How does maximum speed vary with altitude?

This question isn't clear because "speed" means different things to different people, and this is also engine specific.  Every aircraft will have a different speed vs altitude profile.  In general...

Mach will go up, as you increase in altitude.  This is because the lower density air lowers the speed of sound (in an absolute sense).

True airspeed will go up as you maintain a constant indicated airspeed.  Conversely, maintaining a constant true airspeed will increase your indicated airspeed as you descend.

Normally, a jetliner should not be seeing true air speed at sea level.  For jetliners, even if your engines are good enough to get you there, the aircraft has an indicated airspeed structural limit that keeps you from accelerating.  You can keep the same speed (at structural limit) through higher altitude, which increases your TAS.

There is a chart here which should help show the relationships between altitude and speed:

A simplified chart for determining Mach number and true airspeed from airspeed-indicator readings:


Airfoil and Wing Analysis for Transonic Speed (M = 0.85 and Re = 42 million)

I am working on a project where I have to design a plane which has to
  cruise at 0.85 Mach at 35,000 ft. While working on the wing design, I
  quickly discovered that 6 series airfoils, with a good amount of
  thickness ratio, have a low Critical Mach Number. So, I started
  exploring Supercritical Airfoils (SC Airfoils). For doing analysis of
  SC airfoils can XFLR5 be used?

This is a direct copy-paste from XFOIL manual which is the underlying library for 2D analysis in XFLR5

One should always be wary of trusting solutions which show regions
of supersonic flow.  Such flows can be reliably predicted only with
a truly nonlinear field method (such as the MSES code).  As a rule
of thumb, if the maximum Mach number doesn't exceed 1.05 anywhere,
shock losses will be very small, the Cp distributions will be
reasonably accurate, and the drag predicted by XFOIL is likely
to be accurate.

  Moreover, Aspect Ratio and Sweep also influence Critical Mach Number.
  Is there are a relationship that connects these three entities?

AR does not influence the Critical Mach Number. However, sweep does
A direct copy-paste from old USENET message archive.

The airfoils on a swept wing behave as though they were flying at a
reduced speed, reduced Mach number, and reduced dynamic pressure.

effective speed = V cos(L)
effective Mach  = M cos(L)
effective q     = 0.5 rho V^2 [cos(L)]^2

where L is the sweep angle, and V and M are the airplane's speed and
Mach.

  Lastly: is there a credible source that compiles the analysis of
  Transonic Airfoils (Supercritical Airfoil)? (Similar to Theory of Wing
  Sections by Abbott and Doenhoff, which contains data for conventional
  airfoils)

NASA supercritical airfoils: A matrix of family-related airfoils

  I tried using XFLR5 for the analysis at Re = 42 million and 0.85 Mach
  Number. But, solution diverges

There was 2 issues I could see in the given dat file.

finite TE gap and coarse coordinates.
I fixed this in XFOIL by going to GDES menu and setting the TGAP to 0 with the blending distance 1.
And then used MDES to filter out glitches in the UE distribution.
Mach effects.
Say your plane is flying at CL=0.6 but what happens when the MACH increases with constant alpha is, it improves CL. So the non-converging solution actually refers to a very high CL in the ranges of 3-4 which is not realistic and cp is high above the critical CP. Please refer to the images attached. when you directly use cl or reduce alpha accordingly you can get this sorted as shown on the last pic.


What type is this aircraft and engine at Munich airport?

The fact that it is a single deck, twin engined aircraft with that distinctive wingtip confirm that it is an Airbus A350, operated by Lufthansa. The -900 series is the only A350 in service at this time, and Lufthansa has not ordered the -1000 series.

The A350 is powered by Rolls Royce Trent XWB engines.


What is the difference between a concourse and a pier?

In terminal designs, the main difference between a pier and a concourse is that a pier is connected by terminal buildings on one side, but a concourse has separate terminal buildings connected by transportation either under or over ground.

In practice, the terms "terminal" and "concourse" are often used interchangeably, with "satellite" being used in that specific case, and "piers" being referred to as terminals or concourses.

Simple

Small terminal with a few gates.

Linear

Longer than a simple terminal, with gates on one side. May be curved or straight.

Concourses

Standard terminal with gates on one side, with additional linear terminals connected underground or by buses (occasionally by a bridge), each with gates all around. Each is generally designated as a different concourse.

Standard

A main terminal area, with long extensions having gates down one side. May be designated into different terminals depending on the size.

Pier

Like concourses, extensions with gates down both sides, but connected on one end by additional buildings. Each pier may be designated as a separate terminal.

Satellite

A main terminal area connected to one or more smaller terminal buildings with gates all around. Generally connected underground, occasionally by a bridge. Each satellite is generally designated a different terminal.


What are the differences and benefits between/of time-based vs distance-based separation?

There is more information available from NATS on the website on Time Based Separation [TBS] and from a very descriptive Infographic on TBS PDF. While Heathrow (EGLL) is the first airport where this is being researched and tested, there are als other works, such as a research paper called Potential Benefits of a Time-based Separation Procedure to maintain the
Arrival Capacity of an Airport in strong head-wind conditions PDF which applies TBS at Charles Des Gaulles (LFPG).

The main focus of using TBS is to increase airport capacity during strong headwind operations, which significantly decrease groundspeed and therefore traffic flow.

None of the papers I have found speak of hazards related to decreasing the lateral distance between flights. The general idea seems to be that the minimal safety gain accompanying lower groundspeeds due to strong headwinds (lower GS means lower closure rate to objects / stationary traffic on ground) can be partially sacrificed and offset by TBS to maintain airport capacity or reduce the impact of the strong headwinds on airport capacity. This sacrifice becomes available since stronger headwinds also aid the dissipation of wake vortex.

The below table is based on own calculations, assuming constant approach speeds.
It should show you how the time to cover a set distance increases with decreasing ground speeds, which means that e.g. it takes longer to cover 3nm to reach the runway threshold. Reducing the lateral separation (the values chosen are deliberately a bit extreme, as none of the papers mentioned a formula to calculate TBS distances) reduces time to cover the distance, but covering 2.5nm at 130 GS is becoming quite close to 3nm at 150 GS.

+-----+-----+-------+-----+
| GS  | 3nm | 2.5nm | 2nm |
+-----+-----+-------+-----+
| 150 |  72 |    60 |  48 |
| 140 |  76 |    64 |  51 |
| 130 |  83 |    69 |  55 |
+-----+-----+-------+-----+


Is the airspeed different between above and under the wings?

In case of a subsonic attached flow over the cambered airfoil (or a symmetric one at an angle of attack), the airflow speed is higher over the wing and pressure is lower. It is the other way around on the lower surface.

Source: virtualskies.arc.nasa.gov

Note that this is just the mechanics of the flow over the wing. This is not an explanation of lift.


What is the reason of having an Angle of Incidence on an airplane?

Strictly speaking, there is no need to set a particular angle of incidence- the wing will decide that for itself depending on the conditions (speed, weight, altitude etc.). What we are deciding is the The mounting angle, which is set for various reasons and is not variable in flight (except for some rare cases) including:

Mostly this is set so that the fuselage is (nearly) horizontal during cruise. This is especially important for airliners- DC-10, especially flew at a pronounced nose high attitude requiring the cabin crew to walk uphill.
Setting the wing at an incidence helps improve the visibility- this is important especially for carrier aircraft, where the pilot requires good visibility and also higher angle of attack (for keeping t/o and approach speeds low).
Setting the wing at an angle helps in keeping the drag low for the given lift. Having the wings at an angle and fuselage horizontal means that the drag is minimized, while the wing has the required angle of attack.

Though the angle of incidence is usually fixed, it can be varied in response to specific requirements. A good example is the Vought F-8 Crusader, which allowed the wings to be pivoted 7° out of the fuselage on takeoff and landing, resulting in a high angle of attack -- reducing the approach and take-off speed -- while keeping the fuselage level and giving the pilot a good forward field of view.

F-8 Crusader with the wing pivoted up during landing. By USN - U.S. DefenseImagery photo VIRIN: DN-SC-88-06695, Public Domain, Link

Another example is the Martin XB-51 which had a variable incidence wing to reduce the takeoff run.


Can European (or more generally, non-US) airspace users airfile IFR flight plans?

Sure you can. This is from ICAO Doc 4444:

4.4.2.2.1    A flight plan to be submitted during flight should normally be transmitted to the ATS unit in charge of the FIR, control area, advisory area or advisory route in or on which the aircraft is flying, or in or through which the aircraft wishes to fly or to the aeronautical telecommunication station serving the air traffic services unit concerned. When this is not practicable, it should be transmitted to another ATS unit or aeronautical telecommunication station for retransmission as required to the appropriate air traffic services unit.
4.4.2.2.2    Where relevant, such as in respect of ATC units serving high- or medium-density airspace, the appropriate ATS authority should prescribe conditions and/or limitations with respect to the submission of flight plans during flight to ATC units.

And this is from SERA (Standardized European Rules of the Air):

SERA.4001(c) A flight plan shall be submitted, before departure, to an air traffic services reporting office or, during flight, transmitted to the appropriate air traffic services unit or air-ground control radio station, unless arrangements have been made for submission of repetitive flight plans.


How does downwash affect angle of attack?

Is it correct to assume that, looking at the flow field closely around a wing, "traditional angle of attack" doesn't matter as much since the large induced flow velocities in front of the wing will make actual "aerodynamical" angle of attack, i.e. the angle at which the oncoming air actually hits the wing (as opposed to the angle between freestream flow direction and chordline) differ significantly?

Yes. Just witness the angle at which slats are pointing down: They are oriented to the local direction of flow which is strongly up at the leading edge when the coefficient of lift is high.

