators sag when unpressurized.  The aileron PCUs however are non-idling, and the ailerons don't sag when unpressurized. If you try to move an RJ's aileron when its hydraulics are off, it is totally rigid.


What are the requirements for a foreigner to become an Air Traffic Controller in Sweden? Background
I am an Air Traffic Controller (ATCo) working in Greece and I'm looking for information about working in Sweden as a foreign ATCO. I speak Swedish and my wife is of Swedish origin (not a citizen though).
My question is...
Is it possible for foreigners who know the Swedish language to get a job in ATC in Sweden? Where can I find vacancies for ATCO positions in Sweden?

Sweden currently has a surplus of controllers, so you are unlikely to find open positions. However, since you are a qualified controller and speak Swedish, you should be qualified to apply for any position that might come up. I suggest you have a look at LFV, which is the largest Swedish ANSP: www.lfv.se/en


Could a pick up truck save a plane with failed landing gear? I recently saw this fictionalized video.

While it's not real, I do wonder if this could be possible, even if not the best way to handle the situation.

For the Boeing 727 that you see there, that whole procedure is impossible with a pickup truck. Given that the 727 is operating at its Operating Empty Weight, 102,900 lb OEW for the 727-200 advanced, and the force on the nose gear is somewhere around 10% at rest, that would put it way over the limit for that particular truck model (Nissan Frontier at 4,690 to 5,816 lbs GVWR). That would be even less pretty for the truck since the force at touchdown on the nosegear should be higher.

Also, according to this thread, the maximum speed for that truck is about 95 mph. Assuming that this Yahoo Answers is correct, the landing speed of a 727 is 130 to 150 knots (150 to 170 mph). This is way greater than the speed of any pickup truck.

I had to make a response like this some time ago as well to the exact same video someone had a question about.

However, you could land a GA plane on a pickup truck if you had the right equipment.

Nitpick on the video: if the plane's nose gear didn't deploy all the way, then truck or no truck, it should have collapsed on impact.


What exactly rotates along the lateral axis? Is 'rotate' mathematically correct? Researching the etymology of 'rotate' (that OED lacks!), I chanced on Anas Maaz's Quora answer:

  What many does not know is 'rotate' is just not a fancy word, but it involves a real physical rotation. When Rotation speed (Vr) is reached, the pilot pulls the control column which deflects the horizontal stabilizer, causing a rotation along the lateral axis of the airplane lifting up the nose wheel.

Am I correct that Maaz's referring to the 'lateral axis' in the diagram beneath?

Even if yes, what exactly ROTATES? How does the lateral axis ROTATE?

Imagine the wing on the y-axis and time on the x-axis, like the graph beneath.
I don't think it's correct mathematically to state that an axis rotates, when it's the function that moves up as time passes?

Yes that is the lateral axis of the aircraft.
The aeroplane rotates about the lateral axis, causing the nose to go up/down. The aeroplane then descends or climbs, causing the aeroplane axes to shift relative to earth axes. But the aeroplane axes are all defined relative to the aeroplane.


What are the heating tubes used during SR-71 airframe manufacturing? I was watching a YouTube video "Titanium - The Metal That Made The SR-71 Possible" when I stumbled upon this frame (at 12:26).
I could see these heating tubes sort of things under the airframe. What exactly are these and what benefit do they add during the manufacturing process to the airframe?

It's basically an open air test oven.
The SR71's skin temperatures averaged around 5-600 deg F, up to 1000 deg F in places.  The titanium structure had to accommodate a lot of thermal expansion from all this heat, and used special skin joints and corrugation of the larger-area skin panels to accommodate the expansion.
The heating system in the clip would have been a text fixture with an array of radiant heating elements to subject a skin assembly to operating temperature to study the effects and validate the design (go to Page 29 of the linked PDF).


Can old passenger airplanes be refitted with latest engines to extend their life and efficiency? I was wondering if this might make sense as commercial aircraft design has not changed significantly (from the outside) in the last 50 years?

You are absolutely right - refitting new engines to older airplanes makes a lot of economical sense. After all, two thirds of fuel economy gains in the last 50 years were due to better engines.

However, some conditions conspire to make this harder than it might look:

The better efficiency was gained by increasing the bypass ratio of engines, which means that the new ones had more diameter for the same thrust. This makes them hard to fit to airframes designed for the smaller engines, and adding a longer landing gear would be too expensive - then it is indeed better to buy a new airframe.
Designs with tail-mounted engines would have center-of-gravity problems with newer, heavier engines. Tail-mounting engines was very popular after the Sud Aviation Caravelle pioneered this, but rules out those designs (BAC One-Eleven, Vickers VC-10, Boeing 727, Illyushin 62, DC-9, Tupolev 134 and 154) for refitting with more modern engines.

Add to this that airframes have a limited life time, and fitting new engines makes sense for only a limited number of aircraft.


Can the signature of stealth fighters be uncovered, and then used for future detections? A recent theaviationist.com article speculates that the reason behind the 'Combat Sent' and other SIGINT aircraft escorting the F-35's over Estonia two days ago was to deter Russia from using its radars—as their locations would be exposed.

Thinking about what harm could the radars have done, I have the following hypothetical questions not related to a particular aircraft / current world events.

Can an air defense system uncover the radar signature of a stealth fighter and use the findings to easily spot that stealth fighter in the future?
Can this information be passed on to the fighters so they can easily engage in beyond-visual-range (BVR) combat with that stealth fighter?

I would like to make some general comments, as having worked on programs stood up to build sensors for various military threats. In these programs we were always attempting to get data on relevant threats. The more challenging the signature the more it drove the sensor technology, hinting at areas of improvement. In the sensor research we often met with the target community. At times we used their services to look at sensor sensitivities. Perhaps not surprisingly, even though they were on our side, they were at times reluctant to talk about certain sensitive areas.

What I learned from this experience is that, if you are the target you hide your signature as much as possible, and if you are the sensor you hide its characteristics as much as possible. Whenever you give away information you risk exposing your vulnerabilities, and each has its vulnerabilities. An example might be controlling creation of photographic images of sensor optics on an air-to-air missile. This is why some "black" programs hold their technology in war reserve (think hidden in hangers), and are only utilized when the threat warrants exposing this new hardware to the adversary.

As another example, I can point out missions during my cruises aboard the USS Nimitz flying light attack. During this time period the battle group often passed by Libya. The carrier stayed out of the contested waters while the destroyers breached the line drawn in the sand by Gaddafi. Other missions included flying electronic surveillance platforms with fighter escorts close aboard the coast. The radars would light up the surveillance aircraft and that data was captured for further analysis.

My assessment would be that, whenever you bring out a new technology you try to cloak it as much as possible. The other side is out there collecting data on you. Sensors improve, and targets find better ways of hiding. A good example of the evolution of threat can be found looking through the literature on surface-to-air missiles. As soon as the Russians came out with a new SAM we came out with a new countermeasure, which drove them to better sensor technologies.

The findings that a collection activity might obtain will more than likely not allow them to "easily" spot the target. This sort of technology is on the "bleeding edge" and gains are typically hard won and incremental. As we get better sensors, they get better at masking their targets.

One last observation which might not be obvious, but definitely quite important, is that the target might use information about the sensor to develop tactical procedures to minimize its effectiveness. If I learn that your sensor is using microwave electromagnetic radiation I will think about hiding in water vapor or rain storms to minimize its effectiveness. It is truly a cat and mouse game.


What is Mode B on the transponder? Mode A selected on the transponder provides identification of the aircraft to the secondary radar; Mode C will add an automatic height read-out of an aircraft. Mode S is a selective addressing. But previously there was a Mode B which nowadays is not in used anymore. What was it used for, and what information was this mode able to provide?

Mode B

SSR modes B and D are not used today, but mode B has been used in UK, Australia and some other countries for some time, and it was compliant with mode A:

  In reply to interrogation in Mode-A and Mode-B information of
  identity, which is set by the pilot, is sent to the ground
  interrogator.

