Why do oblique-wing aircraft suffer from significant aerodynamic cross-coupling? Oblique-winged aircraft (which have one long wing that pivots around a central point, so that the wing is straight at low speeds but strongly-swept - one side forwards, the other side back - at high speeds), while theoretically more efficient than conventional aircraft, have a number of issues which have prevented them from seeing any significant use; one of these problems are their various unusual cross-coupling modes, including a significant tendency towards pitch-roll coupling (where pitching up or down makes the aircraft roll to one side or the other, and vice versa) and a need to hold a significant bank angle just to maintain coordinated flight and keep from slipping sideways (even with no thrust asymmetry), all of which become increasingly severe at high sweep angles.

For example (emphasis added):

  McMurtry found the handling qualities were very close to the simulator, but that the aircraft’s asymmetry resulted in unusual trim requirements, asymmetric stall and inertial coupling. [...]  For instance, the AD-1 required about 10° of bank in order to trim the aircraft with no sideslip at 60° wing sweep.

  [...]

  As hoped, the low-cost, low-speed, low-technology AD-1 successfully demonstrated the concept of a manned oblique wing aircraft, sweeping the wing to a maximum of 60°.  The aircraft experienced cross coupling between pitching moment and aileron deflection.  This pitch-roll coupling and the aeroelastic effects contributed to unpleasant handling qualities at sweep angles above 45°.  [...]  [A Summary Of A Half-Century of Oblique Wing Research, pages 20-21.]

  The proposed wing pivot was also to be canted so that at 0° sweep, the wing was canted to 0°, but at 65° sweep it would be canted to 10°.  The wing incidence would also have been increased (a standard feature of the F-8) with higher sweep angles.  These features were expected to reduce aerodynamic cross-coupling, such as the necessity to bank the whole aircraft to trim sideslip, as found in the AD-1 flight testing.  [...]  _[A Summary Of A Half-Century of Oblique Wing Research, page 23.]_

  Flight tests of AV1 and AV2 proved the design to be controllable in both pitch and roll.  Noble states, “The elevon mixing is standard symmetric and the aircraft responds equally to left and right inputs.  There is some slight roll coupling associated with up and down elevator.  I believe this can be eliminated with fine tuning of the elevon mixing.”  _[A Summary Of A Half-Century of Oblique Wing Research, pages 32-33.]_

Why do oblique wings come with these strange cross-couplings?

With the wing swept forward, a positive lift force on that wing will bend the wing upward and locally increase the angle of attack, especially to the outboard of a wing. The same thing happens on the wing swept backwards, only an upward deflection from lift force will reduce the local angle of attack, likewise to the greatest extent toward the wing tip where it is deflected upward the most. One wing will end up generating more lift than the other, which will require a rolling input from the aileron to cancel out. One could attempt to balance this out with different twist profiles of each wing, but that would only solve the issue in one g flight.

When, for example, the elevator is deflected to induce a positive pitching moment, the angle of attack on the wing increases and lift force increases. The increased lift generates an increased deflection of the wing. However, as outlined above, the impact on the local angle of attack due to that increased deflection is opposite between the left and right wings. This produces a coupled roll effect.

Finally, the asymmetric lift and therefore induced drag, in addition to the use the ailerons to counter this rolling tendency, will produce adverse yaw. To eliminate the sideslip you would need to use substantial rudder input, or a bank in the opposite direction of the adverse yaw induced sideslip.

Edit:

For a really simple visualization of the wing deflection inducing a local angle of attack change, take a piece of paper and hold it flat in the air in front of you, with the long edge facing you but turned away by 45 degrees (imagine you are looking at the leading edge of a 45 degree swept wing).  Bend the short end of the paper sheet that is farthest from you (the wing 'tip') upward. Your perspective is aligned with the freestream aerodynamic flow, and by bending the tip upwards you can now see the top of the outboard wing. For a normal rear swept wing this produces washout and unloads the outboard wing. Now imagine the short end of the paper sheet that is closest to you is the wing tip. Hold the wing 'root' fixed and bend the new wing tip up. The upward deflection increases the local angle of attack at the wing tip, rather than decreasing it as before for the rear swept wing.

As an aside, this is also the reason why forward swept wings are often aeroelastically unstable. An increase in the lift force produces more lift force due to the deflection, a positive feedback that can much more rapidly lead to divergent flutter!


What is the MEL (minimum equipment list)? What is a minimum equipment list for an aircraft? How are MEL and MMEL related? Who creates the list, and how do they decide what is on it?

As mentioned here:

  A minimum equipment list is a list of equipment that must be installed
  and operable for the aircraft to be considered airworthy. It is
  aircraft-specific and spells out which pieces of equipment may be
  inoperable while maintaining airworthiness. If something is found to
  be inoperative, the pilot goes to the MEL, finds the entry for that
  item, and determines if the airplane must be grounded until that piece
  of equipment is fixed.

A Master minimum equipment list is:

  An MEL for a specific aircraft originates from a master minimum
  equipment list (MMEL). The MMEL is a list of all equipment on an
  aircraft type; it details which equipment is allowed to be inoperative
  without grounding the aircraft. (Think of a MMEL as a general, broader
  MEL.)

FAA publishes MMELs by manufactures.


In the B737, does an IAN NPA require that the G/P is 3° or can the G/P angle vary? For an IAN Approach to be recognized and work in a Boeing 737, does the Approach G/P need to be exactly 3° or can it vary?

The G/P can deviate from the typical 3 degrees, but not much.
For any non-precision approach (NPA) flown with VNAV or Integrated Approach Navigation (IAN), the navigation database needs to contain a stored glide path (GP) angle or a suitable waypoint (runway or MAP):

VNAV should be used only for approaches that have one of the following features:

a published GP angle on the LEGS page for the final approach segment
an RWxx waypoint coincident with the approach end of the runway
a missed approach waypoint before the approach end of the runway, (e.g.,
MXxx).

[...]
The FMC
obtains the GP angle displayed on the LEGS page from the navigation database. [...] A GP angle
is coded in the navigation database for nearly all straight-in approach procedures.
[...]
This GP angle is normally defined by the regulatory authority responsible for the
approach procedure and provides a continuous descent at a constant flight path
angle for a final approach path that complies with minimum altitudes at
intermediate step-down fixes. The typical GP angle is approximately 3.00°, but
can vary from 2.75° to 3.77°.

(Boeing 737 NG FCTM 5.45 - Approach and Missed Approach, emphasis mine)
As you can see, the GP angle can vary between 2.75 and 3.77 degrees. For IAN, this G/P information is passed on to the AFDS by the FMC. The list of IAN restrictions does not limit the GP angle:

IAN Requirements and Restrictions

airplanes must be equipped with FMC U10.5 or later and IAN FMA
displays
dual or single engine approaches are authorized
waypoints in the navigation database from the FAF onward may not be
modified
raw data monitoring is required during localizer based approaches.
During FMC based non-ILS approaches, raw data monitoring is
recommended when available in accordance with the techniques
described in the Non-ILS approach section in this chapter
QFE operation is not authorized
cold temperature altitude corrections are not permitted
RNP appropriate for the approach must be used
the autopilot is required until suitable visual reference is established when
performing an approach requiring an RNP of 0.15 or lower.

(Boeing 737 NG FCTM 5.60 - Approach and Missed Approach)
The FCTM does however says the following about when an approach can be flown with IAN:

For FMC based approaches, a proper series of legs/waypoints describing the
approach route including an appropriate vertical path or glide path (GP) angle
must appear on the LEGS page. A GP angle displayed on the LEGS page means
the vertical path complies with final approach step-down altitudes (minimum
altitude constraints). A glide path angle suitable for an IAN approach is one that
approximates 3° and crosses the runway threshold at approximately 50 feet.

(Boeing 737 NG FCTM 5.61 - Approach and Missed Approach, emphasis mine)
It does not say if the full range from 2.75 to 3.77 degrees is considered approximately 3.00°, but it certainly does not have to be exactly 3.


What is the difference between a tail slide and a whip stall? Can someone tell me what the difference is between a tail slide and a whip stall?

Whip stalls are prohibited as far as I know in the C152 Aerobat but watching a video of a whip stall it looks exactly the same as a tail slide. The thing is that my aerobatic instructor taught me how to tail slide and I distinctly remember him telling me to make sure that I held the elevator neutral or it could snap off or be damaged.

A "tail slide" occurs when an aircraft is nose up and loses all forward airspeed. Gravity then pulls the plane "backwards" towards earth.  The "whip" occurs when a sinking aircraft is flipped nose down by the airflow as it assumes a more aerodynamicly stable orientation.

Aircraft will generally nose down when they sink due to more area behind the center of gravity than ahead (as viewed from underneath the plane).  The horizontal stabilizer, farthest from the CG, is the main contributer, especially with down el.

Note that with canard and delta designs, it is the back of the main wing serving this function.

The issue is that when one tail slides, the increased vertical sink rate turns a gentle nose down into a hard "whip", greatly increasing stresses on the aircraft.  There for it is imperative to make sure the plane can handle this and not to tail slide too long.

For hang gliders and "flying lawn chair" type aircraft, a hard whip can be induced by pulling forward too aggressively in stall recovery (deltas have big tails), resulting in a full blown forward tumble.

Definitely something to discuss with your acrobatics instructor.


How can I figure out critical altitude? In Vmc situation (minimum speed where pilot can maintain altitude), How can pilot figure out the Vmc? And also, why critical altitude is important to know?

Vmc is not the speed in which you can maintain altitude, it’s the minimum speed in which you can still maintain directional control with an engine inoperative in a twin.  Vmc is a design specification and not a maneuvering speed which is established by the manufacturer according to the design requirement of the aircraft.  The actual minimum control speed for a given situation, Vmca, varies based upon a number of factors, including density altitude, loadout, aircraft configuration, etc.  Vmca can be determined by doing a Vmc demonstration, which is a required maneuver to be demonstrated for an airman certificate operating multi engine airplanes.

Critical altitude is the altitude at which above this a turbocharger or supercharger can no longer maintain the maximum design operating manifold pressure for a reciprocating engine.  It is an important factor if you need to know the maximum power output of your engine at any given altitude for climb requirements, ability to attain and maintain an altitude to clear terrain and obstacles, etc., no different than knowing the available power for an normally aspirated engine.