Typical landing configuration of an airliner wing, from an article by A. M. O. Smith, McDonnell-Douglas, in Journal of Aircraft, Vol 12 No 6, 1975. As always: Converging streamlines indicate accelerating flow and falling pressure while diverging streamlines show decelerating flow and rising pressure.
Note that the double slotted flap here is instrumental in inducing this steep local flow angle: Without it, the wing would not produce nearly as much lift and the suction on the upper side would be much weaker, causing less local bending of the direction of flow.
Note as well that the angle of attack of the airfoil is 0° while the streamlines entering the drawing on the left already have a marked upwash angle. The same happens in reverse at the right side where the flow shows a distinct downwash. This is a 2D simulation and at infinite distance to the airfoil the direction of flow is strictly horizontal. On both sides, because this airfoil does not produce induced drag in 2D flow (an effect also known as d'Alembert's paradox).
On a real wing, however, the tip effects reduce the lift curve slope so the local wing section will show a lower lift coefficient at the same geometrical angle of attack. Now suction and upwash are reduced (but still exist) and the air flowing off the wing leaves it with an added downward speed component. The far-field flow pattern now does not any more have the symmetry of equal up- and downwash. Instead, the downwash angle is increased to twice the magnitude of the reduced upwash angle because the influence of the free vortices in the wake must be added. The result is a backward tilt of the sum of all pressure forces acting on the wing which we call induced drag.


What are the radio characteristics of communications between tower and aircraft?

They are omni-directional:  Anyone, in any direction, can hear the broadcast.

They are analog (non-digital), AM signals, so there are no bits, and thinking in terms of kbps or data-rate doesn't make sense.

These have similar transmission power to many AM radio stations.  Do you have safety concerns about local AM radio stations that broadcast baseball games?

Finally, the tower does not "control" the airplane.  The pilot on-board is the one in control.  The tower may simply give traffic directions, such as what heading to take, or what altitude is appropriate.


Is a slip an intentional consequence of a cross-control?

You are right: A slip is a cross-controlled (and uncoordinated) situation. It's exactly what your instructor was teaching you to avoid during your early lessons.

As far as nomenclature goes: all slips are cross-controlled maneuvers, but not all cross-controlled maneuvers are slips: On climb-out in most piston singles you will be stomping on the right rudder to offset the plane's left-turning tendencies, and you will probably notice that you're rolling in a tiny bit of left aileron (which offsets the banking tendency introduced by the rudder) - your controls may be very slightly crossed, but the result is coordinated flight (a centered ball).

The real difference here between an "unintentionally cross-controlled situation" and an "intentional slip" is the intentional bit: When you are knowingly slipping the aircraft - for example to bleed off altitude on a high final without increasing your airspeed - you're aware that you're flying in an uncoordinated and aerodynamically unstable condition. You're aware of the risks, and are hopefully monitoring the aircraft carefully to avoid a stall/spin.

For a little more insight, faaflighttest.us has a reprint/copy of an Aviation Safety article from April 2006 which is worth a read, and possibly an hour in the air practicing some of what they discuss with your instructor so you can get the "feel".


What's the difference between the "100 above" and "100" GPWS above callout?

"100 above" refers to being 100 ft above the decision height, which is the height at which the pilot must initiate a missed approach if they do not have the required visual reference. The equivalent call on a Boeing is ‘approaching minimums’. The decision height or minimums will commonly be around 200 or 300 feet, but it is airport specific, so when you hear the "100 above" call, you are 100 ft above 200 or 300 ft (300 ft or 400 ft above the ground respectively) - or whatever the decision height is for your specific approach.
The "100" call means that you are 100 ft above the ground.
The audio clip in your video is a bit confusing, and probably not taken from a real flight. During a real approach, you would expect to first hear "100 above" followed by "minimums", since "minimums" is the call made at the decision height.


Can anyone identify this pusher plane from apparently the 1930s?

That is a Stearman-Hammond Y-S1, registration NC15524, construction  number 309.
The remains of the aircraft are now owned by the Dutch aviation museum  Aviodrome. They are planning to restore one of the four Stearman-Hammonds they have into flyable condition. This project is running since 2007, and is progressing very slowly.

source: Smithsonian National Air and Space Museum


Can the new multi scan weather radar on the A320 see low level cumulus?

I'm not sure where you got your info from, but here is what I know of the Multiscan radars from Collins and Honeywell:

Multiscan displays precipitation from the highest tilt to the lowest so if the cloud is dense enough, even if it's a low level cloud, it will still cause returns. Keep in mind - just because the clouds are cumulus doesn't mean it has enough density to cause the WX-Radar to display any returns.

If you're talking about the dark-cockpit-concept, then yes, at certain high altitudes, the clouds will not show on the radar. This is because the OEMs want us pilots to only see what will pose a potential risk of turbulence and safety while at that particular altitude. If you want to see what's directly underneath your aircraft, the radar should be set to manual mode. However, this only happens during cruising at high altitudes.


Airplanes do not use their rudder to turn; they do it by banking into the direction of the desired turn, then the wings "lift" the plane in that direction.  The rudder is only used to maintain lateral trim, so that the airflow doesn't impinge on the side of the fuselage and drag is minimized.  To initiate a turn, the pilot uses the stick or yoke to actuate the ailerons on the wingtips, causing the wing on the inside radius of the turn to go down and the wing on the outside radius of the turn to go up.  A slight back-pressure of the stick or yoke raises the nose, causing a "climb" to be initiated, but since the plane is banked toward the center of a circle, the "climb" doesn't cause a change in altitude, instead it causes the plane to fly a circular path.  Small, simultaneous changes in engine power and rudder deflection are needed in order to maintain proper trim and constant altitude in what is called a "coordinated turn".

Helicopters turn in a manner analogous to that of an airplane.  In forward flight, the cyclic stick (between the pilot's legs) is pushed slightly in the direction of the desired turn, causing the rotor disc to tilt, just as the wings of an airplane are tilted in a banked turn.  The collective lever and throttle (at the pilot's left side) are adjusted as needed to maintain altitude and the rudder pedals are adjusted to maintain a coordinated turn.  Since the fuselage of a helicopter is suspended from the rotor hub like a pendulum, as the helicopter enters the turn, the fuselage tends to be flung outward, so it banks, just like the fuselage of the airplane that is rigidly connected to its wings.  (In a hover, with no forward airspeed, a helicopter can turn in any direction with the rudder pedals alone, something an airplane cannot do.  In this special case the fuselage remains level.)

To change altitude, it is necessary to add power to climb or reduce power to descend.  This is true of any heavier-than-air aircraft, as, for a constant velocity, increasing altitude implies increasing potential energy and decreasing altitude implies reducing potential energy.  In helicopters in forward flight, a climb is accomplished by first pulling back slightly on the cyclic stick to raise the nose and establish a climb attitude, then raising the collective lever and twisting the throttle grip on the collective lever to maintain constant rotor speed.  As the collective is raised, the pitch of the main rotor blades increases, giving them more "bite" on the air and, therefore, more lift.  A skilled helicopter pilot will perform these actions in a coordinated manner, such that everything appears to happen simultaneously.  In the special case of a hover, a helicopter can climb and descend with the rotor disk in a constant horizontal attitude.  As described above, the collective lever is raised and power is increased with the throttle to put more energy into the system, raising the aircraft; to descend the collective lever is lowered and power is decreased with the throttle.  Directional control in a hover is maintained entirely with the pedals; if the helicopter is the most common type with an anti-torque tail rotor, moving the rudder pedals requires the throttle to be increased or decreased slightly to compensate for the varying power consumed by the tail rotor and to maintain constant altitude.

(If this sounds complicated to you, you're right.  Rotary wing flight is considerably more difficult than fixed wing flight, and it took decades longer to perfect helicopters after fixed wing aircraft had taken to the skies.)


Is the Reynold's number a function of the fluid flow stream or the obstruction?

Maybe it helps to see the Reynolds number as the ratio of inertial to viscous forces. High flow speeds mean high inertial forces, and so lead to high Reynolds numbers. Viscous effects are strongest when the airflow hits the obstacle, and become less strong the longer the air flows around a body, so a short flow path has a low Reynolds number.

In the end it does not matter if the length of the flow or its speed drive the Reynolds number up. It is the combination of both. I am neglecting the gas properties here, because I assume we talk about air at moderate temperature and do not want to make the issue more complex.

Airfoils have no Reynolds number per se, flow conditions have. The Reynolds number of a measurement can be doubled by making the chord of the airfoil twice as long at the same airspeed, or by doubling the airspeed at the same chord. The wing of an airplane operates in a range of Reynolds numbers, depending on flight speed. That is why polar diagrams normally contain the results of several test runs rsp. computations, each at a different Reynolds number.

Sometimes you see airfoils which are optimized for a particular Reynolds number. To avoid early separation, the recompression area of an airfoil must become proportionally longer the smaller the Reynolds number is at which the airfoil is supposed to function well. At higher Reynolds numbers such an airfoil will still work, but will produce more drag than one which is optimized for a higher Reynolds number. The Reynolds number allows the aircraft designer to pick an airfoil that works well at the combination of speed and chord length for which that airplane is designed.


What is the difference between frequency wild AC system and other AC systems?

Frequency wild Unregulated AC is generated using alternators.
They were more common before constant speed drives were added to aircraft beginning in the 50's.

The alternator runs at various RPM (which varies the frequency) since it is driven by the engine.
— Wikipedia

The difference is that regulated AC is of fixed frequency. Better for motors, pumps, etc.
Unregulated AC was used for things like windshield wipers and windshield heating. DC inverters were used when regulated AC was needed.

Edit: On a plane like the ATR 72, it is indeed referred to as AC Wild.
It is used for heating the windshield, propellers, and probes. Among other systems that don't require regulated AC.

(Source)

ATR notes that discuss AC Wild can be found here. (Courtesy of FreeMan.)


Which "warnings" are issued when a military aircraft is going to enter foreign airspace?

This depends on the rules of engagement for a country and how the intercepting aircraft are instructed to act.