Source: Radar Theory for Area/Approach Radar Controllers

Modes A and B are used with a squawk code of 12 bits, offering 4096 different identifiers. In both modes the code is selected on the SSR transponder panel, using 4 octal digits (0 to 7).

SSR transponder panel, source

For a transponder handling multiple modes, the correct mode must be selected in parallel of the squawk code.

The interrogator (SSR ground station) can request identifiers using one mode at a time, and receive answers from transponders set to that mode. Aircraft using code 1234 will only reply when interrogated using the mode their are currently set to.

All legacy modes, military and civil and mode S

SSR interrogation modes are identified by the timing between pulse P1 and P3 (P2 is used to reject interrogations from secondary SSR lobes).

SSR modes identification, adapted from source

While mode A is compliant with military mode 3 (both often referred to as 3/A), mode B is purely civil. Its purpose is assumed to be an extension of the mode A, giving 4096 additional codes to ATC.

ICAO Doc 9684 which is the reference for SSR doesn't even mention modes B and D anymore.

Mode S specificities

To be exhaustive, let's mention Mode S visible at the bottom of the previous image.

Mode S (for select) allows to interrogate transponders one at a time when necessary, to prevent FRUIT. In addition of the squawk code, mode S transponders are given by authorities a unique (and quasi-permanent) 24-bit address. This address is used by SSR interrogators to specify which transponder is the recipient of the interrogation.

The address is sent in a datablock which modulation is not a pulse like P1 and P2, but a burst of carrier which phase is reverted when bit is 1 (see a detailed explanation in section Mode S of this answer).

This burst is led by a synchronization of phase reversal (SPR) burst, at the timing of legacy pulse P6, which role is to prepare the receiver to detect possible phase reversals events, hence to decode bits at the right time. In other words this is a clock synchronization. The data themselves can be a series of 56 bit when only the address is required, or 112 bits when the "interrogation" carries an ADS-B message too (with this message, we are entering a data link mode, this is where the ATC future lives).


How would a detachable cabin module work? Ars Technica recently reported that Airbus filed a patent for a detachable cabin module that would speed loading and unloading of passengers (otherwise known as self-loading freight).

This does seem like it might improve turnaround time since passengers are rather slow to self-load and self-unload. That's not my question (which is in fact an earlier question).

My question is that I had always imagined the strength of an airframe was due to its long cylindrical shape. This is both in terms of it as a pressurized tube as well as lateral strength from bending forces. The detachable cabin module would seem to defeat all of these advantages. Am I correct in this understanding? Are there ways that the Airbus patent mitigates them?

You are right, this patent looks like it will not work in reality.

Fuselages have to carry two primary loads:

Pressurisation loads, which cause uniform tension in lengthwise and circumferential direction, and
bending loads from inertial and control surface loads, which create both tension and compression loads in lengthwise direction, plus some shear.

Reducing the height of the fuselage will drive up the strain from the bending loads, so more material is needed to transmit them. Also, the bottom of the passenger compartment and especially the corners at the lower edges will experience much higher loads than a cylindrical fuselage ever would under pressurization. Again, to make this work needs a much beefier structure.

The loads could be kept the same if the passenger compartment is tightly bolted to the remaining aircraft structure such that the combination of "flatbed" fuselage and passenger compartment can be considered as a contiguous structure, but this would need redundancy in the connections and would make fitting the passenger compartment non-trivial. Airbus is arguing in this direction, but I wonder how much time can be saved and how much maintenance time is added if the primary structure of the fuselage is torn apart and re-joined at every stop. Note that you not only need to connect the parts at each end, but to transfer pressurization loads you need to bolt them together along the full length of the passenger cabin as well.

Today's airliners are already designed to survive partial depressurization, such that the lower fuselage is depressurized without the pressure loads in the upper part causing damage. Therefore, the fact that both parts can be split and can depressurize independently of each other does not add additional load cases. However, the structure today is dimensioned to sustain those loads only for a short time. Pressurizing only the passenger compartment or not joining the parts along the full length for every flight will need a much stronger structure than what is used today, so the weight impact of a simpler way of joining both parts will most likely result in an economically unattractive outcome.

I did not go into detailed calculations, but I strongly suspect that this idea will not be adopted by airlines. Besides the structural impact, such a way of loading passengers would complicate logistics, since a fitting passenger container plus facilities for mounting it on the aircraft must be available at the departure and destination airfields.

Lastly, proposals like this are not exactly new, which is why the first claim extends over 73 lines to be narrow enough to not conflict with earlier patents or publications. If there had been some value in this idea, we would had seen it being adopted long ago.

In helicopters, however, which fly at altitudes where no pressurization of the passenger compartment is needed, this idea has been tried out before. See below a picture of the Sikorsky S-64 Skycrane carrying a container (picture source) in which passengers could be carried.


What job opportunities are there for low time pilots in Malaysia? Does a new pilot who has just obtained ATPL license has job opportunities?

When I search at pilot job offered on Internet, they mostly required more than 3,500+ flying hours. What I knew about those who obtained ATPL license have flying hours aroun 1500 hours. It mostly includes aviation academy training used turboprop engine plane and they are very light weight. The jobs ads require those pilot who have flown aircraft more than 40,000 lbs.

Will a new pilot get an opportunity to get such a job? How does new pilot get their job under these kind of conditions?

Malaysia I suspect would be very similar to where i started working a few years back in Africa. In countries (continent) like that you really need to be there if you are a low timer. Hanging around the airports and meet people who could help you with finding your first opportunity forget about sending CV's from miles away hoping for an opportunity, key here is to be pro active and make yourself known locally....


What will the 737 MAX 7, 8, and 9 ICAO IDs be? Since the ICAO identifiers B737, B738, and B739 are already taken, what will the ICAO identifier for the 737 MAX 7, 8, and 9 be? Would it possibly be B73M7 etc?

The new Boeing 737 MAX is based on the Boeing 737 Next Generation family, which it will replace. Also note that 737 MAX is still under development and is scheduled to release in 2017.

Update:

(Source)

The first 7 is omitted and the suffix is now an M for Max, e.g. B37M for the Boeing 737 MAX 7.

Wikipedia has complete list of ICAO codes here.

ICAO Aircraft Type Designators: Search for Boeing and you will find 737 MAX in the list.


What are the pros and cons of single-engine vs. twin-engine? Why would someone prefer to own a single-engine vs. a twin-engine airplane?

What are the pros and cons of each?

In the light aircraft world there are a few trade-offs to consider.  The short version:

Single-Engine Pros:

Simplicity
One engine means fewer controls, a simpler fuel system, simpler vacuum system, simpler electrical system, etc.
Operational cost
Only one engine to feed, so you spend less on - fuel & oil per flight.
Maintenance cost
Only one engine to maintain - One set of spark plugs & cylinders, One engine to overhaul.

Single-Engine Cons:

No redundancy: You have one engine, if it fails you're a glider.
Singles generally tend to have less spacious cabins than twins.

Twin-Engine pros:

Redundancy
If you lose an engine the other will give you a chance to get to a landing site.
Cabin space
If you step up beyond "twin trainers" you can get a very nice cabin in a piston twin.
Equipment
Again, if you step up beyond the "twin trainers" twin-engine planes tend to be "traveling aircraft", so they often come with nice features like deice/anti-ice equipment.

Twin-Engine cons:

Operating costs
Two engines to feed (fuel & oil costs double).
Maintenance costs
Two engines mean twice the cylinders, twice the spark plugs, etc.
Complexity
The redundancy you can get in a twin comes at a price: Synchronizing the propellers, a more complex fuel system (crossfeed), paralleling alternators, vacuum system failover, etc.
Short and Soft field performance can be lacking.
Failure of an engine can be problematic.

In addition to the operational considerations there are some other practical items to consider: twin-engine aircraft require a multi-engine rating (which means more training time). Twins will also generally be more expensive to purchase/rent (and may be more difficult to rent when away from home), and will likely cost more to insure (particularly for low-time pilots).