What geographical area does a METAR cover? According to the FAA's AC 00-45G section 3.1, a METAR is defined:

  Aviation Routine Weather Report (METAR) is the primary observation code used in the U. S. to
  satisfy World Meteorological Organization (WMO) and International Civil Aviation Organization
  (ICAO) requirements for reporting surface meteorological data. A METAR report includes the
  airport identifier, time of observation, wind, visibility, runway visual range, present weather
  phenomena, sky conditions, temperature, dew point, and altimeter setting. Excluding the airport
  identifier and the time of observation, this information is collectively referred to as “the body of the
  report.” As an addition, coded and/or plain language information elaborating on data in “the body
  of the report” may be appended to the end of the METAR in a section coded as

  “Remarks.” The contents of the “Remarks” section vary with the type of reporting station. The
  METAR may be abridged at some designated stations only including a few of the mentioned
  elements.

What doesn't appear to be defined is exactly what area the METAR covers. It's understood that a METAR is a reporting for a particular station's current conditions at a particular time, but is there a more discrete definition for where the METAR's conditions can be expected?

The definition of a TAF specifically states that the forecast conditions are expected in an area around the reporting station with a radius of 5 statute miles. The METAR has no such specification that I can find.

Geographically METARS are for a single specific location (the station identified in the report).

An automated METAR with no human supervision covers the conditions within approximately 6 inches of the weather station reporting it (for example, a badly positioned AWOS/ASOS station may be shielded from wind in some directions which affects what it reports).

A manual (or manually augmented) METAR covers the conditions the weather observer can physically see (clouds, precipitation, visibility, etc.) in addition to the ones they can measure (temperature, dew point, barometric pressure), which are valid for the point of measurement.

Practically speaking if the weather station is positioned well a METAR (like a TAF) is probably fairly representative of conditions within 5 miles, assuming there are no geographic features such as mountains which may mess with the weather. The data is still still "useful" within about 20 miles (e.g. KMTP uses the weather reported by KHTO, about 17 miles away), though you should obviously cross-check it - in that example the winds at Montauk (right on the water) can be noticeably different from the winds at East Hampton (further inland).

Some information is broadly useful over a wider area. FAR 91.121 requires you to set your altimeter to the value reported by a station "within 100 nautical miles" of your position when you're operating below 18,000 feet because things like barometric pressure tend to affect relatively large areas.


Lift to drag ratio of airliners I don't know if this is the right place to ask, but I am looking for lift-to-drag ratios (L/D) of different airliners. I have found a paper [1] which has some estimates of L/D for some jetliners, but it's a limited sample
Table 3. Estimated aerodynamic characteristics of jet airplanes (L1011, a few Boeing, Douglas and Airbus). This question also looks promising, but delivers airfoil numbers, not aircraft numbers, sadly. Is there any online resource or a place that might point me in the right direction?

  [1] Historical evolution of air transport productivity and efficiency
      Rodrigo Martínez-Val, Emilio Pérez and José F. Palacín
      Universidad Politécnica de Madrid, Madrid, Spain, 28040

In quotable literature, I recommend „Aerodynamic Design of Transport Aircraft“ by Obert. In other useful stuff, there is a very useable (but not proven by Boeing) aerodynamic model of a 787 available on the Piano X website.

I‘d add that for any really useful analysis other than level flight, getting hold of a decent engine model giving maximum and minimum (net) thrust as well as fuel flows throughout the operating range will, in my experience, prove much harder than the aerodynamics...


Can a commercial cargo aircraft be converted into a commercial passenger aircraft, and vice versa? Just saw a video in a news, a military cargo plane carrying people stranded in some country. The seats were like in a commercial airliner. This led me to ask the question.

Would it be possible to see some examples where it was done, along with the procedure?

There are such a thing as "Quick Change" aircraft, allowing entire 'pallets' of chairs to be exchanged for cargo.

Image courtesy of canalblog.com

Image courtesy of airliner.net

Nice film to show the process.

Military transport aircraft can also be fitted out with stretchers or seats, or whatever other combo of stuff you want. To the best of my understanding these lock in like any other cargo pallet in the floor. They are less concerned with aesthetics and more with practicality:

Image courtesy the DOD via Wikimedia

Image courtesy of aarcorp.com


Upon landing (uncontrolled field), can I exit the runway via a crossing runway? I understand at a controlled airfield it's a no no unless directed by atc to do so.

If you are at an uncontrolled field you are responsible for maintaining situational awareness of other traffic and using UNICOM/CTAF to broadcast your intentions, find out if anyone else is going to be using the crossing runway, look both ways before crossing and then cross.


Can jet engines be used for VTOL civilian aircraft? Why do almost all civilian VTOL aircraft existing today use propellers (or rotors) for take off and landing and not jet engines (as the Harrier does)? Is there a technical reason for this?

Can?

Of course they can.

Does it make sense?

No, it does not.

A VTOL needs thrust at least equal to its weight. That is much more than a fixed-wing aircraft (HTOL) needs. Airliners have lift-to-drag ratio around 18–20. That is in clean configuration; in landing configuration it might be about half (~10) and we need twice as much thrust in case an engine fails, so around ¼ weight. So a VTOL would need 4 times as much thrust as a HTOL.

Likely more, because the craft would still have to be capable of reasonably controlled landing in case of engine failure. And while rotorcraft can do that by autorotation and pilot of Harrier or F-35B can simply eject, jet-powered civilian VTOL would only have the remaining engines for it.

Now fortunately thrust and power are only indirectly related. To create thrust, force must be applied to the air. This force accelerates air, giving it some momentum and some kinetic energy. Thrust is (change of) momentum per time and momentum is mass time velocity while power is (change of) energy per time and energy is ½ mass times speed squared.

So we can produce a lot of thrust with little power by accelerating a lot of air by a little. Which means large propeller: a rotor. That's how helicopters get enough lift/thrust with reasonably small engine.

Unfortunately rotors are not very practical for flying fast. One would also be rather difficult to tilt. That's why V-22 Osprey and Augusta-Westland AW603 use a compromise that is somewhere between; large enough to still have good efficiency in hover and small enough to be possible to tilt.

What would definitely not be efficient for hover is a jet though. Jets are small and can't affect that much air; what they can is accelerate it a lot. That is necessary for flying fast, but it is terribly inefficient when flying slowly.

Military jets don't care. They want so powerful engine to fly very fast anyway and they don't care for efficiency much as decisive edge in combat is worth a lot of money. But for civilian transport the small advantage of being able to land closer to destination would not outweigh the disadvantage of being several times more expensive to operate. Flying the long distance with HTOL to a nearby airport and, if the destination is not reachable by car, hiring a helicopter just for the final part of the trip ends up being a lot cheaper.


How can you tell what position the flaps are in whilst on a commercial aircraft? I am flying with thomson soon and want to be able to tell what position the flaps are in. How can i figure this out?

If you are seated behind the wings, but close, you can usually see black markings (thick line and a number) near the inboard side of the engine pylon which indicate their extension. Some might not be visible when the wing is in cruise configuration.

Same goes for leading edge slats, only you'd need to be seated near the overwing emergency exit to see those.

I've personally seen them on both Airbus and Embraer jets (no recent flights on Boeing airframes so can't confirm if they mark them too).

(Source)


Does the FAA generally consider a door opening in an unpressurized cabin during flight to be an emergency? Sometimes doors pop open on older training aircraft during flight.

I've read some indications from poorly-documented sources suggesting that this is considered an emergency, but it doesn't appear to be addressed in any flight manuals, the AIM, or the FARs.

Is there any publication from the FAA that indicates a stance on the issue as it pertains to it being considered an emergency or not?

A door popping open is not an emergency in itself, it has to cause a threat to the safety of the flight or those on the ground to be an emergency. I've had doors pop open in a light aircraft in flight and it has been a non-event every time, I didn't mention it to ATC as there was no reason to, I just told maintenance once I was on the ground. I've never seen a FAA or other country authority publication saying it was an emergency.

Regs generally don't get that specific, what is life threatening on one airplane may be a non-event in another. The POH and airplane specific training is typically where emergency events are specified.


Between land and sea, which route is preferred? Suppose that you have to connect two destinations like Phoenix and Anchorage, which are 5850km apart. Now, you have two choices: to either fly over the land, or completely fly over Bering Sea (Arctic Ocean). So, what would be the better route to choose? Currently, the route is partially over the sea and partially over the land.

Also, this is just a case, in general out of land and sea, which route is preferred? One common argument is that you are safer by flying over land in case of an emergency, but are there any aerodynamic concerns also involved?

At the cruising altitude of most airliners, there is no noticeable aerodynamic difference between being over land or over sea. At lower levels, being over sea would typically offer less turbulence, in part because as the ground warms you can encounter rising air thermals.

Generally, airliners fly whatever route provides the shortest time, which is not always the shortest distance, often called the Great Circle route. The GC route is not always quickest because of jetstream winds.

One thing to consider when choosing between an over-land or over-sea route is the distance to another airport should an engine fail. You might have heard of ETOPS. Basically, twin engined aircraft must fly within a certain distance of a suitable landing site - often 2 or 3 hours of flying time on one engine. In the middle of the ocean the quickest route may take the aircraft outside of this range, so it will have to fly closer to land, or even in certain remote parts of the world, still over land.

Another consideration is life rafts. They are legally required when flying more than 25 miles (if I recall correctly) out to sea. Life rafts add weight and therefore cost so some airlines have actually removed life rafts from their aircraft, or at least only have a few of their jets allocated for flights over water.


Do codeshare & alliances conflict with competition? I am aware of the advantages that codesharing has on airlines such as
- allowing airline to expand operations without physically flying to that destination

creating perception of large flight network
reduced operation costs from seat transfers e.g KLM & Kenya Airways on the NBO-AMS route

However my concern is ;

are there any other further benefits ?
do codeshares reduce competition?
can codesharing airlines compete ? (e.g Star Alliance members, Lufthansa & Thai Airways)
do alliances reduce competition ?
can airlines of the same alliance compete ? (e.g Oneworld members, British Airways & American Airlines)

Alliances are the airline industry's way of corporate mergers. Since national airlines are still regarded as a symbol of national pride and often are part-owned by the respective governments, eradicating the proud name from the industry would be too hard to swallow. Therefore, alliances are formed in which tariffs and schedules are harmonized. The effect for you, the customer, is the effective removal of competition, resulting in monopoly pricing on routes which are served exclusively by carriers of the same alliance.