Radio Contact

The intercepting aircraft will try to establish radio contact on the distress frequencies available. 121.500 MHz or for NATO aircraft 243.000 MHz.

Visual Contact

Unless there is an imminent threat, the intercepting aircraft can position next to the intruding aircraft and establish visual contact with the pilots. There is also standardized procedures, such as the infamous rocking of wings.

(Image Source: www.cfinotebook.net)

Flares

Unless at high altitude, ground based units can shoot flares to attract attention and inform aircraft that they are entering prohibited or restricted airspace.


What is the role of baffles in Fuel Tanks?

They help in keeping the fuel from sloshing around during maneuvering. Some sloshing is impossible to avoid, but a reduction is extremely helpful in avoiding unwanted dynamics.

Consider a sudden pitch-up into a climb, or a sudden acceleration when the afterburners are lit: Now the fuel will rush backwards, shifting the center of gravity backwards. If this happens suddenly and the fuel starts oscillating, the aircraft would fly a wavy flightpath without pilot input. Now add the input of the pilot who tries to keep the flightpath steady, which is delayed by maybe 0.2 seconds, and add the latency of the control system, and you might have ideal conditions for pilot induced oscillations.

Some combat aircraft don't use baffles, but a coarse, sponge-like foam which takes up maybe 2% of the tank volume but dampens fuel oscillations very effectively.


Why do only some aircraft require tail anti-icing?

Some airplanes don't need the horizontal stabilizer to have anti-ice because it was shown during flight testing that there were no adverse effects from having ice on the tail.

This is mainly because the horizontal stabilizer/elevator has been designed so that even with a degradation of lift due to the ice, it still produces a sufficient tail down force to maintain control, even when approaching the stall speed.

The airplanes that do have anti-ice usually either have smaller horizontal stabilizers or (on some older designs) couldn't get the FAA to sign off on a design without it.  Some designs are also more prone to collecting ice (based mainly on the radius of the leading edge) so must demonstrate sufficient handling qualities with even more ice, which may not be possible.


Is it possible to fly backward if you have really strong headwind?

Yes, certainly! If your airspeed is lower than the speed of the headwind, the aircraft will fly backwards relative to the ground.

Example videos:

from ground (noisy, better turn down your volume)
from cockpit

However, note that headwind cannot cause a plane to fly backwards through the surrounding air. Constant wind does not affect airspeed.


Do the FAA's InFO publications carry any regulatory or legal weight?

An InFO (Information For Operators) document is an informational document containing recommendations. Quoting directly from FAA Order 8000.91:

  Much like a Safety Alert for Operators (SAFO), which contains critical safety information, an Information for Operators (INFO) message contains valuable information for operators that should help them meet administrative requirements or certain regulatory requirements with relatively low urgency or impact in safety. INFOs contain information or a combination of information and recommended action to be taken by the respective operators identified in each individual INFO.

They are not regulatory changes/obligations (like ADs) or formal notices of violation (administrative actions) - they're just a "friendly hint" that the FAA is concerned about something and would like operators to be aware of it and take action if the operator's analysis determines it to be necessary.
If the situation addressed in an InFO or SAFO document doesn't improve it may wind up prompting a regulatory change.

The InFO and SAFO programs are very similar, so FAA Order 8000.87A covering the SAFO program may also be of interest.


How did the controls work for variable-pitch props?

Control by wire? As in electrically?

Many of the small planes like Cessna Cardinal use an engine like Lycoming O-360 (4 cylinder) use a spring to return the blades to a flat pitch, and engine oil pressure to change the blade pitch. The control knob in the cockpit mechanically allows the oil pressure to change the pitch.  In other small planes, like a Bonanza, there is a set of counterweights instead of a spring to accomplish the same. No electric control at all.


What is a "departure" in the context of flight testing / maneuvers?

Departure means that the aircraft has departed from controlled flight. From FAA news:

  A Loss of Control (LOC) accident involves an unintended departure of an aircraft from controlled flight

In this video about flight testing F-16s, the test pilot says 'intentionally departed from controlled flight' and later 'the aircraft is prone to departure if mishandled' and 'the aircraft departed in yaw'.


Traffic pattern (circuit) phases are named so that they do not cause confusion1. I think of it as the leg which is taking me towards my landing/home base. (It is just a memory aid.)

However, the following reason is the best one which I found:

  This leg represents the foundation for the final approach, that is,
  the base from which the final approach is initiated.

Another reason is as follows, although it is similar to a memory aid:

  ... base leg is called “base” because it is really the foundation of
  your landing (the “base” of a house is its foundation – and if it
  isn’t right, the whole house will never be right) – if the base leg
  isn’t right, your whole landing will suffer for it.

1: Although, upwind leg, crosswind leg and downwind leg relates with the wind direction too. But if wind is perpendicular to the runway, downwind leg is not actually downwind, but it is still called the downwind leg.


When rearranging the lift force equation to solve for lift coefficient instead of lift force, why is the numerator often given as F not 2F?

The definition of the coefficients differs between countries/regions. The factor ½ is used in most of Europe and in Russia, because it is part of the dynamic pressure equation. In the USA the 2 is incorporated in the coefficient.


Why were DHL611 and Bashkirian 2937 on a collision course?

The Airways that the planes fly on criss-cross the sky and there are many, many intersections where there can be a potential for 2 aircraft to meet.

So yes, it is normal that aircraft are routinely flight planned to be on "collision courses". The flight plans don't keep aircraft separated. They are used so that Air Traffic Controllers can know the exact routing and plan where any conflicts may occur. It is normal and safe as ATC can then keep the aircraft separated using radar.

Generally aircraft flying eastbound fly at odd altitudes and westbound aircraft fly at even altitudes. This works quite well but we can still have 2 westbound aircraft both flying at 36,000' on a "collision course" because westbound can mean any track from 180 degrees to 359 degrees.

In this case the 2 aircraft were crossing paths at about 90 degrees to each other at the same altitude.

Normally they would be constantly monitored and the ATC controller would simply have one aircraft climb or descend to avoid the other aircraft. Another common practice is to tell one aircraft to turn slightly off course to avoid the other aircraft.

On this day the ATC system and the controller were both overloaded and a number of factors contributed to the controller not seeing the conflict until it was almost too late. At the last minute he told one aircraft to descend to avoid the other. This action alone would have been fine but the order was given far too late.

The Traffic Collision Avoidance System on the aircraft also saw the conflict and automatically issued an instruction for one pilot to climb while instructing the other aircraft to descend.

The controller did not know that the TCAS had also spotted the conflict and his  command to descend was contrary to the TCAS climb command. The pilot chose to follow the controller's instruction instead of following the TCAS climb instruction. The other aircraft was also following his TCAS instruction to descend so both aircraft were now descending while still on a conflicting flight path routing.

TCAS is there as a safeguard when the ATC system fails. It should always take priority over an ATC instruction.  It is clear that the ATC system failed these pilots and TCAS operated as it should.

As pilots we spend so much time following ATC instructions that I can see how it would be difficult to ignore the controller and instead follow the TCAS instruction.


What are the differences between battens and ribs?

A flexible airfoil's battens have the same purpose on a sailboat, windsurfer, hang glider, iceboat, landsailer, 17th century windmill, or anything else.  The mast or spar adds stiffness to one dimension (transverse to  airflow), the battens to the other (parallel to airflow).
On a rigid airfoil covered by fabric or film, ribs have the same purpose.  (Some recent sailboats have such rigid airfoils instead of flexible sails, too.)
The main difference between ribs and battens is that battens usually stiffen only the aft part of the foil.  On something that sails on both port and starboard tack, that lets the front part of the foil flex appropriately.  (If you found a hang glider that dared to fly inverted, the same would apply.)  On a hang glider that doesn't fly inverted, leaving the front part unbattened avoids stress risers and, at any rate, it's only the aft part that would be prone to fluttering like a flag and contributing no lift, were it not stiffened.


Is there an offline airplane route software similar to the in-flight entertaiment map?

1) The software from various airline companies seems to be more or
  less of a standard. So they should work in a similar fashion. Which
  input is needed in order to make it work (i.e., altitude, lat/lon
  coordinates, temperature)?

"Inputs" from the aircraft devices, like pitot, anemometer, gyro, etc. ? I don't know. DeltaLima answered that for you. What you see on the screen is definately a software rendering, that means an operating system is running, and displays a frame on that screen device. The graphic is software-generated (the map, the plane, its orientation, the text, the sun, the clouds, zooming, etc.)

So, at some point, the inputs signals (altitude, cruise, heading...) has to be converted in computer bytes that will be handled by hardware devices of a computer, buffered in memory, then used by "some software" in the way they have been designed for.

Option 1 :

There's a central Operating System (OS) that holds all the computable datas and feeds all onboard software on demand including FMS, IFE, etc.

analogic device -> convert to computable data (bytes) -> central OS
                                                        / | \
                                                      /   |   \
                                                    /     |     \
                                    Cockpit softwares    IFE    Cabin Softwares

Option 2 :

There's no central OS, but multiples ones, and a direct star-type (tree) data pipeline.

analogic device
 |_ convert to computable data (bytes)
     |_ some software on some OS
     |_ some software on some OS
     |_ IFE softwares on IFE OS
     |_ some software on some OS

Option 3 :

IFE Software is a included in one OS that also run other softwares

analogic device
 |_ convert to computable data (bytes)
     |_ OS1 (cabin)
     |   |_ IFE softwares
     |   |_ Inflight intercom software
     |_ OS2 (cockpit)
     |   |_ Front Panel softwares
     |   |_ BlackBoxes softwares
     |_ HUD software (of packed software in hardware)
      ...
      ...