Why are contrails not visible from the cabin? If you observe a plane flying whilst on the ground, plenty of times you will see contrails:

Why are they not visible from the plane itself when sitting in the most rear part of the cabin even by trying to look towards the rear of the plane fuselage? They seem to be quite close to the engines.

Knowing that they form due to the heat, they should be seen quite close to the engine.

The distance between the engine and the start of the contrail depends on:

the engine type
the temperature of the outside air
the moisture content of the outside air.

Usually the contrails don't form before the tail of the aircraft, but in extreme cold or humid air contrails can be seen forming just behind the wing.

On larger aircraft you have a better change of seeing the contrail from inside the aircraft. On the 747 you can often spot them for the window of the rear passenger door.


Gear reduction on large turbofans Do engines like the GE90 and Trent 1000 use transmissions (planetary gears?) for RPM reduction on the fans, or are they welded directly to the same shaft as the low pressure turbine?

Those with gear reduction are called geared turbofans (GTFs). In general, you can check the Wikipedia article on any engine, see its classification and specifications, and also check the linked citations.

The GE90 and Trent 1000 are not geared turbofans. Wikipedia lists only 5 geared turbofans, with the biggest (a recent engine; first run 2008; entry into service 2016) powering the A320neo family (narrow-body jetliner) and other regional jets.

Because of the immense torque of large turbofans (thrust >200 kN), it is not yet feasible. It took 20 years to develop the ~100 kN PW1000G.$^1$ It took that long to ensure the new components are maintenance free (apart from oil changes). So for the time being GTFs are outliers, and when a turbofan is one, it would be stated.

Prior to the PW1000G, there is the ALF 502/LF 507 from the 80s that still powers the 146/RJ. The ~30 kN GTF was designed/chosen for its relative quietness for operating from noise-sensitive airports.

Note: the fan blades are not welded, rather connected/attached. You can watch a fan removal here (noisy video).

$^1$ http://large.stanford.edu/courses/2014/ph240/suresh1/docs/pwspecs.pdf


Why do my flights always use runway 24L at LAX airport? I have taken several flights (in and out) from LAX (Los Angeles International Airport), both domestic and international and have flown in variety of commercial planes (727, 737, 747, 757, 777, 787, A319, A320, A330, A380, turboprops, etc). at various times of day but have always landed/taken off from runway 24L. According to Wikipedia, LAX airport has the following runways.

Runway     Length       Surface
          ft      m
6L/24R  8,926   2,721   Concrete
6R/24L  10,285  3,135   Concrete
7L/25R  12,091  3,685   Concrete
7R/25L  11,095  3,382   Concrete

I'm sure other runways are also used but why does 24L seem most preferred?

Two words: sample size. Eventually, you will use another one :)

There are only 4 runways, and they are all used heavily.

LAX normally has a westerly traffic flow, with aircraft landing/taking off toward the ocean.

How the airport operates:

  At all times, the inboard runways shall be preferred to the outboard
  runways for departures

and

  well over 90% of all operations use the preferred runways

Quotes from this document. Nice graphics can be found in there, too.


Why were the F-35B lift fan doors changed after flight testing? Why were the F-35's lift fan doors redesigned from those used during flight tests? The original design had two main doors which opened to each side, rather than the “car hood” style door used in production F-35B aircraft.

It was due to flow issues. Here's an excerpt form Air & Space Magazine

  We discovered a few things that would need to change before the
  aircraft entered production. On the X-35B’s STOVL variant, the doors
  above the lift fan had a bi-fold arrangement: They folded and slid
  outward, creating an opening for air to enter the fan. However, when
  the airplane was slowed to a hover, the air flowing across the top of
  the aircraft would not on its own make the turn into the fan
  efficiently; it needed to be guided. On the production F-35B there is
  a different arrangement: Instead of two doors sliding open, a single
  door, hinged aft, lifts up and acts like an air scoop, helping to
  funnel the air into the fan. But the new design of the lift fan cover
  created additional challenges for the two auxiliary air inlet doors
  that sit right behind it. Not all the air is channeled into the fan,
  and at higher speeds, the air that flows around its raised cover is
  turbulent, causing the open doors on the auxiliary air inlets to
  vibrate. The hinges on those doors, therefore, had to be strengthened.

You'll also note that the Auxiliary Lift Fan doors changed (bigger, hinged on outboard edges) as a result of the main door change.


When is VOR navigation mostly used? What pilots use VOR navigation: GA pilots, commercial pilots, military pilots? When is it most used?

Im not sure on military but its still used by GA and Commercial pilots frequently. Although as has been noted in the comments it is slowly being downsized. It should also be noted that Commercial planes and GA planes use the same facilities and the same ATC. Once in the air a C172 may be given the same instructions as a 777 with the only differences being altitude and speed.

On one hand some VOR's are used for approaches to some airports. There is an effort to replace these with GPS approaches but there are still active VOR approaches out there today so anyone flying IFR to these airports needs them.

They are used for all sorts of IFR navigation and are often the terminal points on an airway, so you may be cleared after departure direct to a VOR.

VOR's can be used as a fixed point for a hold in IFR flying.

VOR navigation (and the use of paper charts for that matter) is still taught during PPL training since GPS is not necessarily installed in every plane.

Some FMS units use RNAV as a backup to GPS or inertial navigation.

This AOPA article covers most of their uses.

Personally, I am a big fan of GPS and fly direct when the airspaces and clearances allow for it (VFR). That being said, the planes I rent generally have dual Nav/CDI installations which I frequently use to cross check my position with the GPS.


Why would a canard wing be more prone to icing than a stabilizer? Someone said a canard wing is more prone to icing than a stabilizer. If true, what would explain this?

It has to do with the local speed around the airfoil.

In a canard configuration, the forward wing has to produce more lift per area than the main wing in order to have natural longitudinal (or pitch) stability. This means the suction is more intense, and the pressure on the upper side of the airfoil right behind the leading edge is lower than on the main wing. Also, the angle of attack of the wing is higher, presenting a larger area to the oncoming flow. Large droplets of supercooled water will simply splash on the leading edge and freeze there instantly, causing clear icing.

In a conventional aircraft the tail surface will provide very little lift, and in many cases even produces a small downforce. Now the main wing shows lower pressure on the forward part of the airfoil, just where ice collects the most. In the special case where the tailplane is in the wake of the wing, some of the water droplets will have frozen to the wing already and less water will be available to cause tailplane icing.

The capacity of air to hold water depends not only on temperature, but also on pressure: The lower the pressure, the more likely it is for condensation to happen. Condensed, super-cooled water, or liquid water hitting a cool surface, will form ice. Now you have both components needed for the explanation.

Pitch stability is the explanation for the higher propensity of canard wings to collect ice than conventional tailplanes!


Do airplanes have any kind of fire insulation? Commercial airplanes sometimes suffer from a bird strike or something else that could ignite a part of the engine. So apart from noise insulation, do airliners have fire insulation?

Yes, aircraft have fire-protection systems as well with many of the materials being of such a sort.

From boeing.com about the fire-resistant/fire-suppression systems on their aircraft. You can see that most of the aircraft is covered in these materials. Emphasis mine.

  Interior components of Boeing airplanes meet flammability requirements prescribed in Title 14 Code of Federal Regulations (CFR) Part 25. These components include:

- Interior ceiling.
- Interior sidewall panels.
- Partitions.
- Galley surfaces and structure.
- Exposed surfaces of stowed galley carts and standard galley containers.
- Large cabinets and cabin stowage compartments.
- Passenger seat material.

  For materials in areas not covered by the CFR requirements, Boeing design guidelines are used to identify additional flammability, smoke, and toxicity requirements.
  The standards for flammability of insulation blankets have improved over time. A recent requirement change calls for the enhancement of the fire-protective features of insulation blankets in the event of an in-flight or post-crash fire. The latest standard increases protection by minimizing the contribution of the insulation blankets to the propagation of a fire. Thermal/acoustic insulation installed behind cabin interior panels with the appropriate fire-resistant properties can delay the onset of fire into the cabin in the event of a crash (see fig. 1). The insulation blankets, along with the airplane skin, must be capable of resisting burn-through from a fuel-fed post-crash fire next to the bottom half of the fuselage for a minimum of four minutes to allow passengers to evacuate the airplane before burn-through can occur.