This can be compared to the car industry: Chevrolet is not competing with Pontiac or Cadillac, regardless of what the advertisements say. You might think that the wider choice of brands is a benefit, but in the end the benefits are mainly on the side of the industry.


What are the advantages and disadvantages of tail wheels that cause modern airplanes to not use them? Photos' source: Cessna 140 and Cessna 150.

According to Wikipedia here, the Cessna 150 is successor of Cessna 140. Both have two-seat capacity and a single engine.

Why was the Cessna 150 changed to tricycle landing gear from the "old school" tail wheel? If we consider that the two airplane models have the same weight and capacity, then what are the advantages/disadvantages of tail wheels vs. tricycle landing gear? Why are so few new tailwheel aircraft produced?

Note:
I mentioned the Cessnas here solely because I knew their story better, so I can more easily compare them. This question should not be considered specific to only Cessna's products.

A tailwheel is a good choice for operation on unprepared surfaces with aircraft that have low wing loading and need to be as light as possible. Two main wheels and a small tail wheel weigh less and cause less drag than a tricycle gear, especially if they cannot be retracted. On the Bo-209 Monsun only the nose gear was made retractable because, being positioned right behind the prop, it caused 40% of gear drag all by itself.

On the other hand, high wing loading aircraft with their high landing speed would need very long landing runs due to their inability to brake hard. A rejected take-off close to the decision speed would be impossible without crashing through the airfield perimeter. Braking too hard with a tailwheel configuration will cause a headstand.

To summarise the reasons given in this answer:

A tricycle gear offers better visibility on the ground.
A tricycle gear allows full brake application.
A tricycle gear makes loading and unloading easier because the fuselage is horizontal.
A tricycle gear has less drag during the initial stage of a take-off run.

Cessna simply shifted priorities between the 140 and the 150.


What are ISA +10, ISA -15 and ISA -30? I have found out that an ISA atmosphere is:

15 °C temperature
5 °C dewpoint
1013.25 hPa pressure

What are the conditions of an ISA +10, ISA -15 and ISA -30? I know the temperature, but what are pressure and dewpoint?

I need to create those conditions to do some flight testing in a simulator.

First of all:

  I have found out that an ISA atmosphere is:
   15 degC temperature 05 degC dewpoint 1013.25 pressure

Not entirely correct. The temperature and pressure are ISA values at mean sea level. These alter with altitude.
Dewpoint is not defined in ISA, as ISA does not contain dust, moisture and water vapor.
ISA-30 atmosphere means temperature deviation from ISA temperature. This means that on a given altitude, temperature is 30 degrees colder than in ISA. Respectively ISA+10 states 10 degrees warmer.

Below is a table of ISA values.

For more physics for calculations, here is a very nice pdf on the matter.


Are radio channels used by airliners half-duplex channels? Can a pilot transmit and receive information at the same time? Or can he either transmit or receive but not at the same time?

I thought communication channels were half-duplex because this reduced the chance of the information by trasmitter (say the ATC) being overlapped by information from the receiver (the pilot) in case he possibily accidentaly pressed the PTT button. Is that a right assumption or not?

They are half-duplex because all parties share the same frequency. Only one party can transmit at any time.

Full duplex would require two frequencies and therefore third parties would have to monitor both frequencies simultaneously in order to hear both sides of the conversation. Half-duplex operation on a single frequency is inherently 'open-channel' - everybody can hear everything transmitted by anybody else on that frequency. That's the way it's always been done in aviation and in many other fields.

(Answer taken from comments by https://aviation.stackexchange.com/users/13124/transistor and https://aviation.stackexchange.com/users/15152/fiddlesticks)


Is it hard to hover a coaxial rotor helicopter? Convetional rotorcrafts are hard to hover as when we increase collective, even tail rotor pitch needs to be compensated. But in coaxial design, will just increasing the collective do the job as torque doesn't change?

The difficulty in hovering a helicopter lies in the fact that hover is unstable. Any tiny divergence will increase over time, so you have to keep adjusting the controls all the time. And this does not differ between single rotor with anti-torque and dual-rotor.


What is the cart and cable connected to this plane? During boarding I noticed many times that planes are "connected" with some orange cable.

It's curious to know what is it. Does anybody knows?

On further investigation, thats an A320 rather than a 757, and the A320 Airport and Maintenance Planning document shows that that hookup is the ground power port (page 206).

The stairs are for passing documentation up to the cockpit side windows, which open.


What is the difference between an APU and an ATS? Is there a big difference between what an Auxiliary Power Unit (APU) and an Air Turbine Starter (ATS) system actually does?

My understanding is that an APU provides power for the electrical systems of the aircraft and pneumatic pressure for air-conditioning and starting the main jet engines. An ATS provides starting power (compressed air) to a gas turbine engine.

Is the only difference that the APU also generates electricity for on-board electrical systems?
Would you ever have an APU and an ATS on the same aircraft? What are the use-cases for each product if they are mutually exclusive?

The ATS is powered by the APU, so you typically need both. It's a bit different for some fighter jets though.

See: How is bleed air used to start a jet engine?

The battery is used to start the APU, the APU then provides electrical and pneumatic power.

The pneumatic power is used for air conditioning, engine starting, pressurizing the hydraulic reservoirs, and other things.

The ATS is a turbine that needs to spin, it is spun using this high pressure air from the APU. When the ATS spins, it spins a shaft connected to the engine, which then spins the engine just enough to compress air and start.


FADEC or EEC for PT6-42A...is it feasible? In a response to a You-tube clip showcasing Piper's M600 single turbo prop, a viewer dismissed it as an inferior engine compared to a GE (probably an H-80 series) due to lack of FADEC or Electronic Engine Control (ECC) which GE engine has but lacking in PT-6.

Since controlling the PT-6 on the above aircraft does not seem to require a big workload from the pilot, I am wondering is there any necessity to equip this engine with FADEC, and whether the cost/benefit ratio is justifiable?

It is quite feasible and available on other PT-6 family engines. The PT-6C engine is available with FADEC but appears to be more for rotorcraft. That being said its fundamentally a similar engine and the systems could be migrated to the common PT-6A.

The question of:

  I am wondering is there any necessity to equip this engine with FADEC

This is somewhat of an opinion based question depending on what you personally feel is necessary. FADEC allows a pilot to run an engine at optimal performance all the time without needing to babysit the various engine parameters and settings. Some see this as a great benefit (taking something off your plate in a demanding airplane) while the more classically minded prefer to have finer control and operate their engines in various ways that may be considered less than optimal but appropriate for the mission.

The cost of FADEC is putting the engine control in the hands of a computer, which many are slowly becoming comfortable with. One thing to consider on smaller planes like the M600 is backup power systems. Large planes have the luxury of things like an APU and  a RAT as well as lots of alternators and battery banks to mitigate power failures across 4 discrete power sources. The M600 has only its alternator and batteries as a backup so in the event of a power loss you have less to fall back on and may lose the engine.

  whether the cost/benefit ratio is justifiable

Without knowing P&W's financials its hard to say. They are clearly selling lots of PT-6's and many might say theres no reason to change something that works. Since they clearly have a FADEC system for the PT-6C I would imagine a great deal of the work has been done and the system could be migrated to the PT-6A at least in part but you would need someone from PW to confirm that assumption.

Just a bit of a history note the PT-6A is a fairly old engine and has been around since the early 70's a time long before FADEC was small enough for something like the M600. On the same note the PA-46 first flew in '79 a bit before FADEC was viable on a small scale. There is some reality to keeping up a certified config once its certified.


What forward airspeed produces maximum rate-of-climb in helicopters? Lift in a helicopter increases with airspeed (from zero IAS up to some Vmax).  But in contrast to fixed-wing aircraft a helicopter can generate lift at zero airspeed.
Is there a formula for the airspeed at which a helicopter achieves it maximum rate of climb?

There are a couple of things at play here:
First, the increase in overall lift of a helicopter as it correlates to an increase in forward airspeed actually has a lot to do with the establishment of Effective Translational Lift (ETL) or the airspeed at which the entire rotor system realizes the benefit of the horizontal air flow. This happens when the helicopter's rotor disc moves completely out of its own downwash and into undisturbed air.
Second, beyond the ETL improvement, forward airspeed in a helicopter only produces an increase in asymmetrical lift, rather than the the nice, tidy symmetrical lift dispersal of fixed wing aircraft. This is because of a principle known as (surprise, surprise) Dissymmetry of Lift. The forward airspeed is added to the rotational speed of the advancing rotor blade on one side of the aircraft and subtracted from the retreating rotor speed on the opposite side of the aircraft. While this is counteracted to some extent by mechanical adjustments (flapping, feathering, leading-lagging) to equalize the inherent rolling tendency, this means that the forward speed alone, as a catalyst for lift production, is not the be-all and end-all.
You are correct that a helicopter, under the right conditions and possessing a surplus of available power can generate lift at zero airspeed. This is why a helicopter, under these conditions, effectively has a Vx of zero, since its best angle-of-climb over distance would be a straight vertical climb.
For Vy, the best rate-of-climb over time performance, it is important to plan for torque settings at level flight. Climb performance charts show the change in torque, above or below torque, required for level flight under the same gross weight and atmospheric conditions to obtain a given rate of climb or descent.
Here is a sample calculation taken from the FAA Helicopter Flying Handbook:

Determine  torque  setting  for  cruise  or  level  flight  using  Figure 7-7.
Pressure Altitude = 8,000 feet,  Outside Air Temperature = +15 °C,  Indicated Airspeed = 80 knots,  Maximum Gross Weight = 5,000 lb.

With this chart, first confirm that it is for a pressure altitude of 8,000 feet with an OAT of 15°. Begin on the left side at 80 knots indicated airspeed (point A) and move right to maximum gross weight of 5,000 lb (point B). From that point, proceed down to the torque reading for level flight, which is 74 percent torque (point C). This torque setting is used in the next problem to add or subtract cruise/descent torque percentage from cruise flight.
Determine climb/descent torque percentage using Figure 7-8.
Rate of Climb or Descent = 500fpm,  Maximum Gross Weight = 5,000lb.