There are other Options, each one could be better called "onboard System"

By the way, if, by "inputs", you meant "how to make that tiny plane move along the map", there are thousands of ways doing it, but the schemas is the same :

draw a map (use some gree/blue/brown color to make it look good enough)
draw a path using points locations on a X-Y (invisible) grid and line segments
slam a top view of a plane on all of that at a precise X-Y location (and don't forget to rotate the plane icon in the correct heading or your passengers will start to panick)

No matter how beautiful the flightpath could be rendered, remember it's a computer generated graphic, and doesn't need to be accurate at all. For example, refreshing could be set at an interval of 5 seconds instead of real time (5 seconds is good enough even for steep turns on final) The map resolution doesn't require a milimetric precision. 1000ft or 500ft lateral precision could suffice, and even if the displayed (text) altitude is in feet, the 3D rendering doesn't care about that at all : it uses a fraction of that altitude, then accordinly shifts the position of the aircraft model on the screen to give you the impression the aircraft is at an higher altitude...

That the logical way the flightpath software requires. Accuracy and physics computations are not required, unless you want to create a game or even better, a simulator (in terms of reproducing nearly-real behaviour in a virtual world) GoogleEart's Flight game for example isn't as precise as many other games out there (Lock On, X-Plane...)

But for all of those, the "inputs" are all binary datas stored in memory and used as variables by the software. Turning a plane isn't dependent on a heading value. It's one block of the software code that reads the heading variable, then rotate the object (the plane) by the corresponding angular value, then another block of the software code draws (renders) the object on a screen buffer. Finally, the screen buffer is passed to the displaying device (your IFE screen) and each pixel of that screen shows what has been calculated by the software for the human eyes.

In the end, what you see on a screen - ANY screen wether it's your TV, the IFE, your computer screen when you read this answer, your phone screen - is a series of slideshows pictures rendered by the screen device one image after another. But it's done so fast that the human eyes (or brain actually) think what is shown is really moving. It's not, but looks like.

  2) Is it possible to acquire one version of this software and run it
  offline as a simulation, giving as input the needed information?

I tend to say yes ! but wait !

Good luck finding that software though. You should ask someone working for an IFE system provider for aircraft manufacturer.

And note one thing : Such softwares could be packed in some hardware components having an inbuilt memory card, this preventing anyone from copying that very software while it is successfully loaded in the IFE OS just like any other software loaded from disk/CDs/USBs... Of course you can't unplug the hardware device from the IFE hardware motherboard, and plug it on your computer motherbord, that wont make it.. Just like your phone (wether android or iphone or whatever) the firmware is burried deep in one hardware component. You can't easily copy a firmware from an iphone to a nexus.. :P

But at one point, those softwares has to be created on a computer, then, have their "saved as files" state in the computers of the enterprises creating them.

As you could have guessed, chances to get a copy of such software is pretty.. non existent, not because it's nearly impossible, but because such software are copyrighted, and/or the (hardware) system supposed to work with aswell. A flightpath IFE software developped by a company may fail to work on your computer if you don't have the same OS architecture as the company's, and/or even some hardware devices that may be required to input or output valid binary datas read/written by the software.

Someone who has a copy of such software wont tell you he has...

I can tell you the Air Austral IFE is Linux based Operating System. The flightpath map displayed, the movies/music/games entertainments are all running on that OS. While the IFE on some Air Madagascar planes could be Linux based aswell, the softwares (the rendering and graphic layout) are different, meaning that those are not the same softwares as the Air Austral ones (or other versions)

  3) Is there a freeware solution to this? I'm more interested on
  information like the geography, cities, countries and borders.

An authentic flightpath software like those onboard aircraft ? Nope ! Never seen one in the items library of a store, and never found a page using google of a page "FlightPath Software Download".

A software that shows earth, oceans/lakes, country boundaries maybe ? Yes !

GoogleEarth/GoogleMaps
Nasa Worldwind
Bing Maps
Many little and less widely used software on SourceForge using rendering libraries and web hosted databases
...

A software that allows you to draw/show paths (and objects if possible)

GoogleEarth
Cesium
Nasa WorldWind
Marble
...

A standalone software that shows you (simulated) realtime flight of an aircraft over a map or 3D Earth ?

Anyone who tried creating such software ended making it a web-based application that you can access only through a navigator (like FlightRadar) Otherwise, it would be the decision of an enterprise or organism to hire developpers to produce such software for internal business purposes ONLY !

display aircraft on a radar screen
display meteorogical objects evolution on a map
display satellites movements at a space center on a large screen
(^^ purposes like that that has nothing to do in public domain)

That's exactly what IFE hardwares/softwares is about : have a bunch of engineers/programmers, build the devices/softwares, sell them to airlines/aircraft manufacturers. Panasonic, Thales, LiveTV of JetBlue, Rockwell Collins... Don't expect them to upload their baby on a website and allow you to download them..

So far, I said "anyone who tried..." ... ... failed ! Because building such software, to be accurate enough and interresting for you and me, would require a massive amount of datas :

flightpaths are not straight lines from airport to airport. You can spot this with FlightRadar, then you'll need each airport data, with their SID/STARS; you'll need waypoints, you'll need routes (those you can see on a real jepessen map) etc.
of course, you'll have to maintain those datas up to date.
those datas are NOT freely available, so if you're a developper that paid acquiring them (or a company that employs developpers) you should find a way to make profit on it. That's what Microsoft made with Flight Simulator, what Lockheed Martin is doing with Prepar3d, X-Plane, etc. Games are selling well, but a simple "show my flight plan" software isn't interresting enough to make profit on it. For you or me perhaps, but not at all for average people.
And because the datas has to be updated, a standalone application that doesn't connect to the web would prove impossible to achieve, as it will be obsolete soon enough and would require anyway an internet connexion and mostly a registering (payware) for stream updates. Most applications that does this are web-based applications, not downloadable ! This is even more not standalone if you are thinking of displaying an actual, flying, aircraft (like FlightRadar) - And you'll have to purchase access rights on some radar centers... and implement the programatic logic to handle those streams of datas at runtime.
What about maps ? If the plane is flying above the city of New-York, what are you going to display ? A greenish-brown landscate ? A text showing "New-York" ? What about the shores (or how the software knows there is nearby ocean and land ?) Yes, you must have a landclass/waterclass database. The way the datas are stored highly depends on the way the software displays them. You need raster (images) tiles of the earth if you want photoreal-like terrain, or vector-based datas for computer generated landscapes. The later is lighter in terms of required disk space, but vector-based datas ar good for flat terrains (projections) To have datas for mountains, valleys, etc. you'll need anyway discrete values of land elevation around the globe. The disk space used quickly rockets... So don't expect from someone the task to compile all those datas for you.. and there are awfull voids here and there (in far far away land on this earth) that you can't retrieve freely on the web (well, there are 100 meter resolution terrain elevations available for free out there, but higher resolutions are very hard to get for the entire planet)
That's why on flightplans IFE, you can't zoom in and see roads on the map ;)

I've started developping my own "show aircraft routes and display realtime flying tiny aircraft over a map" years ago, still, it's stucked as a "Work In Progress" due to the massive amount of new issues occuring and the interlacing complex features I would like it to have (Compatible with the game simulators mentionned above)

There's almost one new created airline every week, one new aircraft delivered every day, updating the - offline - database of that program occurs on a daily basis, weekly at longest, and I still need internet for that ! So, the day I may release that application would never come before I die. Just to pick an example, the application is developped as a Windows Forms interface. There's a better interface provided by the dotNet Framework called WPF. Upgrading the application to this new architecture would be a must before releasing it, and I have no time to do that. The database weight several Gigabytes (as expected : Maps and graphics...)


How exactly does acceleration of air occur just by having a convex
  underside

Just like the curved forward surface of a wing does it. Imagine for a moment that the air flows along a straight path: Now it would "see" that the surface below it curves away from its path of travel, and if that path would remain unchanged, a vacuum between the wing and the air would form. Reluctantly (because it has mass and, therefore, inertia), the air will change course and follow the wing's contour. This requires lower pressure, to make the molecules overcome their inertia and change direction.

Near the tip the flow has a sideways component, flowing away from the center on the lower side. This sideways flow, coupled with the sideways curvature on the lower wingtip, creates suction. This area of lowered pressure will suck in more air from inside. In essence, the curved lower side of the Hoerner wingtip extends the suction which is found on the upper surface of the wing to the side of the wingtip.

  As we can see, the effective wingspan using Hoerner Wingtips can be
  increased without increasing geometric wingspan.

Well, that is at least what Hoerner claims. Maybe it does move the peak suction of the wingtip vortex a little outside, but this has no effect on the flow field behind the wing and the eventual location of the trailing vortex. Below is a graphical overview of different wingtip shapes and their sideways location of the core of the tip vortex.

Wing-tip shape and tip-vortex location for a family of wings (picture source)

However, this is not relevant for induced drag. In the end, the strength and width of the rolled-up vortex sheet behind the wing indicates the amount of induced drag. Induced drag itself is the horizontal component of the resulting pressure forces on the wing (see here or here for more on that). The tip vortex is sucked into that vortex system regardless of its initial location. See below for an illustration:

B-747 with contrails (picture source)

You can see that the outer contrails of this Boeing 747's engines wrap around the contrails of the inner engines. This shows how the air is pushed down in the wake of the wing and that the centers of the vortices are slightly inboard of the contrails of the outer engines.


How do runway declared distances affect my takeoff distance?

Actually, the take off distance given in runways will be greater than the runway distance as it will include the distance to overcome a predetermined obstacle.

Source: tc.gc.ca

The balanced field length is dependent on the aircraft configuration among other things and is usually available in AFM. The airfield lengths, on the other hand are made available so that operators can decide which aircraft to operate based on the runway lengths available.

In general, the ASDA should give you the available distance for taking off as it gives the required field length available in case you decide to abort the takeoff for whatever the reasons.


Is the true altitude always lower than the indicated altitude as long as a correct local altimeter (QNH) is set on the altimeter?

No, the true altitude can also be higher than the indicated altitude, if the temperature is higher than expected from the International Standard Atmosphere (ISA). This case is usually not as important as the opposite case because flying at higher true altitude does not create a risk due to terrain clearance.