You can also find out much more in that webpage about the various active and passive fire-suppression systems in aircraft.

The FAA Regulation about these systems can be found here in PDF format.


Pre-50's, how was electricity generated without using an Integrated Drive Generator? The question How do airplanes regulate the output frequency of AC generators? here on SE already answers why we need an IDG with a constant speed drive on airliners.

  [IDG's] have been used in aircraft electrical power generating systems since the 1950's.—Wikipedia

Before the 1950's, in the early jet airliners, turboprops, and big piston planes, how did they manage the same task of regulating the AC frequency?

Or did they rely purely on DC via alternators and a bank of batteries? I'd think old big planes had plenty of pumps, etc., to require AC.

Inverters, it would appear, were not uncommon. I get the feeling that the quality of the (unregulated) AC from alternators was insufficient for instrument applications, and was primarily used for less sensitive systems. For more stable AC, it appears that the inverters were used.
B-17G

AC System
Alternating current for the autosyn instruments, drift
meter, radio compass, and warning signals transformer is furnished by
either of 2 inverters, one of which is a standby for the other. One
inverter is under the pilot's seat and the other under the copilot's
seat. A single-pole, double-throw switch on the pilot's control panel
controls the DC power to the inverters and selects the inverter to be
used. In the "NORMAL" position the left-hand inverter is on and in the
"ALTERNATE" position the right-hand inverter is on. Source

Anecdotes on PPRUNE also seem to offer support for this.

B-47 Stratojet
Finally, a decent source. Two (unregualted) AC alternators for 'windscreen deicing and some radar equipment', with 2+1 spare single-phase inverters for other AC users.

KC-97 Stratostanker
Has 6 generator dials, and 4 inverters on the flight engineer's station, looking at a panel diagram and the bottom right of this photo. That being said, there are two switches labelled alternators on the AC section of the panel. They only seem to offer "NESA Bus" (B-47 manual seems to imply this is for window deicing) or Bus No. 2, depending on how the right switch is set. The left switch is for the voltmeter/ammeter dials.

Credits to brewbooks on Flickr


When evacuating after a rejected takeoff, what is the minimum distance you should be from the plane and which direction should you run? In case of a rejected takeoff, and due to a malfunction or engine explosion, how far do you have to distance yourself from the plane and in which direction would you run towards?

In my opinion (please correct me if im wrong) this is what i think, for direction, the most logical sense would be upwind incase of engine fire. for distance, during our training we were told 1 wing span...?

Please include some official references like FCOM, FCTM or other similar documents!

Thank you for taking the time to read my question too!

First and foremost, you follow the directions of the flight crew and any emergency services you encounter. This takes precedent over anything anyone else tells you. Failing to do so may put your life and the lives of those around you in danger.
That said, lets break this down into the possible sources of danger...
Brakes and Wheels
If you're evacuating from an aircraft there's fair chance the brakes and wheels are extremely hot and possibly overheating. If that's the case there is a danger of them exploding. We have a good answer about how firefighters deal with these, which says to avoid being directly in line with the front or sides of wheels and to prefer being at a 45° angle to them.
Based on this, you should avoid being close to tires in general and ideally keep to a path that puts you at 45° to the wheels until you are at least 80 m away (263 ft). The most important thing is to avoid being directly inline or to the side of the tire.

Engines
The engines are normally shutdown asap after an emergency landing, however they could still be running. If the engine has been shutdown and isn't on fire it should pose no threat to your safety, though for good measure I'd avoid putting myself in line with the turbines (or generally to the side of the engine), because if they break/explode it will not be contained.
If an engine is still running, on an airliner you'd normally avoid evacuating on that side of the aircraft as there is a danger of being sucked into the front of a running engine or being hit with the hot air coming out the rear. Again I'd avoid being to the side of the engine in case of turbine disc failure.

Fire and Smoke
Fire and smoke can be spread by the wind, so as others have said, I'd prefer to be upwind to avoid a lung full of toxic smoke and being in the path of a fire. Though I think this is very much a secondary consideration. I'd be much more worried about the tires.
Explosions
If there is a general explosion on the aircraft debris could fly in any direction so there is no preference for a safe direction to run in this case. For distance I'd suggest an absolute minimum of the 80 m as per the brakes, but why would you want to be closer than necessary to a potential explosion? Keep going and DO NOT stop to take a selfie or video until you come across some emergency services, who should guide you to a safe location.


What causes tail-down force? What phenomenon causes aerodynamic tail-down force? Why will there be more downforce on the tail when an aircraft picks up speed? What happens if the aircraft goes too slow - does this means less downforce on the tail? Do the center of lift and center of gravity play any part in determining the downforce on the tail?

The phenomenon is called trimming.

This is the process by which the pilot adjusts the lift at the tail surface to shift the center of pressure right above or below the center of gravity and removes any stick force so the airplane will keep this attitude unaided.

If the center of gravity (CG) is right above or below the aerodynamic center (AC), the aircraft's longitudinal static stability is indifferent. Move it forward and stability increases, move it further back and the plane becomes unstable. The aerodynamic center is where the angle-of-atttack-dependent lift forces act, and in an indifferent configuration the lift coefficient on wing and tail will be equal. Increasing stability thus means to reduce lift at the rear surface, and if a lot of stability is desired, lift at the rear surface will become negative.

When the airplane is hit by a gust (change in vertical wind speed), the angle of attack at both wing and tail will change by the same amount. Now it is important how much lift changes relative to the lift before the gust. If the lift coefficient on the wing is larger than on the tail, the same amount of angle of attack change will cause a relatively smaller change of lift on the wing than on the tail, and the center of pressure will move such that it causes a correcting pitch moment.

By selecting the proper center of gravity location, the pilot can adjust the level of stability he/she desires. To stay at the desired attitude in flight, he/she now has to move the incidence of the empennage (or the elevator deflection) to a point where any pitch moment disappears. The consequence is a downforce on the tail if that is what it needs to trim the aircraft with the given center of gravity location.


In general terms, is a Constant Speed Propellor equiped aircraft more expensive to maintain? I've been looking at possibly buying a used Mooney M20 or a used Cessna 172.  One of the big differences between the two is that the Cessna is a fixed prop, and the Mooney is a constant speed propellor.  While CSPs sound really nifty and efficient...I can't help but wonder if they would be more expensive to maintain.  Both in terms of regular maintenance and just the fact that it has more parts that can break.

Is a CSP more expensive to maintain than a fixed pitch prop?  And why? If you can include some numbers that relate to the planes mentioned, that'd be cool too...

Yes, they are more expensive because they are more complex.  I would not avoid buying a plane simply because it has a CS prop because, in the grand scheme of airplane ownership, CS prop maintenance has a small impact on your hourly cost.  Maybe ~$1/hr for a piston single with repetitive ADs.

Now, the maintenance of a CS prop can range from a simple visual inspection of about \$100 USD to a complete replacement of the prop (up to \$15,000 USD, maybe more for large props).  An owner will likely not do an "overhaul" when the prop reaches TBO but will instead opt for an IRAN (Inspect, Repair As Necessary).  This is typically a little cheaper than a full blown overhaul and, from a practical perspective, extends the life of the propeller as an overhaul would.  At some point you'll end up grinding blades and balancing the prop but that's not annual maintenance.  I've seen the cost quoted at around \$1,500 USD for a high performance piston single IRAN which comes to less than $1/hr for a private owner.

All in, a CS prop only adds a small amount to the hourly operating cost.  In my opinion, the performance gains are worth the cost in most cases.