With this chart, first locate a 500-fpm rate of climb or descent (point  A), and then move to the right to a maximum gross weight of 5,000 lb (point B). From that point, proceed down to the torque percentage, which is 15 percent torque (point C). For climb or descent, 15 percent torque should be added/subtracted from the 74 percent torque needed for level flight. For example, if the numbers were to be used for a climb torque, the pilot would adjust torque settings to 89 percent for optimal climb performance.

As you can see on chart 7.7, the green line represents the Vy speed and is relatively low, because it represents the speed at which the lowest percentage of maximum torque is required to sustain level flight in the given conditions, and thus, the speed at which there is the maximum amount of surplus torque available to climb, all while remaining in ETL flight.


What to look for when purchasing a headset I'm currently a student pilot, and I'm starting at an aviation university this fall to major in commercial aviation. Needless to say, I will be flying a lot. I'm looking to upgrade to a nice headset from the pretty basic one I have now, and I'm looking for a bit of guidance. I'm 6'3, so I wouldn't say I have a small head, and I've had problems in the past with headphones feeling cramped on my ears, which I don't want to be the case with my headset. I plan on having this headset for a while, at least all the way through college and hopefully several years after that.

First of all, is noice-cancelling actually that important(in terms of health and comfort)? I know there are a few decent headsets are out there w/o noice cancelling, and they are quite a bit cheaper than noice cancelling headsets.

I've been told about a couple commonly used expensive headsets that are pretty widely used across the industry. However, these headsets are $850+. At this price level, does the price of the headset accurately reflect quality, or is everything more personal preference? What are some of the pros and cons of the more popular headsets, and what should I look for when purchasing one?

I've had problems in the past with headphones feeling cramped on my
  ears, which I don't want to be the case with my headset

Do your best to test them out before you buy. Some pilot shops will let you put them on your head and if you are nice to the people that lurk around the airport they may lend you one for a flight to try out. I find that the Zulu's have bigger ear cups than the Bose but I don't have any hard numbers on size this is personal experience wearing them.

  First of all, is noice-cancelling actually that important(in terms of
  health and comfort)

Yes, just ask any one that flew in the pre-headset days. I personally find that I can comfortably fly for longer times with an ANR headset. Another thing to note is that the ANR head sets tend to not squeeze your head quite like the passive headsets I find this to be the real gain in comfort.

One of the big functional differences is that some of the LightSpeed headsets do not cary a TSO, I belive all of the Bose do. This may impact your ability to use them for commercial purposes in the future. You can find a discussion on it here.


What are the aileron lengths of commercial and military aircraft? I would like to know the aileron lengths for different aircraft, such as:

Cessna 150
Airbus A380, A320
Boeing 747, 777, 787
Lockheed Martin F-22 Raptor

I have tried to search online, but was unsuccessful in finding the values. I also tried searching in some books but couldn't find the values there either.

Any help guiding me to the books or any resources which would give me the values would be highly appreciated.

This is quite difficult to answer as some modern supersonic fighters  don't have just ailerons, but 'elevons', flaperons, tailerons and most of the time combine the movement of several control surfaces by computer to generate a desired effect.

Large civilian aircraft tend to have more than one pair. A relatively smaller set closer to the wing tips for slower speeds and a general large aileron closer to the wing root.

I would suggest you get a top-view of these aircraft. When you find our the length of the aircraft or wingspan you could derive it yourself through approximation.


Do airline dispatchers pre-block seats for weight and balance on large wide body aircraft? I was reading a Oman Air flight review and one of the bits intrigued me:

  I had earlier used ExpertFlyer to monitor both how busy the flight
  would be and the seat map. I selected a window seat when I checked in
  and noticed there were plenty of empty seats around me. On the day of
  the flight, I noticed the two seats next to me were showing as
  blocked, which I assumed was part of a weight and balance exercise.

This review is for a wide body aircraft (Boeing 787 Dreamliner) and I am just wondering if Dispatchers ever block seats for weight and balance like the review writer refers to.

I mean I have often seen large blocks of seats not available for selection sometimes in the online interface but I always assumed this was for commercial reasons and not technical. In smaller flights I have seen passengers being moved around for weight and balance but is this also done in the large wide body aircraft?

Does anyone know?

Yes sometimes, because with most aircraft you can't fill all of the seats and all of the fuel tanks and stay under max gross weight, meaning the airplane can't achieve its potential maximum range with a full cabin and on long range city pairs some sort of passenger/fuel horse trading may have to go on.

So on flights where the airplane needs more range that it has with a full cabin, it may be forced to limit seats filled to accommodate the extra fuel, and it will distribute the empty seats throughout the loading zones to prevent load concentrations for balance reasons.

You will also see seats blocked off to accommodate dead heading crew, but if you see a significant number blocked off in each cabin loading zone, and the seats are still empty when you board and its a long flight, it was probably due to fuel requirements.


What aircraft are these, and why are these engine sizes so different? These glamorous vintage images were once famous in magazines and used by airlines to advertise their products, and yes, flight attendants clothing was like this! Old and more relaxed times, though flying was for the rich.

Left: Canadian Pacific, 1968.
Right: Pacific Southwest -  San Diego Air and Space Museum Archive, likely end of 60s too.

With a flight attendant showing the scale, it's pretty clear that the size of the engines are completely different; they are likely of different designs (turbojet/turbofan) in spite of being of the same period. What are theses engines, and the aircraft that used them? Why so different choices (what did that change for the airlines)?

(Left), (Right)

On the left is the Pratt & Whitney J75 (JT4A) on a DC-8, with ram air inlets1.

On the right is an RR RB211, possibly on an L-1011. The EPR probe (I think) can be seen protruding from the inner lip at the 12 o'clock position, plus the snubber2 holding the blades together.

The former is a turbojet, that's why it's narrow. The latter is a high-bypass turbofan, that's why the big fan.

We can't directly compare both engines. The RB211 is 14 years younger, and much more powerful, suited for big planes unlike the narrow-body DC-8. The RB211 wouldn't fit under the wings of the DC-8. Different planes, roles, and engine requirements.

(Source) DC-8 with the high-bypass CFM56.

The DC-8 was later retrofitted with high-bypass because it's better for fuel economy and noise—the CFM56 to be precise—without the DC-8 getting the retrofit, the CFM56 would not have become as ubiquitous as it is now. The project needed civilian customers to survive3.

First run years: J57 (1955), RB211 (1969), CFM56 (1974).

L-1011 (top) and DC-8 (bottom). Eyeballed the scale.

1 Accessories on the Pratt and Whitney engines are mounted beneath the high-pressure compressor casing. Two drum-type oil coolers are provided, each fed with air from individual ram intakes in the lower part of the nacelle.— Flight
2 A snubber (or clapper) is a damper used to prevent blade flutter on narrow-chord fan blades, at the cost of reduced efficiency. Hollow wide-chord blades are more stable and do not need snubbers.
3 The CFM56 featured article on Wikipedia is a must read.


How/Where can I get additional weather information for flight planning? METARs, TAFs, the area forecast (FA), and flight service briefings are great tools, but if I'm planning a flight with an overnight stay (or a flight that itself might take me a couple of days of travel time) I often feel that the weather picture I'm getting is a little too short-term for my liking.

Aside from watching The Weather Channel are there any supplementary weather sources I can look at which are useful for aviation/flight-planning purposes?

I'm interested mainly in resources for the USA but answers for other countries would be interesting too.

In Europe, several national meteorological services provide additional info (sometimes on a paid basis). Some other services cater e.g. especially to glider pilots. Examples:

http://www.flugwetter.de/ ($)
DWD - Deutscher Wetterdienst Luftfahrt  (free)
http://www.meteoschweiz.admin.ch/web/de/services/aviatik.html ($)
http://www.nzz.ch/wetter/thermikprognose/ (free)
http://wetter-jetzt.de/  ($)
...

There are also a number of private websites run by clubs and/or enthusiast pilots, e.g. http://www.schaenissoaring.ch/wetter_berichte.cfm?SelectedMenuID=154 or http://www.alpenstreckenflug.de/wettera.htm which offer excellent (curated) information, sometimes on very special conditions like mountain waves etc..


Why does this shockwave not touch the body that creates it? In the shock wave of the next image (in a blunt body)

Why the wavefront is in front of the object? It does not assume that if the object travels faster than the speed of sound wave front should be just touching the object that causes it ?

After passing through the shock wave, the air still has to evade the body. If the shock-wave touched the body, the air would have to escape through infinitely thin space at infinitely high pressure and it obviously can't do that.

So behind the shockwave there is a boundary layer (called shock layer) in which the air is moving with the body and flows, under high pressure, to the sides where it can escape.

The blunter the body, the thicker the boundary layer, because the air must cover longer distance to the sides and takes longer to do that.

Of course the high pressure between the shock and the body causes a lot of drag, which is why sharp nose and leading edges are preferred for supersonic flight.


How to correct these garbled tail numbers? I have a collection of tail numbers of US aircraft which flew a bunch of domestic flights in the 2000's, but some of them seem to end with @ or @@, or @@@, which are obviously not a valid tail number characters. My question is - are these prefixes of tail numbers with the last character/digit removed? Or are these valid tail numbers if we just drop the @'s?
Examples:
N71@@@
N9346@
N93@@@
N795@@
N796@@
N797@@
N400@@
N401@@
N402@@
N403@@
N3744@
N405@@
N406@@
N404@@
N407@@
N3753@
N3752@
N3756@
N408@@
N409@@
N411@@
N3759@
N3765@
N3766@
N3767@
N3768@
N410@@
N412@@
N413@@
N414@@
Now, I could go look up these numbers in the registry. But - that would look up current data, while I need data from 2001 and 2002. Today, some of these numbers (when dropping the @'s) are registered to private individuals, some to non-airline companies, and some aren't registered at all.
Note: The carriers owning these specific numbers in the list above were Southwest Airlines and Delta Airlines.