In a standard atmosphere, the indicated altitude
reflects the true altitude above the mean sea level
(MSL) and therefore provides a reliable indication of
terrain clearance.
Whenever, the temperature deviates significantly
from the standard temperature, the indicated altitude
correspondingly deviates from the true altitude, as
follows:

Extreme high temperature: the true altitude is higher than the indicated
altitude,
Extreme low temperature: the true altitude is lower than the indicated
altitude, thus creating a lower than anticipated
terrain separation and a potential obstacle clearance hazard.

(Airbus - Approach-and-Landing Briefing Note - 3.1 - Altimeter Setting - Use of Radio Altimeter)


What item is a pushback operator holding up after pushback?

It's a steering bypass pin which isolates the nose wheel steering and enables the tug to turn the aircraft.

The operator holds it up so that the captain can see it to confirm that it has been removed.

This Wikipedia article refers to it

On some types, there may also be a "downlock" pin which prevents the nosewheel gear from retracting whilst the aircraft is on the ground.  In this case, the captain is looking for 2 pins being held up.  More often though, the ground lock pin(s) are removed before pushback and stowed onboard.


How are spacecraft and aircraft traffic coordinated?

A TFR will be issued for the area that the spacecraft is expected to need for safety. Here is an example.

  No pilots may operate an aircraft in the areas covered by this NOTAM (except as described).

No exceptions are described, and the airspace is patrolled (at least for manned launches). As with errant sea vessels, a launch may have to be scrubbed if the safety area is compromised.

The company or agency in charge of the space flight operation would need to contact the FAA to coordinate such a TFR.

For space shuttle landings, Special Use Airspace was established. The shuttle landed in a pretty steep profile, so it only spent a relatively short time in controlled airspace. ATC coordinated directly with NASA while the shuttle was transiting the airspace.


Where is the generator in a large turbofan of a commercial airliner?

Image source

The generators are usually driven from the Accessory Gearbox. This CAD drawing is from the company that makes accessory gearboxes for the Rolls Royce Trent engines driving the A330 and A350, and it shows the connection of the accessory gearbox to the high pressure rotor.

This picture from the book The Jet Engine by Rolls Royce shows the same arrangement: a bevel gear drives off of the HP spool, then drives another bevel gear connected to the crescent shaped Accessory Gearbox underneath the engine.

The gearbox itself is shown above (from the same book), with the accessories connected to the LP spool:

The Engine Driven hydraulic Pump
Fuel pump
Oil pump
Pneumatic starter, requiring the highest loads and therefore dimensioning the gear strength.
The IDG: Integrated Drive Generator which provides the AC power.

The accessory gearbox contains the engine starter as well, pneumatic for older types and electrical for bleedless engines. The engines are started by turning the HP rotor, and the accessory gearbox must be connected to the HP rotor for this purpose. For this reason, the HP rotor drives the generator and other accessories, although it would be more beneficial for fuel economy to use the LP compressor. Source: The Jet Engine issue 5 by Rolls Royce.


Can we use ECI coordinates for an airplane?

The basic reason is the goal in navigation.  For an airplane it is to deliver the aircraft to a particular destination on the earth's surface, potentially also avoiding certain areas fixed to the earth's surface.

If you do all your navigation calculations in ECI, you then have to keep track of the earth's rotation to translate that to a point on the surface.  If you do all the navigation in ECEF, then that translation is unnecessary.

Both methods are mathematically valid (for ECEF, you have to have some method of removing or ignoring the non-inertial frame effects like Coriolis; for ECI, you have to keep track of earth rotation).  I don't have any direct evidence, but I suspect that since ECI was not an option historically, the current solutions are robust and no one is looking to create a radically different method for unknown benefit.


If a commercial airliner enters into a spin at high altitude, is it possible to recover?

How to stabilize a spinning aircraft

Stabilizing a spinning aircraft is fairly easy. If the center of gravity is not too far forward, the rudder is deflected in spin direction and the elevator fully up (adjusting the horizontal tail trim helps, too), the spin will continue until the aircraft hits the ground. In some cases, the spin might oscillate between two stable modes, and the aircraft will continuously pitch up and down between them. An airliner with its high wing loading will loose a couple of hundred meters per turn in a spin, but will not accelerate once the spin has fully developed. Increasing air density on the way down will actually decrease the rate of descent.

But I guess you wanted to ask something entirely different.

Ending a spin

Ending the spin is equally easy and is performed by the opposite commands which stabilize the spin. Proving that the aircraft will easily come out of a spin is actually part of certification requirements. The stable rate of descent gives the pilots ample time to end the spin, and even if the engines have failed, emergency systems will ensure that the pilots can control the aircraft. The risk that an otherwise normal flight will end in an unrecoverable spin is practically zero.

I would actually prefer that the airplane enters a spin. If the center of gravity is in the forward part of the allowed range, it is more likely that it will enter a spiral dive after stalling. Now the pilots need to react quickly and with measure, because drag is much lower in a spiral dive than in a spin, and the resulting higher speed poses a real danger. For that reason the flight control system in the type of aircraft you mentioned, the A-320, will prevent the aircraft from entering a spiral dive in the first place.

What if something snaps?

So far, we have assumed that all systems work. What if something snaps? The surprising answer is: Mostly nothing. A fundamental design principle for aircraft demands that primary structural parts or equipment can snap without the aircraft breaking apart. This is called redundancy: Every primary structure and every flight-critical piece of equipment is duplicated, so if one part breaks, the other, redundant part will continue to carry the loads and the flight can be terminated as planned. Regular inspections make sure that such damage is caught quickly, and the manufacturers continue to improve the existing fleet once a weakness has been spotted.

Even if something snaps, it is extremely unlikely that the aircraft will enter a spin, and even if this happens, it is easy to end. Your fears are unfounded.


The closest question to this currently is Why aren't modern helicopters quieter?, which one should read first. That regards sonic stealth.

As for radar stealth, any rotating part, visible from the outside, is a major source of unavoidable radar reflections. In 4th generation jet fighters, the engine faces generally contribute more than half of the total RCS (radar cross-sections). Very simple tweaks like installing radar blockers into the intakes can reduce the RCS several-fold.

The rotor is a very large rotating part. Even if made completely radar-transparent, the rotor will accumulate dirt and grime from the air, which have some reflectivity. On a properly stealth-shaped surface, this will be deflected away from the mainlobe. But a rotor, constantly rotating and flexing, has an uncontrollable shape.

This means helicopters can never reach B-2 or F-22 level of stealth. They can get lesser RCS reduction, along the lines of the Silent Eagle proposal. This could be useful, but helicopters fight at much shorter ranges than fighters or bombers.

The F-15 is expected to be visible to high-end opposition fighters out to 300-350 km, and the F-22's stealth reduces that to about 40-50 km against most fighters. At that range, radar stealth is defeated by more powerful or longer-wavelength radars, and even an aircraft with no radar cross-section could be detected by IRST/FLIR systems (correspondingly, high-end opposition fighters use all of these). The F-22's weapons have some hit probability at ranges up to ~80 km against fighters and up to twice that against other aircraft, creating useful room to shoot first. Helicopter radars top out around 50 km and their weapons even closer.

The Comanche could never hope to be as stealthy as the F-22. It's hard to guess its best-case scenario, but something like 60-100 km would've been pretty good. Operationally, for a helicopter, such detection range doesn't make much difference.

Helicopters can already hide from THAAD (theater area air defenses, mostly networked long-range SAM such as the Patriot or S-400) by nap-of-the-earth flying, placing surface features between them and the radar, and hiding among the ground clutter. It's only at 15-40 km that a helicopter would come in direct view of short-range air defenses (highly mobile "all in one" transporter, launcher and radar vehicles). A Comanche would still be visible and targetable by radar at this range.

Essentially, a helicopter's stealth would help protect it from enemy fighters. But there's more than one way to protect your aircraft from enemy fighters.

For the US, it's air superiority. The US maintains an air force that, including the USN, is a match for the rest of the world combined for air-to-air combat. It's not by happenstance - US military doctrine prioritizes air superiority, so resources were spent on it.

With air superiority, a helicopter doesn't need to hide from enemy fighters; the risk they face in every mission won't be worth it for such a low-priority target. The Comanche project was started in a wave of excitement with stealth, with an assumption that stealth could be an easy add-on to it. Without stealth considered, it was underwhelming. Radar-stealthing a helicopter proved difficult and expensive, and there just wasn't much to be gained by it.


It's important not to overthink it.  I'll keep it simple.  A coordinated turn means you are keeping the tail lined up with the nose in the airstream.  If you are uncoordinated, you are flying sideways in the airstream to some degree or another; the side of the fuselage is being presented to the airflow.

If you learn to fly in gliders, it's obvious because you have a yaw string on the canopy to tell you that the tail is lined up behind the nose in the airstream, since the string itself allows a direct visualization of the airstream.  In power planes with a propeller up front, you are forced to use an inclinometer to tell you the same thing, because it detects the lateral acceleration relative to the vertical axis that is present if the tail is not lined up behind the nose.

Practically speaking, whether you are banked in a turn or in level flight, it really doesn't matter.  The fuselage is lined up in the airstream or it's not.  The ball is centered, or it's not.  Just use your feet to keep it lined up as required using the skid ball.


Who decides whether an airline docks at a jetbridge or parks at a remote stand?

This can depend on all kinds of things, so it's hard to give a definitive answer:

Many low-cost airlines park at remote stands as this is cheaper than parking at a jet-bridge.
Some airports have jet bridges that can't handle particular aircraft types (often large aircraft at small airports) and therefore these aircraft must park at remote stands.
Related to the above, some airports have no customs facilities for jet bridge arrivals (or vice-versa) and therefore international flights must be parked at a remote stand and passengers bussed to customs/immigration.
Sometimes a flight will be allocated a jet bridge in advance but then suffers from a delayed arrival. By the time it gets to the destination the jet bridge that was assigned to it is taken by another aircraft. In this case, the aircraft may have to be parked at a remote stand.
As you say, bad luck can play a role. Airports have a limited number of jet bridges and sometimes there's just no space for your aircraft.
Lastly, in my experience it is particularly common to get a remote stand if you arrive late in the evening: Some airlines like parking remotely in this circumstance as it's cheaper than parking overnight at a jet bridge.