Which sub-scale setting may a pilot use on an instrument approach in South Africa--QFE or QNH? I saw this question in my Instrument Rating exam and I need to know which answer is correct, A or B.
Question: As far as the vertical reference of an aircraft during the final approach segment of an instrument approach procedure is concerned, a pilot:
A. may only use the local QNH setting on the altimeter sub-scale
B. may use the altimeter with QFE on the sub-scale
So far I have looked in ICAO 8168 and the SA CATS and CARS 2011 and this is what I found:

This excerpt is from the SACAA CATS and CARS documents:

8 Approach and Landing
8.1 A QNH altimeter setting shall be made available to aircraft in approach clearances and in clearances to enter the traffic circuit.
8.2 A QFE altimeter setting, clearly identified as such, should be made available in approach and landing clearances on request.
8.3 The vertical positioning of aircraft during approach shall be controlled by reference to flight levels until reaching the transition level below which vertical positioning shall be reference to altitudes, except as provided for in paragraph 8.4.
Note: This does not preclude a pilot from using a QFE setting for terrain clearance purposes during the final approach to the runway.

I have never been given a QFE to conduct an instrument approach in S.A. It does happen in some countries but then everyone has to use it. If you look on an approach chart you will see the vertical profile is given in terms of height above sea level so you wouldn't be able to conduct the approach with QFE set. Edit: The SACAA charts do show height of the waypoints of the vertical profile, but Jeppesen doesn't. The MDA and DH are given terms of elevation and height, but this is to use in conjunction with the radio altimeter. QFE might be used by aerobatic pilots or some military operations but I think its use is rare. Having said that, the AIP does appear to allow the use of QFE on a second altimeter during the final approach see note(2) and point (f) below. Therefore it appears that B is the answer the CAA is looking for. Using the word "only" in answer A would make it incorrect.
Please look at the AIP here with particular attention to the Altimeter Setting Procedures, I will try elaborate in due course.
Firstly the AIP says

The altimeter setting procedures in use conform to those contained in
ICAO Doc 4444 ATM/501 Procedures for Air Navigation Services and ICAO
Doc 8168 OPS/611 Procedures for Air Navigation Services - Aircraft
Operations

Then it says

During flight in the vicinity of an aerodrome at or below a fixed
altitude called the transition altitude, an aircraft shall be flown at
altitudes determined from an altimeter set to sea level pressure (QNH)
and its vertical position shall be expressed in terms of altitude.

Further down below it says

On reaching the transition level at least one altimeter within the
aircraft shall be set to the QNH and thereafter the vertical
positioning of such aircraft shall be by reference to altitudes.
Note 1: On reaching the transition level, pilots shall reset their
altimeters to the QNH without requesting Air Traffic Control (ATC)
permission to do so and without notifying Air Traffic Control (ATC)
that the change has been made.
Note 2: This does not preclude a pilot from using a QFE setting for
terrain clearance purposes during the final approach to the runway.
f)   During the approach to land, terrain clearance may be determined
by using either a QNH altimeter setting (giving altitude) or a QFE
setting (giving height above the QFE datum)

I believe the intention is to allow the pilot the discretion of using a second altimeter with the QFE set, but when you talk to ATC it will be with reference to the altimeter having QNH set.


Are cargo holds pressurised these days? This question is only regarding aircraft that seat more than about 100 people.

Are cargo holds pressurised these days, what is the situation?

Are only some pressurized, most, of every single one? Does it depend on the carrier, operating region or are there other variations that matter?

Related questions:

Are cargo holds heated?
Do they have lights? (Aren't some lit for animals?)
I've noticed random mentions of "some" of the sections being pressurised. Is this correct? It would seem to me that, if indeed, only some are pressurised then, of course, you absolutely could not rely on your luggage being in a pressurised area.
and what about Fedex-type cargo-only aircraft?

To be clear, I ask this question re "today" because I noticed when googling on this, there is a lot of information, but only old information (say, 10 yrs old plus). That is a recipe for confusion and urban myth, so the total facts from you experts would be great.

(Possibly, it would be difficult to secure accurate online references for this - my quick searching anyway only revealed patchy, out-of-date looking stuffs. Note for example, the Wikipedia article on Cabin Pressurization has only one poor, no-referenced sentence on the whole matter!)

Note for example... Spray bottles (pressurized) in the checked luggage?

Aircraft design has not changed that much in the last 10 years. In fact, most aircraft in production 10 years ago are still in production.

The cargo holds in typical airliners are indeed pressurized. Take a look at a cross section of an airliner (A380 here):

The round shape of the fuselage outline is very efficient at withstanding pressure. Because of that, everything within the fuselage shape is pressurized. This includes the cargo hold below. Only cargo holds located behind the aft pressure bulkhead would be unpressurized, and these are mainly found in smaller aircraft.

The floor of the passenger cabin is not designed to withstand that pressure, because the flat surface would need to be much heavier to do so. Decompression events are considered by regulations though, so there are vents that allow the pressure to equalize. In older aircraft without these vents, a decompression event can cause the floor of the cabin to collapse, as in this incident (and because it wasn't addressed, this one too).

  Are cargo holds heated?

Sometimes. This will depend on the aircraft type. There are many threads over on the TechOps forums about this. The temperature can be adjusted depending on the cargo load.

Although the air outside the plane is cold, the walls are insulated (also for fire protection), and being pressurized along with the cabin helps too. Even in unheated cargo holds, the temperature should be above freezing. For example, the cargo bay of a 767 stays above 7˚C but with heat stays above 18˚C.

  Do they have lights? (Aren't some lit for animals?)

They do have lights, mainly for ground crew during loading and unloading. See this Boeing page about fire protection where it mentions that "another safety feature on many newer airplane models prevents the cargo lights from operating in flight".

  and what about Fedex-type cargo-only aircraft?

These aircraft are still based on the airliner models (and sometimes converted from airliners). Therefore, they have the same pressurization system. The cabin is still pressurized, or they would have to put in some type of bulkhead behind the cockpit.

This includes the cargo version of the 747 (both upper and lower areas are pressurized).

The rare exception would be aircraft like the 747 LCF (Dreamlifter). The cargo section is not pressurized, and there is a bulkhead behind the flight deck section.


What is the typical sink rate during a well-executed carrier landing at the moment the main wheels touch the deck? What is the typical sink rate during a well-executed carrier landing at the moment the main wheels touch the deck?  And how has this changed over the decades?

A lot. About 3-4 m/s (600-800 fpm). This is the speed at which 'normal' landing gear begin to break.

For carrier landings, there is no flare, so the touchdown speed equals to the sink rate on the final approach. The procedure in the linked question mentions 700 fpm.

We can arrive at a similar figure if we consider the standard carrier glideslope 3.5° (pdf) and the typical approach airspeed of ~120-130 kn. If we allow 20-30 kn for carrier speed and headwind, then for 100 kn (51.4 m/s) closing speed we get 51.4 m/s * sin 3.5° = 3.14 m/s ≈ 620 fpm.


Why is the Concorde, and not other aircraft, getting this fog/condensation, over the wing, at *takeoff*? Several pictures of the Concorde taking off, show a cloud of water droplets (fog) above the wings.  Like this:

I assume the fog is created due to low pressure over the wings. But, doesn't the size location and visible turbulence of this pattern suggest that the wing is stalled?  It certainly doesn't look like laminar flow.

I don't recall seeing this on other aircraft at takeoff.
What's going on?
Is it unique to the Concorde?
Is it exacerbated by ground effect?

This Wikia entry says:

  ...the undercarriage had to be unusually strong. This was due to the unusual loadings due to the high angle of attack that Concorde needed to take-off due to its delta-wing.

The "high angle of attack" would support my guess that the upper wing is stalled?
I could understand a stall if the aircraft was landing, but these seem to be near stall during takeoff?

You're correct that the condensation is due to low pressure, but the wing is not stalled. The highly swept delta wing of the Concorde created lift be what is called vortex lift. From my answer in that question:

  In the case of wings having sharp, highly swept leading edges like delta wings, the leading-edge separation vortex phenomenon occurs at subsonic speeds. However, the separation does not destroy the lift as in the case of low sweep wings; instead, it forms two vortices which are (nearly) parallel to the wing edges.

The centre of the vortex is a low pressure region, where condensation forms. In this figure, you can see the vortex formation in a model of Concorde.