Having just N71 could be a valid tail #, but airlines almost always use all 5 characters. Most likely, the "@" characters are masking the actual characters in the full tail number.
A site like PlaneSpotters dot net may help, but the search on
https://www.planespotters.net/photo/search?tag=N405
gives both 405DX (Delta) and 405WN (Southwest). Further research may reveal if either of those was only active outside your target dates, but there will probably be cases where neither can be ruled out.
In this case, the current N405DX is pretty new, while N405WN first flew in 2001 (link), so that's probably it. But there could have been a different Delta jet flying in 2001 using that registration, so again, more research required to rule that out entirely. Searching Google on the full tail number is often useful that way.
Anything with three masked characters is probably beyond resolving, but with only one or two it may be possible.


Has a commercial jet airliner ever stalled on takeoff / at low-altitude? In general aviation, a common reason for fatalities is low altitude stalls on takeoff or practising go-arounds.

Has there ever been an incident where a commercial jet airliner stalled on takeoff / at low-altitude?

(Excluding the one where the tank wasn't secured, pretty sure that wasn't commercial anyway.
Also excluding the MAX 8 incidents).

There are two fairly recent airliner crashes I can think of that were the direct result of an aerodynamic stall, one is Air France 447 and the other is Colgan Air 3407.
AF447 was at cruise height when the incident began, so that one does not fit with your question.
Colgan3407 was landing, so was pretty much "low level" (Does "jet" permit a turboprop?).

Following the clearance for final approach, landing gear and flaps (5 degrees) were extended. The flight data recorder (FDR) indicated the airspeed had slowed to 145 knots (269 km/h).[3] The captain then called for the flaps to be increased to 15 degrees. The airspeed continued to slow to 135 knots (250 km/h). Six seconds later, the aircraft's stick shaker activated, warning of an impending stall as the speed continued to slow to 131 knots (243 km/h) [...]
In its final moments, the aircraft pitched up 31 degrees, then pitched down 25 degrees, then rolled left 46 degrees and snapped back to the right at 105 degrees. Occupants aboard experienced g-forces estimated at nearly 2 G's. The crew made no emergency declaration as they rapidly lost altitude and crashed into a private home at 6038 Long Street,[24] about 5 miles (8.0 km) from the end of the runway

Going back in time, we can probably find a few more - some of the older examples may stretch what you consider "commercial aviation" by todays standards, and some most certainly were not "Jet" engined, but for the sake of completeness here goes:

1923 Daimler airways DH34 crash

The flight was a scheduled domestic passenger flight from Croydon to Manchester. It was also reported to be carrying mail,[2] although this was later denied by the General Post Office.[3] While flying over Buckinghamshire, a storm was encountered. Witnesses stated that an engine stopped, but was then restarted. It appeared to them that an emergency landing was going to be made at Ford End, Ivinghoe when the aircraft dived to the ground and crashed at Ivinghoe

Another DH34 a year later

The aircraft took off from Croydon Airport on a scheduled international passenger flight to Le Bourget Airport, Paris. Witnesses described the aircraft as flying low over Purley before nosediving to the ground

American airlines flight 1

American Airlines Flight 1, dubbed "the New Yorker",[1] was a regularly scheduled, multiple stop flight from La Guardia Airport to Chicago Municipal Airport. It had intermediate stops at Newark, New Jersey; Buffalo, New York; Detroit, Michigan; and South Bend, Indiana. On October 30, 1941, on the flight's leg between Buffalo and Detroit, the American Airlines Douglas DC-3-277B operating the route crashed into a wheat field approximately one half mile east of the town of Lawrence Station, Ontario

Llandow air disaster

the Avro Tudor was approaching runway 28 of Llandow aerodrome at an abnormally low altitude with the undercarriage down. The pilot attempted to correct the descent by increasing the power of the engines and brought the plane up. The aircraft rose steeply to 100 m (300 ft) attaining a nose-up attitude of 35 degrees to the vertical, and then the aircraft stalled

Aeroflot Flight 513

The aircraft was de-iced before takeoff. In the cockpit, the check captain observing the trainee's performance sat on the right; the trainee sat on the left. The captain and first officer remained in the cabin and did not assist the trainee and check captain during takeoff. Flight 513 took off from the runway at a bearing of 100°. At an altitude of 40–50 meters the angle of attack increased to the point of causing a stall. The Tu-124 never recovered from the stall and crashed into a field of snow

Delta flight 1080

During the takeoff, the Lockheed L-1011's left elevator became stuck in a fully upwards position, leading to the aircraft pitching up aggressively and causing the aircraft to lose speed and nearly stall. The pitching force, unable to be overcome by fully pushing the control column down, was counteracted by reducing the thrust on the L-1011's wing engines but not the tail engine. The differential thrust pitched down the nose of the airliner and allowed the pilots to land the aircraft.

China Eastern Airlines Flight 5210

On 21 November 2004, just two minutes after take off, the Bombardier CRJ-200ER fell from the sky and crashed into a lake in Nanhai Park, next to the airport, killing all 53 people on board and 2 more on the ground. [...] An investigation by the Civil Aviation Administration of China (CAAC) revealed that the plane had not been de-iced by the ground crew while it was parked on the tarmac.

Regional Airlines Flight 9288

On approach to Varandey Airport, the crew allowed the An-24RV's speed to drop and its nose to rise until in stalled. At 13:53, the aircraft struck a hill, crashed about 5 kilometers (3.1 miles) from the airport, and burned, killing 28 people (two crew members and 26 passengers).

West Caribbean Airways Flight 9955

The aircraft, a Let L-410 Turbolet, had just taken off from El Embrujo Airport at 9:50, when the left engine flamed out. The crew continued with the takeoff, but the speed of the aircraft decreased rapidly. The aircraft then banked dangerously too far to the right and stalled. The aircraft crashed into a mangrove forest, located just 113 metres (371 ft) from the airport runway.

Turkish Airlines Flight 1951

The crash was caused primarily by the aircraft's automated reaction, which was triggered by a faulty radio altimeter. This caused the autothrottle to decrease the engine power to idle during approach. The crew noticed this too late to take appropriate action to increase the thrust and recover the aircraft before it stalled and crashed.

TransAsia Airways Flight 235

Two minutes after takeoff, the pilots reported an engine flameout. Flight 235 climbed to a maximum height of 1,510 feet (460 m), then descended. The other engine, still working, was shut down mistakenly.[1][2] Immediately before crashing into the river, it banked sharply left and clipped a taxi travelling west on the Huandong Viaduct

So, yeah, quite a few actually!


Does transition to SBAS or GBAS navigation require alignment of the intermediate segment with final segment? I am studying how the Satellite Based Augmentation System (SBAS)  or the Ground Based Augmentation System (GBAS) affect the Instrument Approach Procedures, and I noticed that in ICAO's Aircraft Operations manual (8168, Volume II) it states:

  Part III, Section 3, Chapter 5, 5.3.1: "General. In the intermediate approach segment any sensor can be used; however, the transition to the SBAS navigation shall be made 3.7 km (2.0 NM) prior to the FAF.

  Part III, Section 3, Chapter 5, 5.3.2: Alignment. The intermediate approach segment of an SBAS procedure shall be aligned with the final
  approach segment.

  Part III, Section 3, Chapter 6, 6.3.2: "The intermediate approach segment of a GBAS procedure shall be aligned with the final approach course."

Does the above quotes imply that the intermediate segment must be aligned with the final segment 2.0NM prior to the FAF, in order for the aircraft to be able to switch to SBAS navigation?

In addition, concerning both SBAS and GBAS, I would like to make sure if the rule concerning the alignment of the intermediate segment with the final segment implies that the whole segment (each leg of the intermediate segment) has to be aligned with the final, rather than only the last leg.

Could someone point me in the right direction?

I've never seen an SBAS approach that had more than one intermediate segment. In all cases, the IAF/IF is aligned and more than 2 NM from the FAF. There may be multiple IAFs that feed directly to the IAF/IF. A good example is the PSM RNAV (GPS) RWY 16:

As you can see, ITAWA is aligned with PUDYE.  You can get to ITAWA via SATAN, ITAWT, or directly. Arriving directly at ITAWA requires flying the procedure hold to establish the final approach course.

GBAS (or GLS in the US) is intended as a functional replacement for ILS and as such one of the goals is that GLS mimics the behavior of an ILS. As a result, the GLS procedures are designed to be flown just like an ILS approach.

You can compare the GLS and ILS approaches to Newark Rwy 04L below:

If you compare the two, you'll see the path from the FAF HOWYA to the MAP is identical. The IFs are slightly different as the GLS IF EMBAY is a pure extension of the glide path out to 2800 elevation. The ILS path is slightly different vertically between the IF and FAF due to having to identify the IF via the DME.


Why descent below DA/DH or MDA could only be accomplished using natural vision? I'm curious why descent below DA/DH or MDA could only be accomplished in visual approach conditions? Is there any regulation to indicate that?

The FAA states it pretty explicitly in their definitions:

  Decision Altitude (DA). A specified altitude in the precision approach at which a missed approach must be initiated if the required
  visual reference to continue the approach has not been established...

and similar for MDA

  Minimum Descent Altitude (MDA). The lowest altitude, expressed in feet
  above mean sea level, to which descent is authorized on final approach
  or during circle-to-land maneuvering in execution of a standard
  instrument approach procedure where no electronic glide slope is
  provided.

To over simplify it a bit: As noted in the comments, ignoring the not in use CAT IIIC approaches the whole point of an instrument approach is to get you down to a specified point where you should be able to see the runway. If you can't, then the weather has degraded and you must either try again or go to an alternate. The big idea being that at some point you need to be able to see the runway in order to land on it. Depending on how precise the equipment you are using is (ILS, GPS, RNAV, VOR, (AFD dare we say)) the height you can get down to without being able to see the runway changes.

As for why there are minimums its likely historical, and at least in part practical. Not all approach systems/options take you right down to the threshold like say a circling approach and its possible to fly a poor approach that is within limits and not be directly in line with the runway when you pop out of the clouds. Thus you will need some time and distance to align visually, so minimums exist. On the historical side, early attempts at instrument landings systems we less than optimal to say the least and you would need time to pop out and align.


Would connecting propeller blades with a continuous ring reduce induced drag? Turbine engines are covered, which of course is to contain the process (just like a super/turbocharged engine is as soon as the air enters the intake). But it got me thinking, doesn't this also reduce - or even completely remove - the induced drag around the tips of the fan blades?