As I said, it's hard to give a definitive answer but the above are some common causes for being placed at a remote stand.


What's usually going on in a "hot" military operations area (in the US)?

Military pilot here.

I can certainly understand why there's confusion surrounding MOA's, having been a Private Pilot long before I flew for the military, and I'm glad you asked the question.

The second post you referenced is exactly right. It is extremely common (at least in our training areas) to have  dogfighting and other tactical training with airspeeds above 500 KCAS, and altitude changes of thousands of feet in seconds. No one uses a MOA for instrument approaches. This type of flying occurs multiple times per day, and it's extremely frustrating when civilian aircraft fly through the airspace without talking to ATC. We generally don't have a lot of time to work in the area, so we have to maximize the training we get. Stopping a fight/intercept/practice bomb run because a Cessna wanted to transit the airspace generally results in a reset of the entire scenario and wastes countless dollars and training opportunities.

If there's an emergency we will, of course, understand and please do whatever keeps you safe. Short of that, please, please get flight following, and do not fly through a MOA.


How to calculate the zero lift drag coefficient for a wing with data from airfoiltools.com?

Induced drag is only produced when real lift is created. In 2D flow, when only the airfoil drag is given, there is no induced drag even for discrete lift coefficients. 2D flow assumes an infinite aspect ratio which results in zero induced drag.
Your zero lift drag coefficient should be the airfoil drag coefficient at the actual lift coefficient at the wing station when the airplane is in flight. Do this for several stations over span since the lift coefficient varies over span. Plus the contributions of fuselage, tail and whatever is added to the configuration, of course.


Why do aircraft piston engines consume so much oil?

Automobile engines are not similar. They are liquid-cooled and therefore can be built to much tighter tolerances with regard to thermal expansion and contraction.

Air-cooled aircraft engines must deal with a large range of operating temperatures and oil is consumed due to the relatively looser fit of the piston rings.


What is the best word for lateral translational movement in a rotorcraft?

The term I preferred was "slide"

In our NATOPS manuals, and in our trained procedures in both Search and Rescue ops, and cargo ops, in the Navy the crewman would call for us to "slide left" or "slide right" as he positioned us over the pick up point.  (Easy Left/Easy Right as we got closer to the spot).

The FAA term "sideward hovering" suffices, but it is two words.  If you want one word, slide is about right.


What is the benefit of spoilerons compared to ailerons?

Two reasons often-cited for having spoilerons:

better roll control (especially during stall)
doesn't take up space for flaps

Because of their location, they don't need to take up space which could be used for flaps. For example, on the MU-2, the primary reason for using spoilerons was to allow for full-span flaps at the rear, lowering clean wing area while still having acceptable stall speeds in the takeoff/landing configurations (and therefore shorter landing distances). This is one of the reasons for it to takeoff from runways less than 3000 feet, while having a cruise speed of up to 300 knots.

However, spoilerons have the disadvantage of killing lift, which can be deadly in situations such as a one-engine-out situation in a twin. To compensate for this, usually minimal constant spoiler use is recommended, and mostly rudder is used if an engine fails. If available, aileron trim (despite the name, there aren't any "real" ailerons on such an aircraft) is used to reduce drag if long-term use is expected.

Spoilerons tend to have less of a difference in roll rates at low and high speeds, compared to ailerons -- this could either be considered an advantage or a disadvantage, depending on the situation and whether a pilot is used to one type of roll control or another. This can negatively affect the roll control feel of the aircraft though, as it might feel slightly sluggish at higher airspeeds. They also tend not to provide much (if any) feedback to the pilot, which is a disadvantage.

Note that most large airliners use both spoilerons and ailerons for roll control, to be able to still have an aileron while supplementing large control inputs with the spoilers.


ETH 409: Was the most obvious sign of fatigue being a factor due to one of the pilots calling out “go around” when they were climbing after T/O?

The "Go Around" call was a call for a particular power setting (TOGA) because the aircraft was in a stall condition (rapidly decreasing airspeed, high AoA and stick-shaker). This was not fatigue by the aircrew thinking that they were actually performing a "go around", the captain wanted full-power out of the engines and there is a button that does that.


Why do Airbus and Boeing have different approaches to bleed air systems?

Nobody can tell you for sure without insider knowledge, and insider knowledge would be confidential in this case, but ultimately it boils down to risk management. Every new development carries a risk of budget and deadline overruns and of problems surfacing in operation and the manufacturer needs to be reasonably certain they can put the product to market before they run out of money.

When Boeing started developing 787, the 777 was selling well, so they had the budget reserve to risk more innovation. And it did have some problems in operation with battery fires. Airbus was in worse position when developing A350, so they stuck to more tried technologies to keep the risk lower.


What causes spiral mode instability and how do you minimize it instead of trying to dampen it?

One of the primary causes of spiral instability is the fact that once a turn begins, the outboard wingtip is moving along a larger circumference and therefore moving faster, and tending to produce more lift, than the inboard wingtip.  So the bank angle tends to increase.  Keeping the wingspan small will minimize this effect.  Obviously, this is a trade-off and other desirable qualities are sacrificed.

Re dihedral-- the inefficiency due to more wing area being needed to get the same amount of vertical lift component is rather minimal.  The more significant negative effects of dihedral include a excessive slip-roll coupling that creates excessive roll torque during an intentional slip (eg crosswind landing), bad effects in gusty crosswinds, poor roll response to aileron-only roll inputs, and a tendency toward "Dutch roll" yaw-roll oscillations.

Keeping the vertical fin small will help allow enough sideslip to allow whatever dihedral is present work as effectively as possible to  help minimize spiral instability, but obviously this is a trade-off as well.

For more on the relationship between sideslip and dihedral see the related answers in Is this paragraph about the dihedral effect in the FAA's Glider Flying Handbook correct? or How does the dihedral angle work? .


The compression in the forward part of a jet engine heats the air, and combustion raises its temperature again. In order to achieve efficient operation, this heating must be restricted. If air is heated above approx. 2,000 K, adding more energy will result in dissociation of the gas with less further heat increase. Since thrust is produced by expanding air through heating, raising combustion temperatures above 2,000 K will result in less thrust increase for the amount of fuel consumed. The most advanced engines (F-119 and EJ-200) have turbine entry temperatures of 1800 K, and civilian engines operate a few 100 K below that. The number of 2000 - 2500°C (2300 - 2800 K) you got from a previous answer is way too high.

Oxygen starts to dissociate already between 2,000  and 4,000 K, depending on pressure, while Nitrogen will dissociate mainly above 8,000 K. The 2,000 K figure above is a soft limit, but this temperature is also a challenge for the materials of the combustion chamber and the turbine, and techniques like film cooling are mandatory. As you can see, using pure oxygen would greatly increase the efficiency loss due to dissociation once technology allows to run the turbine at higher temperatures than 1800 K. It would be better for jet engines if air had a lower oxygen content.

To make the best use of the fuel, much more air than would be needed for stochiometric combustion is used in a modern turbojet or turbofan core. The idea is to heat the gas just as much as technically and economically possible.

Too much oxygen does create problems, mainly by raising the amount of nitrous oxides a jet engine produces. However, it helps to achieve largely complete combustion, even if some of the reaction happens not in the combustor, but at later stages. Combustion is never fully complete, and the remaining unburnt constituents are proportional to an inverse exponential function over time.


What is the average number of flight hours that US private pilots require before passing their checkride?

This document from and published by the FAA does include a range although they do not cite their source or where they pulled the data from. It is as official a number as I can find.

  One difference between a part 141 school and a part 61 school is that
  fewer flight hours are required to qualify for a pilot certificate in
  a part 141 certificated school. The requirement for a private pilot
  certificate is 40 hours in a part 61 school and 35 hours in a part 141
  certificated school. This difference may be insignificant for a
  private pilot certificate because the national average indicates most
  pilots require 60 to 75 hours of flight training.

The range most likely varies heavily due to the way people train. I know people who have gotten it in <50 and people who are up around 90 and still have not flown the check ride. It varies largely with how much time you have to train as well as local weather. I was flying once a week (occasionally twice) during my training) and I was somewhere around 63 when I got my ticket.


What happens to a large plane after a successful gear-up landing?

This answer varies wildly depending on the size and type of aircraft involved, but in general:

How is the runway cleared?

They remove the aircraft, then sweep the runway for smaller debris and then inspect the tarmac to make sure the aircraft didn't gouge into it, and tear up chunks.

Is the plane towed away (scraping along the ground)?

No. The aircraft will usually be lifted and placed onto an oversized platform with either cranes or hydraulic  Jacks, and then towed. Dragging it along the tarmac would damage the tarmac (more so then the landing already did) and do further damage to the aircraft.

Are there cranes available in large airports?

Gear up landings (and catastrophic hard landings where the gear collapses) are so rare that no airport (that I'm aware of) has the heavy duty mobile cranes required standing by. It would cost way too much to maintain them and keep operators for them on the payroll when they'd just sit and rust for years and MAYBE be useful for 3 or 4 days. It is much cheaper to hire a crane and operator.

Can a single crane lift and move a large airliner?

Maybe; aircraft manufacturers will often publish manuals for how to recover and salvage the aircraft, and the crew removing the aircraft would refer to those.

Is the plane considered a write-off or can some be repaired and continue in service?

Not necessarily. This part varies depending on the age of the aircraft, how hard the landing was, and a million other factors, but the aircraft can usually be salvaged. The repairs could be astronomically expensive, and the work could take several months if not a year or more.

Where would such repairs take place?