Flow around Concorde in a landing Configuration by Henri Werlé, ONERA, image from efluids.com

A side view of the same thing:

Flow around Concorde in a landing Configuration* by Henri Werlé, ONERA, image from efluids.com

So, basically, the aircraft is still producing lift; only the mechanism is different.

This kind of lift production is quite common in case of highly swept wings. The vortex formation is also found in a number of other cases like chines, leading edge extensions (like this F-18), etc.

Flow visualization over a 1/48-scale model of an F-18 aircraft inside a water tunnel at the Flow Visualization Facility from NASA's Dryden Flight Research Center. Image from wikimedia commons


Does the US export the Mark 77 aerial bomb? The Mark 77 bomb used by US fighter aircraft detonates with an impressive array of inferno when dropped in a cluster which is often done at airshows.

Questions

Is the Mark 77 bomb solely used by the US air force only, or has such ammunition been exported to other countries? Even the typical delivery platform, the F/A-18 Hornet has been exported to a few countries military. According to Wikipedia, it seems like there are no foreign operators of this munition. (given that this is older technology, there is no need to hide its mechanics, and that ammunition usually comes as a package deal with the aircraft)
Are there any Russian, Chinese or foreign manufactured equivalent bombs (guided or dumb) capable of that sort?

EDIT: I know of US's MOAB and Russia's FOAB, but I would like to know if other countries have conventional incendiary bombs

I cannot find any info on the export status of the  Mark 77 with a routine search, and plenty of reports of media attention to its use, which leads me to believe there is not a huge market for it among US allies due to the potential for negative press.

For your second question, the Russian equivalent are the ZAB-xxx series of bombs, where the acronym stands for incendiary airdropped bomb (Zazhigahtel'naya Aviabomba) and the number is the warhead weight in kilograms. It seems the most used version at the moment is the ZAB-500. Variations include the OZAB and OFZAB, where the O is for fragmentation (Oskolochnaya) and the F for high-explosive (Fugasnaya). I have not been able to find a link with info on the  OZAB version.

Similar munitions sometimes come with an RBK (Razovaya Bombovaya Kasseta) prefix, meaning cluster dispenser, for example RBK-500-ZAB, which is a standardized RBK-500 cluster dispenser with over 100 ZAB-2.5 incendiary sub-munitions, for an approximate 500kg of combined warhead mass.


Are there examples of taildragger autoland systems? I have only been able to find examples of auto-land systems for tricycle gear. Is there an equivalent for taildraggers?

I realize taildraggers are more commonly used in VFR conditions, so there may not be a sufficient market draw solely based on the weather argument (as would be the case for e.g. commercial air transport). I also realize that taildragger aircraft tend to cost less, thus making it harder to justify investment in an avionics capability that would likely be expensive. But are there any other reasons why they are so much less common (if they exist at all)? The physics would be different, but modern control systems are capable of handling those...

Follow-Up Feb 23: Since there seem to be no obvious examples - and in light of the constraints identified above and in John K's answer - what would be the simplest (read: cheapest) taildragger auto-land system that would provide meaningful benefit (e.g. prevent ground loops)? For example, would a simple "Heading Hold Mode" augmentation system be helpful? If not, why not?

Fun Fact: The closest to a "landing system" that I could find is Brodie Landing System for the L-4, discussed here.

There aren't any tailwheel airplanes with the level of sophistication that would make it even worthwhile doing.

It could be done in theory but would require a much more sophisticated system because taildraggers are considerably more demanding to land and to control on the ground (they are dynamically unstable while rolling on the ground and want to switch ends - try pushing a shopping cart backwards really fast).  There just isn't a market that would even take an interest in developing it.


What educational qualifications are required to become a pilot in the UK? In order to obtain a pilot's licence in the UK, are any formal educational qualifications (such as school-leaving certificates) required?

As well as any statutory or other formal requirements, do employers of commercial pilots typically require any education qualifications? Or is holding a pilot's licence generally all that is needed?

In the absence of relevant educational qualifications, are there other skills, capacities or demonstrable advantages that would help someone who lacked them, but still hoped to become a pilot?

There are no formal education requirements. However you must be fluent in English. The details can be found here https://www.caa.co.uk/General-aviation/Pilot-licences/EASA-requirements/PPL-SPL-BPL/Common-requirements-for-the-PPL,-SPL-and-BPL/


Can I make an ILS-Approach when I am VFR? When doing a VFR-Flight, the runway to land on is usually approached visually. However, I would like to do some ILS-Training but in VFR.

Is that possible?
If yes: What's the procedure? Do I have to include something in my flightplan? Do I have to notify the Approach/Tower-Controller? In VFR the control zone is approached via pre-defined entry points like Echo or Sierra. But then I can't do an ILS-Approach because there is usually no entry point on runway track.

Sure you can.  Just request to fly the approach using either radar vectors or initial approach fixes, if available.  The tower will usually comply and grant you the approach clearance, adding "maintain VFR" i.e. The approach should be flown under visual flight rules and you cannot use a view limiting device unless you have an appropriately rated safety pilot onboard with access to dual flight controls.

For non towered airports, you will need to listen for traffic in the area on the CTAF and provide position reports often for the benefit of other pilots in the pattern.

Flying practice instrument approaches in VFR is quite common and many pilots do this to maintain instrument proficiency.  It is also common for instrument rated pilots to file a local IFR flight plan to practice as well.


Why do some aircraft stay in airports for a long time? Some aircraft used in long-haul flights arrive at the airport daily at 10 am and stay there until 7 pm. Why do the airlines send their planes and make them stay in the airports for more than 9 hours every day?

There are many reasons. At least one is that the airline may not need that airframe type until later that day (the next flight that requires said airframe). Airlines don't just fly any plane they have on any route they have, this would be terribly inefficient and involve using an airplane that is not needed for a given mission. Take any major airline for example that fly large planes to lots of destination in Europe. Lets say that they have a 777 Arrive at EWR from Paris. The next long haul flight that airline has out of EWR is that evening to Dublin. But though out the day the carrier has 2 commuter flights down to Charlottesville VA totaling about 100 passengers between the two flights. They are not going to use a 777 to move 100 people and some light bags, so the plane sits until its needed again.

All planes (even non commercial) are built for a given "mission" much like you would not buy a Ferrari if you wanted a car that was good on gas.


Where did the standard pressure 29.92" come from? Below the transition altitude (18,000 feet in the US), pilots use local pressure readings to calibrate their altimeters.

Above the transition altitude, pilots set a standard pressure to calibrate the altimeter. The standard pressure is 29.92 inches of mercury or 1013 hPa.

Where did this value come from?

It is called "standard pressure" because 29.92 In-Hg (or 1013.25 hPa) is the standard atmospheric pressure at sea level according to both the ISA (International Standard Atmosphere) and the US 1976 Standard Atmosphere.

Below 18,000ft, local altimeter settings are used because you need to know how high you are above terrain, or whether or not you're at the proper altitude for an instrument approach, or at pattern altitude, etc. In cruising flight above 18,000ft, except for a few places in the world, you are well above any terrain, so errors in actual altitude of even a couple thousand feet is unlikely to cause a problem. In this case, having all airplanes use a common altimeter setting is useful because it 1. doesn't require you to change it frequently as you pass through changes in pressure, and 2. it helps ATC ensure separation without having to inform flights every 10 minutes of a new setting. It makes sense to use standard pressure in this case because it represents a good baseline average, and you may have noticed that reported altimeter settings tend to average around 29.92 anyway.

That being said, there are instances where you may still need to know the en-route altimeter settings for flight planning purposes. Suppose the minimum safe altitude along your route is 19,000 ft MSL and current pressure is 29.42. If you decided to cruise at FL190, you would actually be flying at about 18,500ft MSL, and would be below your minimum safe altitude. Colder than standard temperature can have similar effect as low pressure, and will result in the airplane being lower than the indicated altitude.


On a calm day, what determines the active runway? When the winds are "calm" or variable, and as such there is no wind direction on which to base the decision on what the active runway should be, how is that determination made?