Would it be possible to have a similar setup for a standard prop, it doesn't even have to be stationary, it could be a ring connecting the tips of the prop, spinning with it, like an infinite winglet.  It has the added safety benefit that it'll be visible when the prop is spinning. And I figure if the ring is strong enough to maintain its circumference, the load on the prop should be marginal since it's spinning around its own center of mass.

Is the induced drag on the prop not large enough to warrant any thought, or would such a prop-winglet-ring (I'm sure there's a real name for it, anyone know what I'm talking about?) cause other disruptions of the airflow? Or perhaps there are other reasons, like it would simply be too hard to solve for constant-speed props?

What you're talking about does exist, they are called Q-Tip Propellers.

Remember that a propeller blade is just an airfoil - like a wing - and the basic aerodynamics are no different than a wing. But the rotation of the blade creates more phenomena than a wing, in particular the helicoidal vortex one sees behind the prop and causes all sorts of propeller effects.

In theory, nothing would prevent us from having winglets at the prop tips :
the advantages would be

Making the prop more efficient by reducing the induced drag (same as a winglet on a wing )
Reducing noise
Keeping the propeller tip speed subsonic by decreasing its length

The big problem is in aerodynamic stresses, and as far as I know there have been some quite spectacular failures during testing, so the solution is now to give a greater sweep to the tips (see that as equivalent to the 777 wing tip compared to the 787 for instance). Try and find articles on the Hartzell Q-tip.

As naval propellers are wider and capable of deealing with bigger torque stresses, modern ones do have winglets. You could find some pictures on the web.


Do the A-320 landing gear extend if the mechanism is activated above a limiting speed? I've been told that on the A-320, if the landing gear lever is lowered with the airspeed above a limiting speed, the system will not extend the landing gear. Further, once the aircraft is slowed to an appropriate speed in this condition (i.e. lever down but wheels up), it is necessary to cycle the landing gear lever up and then back down in order to extend the landing gear.

Can anybody confirm or deny, ideally with a reference, if this description is accurate, and if so, what indications accompany such a lever-down/wheels-up condition.

The hydraulic pressure of the landing gear is shut off above 260 KIAS. The maximum gear retraction speed as for the placard in the cockpit is 210 and the max extension speed is 250 KIAS if I'm not mistaken.
From the FCOM 1.32.10
(download from here for example: http://www.smartcockpit.com/plane/AIRBUS/A320.html)
A320 Landing Gear, page 3
OPERATION OF GEAR AND DOORS

NORMAL OPERATION
The green hydraulic system actuates all gear and doors. When the aircraft is flying faster than 260 kt, a safety valve automatically cuts off hydraulic support to the landing gear system. Below 260 kt, the hydraulic supply remains cut off as long as the landing gear lever is up.

The logic is then described in a diagram in the FCOM (see link above).

If ( ADR1 < 260KT || ADR3 < 260KT || MLG on gnd ) && ( LG lever down || safety valve already open )
-> open safety valve

If I interpret this diagram correctly you can leave the gear lever down and the gear should start to extend once below 260 kt because the second side of the and gate just requires the gear lever to be down.
On the contrary if you want to retract the gear and you fly faster than 260 kt then the safety valve closes. Even if you fly slower than 260 kt it will stay closed because the second half of the and gate is not fulfilled. You need to move the gear lever down, then the safety valve opens and then you can move it back up and the self maintained state of "valve already open" will allow you to retract the gear after this cycle of the landing gear lever.
Part 2 of the questions regarding the indications
Indications are as follows:

3 red lights above the gear lever as long as landing gear is not locked in the selected (down) position according to A320 FCOM LANDING GEAR page 18, GEARS AND DOORS, LANDING GEAR INDICATOR PANEL
A red down arrow next to the landing gear lever when the gear is not locked down when the aircraft is in the landing configuration (page 19)
6 red triangles on the ECAM wheel page (3 gears, two redundant landing gear control units) (page 21)
An amber pulsating L/G CTL legend on the ECAM wheels page when the landing gear position and gear position do not agree (page 22)
GPWS audio call out and associated red light outboard of each PFD (see below)
ECAM warnings as follows

ECAM warnings according to the WARNINGS AND CAUTIONS section of the FCOM LANDING GEAR chapter, page 23:

red flashing master warning and cricket aural warning: GEAR NOT DOWNLOCKED on the upper ECAM display, ECAM WHEELs page is called automatically on the lower ECAM display with the indications stated above
red flashing master warning and cricket aural warning: GEAR NOT DOWN, when below 750 ft radar altitude with both engines below 75% N1. Other conditions require flaps to be extended to 3 or FULL but above 260 kt you probably would cause structural damage if you were to attempt that.

From the A320 FCOM Chapter 34 NAVIGATION, GPWS, page 6.

Two aural warnings may be triggered, depending on the area: "TOO LOW-GEAR" [below 190kt and 500ft RA] or "TOO LOW-TERRAIN" [above 190 kt, up to 1000ft RA]

If you are flying faster than 260 kt the GPWS should create a "TOO LOW-TERRAIN" warning.


How and where is the ILS Glide Slope DDM to dots conversion defined? The Instrument Landing System (ILS) gives output in terms of Difference in Depth of Modulation (DDM) to the avionics. But all display uses dots, also TAWS system definitions also uses dots as reference.

There is a generic convention that 0.175 DDM is 2 dots. This is why one dot is 0.0875 DDM and this value is the range for boundary used in ILS. Is there any formal reference for this conversion? I am looking at ICAO Annex 10, but could not find any relations yet.

The problem you're having is that there is no standard definition of dots. There are various standards that come into play and I'll try to explain how they fit together.

As you noted +/- 0.175 DDM is specified as "full-scale deflection."  This is in Annex 10 and, as mins noted, DO-192 for glideslope. It's also in DO-195 for localizer.

But what is 0.175 DDM and why is it referenced as "full-scale"?  DDM is the Difference in Depth of Modulation of the 90 and 150 Hz audio tones that comprise the LOC and GS signals. The significance of 0.175 DDM is it defines the value at the limits of the course deviation display (full scale).

Annex 10 requires the installation of an ILS requires (for most runways) the radiation pattern of the localizer to place full scale at the threshold of the runway at +/- 350 feet. With the LOC antenna at the other end of the runway, this sets the angular deviation guidance for the approach.

This standard means that shorter runways will have a wider LOC beam than a longer runway. There are minimum and maximum angular widths, so very short runways (<~3000 ft) will all have the same maximum width and very long runways will have the minimum width.

Back to the ILS MOPS (DO-192 and DO-195). The MOPS has a standard for display of deviation in terms of centering accuracy (0.0 DDM) and linearity of the display out to "full-scale".  Even if the display isn't part of the ILS receiver it's bound by the MOPS.  In this case (which is common today), the radio is tested to comply at the "output to the display" and must have an interface spec to allow a display to be interfaced. The display - even if it's part of a glass PFD - has to meet the MOPS. And after installation they have to be tested to show they work together within the spec.

But what about the dots? The MOPS don't specify what the display looks like. The requirement is that the pilot can discern the center, the limits, and that the indication is linear within a few percent. Dots are not specified or required.

You identified a generic convention of 2 dots at full scale. I would agree this is common on air transport aircraft. But the GA aircraft I learned to fly in all had 4 dots on the CDI at full scale. And to be even more confusing, many of the new RNP deviation displays don't have any dots, just a bar with a center and end marks with the steering line on top.

In over 20 years of working in avionics, I've never found anyone who could say definitively why 2 dot displays became standard. My suspicion is that flight standards (and airline op specs) typically call for a missed approach if the deviation exceeds half scale. The 2 dot display puts a dot at that point and at full scale. It makes for good human factors. Getting the needle inside the outside dot means you've captured the signal. After getting on course, a deviation out to the first dot means it's time to go missed.


Is there a good online source on rotating detonating pulse engine? I can't find a clear presentation that would explain the workings of a rotating pulse detonation engine graphically, only useless (to a layman) cfd simulations of the combustion chamber. Please share if you know a good online source to learn about this design.

The idea seems fairly simple. Here's a [icture from one article and an explanation from another.

From Ars Technica

Micro-injectors squirt a pressurized mixture of fuel and air into a combustion chamber the shape of a long ring (annular cylinder). Then, this gas mixture explodes, with the explosion spinning around the circumference at supersonic speeds. The high pressure produced by the detonation forces exhaust gases upwards (in this diagram) and out of the chamber, where they expand through a nozzle and produce thrust that can be used to push a turbine or aircraft.
Unlike a PDE, which requires repeatedly initiating detonations, an RDE only requires one detonation that spins around and around the chamber, continuously producing thrust.

So there's no conventional turbomachinery. The detonation wave moves in a circle round the chamber, jumping from injector to injector. The only moving parts are the fuel injector pumps. I'm not sure how you throttle it, though.


Will I not be able to descend with an airfoil that stalls at -0.5°? This stats says this airfoil stalls at -0.5°,does this means if I have this airfoil as a wing ,I won't even be able to  have a descent aoa?

!

Don't believe everything that a computer tells you.

This airfoil is listed in the airfoiltools site, and I took the liberty to copy its XFOIL results here:

I selected the Reynolds numbers 500,000 and 1,000,000, which should be at the low end for a man-carrying aircraft. As you can see, the stall angle of attack is at 15° and the maximum lift coefficient is between 1.7 and 1.75. To achieve a lift coefficient of 1.89 with that airfoil will need a much higher Reynolds number, but max. L/D is already around 120.


What happens if a passenger passes through the boarding gate, but doesn't get on the aircraft? At one time, I was bored because of a delayed flight and that question arose in my mind just for curiosity. I haven't observed any mechanism that checks or counts the passengers who get on the aircraft, so there is a possibility of abuse.

I guess airport security is responsible for that. The passenger may try to get on another aircraft or may get in the airport through a different gate. It must be a bigger problem at international airports and terminals.

If the passenger is caught, what does airport security do, just arrest? What does the airline do if they realise the incident before or after the take-off?

Passengers are counted before the plane leaves the gate. Since everyone's boarding pass is scanned at the gate, they know how many passengers should be on board. If that number does not match, it may be a red flag that someone has sneaked out.