This also depends. If the airline has a hanger at the airport where the plane is being salvaged from, the answer is simple. If not, that's when it gets tricky and expensive. Usually this means renting hangar space (which is SO much more expensive then any other commercial space) and either flying in your own mechanics or hiring a local crew. The necessary repairs and inspections would be done to get it to the point where it could safely fly back to an airport where the airline has the personnel and equipment to finish repairs.


Should an aircraft registration match the flag next to it?

That's because the aircraft (some of them at least) are leased from Aerotron Ireland Ltd., an aircraft leasing company in Drogheda Ireland.

I only looked up one. It was EI-FVA.
Here it shows it being registered to Aerotron on Aug 23, 2016.
And here it shows it entering Blue Panorama's fleet on Dec 8, 2016.


Is it useful for twin propeller airplanes to have twin vertical stabilizers, in line with the propellers?

The vertical tailplane is dimensioned as a tail volume: total vertical area times the moment arm to the CoG, to tackle the following issues:

Control after engine failure.
Lateral Stability.
Crosswind landings.
Spin recovery.

The vertical tail is subject to side forces, and the aircraft structure must be able to withstand the resulting reaction forces. The fuselage is very suitable for absorbing these by applying counter-torque, and is the go-to place for mounting the fin.

A large, long fin has a relatively long moment arm, and it can be beneficial to divide the vertical tail into multiple surfaces, provided there are suitable mounting spots. And behold, the P-38 had two tail booms and a horizontal connection bar, and the natural mounting method is two fins each at the end of a boom.

The fins are indeed in the prop wash and this does help their efficiency - in straight flight. At a sideslip, large angle of attack or in a spin, the prop wash will be directed away from the vertical tails, and these will be dimensioned as if there was no prop wash at all.


How to determine if an aircraft type is currently under investigation?

All aircraft are constantly under investigation. Service bulletins and airworthiness directives are issued for every certified plane and engine. These are optional and mandatory checks and maintenance to be done in additional to the normally scheduled maintenance. Every accident and important incident is investigated by the NTSB, with a public report available online.

There is a period of time before an airworthiness directive is made final that might be considered an investigative phase, but information on this is generally not publicly available. One notable example pertains to the rudder controls on the Airbus A-300.


What is the difference between aviation "accident" and "incident"?

There are indeed only two 'official' classifications of aviation incidents, which are defined in ICAO Annex 13.

  Accident. An occurrence associated with the operation of an aircraft
  which takes place between the time any person boards the aircraft with
  the intention of flight until such time as all such persons have
  disembarked, in which:

  a) a person is fatally or seriously injured as a result of

  being in the aircraft, or
  direct contact with any part of the aircraft, including parts which have become detached from the aircraft, or
  direct exposure to jet blast,

  except when the injuries are from natural causes, self inflicted or
  inflicted by other persons, or when the injuries are to stowaways
  hiding outside the areas normally available to the passengers and
  crew: or

  b) the aircraft sustains damage or structural failure which:

  adversely affects the structural strength, performance or flight characteristics of the aircraft, and
  would normally require major repair or replacement of the affected component,

  except for engine failure or damage. when the damage is limited to the
  engine, its cowlings or accessories: or for damage limited to
  propellers, wing tips, antennas, tires, brakes, fairings, small dents
  or puncture holes in the aircraft skin: or

  c) the aircraft is missing or is completely inaccessible.

  Note I.-- For statistical uniformity only, an injury resulting in
  death within thirty days of the date of the accident is classified as
  a fatal injury by ICAO.

  Note 2.-- An aircraft is considered to be missing when the official
  search has been terminated and the wreckage has not been located.

And then there is

  Incident. An occurrence, other than an accident, associated with the
  operation of an aircraft which affects or could affect the safety of
  operation.

They do also define 'serious incident', but simply as an incident which was nearly an accident; which is still an 'incident' by the definition above.


How long will an aircraft need to fly before it can land without dumping fuel?

Does it vary so much between aircraft types, distances,
  passenger/cargo weights and other conditions that it's not simply
  possible to say what is typical

This is pretty much the case. For example in a lot of smaller trainer aircraft you are more or less always in the CG range to make a successful landing. On the contrary in larger aircraft fuel burn schedules are extremely important to simply keeping the aircraft airborne.

Dumping fuel is generally not idea from a strictly business standpoint you are basically throwing money out of the aircraft which no airline wants to do on purpose.

What varies heavily between all flights is that at some point its up to humans, and humans make mistakes. Fuel loads may be calculated automatically but some guy still needs to usually shut the pump off at the right time, the pilots need to fly the plane as prescribed and the conditions need to be as expected. Any variable in this could create an issue for you.

For example lets say you take off and get an unexpected tail wind of 100 Kts for the whole ride. Lets also say that you loaded up as much fuel as you could on account of you were flying to a smaller airfield and you were unsure they had enough fuel on hand to get you home. You knew you could bring enough to get there and back but you would be right at the limit of the load config. That 100Kt tail wind is going to save you a fair bit of fuel you intended on burning and due to unforeseen things you may now be over weight.

Now lets say you are flying short haul in a 737, you are doing a 30 min hop and its the end of the day. You have the the required fuel plus reserves so you are flying pretty light. A light passenger load and not much cargo to haul, you may be in range for the whole flight.

Those are just two of a very long list of examples of different load configurations that can lead to always in range, or substantially out of range situations.


Under what conditions can a non-RVSM aircraft or pilot fly in RVSM airspace?

According to the AIM Section 4-6-8, the language to use with ATC is simply, "Negative RVSM."  You have to say it on the request for an RVSM flight level, flight level change, or climb/descent through RVSM airspace.  You must also say "Negative RVSM" on check-in if currently at an RVSM flight level.

NBAA provides a guide for navigating the RVSM approval process.  This guidance is fairly new, and indicates that approval through a prior operator of the airplane is acceptable.  Your FSDO will make the authorization.

See also FAA RVSM Documentation


Do flight attendants do pinch-hitter training?

This is generally not standard practice. For one pilot training is particularly costly (even just in small planes) and would simply not be worth it to the airline, which is why there are two flight capable people in the cockpit to begin with.

Historically speaking there is evidence to the fact that it would not alter the outcome of the scenario. The closest instance of this occurring was possibly Helios Airways Flight 522 which lost cabin pressure at altitude incapacitating the crew and most aboard. Flight attendant Andreas Prodromou who held a UK Commercial Pilot License (but was not trained on the B737) was able to remain conscious via a portable air pack and make his way to the cockpit and into the captains seat. However he was un-able to take positive (meaningful) control of the aircraft due to lack of experience investigators eventually concluded after the crash.

There have been some successful talk down landings of substantially smaller aircraft.

FWIW, most modern airliners are sophisticated enough and have the controls concentrated enough that they can be flown single handedly in a manageable fashion.


What ISA temperature to use in the density altitude calculation?

Calculating Density Altitude

Density altitude in feet = pressure altitude in feet + (120 x (OAT - ISA temperature))

Altimeter Pressure altitude is determined by setting the altimeter to 29.92 and reading the altitude indicated on the altimeter.
OAT stands for outside air temperature (in degrees Celsius).

ISA stands for standard temperature (in degrees Celsius).

Keep in mind the standard temperature is 15 degrees C but only at sea level. It decreases about 2 degrees C (or 3.5 degrees F) per 1,000 feet of altitude above sea level. The standard temperature at 7,000 feet msl, therefore, is only 1 degree C (or 34 degrees F).
For example, the density altitude at an airport 7000 feet above sea level, with a temperature of 18 degrees Celsius and a pressure altitude of 7000 (assuming standard pressure) would be calculated as follows.

18 – 1 = 17
17 x 120 = 2040
2040 + 7000 = 9040 feet Density Altitude

This means the aircraft will perform as if it were at 9,040 feet


What are the US regulations governing experimental aircraft?

Your first question

  There are more experimental class aircraft in the USA than any other. That being said, what type of requirements does an experimental aircraft have to meet.

is very broad and should probably be its own question.

As to your second question

  Also, are there ways around certain regulations?

The short answer is "Yes".  You can apply to the FAA for a waiver of specific regulations.  It's commonly done for aerobatic contests to permit aerobatics below 1500', within 4 nm of an airway, etc.  The waiver is of extremely limited duration and calls out the regulations to be waived.  Air shows also have them.

I haven't personally requested such a waiver but I've seen them.

The FAA probably permitted the overflight because the pilot was able to show it was safe, would do no harm, and wouldn't cause them any problems.


How many hours of test flight does a general aviation aircraft go through before delivery?

All manufacturers have a quality system in place, and they also have a production baseline (which is a result of certification). so, as long as the quality system is utilized, there is technically no need to fly each manufactured airplane. Type certification validates the production baseline (and that each manufactured plane is within the tolerances acceptable for the certification authority or requirements).

However, depending on company manufacturing know how (or lack of) there might be several ground tests (structural, engine, systems checkout) before delivery.

On the other hand, for many military aircraft there's a factory checkout flight, to check and confirm the performance of each frame and propulsion system. This is more useful for risk reduction (to avoid customer complaints and to check the complex system in the air).


Is there a relation between the Mode 3/A Code and the ICAO aircraft address?

There is no relation between the ICAO 24 bit aircraft address and the 12 bit Mode A code. The 24 bit address is unique to the aircraft and stays with the aircraft as long as the aircraft has the same registration. The Mode A code changes from flight to flight and often during flight.

The Mode A code is assigned by ATC based on a local or regional assignment scheme. There are Mode A codes that can be used by many aircraft in one area simultaneously, such as VFR codes (1200, 7000) for flights that are not under air traffic control. For IFR flights, the Mode A code is unique for the control area at that given time. When aircraft move from control area to control area, a code conflict can occur when two aircraft with the same code end up in the same area. In such a case one aircraft will be given a new code.
There are various schemes to prevent code conflicts from happening too frequently. See this answer for more info.