Almost every airport that handles frequent traffic will have a Preferential Runway Use Policy. That policy will cover things such as which runways to use for departures and arrivals when parallel runways exist, and the preferred direction even in calm winds.

It's based on the best noise abatement routes/gradients, taxi distances, terrain, etc.

So much so the preferred direction can be used in tail-wind up to a limit (8 knots component in certain places for example). Basically it's the usage that was envisioned when the airport was designed (which does take into account the historic wind data).

  Westerly Operations is the normal traffic pattern used at LAX during the daytime (6:30 am to
  midnight) throughout the year. Aircraft approach the airport from the east and depart the Airport
  to the west due to the prevailing westerly wind. This procedure routes louder departing aircraft
  to the west over the ocean, while arriving aircraft fly from the east to the west over the
  communities on the eastside of LAX, including the Cities of Los Angeles, and Inglewood, and
  the communities of Athens and Lennox.

Above (source) being the normal traffic pattern, it will remain so during calm winds.

Related:

In calm winds, which runway should I select at an untowered airport?
Why would a commercial flight be landing with a tailwind?


Is Vy closer to Vbg with a variable pitch prop My experience with fixed pitch props is that Vy is generally higher than Vbg due to engine efficiency issues. If I set my prop pitch so optimal engine rpm is at Vbg, will Vy = Vbg?

Too long for a comment:

My POH does not show Vbg, only the 'normal' V's listed below, and even then only some of these are indicated on the Airspeed Indicator with the Green, Yellow, Red color bands. How would you determine Vbg? Experimentally?

VA,  Maneuvering Speed is the maximum speed at which you many use abrupt control travel.

VFE,  Maximum Flap Extension Speed is the highest speed permissible with wing flaps in a prescribed extended position.

VNO,  Maximum Structural Cruising Speed is the speed that should not be exceeded except in smooth air, then only with caution.

VNe,  Never Exceed Speed is the speed limit that may not be exceeded at any time.

VS,  Stalling Speed or the minimum steady flight speed at which the airplane is controllable.

VSO,  Stalling Speed or the minimum steady flight speed at which the airplane is controllable in the landing configuration at the most forward center of gravity.

VX,  Best Angle-of-Climb Speed is the speed which results in the greatest gain of altitude in a given horizontal distance.

VY,  Best Rate-of-Climb Speed is the speed which results in the greatest gain in a given time.


Why cycle the magnetos before engine shutdown? Inspired by this question asking about shutting down a light aircraft engine I realised there is another ritual which I do, but have long forgotten the reason why.

Having parked a light aircraft, and switched off the electrics, with the engine still running I always check left and right magnetos, similar to the magneto drop check done as pre-flight checks.

I don't really pay much attention to the RPM, I do it because I've always done it. It's not (as far as I remember) on my shutdown checklist - and I'm starting to wonder why I do it.

Is this necessary? Something in the back of my mind is saying "prevents mag fouling". What is the real reason to cycle Left & Right mag pre-shutdown?

This is a hot-mag check to verify that when you shut the engine down, you are really putting it in a safe(r) state. Think about how the mag-switch works, when you switch to R or L, it grounds the P-Lead on the other side (so switching to R grounds the P-Lead on the Left side). If you don't see the RPM drop, you may have a hot mag on that side.

An un-grounded mag is a dangerous situation in that it can cause the aircraft engine to turn over if you accidentally (or intentionally) move the prop when you are parking the aircraft, or when somebody walks by and runs into your prop.

So during this mag check you are only looking for an RPM drop or a change that tells you that the mag is shut off.


What is this very isolated, modern aviation/heliport in Northwest Antelope Valley, California? There's a modern facility at 118 53' 16" 34 9' 28" or about 9 miles north of Neenach, California. It has 5 hangars - 1 very large, 1 medium, 3 small - with other large buildings and a large water tank (a million gallons?). It sits in the foothills of the south side of the Tehachapi Mountains. The only land access is a meager dirt road, almost a 2 track.

Any suggestions as to what it is?

It appears to be the Northrop Tejon Ranch RCS facility. It is used for testing of radar cross sections. There’s a page here with a little more info on it.


Why did Air France retire Concorde? It was developed by British Airways and Air France in the 1970s as the first supersonic passenger/civilian airplane.

Ultimately the cost of flying the aircraft was more than the revenue that it generated.  The Concorde began a steady decline to retirement after the firey crash of Air France Flight 4590 which killed 113 people, and again suffered setback after September 11th, 2001.  Even with the best safety record in airline history, the crash combined with the terrorist attacks led to passenger fear and low sales. Airlines that operated the Concorde eventually lost money on most flights, and long term operations were simply not sustainable.  To make matters worse, it was very limited to which airports it could fly to in the US: JFK, IAD and DFW. The FAA was opposed to the Concorde for pollution, noise and fuel inefficiency concerns.

British Airways cites mostly political reasons for the ending of its Concorde program.  They claim that the Concorde would have remained profitable had Airbus retained the airplane.  However, there was also speculation that, due to the dwindling numbers of passengers, BA realized that it could make more money carrying passengers subsonically across the Atlantic.


What is the typical voltage used on jet airliners? I have seen several parts that use 28 Volts DC, is this a standard for jet airliners (e.g., Airbuses and Boeings).

You'll see all kinds of power on large aircraft - 28 VDC, 115 VAC 60 Hz (and likely 230 VAC 50 Hz), 115 VAC 400 Hz, 5 VDC (USB power in cockpic/pax seats), 230 VAC, and 270 VDC.

Here's a general idea of what the Boeing 787 Power Distribution System consists of:

(Source)


How can you fly a plane without arms? I hear about Jessica Cox; born without arms, she is the world's first licensed armless pilot.

How can you pilot a plane without arms? How are the controls modified in order to let her fly?

Anyway: great respect for this woman. Absolutely.

Jessica Cox explains it herself in this video:

https://www.youtube.com/watch?v=b2IqpPSF9-U

She's using her legs in place of hands (I.e, using her feet to manipulate the controls, throttle etc) and she specifically flies an Ercoupe which has no rudder pedals.


What prevents companies like Cessna and Piper from connecting multiple alternators to a single engine? Firstly, I'm not even sure if there's only one alternator connected to the engine in a C172-like aircraft. But if that is true, then why don't manufacturers connect multiple alternators to a single? What are the draw-backs of doing so, other than a waste of generated power?

I just want to know whether it is possible - and preferably without any major draw-backs.

Without too much increase in weight and complexity, one more alternator could together produce twice the electricity - this might actually be primarily useful for large air-liners relying more on electricity than hydraulics, like the A380 and the 787 Dreamliner. So, again, why not?

You almost answered the question yourself.

Having more than one alternator would introduce more complexity not only mechanically but also in the electric system (even if they work correctly). Alternators scale really good so that a bigger alternator generates more electric power then two smaller of the same weight.

So the only reason to do that on a smaller air plane is to have a backup if one of them fails.

This brings up two questions:

How often do alternators fail?
what are the consequences of a failed alternator?

For the first question I have a (non representative) experience:
At my airfield 6 SEP plane are are posted. In 25 years I'm flying there we had 1 failed alternator.

For the second question we have to look in the flight instructions and the planes manuals. For smaller planes like those you mentioned they state that you should turn off some electric devices and land on the next reachable airfield. That means there is no immediate risk when an alternator fails.

The conclusion is that we simply don't need more than one alternator in a SEP plane.


How does wing shape impact shock structure? I am trying to understand what the 3D shock structure would be for a wing in supersonic flow. Suppose we have a wing which is made of double-wedge airfoil cross-sections.
If we view the shock structure over the airfoil (side view), it would look like so:

Now, if we look at the top view, we would see the following structure (top view):

Questions:

Is the shock seen in the side view (airfoil shock structure) related to the one seen in the top view?
How would the shock structure change if the airfoil is kept the same but the wing is swept more?
(If you can show some 3D plots for the shock structure, it would really be helpful!)
Can you explain what determines the shock angles in this picture (is it sweep or airfoil shape or both - and which is more dominant)?