The missing passenger can usually be located as using the lavatory, changed seat etc. As a last resort, the passenger's luggage is removed from the cargo hold for security. If things get to that stage, the flight is inevitably delayed for sure.


How many antennae are there on a typical small aircraft, and how are they connected to the avionics? As far as i know there are a bunch of antennas on board of a small (EASA CS-23) plane:

GPS is for GPS navigation in MHz
VHFVOR is for NAV and Glideslope/Localizer
ADF is for ADF
DME is for DME
VHFCOM is for COM

Questions:

So there are three, two or one VHF antenna?
How is the VHF signal split?
If I have two NAV instruments (KN53 and GNS430)
how would the antennae would be cabled to them?

The VOR/Localizer/Glide Slope antenna is usually V-shaped and points backwards on the vertical stabilizer of the plane. On some aircraft, they point forward. They look like old-fashioned wire clothes hanger wire. Newer versions can be blade shaped.  sometimes they are boomerang shaped like the old car phone antennas of the 90s.) They usually have a splitter that allows two radios to use the same antenna. e.g. splitter

COM Antennas are usually a white rod or blade. You can’t split the signal for a COM radio using a rod antenna so one antenna is required for each radio. Not sure about blade antennas.

A GPS antenna is usually somewhat teardrop or ovoid shaped. Garmin GPS You can’t split the GPS signal, so one antenna per device is required. Some ADSB systems have an embedded WAAS chip and need their own GPS antenna.

GPS and COM antennas can be combined.

IFR capable airplanes also have a marker beacon antenna. These can be embedded in the fuselage or external.

A transponder is required in most airplanes and they have their own antenna. These are a small 2" or so stick with a ball on the end. They are mounted on the bottom of the fuselage.

Airplanes in the US at least are required to have an ELT. Old style ELTs and some of the newer ones have a piece of steel with a spring-like coil in them. Newer versions are more aerodynamic.

ADF antennas come in a wide variety of shapes. Some of the newer ones look like GPS antennas.

DME antennas usually look like a shark’s fin.

Composite aircraft often have the antenna embedded in the frame so it is not visible.

This Cessna video describes all of the antennas on a modern C182. I don’t have most of them on my airplanes and the ones I do have look a lot different.


What makes the F/A-XX twin-engined tailless fighter controllable with engines out? There are operational tailless aircraft such as B-2 spirit, X-45.
It seems yaw stability/control at slow speed is achieved aerodynamically, using either split elevons (B-2) or adjacent elevons deflected in opposite directions (X-45) Combined with negative wing tip twist angle, both solutions increase drag / relative swept angle, and by the way, yaw stability. Also note both examples have a very small vertical surface ahead of CoG.

(Left, Right)

On the other hand, looking at this picture of a F/A-XX Boeing concept, it seems vertical surface ahead of center of gravity is quite significant, and forward swept trailing edge is in contradiction with aerodynamic stabilisation using split elevons.

One might think this isn't such a big issue since this concept might use permanent vector thrust adjustments on yaw axis.

Question is : Does it fly with no yaw thrust vectoring?

Edit: One other issue affecting yaw stability is forward swept elevons. Their "split brake" efficiency decreases while sideslip angle increases :

(Source)

Those flying wings even before fly-by-wire (FBW) came along are controlled in the yaw axis by asymmetrically deploying the wing-tip mounted split brakes.*
(Source)
With the engines closer to the centerline the effect of one engine inoperative (OEI) is not as pronounced as on jet-liners with the engines far out on the wings. Even with the lack of a vertical fin, the effect will be less than if the engines were more outboard.
In the Boeing 727 it was discovered that an engine out during takeoff did not have a noticeable effect on the yaw, to the point such planes with close engines had different certification for the V-speeds to allow extra time for the identification of the failure. As well as glareshield mounted lights to indicate an engine failure in a clearer manner.
As you can see in the pictures you have, the engines are close, like the 727. And therefore the standard yaw control via split brake rudders* is sufficient.
As for the F/A-XX, it seems it's still a concept. The second image is also a single-engine UAV.

* The aileron-like device that is split at the wing tips. By creating asymmetric drag, the plane yaws.

Unlike conventional aircraft, truly "tailless" flying wings cannot use a rudder for lateral control as it was absent, so a set of clamshell-like double split flaps on the trailing edge of the wingtips were used. When aileron control was input, they were deflected up or down as a single unit, just like an aileron. When rudder input was made, the two surfaces on one side opened, top and bottom, creating drag, and yawing the aircraft. By applying input to both rudder pedals, both sets of surfaces were deployed creating drag so that the airspeed or the glide angle could be manipulated.
(Wikipedia)


What are the requirements to get your private pilot certificate? I am 15 right now and I want to get the Certificate/License when i am 17. (17 is the age right?)

How much training do I need and is there any written tests etc?

What kind of IFR training will be included/needed?

In the US, you'll need to be 16 old to start training and 17 to get your Private Pilot's license in a piston-powered aircraft. To get your license you'll have to have a minimum of 40 hours total time, including at least 20 with an instructor and 10 solo. You will also have to pass a written test, and get a Medical certificate.

Details:

If you're pursuing a Private Pilot license, per 14 CFR §61.83(a) or (b) you must be 16 years old to get a student pilot certificate (or 14 years old if you're going to train in a balloon or glider.)

NOTE: 14 CFR mean Code of Federal Regulations, Title 14 - Aeronautics & Space.  §61.## means "Part 61" which covers Certification of Pilots and Instructors.

To start training you will also have to pass a medical exam for a 3rd Class Medical Certificate, per 14 CFR §61.23(a)(3). If you're generally healthy, getting a 3rd Class Medical isn't terribly difficult.

Assuming you successfully complete the training, per 14 CFR §61.103(a) or (b) you can get your Private Pilot license for an airplane at 17 years old (or 16 years old for a balloon or glider.)

Per 14 CFR §61.109(a) to get your license you will need to have completed at least 40 hours of flight time, including at least 20 hours of time with an instructor and 10 hours of solo time.

However, many pilots take more than the required minimum 40 hours to get their license. My suggestion is to try to complete your training in as short a calendar time as is possible. By taking lessons frequently (more than once per week) you will minimize the amount of "repeat" lessons, which will help you get your license with fewer hours. If you're only taking a lesson every few weeks, you will end up repeating topics and spending more than the required 40 hour minimum.

Per 14 CFR §61.35, you will have to complete a ground school program (can be with an instructor, or a "home study" course) before you take the Knowledge test (aka: written exam.)

Per 14 CFR §61.109(a)(3), you will need at least 3 hours of "Instrument" flight training to get your Private Pilot license. That's really not intended to be enough for you to fly safely on instruments, but to give you a basic set of skills to get out of a low-visibility situation. If you keep up with your flying, getting in Instrument rating is a GREAT idea, once you have enough hours.

You can look up the specific wording of all of the rules at http://www.ecfr.gov/

Another good resource for student pilots is the Aircraft Owner's and Pilot's Association's Learn to Fly page http://www.aopa.org/Pilot-Resources/Learn-to-Fly.aspx.


Do box-wings suffer from induced drag the same way as normal wings? Most wings suffer from induced drag due to a pressure difference above and below the wing causing air to sneak around the tip, forming a vortex. There are various methods to minimize these effects, such as winglets.

However, looking at the Synergy aircraft as an example, box wings have no wing tips. Disregarding any other parts of the aircraft, are the wings actually free from induced drag? Or are they still causing induced drag, just in a way I'm not able to think of with my limited fluid dynamics experience?

Synergy aircraft with box wings (picture source)

I've read somewhere that a traditional bi-plane design is less efficient due to the wings interfering with each other (apparently something addressed by the synergy aircraft by placing the upper wing further back or something), and the upper wing is actually more of a tail-plane, pushing down, thus further increasing the airspeed in between the airfoils if I understand correctly and eliminating the pressure differential from the top of the upper wing to the bottom of the lower wing, and both wings would of course generate normal drag by cutting through the air, but I'm only interested in the induced drag at this point.

The box wing is only better when you compare wings with identical span. The two wings of a box wing work in different Treffz planes, so the downwash is spread out vertically. The difference in induced drag to a single wing is not big, just a few percent. The friction drag is higher (see below), as is the structural mass, so the box wing needs to create more lift. This makes the induced drag of a box wing effectively higher than that of a single wing.

What is induced drag, anyway? It is the consequence of creating lift over a limited span. The wing creates lift by deflecting air downwards. This happens gradually over the wing's chord, and creates a reaction force orthogonally to the local speed of air. This means the reaction force is pointing up- and slightly backwards. This backwards component is induced drag! Wingtips are not involved and are not causing induced drag. Lift creation is.

If you fly fast, there is a lot of air mass streaming past the wing per unit of time, so you need to deflect the air only slightly. Your induced drag is small. Sames goes for a large span: There is more air which can be deflected, so the induced drag is small.

A box wing needs two slim wings per side, which will have a smaller chord than a single wing of the same surface area. So their Reynolds number is smaller, and their friction drag is higher. Also, the wing spar is less thick and will need to be heavier to carry the same lift!

If you drop the restriction of keeping span identical, the optimum single wing can afford to have more span (due to its better structural efficiency), and away goes the advantage of the box wing. And once you look at the full picture and add structural mass, the box wing never had this advantage in the first place.

Yes, but what about the Synergy?

The Synergy is a clever design with some advantages, but it cannot cheat physics. These are the advantanges:

The pusher prop keeps the airframe free of wake turbulence, so more area can be kept in laminar flow.
The pusher prop sucks the air from the rear fuselage, effectively avoiding separation.
The two stubby tailbooms and fins give great protection for the propeller area on the ground.
The compact layout keeps the stabilizing effect of the propeller small, so maneuverability does not suffer much.
The use of composites and glider airframe technologies reduce friction drag.
The diesel engine consumes cheaper jet fuel and is more fuel-efficient than a gasoline engine.

Note that I did not mention the box wing design?

Here are the disadvantages:

Wing sweep in a propeller aircraft looks cool, but increases drag, because the wing must be bigger to create the same lift.
In total, this configuration has four vertical tails, each of them with its own interference drag and a short chord which, again, increases drag over a comparable single vertical tail.
The stretched-out horizontal tail is also less effective than a smaller single surface with more chord and more distance from the center of gravity.
The compact layout provides little pitch or yaw damping. I wonder what the ride qualities in gusty weather are.