Quite recently (in the last couple of years) IFR flights in Europe can operate under conspicuity code 1000 which is a signal to the flight data processing system to use the Mod S downlinked aircraft ID for identification and flightplan association. This allows many aircraft flying in Mode S surveillance airspace to use code 1000 simultaneously, freeing up Mode A codes and reducing the number of code conflict.


Is rebooting the computer normal before/during flights?

Today's modern airplanes have many computers, and sometimes things get out of sync and don't communicate properly.  During the preflight checks, we check each system according to the recommendations of the manufacturer, and occasionally one of the checks will not pass correctly.  Depending on the system, when a check fails, rebooting the computer may be an option to see if it fixes it (similar to rebooting your home computer).  Other times, the issue is the communication protocol between two (or more) different computers.  Often times in this case, the entire airplane must be powered down in order to reset them all at the same time.  Since these tests aren't performed in the air, the problem usually doesn't crop up there.

In the vast majority of cases it isn't a safety critical item that we are dealing with (where the software testing is more thorough), but rather a monitoring system.  The safety critical items (like flight control software, etc.) will have multiple redundant systems and often have different versions of software on at least one of the computers "just in case".

Generally the flight crew won't tell the passengers when they need to "reboot" a computer unless we have to power down the entire airplane.  That's very noticeable and will cause some people to worry so we like to give you a heads up before all of the lights go out and the air conditioning system shuts down.  That of course is up to the discretion of the flight crew though.


Is there a formula to calculate take off runway distance for commercial planes?

These calculations are quite complex because they take into consideration a lot of different variables, just off the top of my head:

Take off weight, which includes FOB
Runway length
Runway condition
Field elevation
Air temperature
Air pressure
Wind
Engine type and airplane configuration

Based on those (and I am sure many more), you have to calculate at minimum V1, VR and V2, and possibly others.

These calculations are looked up from a table.


Which situations would cause a company to ground or recall a aircraft series?

There are various levels that a grounding can occur at.

Governing Agency: A legal authority can ground an aircraft make/model/serial number range (or one containing a certain part/system) if it deems they are unsafe or there is some question about the airworthiness. The most recent example of this was the recent grounding of the 787's by the FAA after the battery issue was called into question. Governing agencies can also practically ground an aircraft/make/model by issuing a mandatory AD against it; this will practically ground all airframes until proper maintenance occurs. A federal entity may also chose to not certify an aircraft for operations within a country, and/or the manufacturer may chose to not seek certification in that country although some countries allow one off certificates to be issued. In emergency situations the local governing body may shut down and ground all aircrafts in a certain area. The most prominent example of this that I know of is when all aircraft were grounded following the 9/11 attacks; this is generally more of a ground safety issue than an airworthiness issue.
Operator: The owner/operator can ground an aircraft for safety reasons if they belive there may be an issue with the functionality or operation of the aircraft in question. A good example of this is the grounding of the Southwest 737 fleet due to weight and balance issue, this was done by the airline potentially before a governing authority intervened. Another example was ANA's grounding of its 787 fleet due to engine problems.
Pilot In Command: As the pilot in command of a flight and the one responsible for ensuring the safety of the flight you can ground an aircraft for any reason you so chose and be well within your rights. Let's say on your preflight checks something is out of place or looks wrong; in such a case, the pilot can ground the aircraft until the issue is fixed.
Maintenance Persons: Generally when attacked from this angle it's more of a prevention to return to service than a grounding, but if an issue is found during a stage check or annual the aircraft may be pulled until the issue is fixed. There is an interesting article about withholding maintenance signatures here.
Recall: Manufacturers generally provide AD's instead of a full blown recall however some cases may warrant a full blown recall of airframes. When Porsche dropped the TC for the PFM3200 engine Mooney offered a recall of sorts, the owners could either sell the aircraft outright or the engine retrofit would be subsidized.


Is there any cockpit procedure used to avoid shutting the good engine, instead of the bad one (on fire, for example)?

Procedures exist

If you read about the Kegworth crash you will see that the pilot started to follow a procedure he has been trained on but was interrupted by an ATC call and failed to complete the procedure. The procedure involves checking the engine vibration meters and reviewing decisions.

So, yes, procedures exist.

Procedures vary

Procedures vary depending on exact aircraft model and on the airline operating the aircraft. Some airlines use more memory checklists than other airlines. So it isn't possible to list a specific procedure that is used by all pilots of multi-engined aircraft.

Identifying problems

Pilots don't start from the point "we need to shut down engine No 2" they start from some observable event like "smoke in the cockpit" (as in the example incident above)

There are instruments on the instrument panel that show engine temperatures, vibration and other parameters that should usually make it evident which engine is not performing normally.

Here's an example procedure - note that this isn't the one for the airline and plane model in the incident above but it is probably similar

Note the points

Eliminates No 2 engine as source ...
...
Eliminates No 1 engine as source ...


For a plane with a pusher propeller, how does a root-stalled wing affect its performance?

Interesting question. Its far easier to find studies which focus on effect of propeller on the wind aerodynamics. I highly doubt there would be any analytical equation that can give effect of stalled wing on a pusher prop. For one thing, stalled airflow goes out of the realm of what analytical aerodynamics can handle. Even for having an empirical equation, since the stall is highly irregular phenomenon, it would be very difficult to arrive at any empirical equation which can give reasonable correlation for wide enough envelop of wing-pusher configuration. The impact would be highly sensitive to geometric configuration parameters such as location of propeller with respect to centreline, distance from trailing edge and so on. For a specific configuration, one perhaps could come up with a correlation but that would need a good amount of experimental data and it would be of limited utility.

In my opinion, the impact of the stalled wing on the propeller performance is far less than the impact of the propeller on the wing stall. Definitely there would be some impact on the performance on the propeller due to "dirty" incoming airflow on the pusher propeller, but the impact should be small. I base my expectation on the fact that the streamwise velocity would be still much higher than the cross-stream velocity components in the stalled airflow behind the wing, thus giving marginal changes in AoA that the propeller blade aerofoils see. This particular research paper say this in the conclusion:

  The prop-wash effect mainly increased the aerodynamic performance of the wing, but the wing–wash effect was just a small part of this interaction. However, the wing-wash effect cannot be ignored as also mentioned in previous study.

Note that this is valid only for lower Re regime for the particular configuration studies. But I would expect results not too different in other cases.


Is induced drag essentially nothing more than a specific type of form drag?

No, by definition it isn't.

There is drag. Drag is caused by different physical phenomena. According to the cause, it is classified to:

Induced drag

Induced drag is side-effect of generating lift over finite wing span.

Lift is an upward force that the air exerts on the wing. By principle of action and reaction (Newton's third law of motion), the wing exerts downward force of equal magnitude on the air, and since the air is free to move, that force accelerates the air downwards.

Now there are two equivalent arguments why this causes drag:

In the frame of reference of the wing, the wing is not doing any work (because it is not moving), so it can't change kinetic energy of the air, and therefore can't change its total velocity. Since the vertical component of that velocity increases, the horizontal component has to decrease and this requires forward force from the wing. The backward reaction to that force is the induced drag.
In the frame of reference of the oncoming air, the kinetic energy does increase. Therefore the wing has to do some work on the air, which means applying force in direction of its motion. The backward reaction to that force is the induced drag.

The explanations are both equally true, just using different frame of reference.

Induced drag decreases with (square of) speed (for constant lift), because at higher speed there is more air to accelerate, so it only needs to be accelerated by less.

Induced drag is also independent of cross-section. If you cancel lift by moving elevator or aileron up, the cross-section may not change at all (the flap might still be behind the wing), but the induced drag will vanish.

By the way, it follows from this that induced drag of wing at 90 degrees is zero, because it is not producing any lift. The form drag is, of course, huge.

Form drag

Form drag is caused by imperfect pressure recovery behind the body. At the front of the object moving through fluid the fluid slows down, which comes with increase of pressure. In ideal laminar flow the fluid closes behind the object with the same slow speed and same high pressure. But as the object moves faster, the inertia causes the flow to separate and form a turbulent area behind the object where the pressure remains low. This pressure difference cause the form drag.

The form drag depends on cross-section and shape of the object. It always increases with speed.

Interference drag

When complex structure, like an aircraft, is analysed, the form drag of each component (wing, fuselage, empennage etc.) is first analysed independently. But as the components are assembled together, the pressure fields around the parts negatively affect each other and this increase in drag is then called interference drag.

Skin drag

Skin drag is caused by the friction between the surface of the object and the fluid. The skin drag depends mainly on wetted surface of the object. It increases with speed in similar way to the form drag, so for practical purposes it is usually combined with the form drag into one term, the parasitic drag.

Wave drag

As the object moving through fluid approaches the speed of sound, the flow exceeds it as it tries to avoid the object and forms a shock wave. Behind the shock wave the pressure is much higher and this increase is not balanced by any increase on the aft part of the object, therefore causing large amount of drag.

This kind of drag only appears as the flow speed approaches speed of sound, but then it quickly grows larger than the other forms.


Is there an FAA regulation to switch off the air-conditioning during take off?

It is not an FAA regulation to have the air conditioning off during takeoff and/or landing, however some manufacturers require it, in which case the pilot is legally obligated to comply with it.

I have flown two aircraft (both jets) which required it to be off:

Some of the Learjets require the AC to be off for both takeoff and landing because it uses a high draw electrical motor to drive the compressor, and when it cycles off and on it causes electrical spikes which can cause problems with the anti-skid system.
Some of the Dassault Falcon jets require the AC to be off for takeoff (actually, they automatically turn it off, but if the automatic system is inoperative the MEL requires turning it off manually) for performance reasons.  Since the AC is these aircraft is using bleed air off of the engine compressor as a source, allowing the air to continue through the engine instead increases thrust.

I know that many airliners have the option to takeoff with it on or off for performance reasons, and provide performance charts or adjustments for both.