Is the shock seen in the side view (airfoil shock structure) related to the one seen in the top view?

No, not at all. The first picture (side view airfoil) shows an unswept wing in supersonic flow and is not very precise. The shock angles of the LE shock and the TE shock should be identical.
The second picture (top view wing) shows a swept wing at bit above the speed of sound (almost half the speed in the first picture). Here, the flow pattern around the airfoil is fully subsonic (that's why the wing is swept!) and only its most forward tip will cause a shock which is almost straight because flow speed is only a bit higher than the speed of sound.

How would the shock structure change if the airfoil is kept the same but the wing is swept more? (If you can show some 3D plots for the shock structure, it would really be helpful!)

That depends on the ratio between speed and sweep angle. As long as the sweep angle exceeds the shock angle, flow around the airfoil behaves like in the  subsonic case with a stagnation line, flow around the round leading edge and no bow shock or expansion fans. When the shock angle is greater, the airfoil will show a flow pattern like in the top picture. Sorry, no fancy 3D plots!

Can you explain what determines the shock angles in this picture (is it sweep or airfoil shape or both - and which is more dominant)?

The first and foremost factor for the shock angle is speed. Mach 1 will cause a straight shock and higher speeds will bend the shock backwards into an oblique shock with a shock angle $\varphi$ according to the equation $$\varphi = arccos\left(\frac{1}{Ma}\right)$$
This angle will become smaller with increasing bluntness of the body causing it.

Shock angle diagram (from Wikipedia, source). The caption is: This chart shows the oblique shock angle, β, as a function of the corner angle, θ, for a few constant M1 lines. The red line separates the strong and weak solutions. The blue line represents the point when the downstream Mach number becomes sonic. The chart […] is valid for an ideal diatomic gas.
Explanation: M1 = Mach number ahead of the shock, M2 = Mach number past the shock. β is the half cone angle such that β + $\varphi$ = 90°.
Consequently, the angle of the shocks emanating from that T-38 show that it flies at around Mach 1.09. In Schlieren images like this darker areas indicate higher density and lighter ones lower density. The two dark lines coming from the fuselage tip and engine exhausts are the two compression shocks which are audible as a double boom when a supersonic aircraft passes overhead.


Does a pilot follow the PAPI lights in the same way in all categories of aircraft? Since there are different dimensions of aircraft in various approach categories, I wonder what changes pilots in different aircraft have to make when following the PAPI lights. I know that 3° (for example) for one aircraft is the same for all other aircraft, but different categories might have different touchdown points when following the same glideslope due to the MEHT.

Since the PAPI is fixed, what differences are then when flying it in two different aircraft? Does the size of the aircraft change the flight path in any way? Will I always touch down at the same spot?

The lights themselves are fixed, there is no way ATC or the airport operator can simply adjust them to suit the next inbound aircraft. If the pilot needs to they can fly the approach with 3 whites/1 red, but AFAIK that isnt really necessary in most situations (maybe for 747s and other longer aircraft).

It's worth remembering that the PAPI cannot provide precision to the foot. Having 2 white lights only means you are anywhere between about 2.8 to 3.2 degrees on a standard approach, which can be a considerable range when you're a few miles out.


Are tritium runway lights widespread in remote areas? There was some interest in the 80s and 90s in providing runways in remote areas with tritium lighting (which requires no external power, since the radioactive decay in a sample of tritium gas induces radioluminescence in a suitable material). However, apart from this failed trial in Alaska (pdf) and various vague mentions of it on the internet, I can't establish if this every became a viable technology.

Has it become widely-used (perhaps for military airstrips, if not for civilian ones)? And if so, how did the technology improve in the last couple of decades to make it work?

I think it's a major headache to heave radioactive material in remote areas. Soviet union used RTG for lighthouses. A lot of countries were worried about that. You can find articles on the internet. So probably it's not very good idea due to threats.


Why do some airplanes have vertical strakes? A lot of different aircraft have vertical or near-vertical fins below the empennage, or above the empennage and attached to the front of the vertical stabilizer.

What purpose do these devices serve?

The vertical tail surface is like a wing, maybe with a smaller aspect ratio, but it works much the same over sideslip as a wing works over angle of attack. The fuselage, however, is a slender body with a much smaller lift curve slope, but a much higher "stall" angle. In fact, its lift curve slope is nonlinear and tends to increase with higher angles of attack and angles of sideslip. If you want to know more how the lift curve slope varies with aspect ratio, please read this.

This means that at some sideslip (around 15° for most configurations) the vertical tail stalls and its stabilizing side force decreases with further increase in the sideslip angle, whereas the destabilizing influence of the fuselage increases further. The aircraft becomes laterally unstable pretty soon.

To buy a little more stability around the point where the vertical tail stalls, these strakes have been added. They have a high leading edge sweep, so they will produce vortex lift just like a highly swept delta wing, only sideways. The suction force of this vortex will act on the root of the vertical, just were it has the biggest chord. In fact, this helps to keep most of the vertical active, because now it becomes part of that delta wing ahead of it.

The strakes in the upper picture have a different purpose on top, however. They limit flow separation on the rear fuselage in high speed flight by preventing crossflow. Actually, they are an embarrassment for the aerodynamicist, because they show that the lower fuselage suffers from flow separation just when it hurts most.

Most military cargo aircraft have them, because in order to keep the ramp short and to provide enough clearance for rotation, their tailcones are steeper than what the airflow can manage. Among modern military transporters, only the Lockheed C-5 (thanks to @dotancohen for the hint!) and the Antonov-70 can do without them. This was a definite source of pride for the chief aerodynamicist of the Antonov Design Bureau, Oleg Bogdanov.

The lower fin in the second picture was added to compensate the destabilizing influence of the two floats, which will destabilize the aircraft in much the same way as the fuselage does.

Extending the vertical downwards also helps to improve handling qualities and should be done as much as tail strike clearance allows.


How does one determine the elevation on a sectional? (Refer to figure 21) How does one determine the elevation variation of the light tan area between Minot (area 1) and Audubon Lake (area 2)?

A. 2,000 ft to 2,500 ft MSL.
B. 2,000 ft to 2,700 ft MSL.
C. sea level to 2,000 ft MSL.

I want to thank everyone for the help.  I made an 88 on my Drone certification that I took today (10/11/2018)

The answer is A. The FAA chart user's guide is your friend here:

Simply check the scale, that already tells you it's 2000-3000 feet. Then look for contour lines, there aren't any, then it's 2000-2500 feet.


What type of mechanism is used to operate the Embraer EMB 120 front door? I have never worked on this type of aircraft, but I usually come across it in several hangars. I've been wondering on which mechanism this bulk door is lowered and lifted.

Is it by pneumatic system, hydraulic system or by electrical motors?

From the EMB120 Ops Manual:

  FORWARD ENTRY DOOR

  The forward entry door incorporates a folding air stairs and is fixed
  at the fuselage's lower edge.

  The door can be raised manually or hydraulically. For normal
  operation, two hydraulic actuators powered by the green hydraulic
  system or by an accumulator, raise the door. This actuator has the
  capability of performing four complete operations and is controlled
  from the INTERIOR FWD DOOR CONTROL PANEL (ATTENDANT'S PANEL) or from
  EXTERIOR FWD DOOR CONTROL PANEL. Each panel incorporates a pushbutton,
  which energizes a solenoid valve that allows hydraulic power to raise
  the door. A blue light illuminates while the door is moving up. This
  system's circuit breaker is located at the INTERIOR FWD DOOR CONTROL
  PANEL (ATTENDANT'S PANEL).

  The door may be raised manually by an outside ground attendant who
  lifts the door to its closed position.

  The spring accumulator is located in the hydraulic compartment at the
  right side of the fuselage forward section. Once the door is raised,
  moving the red inner or outer handle down will latch it. The hydraulic
  circuit provides a damping function when the door is opening.

  The hydraulic circuit is provided with an emergency valve, which
  allows the forward door to be lowered when the solenoid valve fails.