I would expect that a more conventional layout along the lines of the fs-28 would be even more efficient.

Akaflieg Stuttgart fs-28 in flight (picture source)


Why do we have to do an alternate brake check during preliminary cockpit preparation? As part of the preliminary cockpit preparation in my company’s older A320s we have to perform an alternate brake system check but not in the newer ones as per FCOM Normal procedures as below. I found out that there is a system difference but didn’t fully understand could someone please explain the difference?

  ALTERNATE BRAKING SYSTEM
  Note: The purpose of this check is to
  verify, before the first flight of the day, the efficiency of the
  alternate braking system (absence of “spongy pedals”).
  Y ELEC PUMP
  pb-sw....................................................................................................CHECK
  OFF
  CHOCKS..............................................................................................................CHECK
  IN PLACE PARK BRK
  handle.......................................................................................................................OFF
  BRAKE
  PEDALS.....................................................................................................................PRESS
  Apply maximum pressure on both pedals.
  BRAKE PRESSURE (on BRAKE press
  indicator).................................................................CHECK
  Pressure must build up without delay symmetrically on left and right
  sides for the same application simultaneously applied on left and
  right pedals. With full pedal deflection, the pressure must be between
  2 000 and 2 700 PSI.
  BRAKE PEDALS................................................................................................................RELEASE
  PARK BRK
  handle........................................................................................................................ON
  The parking brake must be on during the exterior inspection to allow
  the flight crew to check brake wear indicators.

On the old system the switching between the normal braking using the green circuit  to the alternate braking using the yellow circuit is done by an automatic switching valve pressure dependant (Automatic selector).The alternate system being very little used, it happened the valve did not switch from green to yellow when loosing the green circuit

please refer to the old system here below

http://www.a320dp.com/A320_DP/landing-gear/images/braking-schematic-d.jpg

On the new system the alternate brake is activated by a specific computer(ABCU) decision and is automatically tested by the computer at power up as soon as the hydraulic pressures are available, please refer here below to the new circuit

https://m.facebook.com/A320Theory/posts/normal-brakingnormal-braking-is-obtained-when-the-green-hydraulic-pressure-is-av/790898894383508/

Hope this has been useful and explains why it was very important to do the testing on the old system


Can pop-out floats be installed post-manufacture on all helicopters? I have seen in many helicopters pop-out floats installed.

Are they the optional, or do they come pre-installed with every helicopter?

Pop out floats can be installed easily on any helicopter with Tubular landing skid type landing gear. Its essentially an auto-inflatable tube attached to the landing gear. Have a look at this:


Does the FAA require a specific grade of supplemental oxygen for part 91 operations? In a recent FAA knowledge exam, a question was asked whether medical grade or industrial grade oxygen can be substituted for the aviation grade supplemental oxygen for General Aviation, Part 91 flight. I have looked in the FAR/AIM for answers. But, there is not a specific regulation prohibiting the use of medical grade oxygen in aviation.
The FAA Safety Brochure OK-09-439 recommends that medical grade oxygen should not be used. Yet, there is a Civil Air Patrol document (another government entity that is regulated in part by the FAA) that refutes this due to changes in the understanding and delivery of supplemental medical grade oxygen.
Does anyone have a clear cut and recent regulation or document that clarifies this question? Please include links to the documentation.

Regulations don't usually include detailed technical requirements or specifications, but AC61-107B - Aircraft Operations at Altitudes Above 25,000 Feet Mean Sea Level or Mach Numbers Greater Than .75 / with Change 1 says (p. 33, emphasis in the original):

Only oxygen that meets or exceeds the Society of Automotive Engineers
(SAE) International Aerospace Standard AS8010 (as revised), Aviator’s
Breathing Oxygen Purity Standard, should be used.


Does EASA approve the use of localizer back course? Does EASA approve the use of localizer back course? Are there some countries in Europe that allow it?

Depends what you mean. If you mean "are they approved at EASA airports?" I can't help except to say that I have never seen a plate for one, flying into most EASA countries into a wide variety of airports.

If you mean "are EASA aircraft and pilots allowed to fly them in countries that do allow them?", I can tell you that in the Radio Navigation syllabus for the EASA ATPL exams there is a learning objective that the student must know what to set on an OBS or HSI to fly a back-beam localiser approach (if you want to check it's LO 062.02.05.02.08 in the new syllabus, 062.02.05.02.13 in the old one which is just about to go out but more widely available at the moment). This implies that flying one in an EASA-registered aircraft with an EASA licence and IR is not prohibited.


How does a two-stage supercharger work? How does a two-stage supercharger like the one found in a Rolls-Royce Merlin work? Does the pilot have any control over it?

Actually yes.  A two stage supercharger used a compressor with two or more compression stages, often with an inter cooler between them, driven by a transmission with different gear ratios to control the compressor speed.  A schematic of this type of supercharger can be seen here on a Rolls Royce Eagle engine.

On the Supermarine Spitfires powered by the larger Griffon engines the supercharger speed had both a LO and HIGH setting and could either be manually controlled by the pilot or automatically controlled.  The HIGH setting was not recommended at lower altitudes to prevent overboosting during operations there.


What is this part on the Me 262? I have seen this part on a me 262 model kit but I don't know what it is... I've make some research on it but I find nothing! Maybe it can be radar, or a weapon? Do you know what is this? And also its function?

They are Werfer-Granate 21 rocket launchers -

The Werfer-Granate 21 rocket launcher, also known as the BR 21 (the
"BR" standing for Bordrakete) in official Luftwaffe manuals, was a
weapon used by the German Luftwaffe during World War II and was the
first on-board rocket placed into service by the Luftwaffe, first
introduced in mid 1943.

On the Me262 they were mounted one each side of the nosegear well.
This is the exact aircraft your model is based on -

Messerschmitt Me262A-1a 'Green 3' of the Geschwaderstab [headquarters unit] of JG 7 prepared
to move off across the concrete surface at Brandenburg-Briest in
February or early March 1945. The aircraft, finished in a relatively
rare application of streaked horizontal lines, has been fitted with a
pair of 21cm WGr air-to-air mortar tubes visible beneath the fuselage
aft of the nose wheel. (source)


Has the number of angle of attack sensors been reduced in Boeing 737 Max, in comparison to the older models? Boeing 737 is flying since 1967. I would like to know first how many ' angle of attack sensors' does it have, and has they number been reduced for Boeing 737 Max, in comparison to the older models like 737 Next Generation or 737 Classic, for instance?

There is already an answer on here about how many the B737 MAX have (the answer is 2), and I've found this Airworthiness Directives which states somewhere far down:

  Do a general visual inspection of the left and right angle of attack (AOA) sensor as applicable, to determine if a certain AOA sensor with a paddle type vane is installed[...]

This brings me to the conclusion that there always have been 2 AOA sensors on the B737, as this Airworthiness Directive is dated to 2012. (To be fair, at least since then there have always been 2 AOA sensors).

As a decrease in the amount of AOA sensors from 1967 to 2012 seems unlikely to me, I'd say there are and always have been only 2 sensors on the B737. Following that train of thoughts, the amount of sensors has thus not changed with the different versions of the B737

Interesting sidenote: The AD is valid not only for B737 but also affects all kinds of 727 and 747 models, which indicates that they also have 2 sensors only, but doesn't necessarily mean that other Boeing aircraft have more!


Why aren't variable swept wings being used in newer military aircraft? After finding out how well the variable swept wing design served the F-14, why hasn't it been used more often (outside of a select few)? Almost all of the fighters after the cold-war era haven't used variable swept wings.

Bonus question: is there any significance to the way the F22/F35 wings are shaped almost like triangles?

The main advantage of variable sweep wings is that they reduce drag over a wide range of Mach numbers.

However, the variable sweep wings are not being used in present day aircraft for a variety of reasons.

The variable sweep wings are heavy and mechanically complex, affecting performance and maintenance.
The swing mechanism takes up too much space inside the wing that could be used for fuel.
It is difficult to position hard-points in the swinging part of the wing, affecting payload.
Advances in aerodynamics and better engines mean that the drag reduction using varying the wing sweep is not so critical anymore.
Also, the newer combat aircraft are not trying to outdo each other in speed, trying to gain the edge using better situational awareness, stealth etc.

However, the most important reason for variable sweep wings falling out of favor is that they are nearly impossible to be made stealthy. One way to make the aircraft stealthy is to reflect the Radar energy away from the source. In order to achieve this, the wings of stealth aircraft are designed in a specific way.

However, if the aircraft have a variable sweep wing, this condition is all but impossible to achieve. If stealth is still required, it has to be through more Radar absorbent materials, which further increases weight and maintenance requirements. The last variable sweep aircraft, B-1B, for example is designed to be a low level penetrator, unlike B2.

Because of these reasons, the variable sweep wings have fallen out of favor.

As for the second question, the wings of F22/F35 has basically a trapezoidal wing configuration, which gives a high performance at high speeds.

However, the main reason for using this type of wings is the excellent stealth characteristics they provide compared with other type of wings. One disadvantage of this type of wings is that their low speed performance is not up to the mark, but this has mostly been overcome by the use of improved control surfaces (like flaps, slats etc).

Interestingly, USN considered a variable sweep F22 for carrier operations, but the project was ultimately dropped.


Can fighter aircraft glide? In case of engine failure, do the fighter jets have such a stable airframe so that they can glide their way through to the nearest landing strip (As it happened in the case of Gimli glider)?

All airplanes can glide, if they couldn't they wouldn't be able to fly in the first place. When you glide an aircraft you are converting height into airspeed, which you can use to move across the ground. How far you can go across the ground for height lost is called the glide ratio for the aircraft. Gliders have a very high glide ratio as their wings are designed to provide lots of lift at low speeds, fighters have a very low glide ratio as they are designed to provide lift at a much higher speed enabling the fighter to achieve high airspeeds efficiently.

So a fighter will glide, it just won't be able to glide that far over the ground. If a fighter has enough altitude to trade for speed and a strip close enough by it can be done (and has been done in the past) by a skilled pilot.