Wikipedia answers this on the Boeing 737 page:

  Most 737 cockpits are equipped with "eyebrow windows" positioned above
  the main glareshield. Eyebrow windows were a feature of the original
  707 and 727. They allowed for greater visibility in turns, and
  offered better sky views if navigating by stars. With modern avionics,
  they became redundant, and many pilots actually placed newspapers or
  other objects in them to block out sun glare. They were eliminated
  from the 737 cockpit design in 2004, although they are still installed
  at customer request. These windows are sometimes removed and
  plugged, usually during maintenance overhauls, and can be
  distinguished by the metal plug which differs from the smooth metal in
  later aircraft that were not originally fitted with the windows.


Which UK airport uses different approaches for noise reduction?

Apparently it's called Runway Alteration. The only hits I got were referring to Heathrow airport, so my guess is this is the only airport that uses this procedure.

Actually, the alternation is in two ways: per part of the day, and per week.
During week 1, from 06:00 to 15:00 runway 27L is used, and from 15:00 until last departure 27R is used.

The week after this, the things are reversed:
From 06:00 to 15:00 runway 27R is used, and from 15:00 until last departure 27L is used.
So the people living under the approach of 27L will have noise in the first part of the day in week 1, and during the second part of the day in week 2.
See the full programme here.

It is influenced by the six factors below (taken from the same source):

  Daytime patterns of runway alternation are different to night-time    patterns: at night there are only a handful of landings and very few take-offs.
  Aircraft land and take off    into the wind, so the wind direction has a big effect on flight patterns.
  Government policy currently favours aircraft taking off    towards the west. Even when the wind blows from the east, aircraft    could
  still fly in from east and take off towards the west. This is because we have a westerly preference.
  A historical restriction means    that aircraft cannot take off towards the east from the northern    runway. This is because of the Cranford Agreement.
  The hour between    06:00 and 07:00 is the busiest time of the day for arrivals so we are   able to use both runways for landings.
  Occasionally we have to suspend    runway alternation for airfield maintenance, in response to bad    weather or when air-traffic control
  want to avoid a build-up of    arriving aircraft.

Actually, alternation also occurs for night flights, but then in 4 one-week periods.


What is the vertical speed limit when altitude mode is engaged?

What you are describing is the managed climb mode of the Airbus, where the Flight Management Computer (FMC) determines the optimum climb speed and the autoflight system will then pitch to maintain this speed.

From the A320 FCOM (Autoflight - AP/FD Vertical Modes):

  Climb Mode (CLB)

  CLB mode guides the aircraft in a managed climb, at either a managed or a selected target speed, to an FCU selected altitude, taking into account altitude constraints at waypoints. The system also considers speed constraints if the target speed is managed. [...]

  The pilot can arm the CLB mode during the takeoff, go around, climb and cruise phases and engage it during the climb and cruise phases.

The vertical speed is therefore determined by the difference of the current and the target airspeed. All protections like stall protection based on angle of attack still apply, which may limit the actual climb rate.

The same logic also applies when descending instead of climbing, where the aircraft will maintain a given descent speed by pitching at idle thrust instead of climb thrust.

Once you get close to the selected altitude, the ALT* mode will take over, which follows a vertical speed instead of pitching for speed:

  Altitude Acquire Mode (ALT*)

  ALT* (green) mode guides the aircraft to acquire the FCU selected altitude.

  ALT* (magenta) guides the aircraft to acquire an altitude constraint provided by the Flight Management.
  Once the aircraft has reached the altitude, the altitude mode (ALT green or magenta) engages.

  Engagement Conditions

  The mode engages when the aircraft reaches the altitude capture zone defined by the aircraft vertical speed
  (among other parameters).

  [...]

  Guidance

  The ALT* mode has internal V/S guidance that is a direct function of the difference between present altitude
  and altitude target. This V/S internal guidance has priority over the target speed. Therefore if a thrust reduction
  occurs in ALT* (Engine out), the vertical speed order will not be modified and a speed variation will occur. The ALT*
  mode does not have a mode reversion that results from speed variation.

  [...]

  ALT* and ALT CSTR* modes have internal protections that decreases the vertical speed when VLS or VMAX is reached (VLS or VMAX becomes the priority target).

  [...]

  The system switches automatically to ALT (altitude hold) when the altitude deviation becomes less than 20 feet.

The vertical speed is therefore only limited by the internal guidance logic, but will be overwritten when VLS or VMAX are reached providing the usual flight envelope protections.


Why was a Lockheed L-1011 chosen by Orbital ATK for its Pegasus launches?

One reason the L-1011 was chosen was that the keel beams on the L-1011 are spaced such that they could create an opening inside the aircraft for the verticle tailfin of the Pegasus when mated to the bottom of the aircraft and do so without jeopardizing the structural integrity of the airframe. This allowed them more clearance than other aircraft although the L-1011 still have to be jacked up to mate the Pegasus.

Second, while the L-1011 is no longer manufactured, there are enough of them still sitting around that getting parts was still an option, and in fact, sometimes entire aircraft were available for purchase.

Third, they were able to acquire their own L-1011 simulator from Delta and they operate it on a regular basis to maintain their flight currency. Since they own it they can make modifications to it to allow them to not only simulate their training environment but also their launch environment.

And lastly (although may not have been a consideration) some of the pilots of the Stargazer were Lockheed test pilots.

For more information, you should watch this short video:

NASA Edge Program - OrbitalATK ICON Program


Is any convention to tell the propeller rotation direction of an airplane?

When you say "clockwise" or "counter-clockwise" it is generally accepted that this would be the view as if you were standing behind the aircraft. Unless it is otherwise stated, this is the convention that you can assume.

If you need a way to tell by looking at the prop, the thicker part is the leading edge. In the first photo you posted, this would rotate counter-clockwise from the perspective of the photographer, or clockwise from the perspective of behind the aircraft. This is, by convention, a clockwise rotating propeller.


Near the poles 'polar grid navigation' is used. By placing a rectangular grid over the pole, aligned with the prime meridian, a local coordinate system is created. Aircraft fly constant grid courses near the poles.

For your example crossing the south pole approaching over the prime meridian the true course will change from 180° (T) to 000° (T), but the grid course will remain 180° (G) all the time.

Grid courses and true courses can be converted easily, this youtube video explains it nicely (although he gets the grid track in the wrong direction in the end, it should be 110°(G) instead of 290°(G))


What are the methods of reducing vortex?

Wing vortices  cannot be reduced. It is a byproduct of creating lift. The various wingtip shapes help to combine the airflow from the top and bottom of the wing and can make the wing act like it is longer and creating the same lift, but the vortex is still generated. The biggest vortices are created by the big airplanes when they are heavy, dirty (lots of flaps, etc. deployed) and slow (i.e. takeoff and landing).  Even my little 4-seat Cessna Cardinal creates vortices.

See Section 3.14 here
http://www.av8n.com/how/htm/airfoils.html#sec-vortices
of this great e-book "See How It Flies"
http://www.av8n.com/how/


Why do most fighter craft have a center stick?

At present, it appears to be a design choice rather than any other thing.

Most of the present generation fighters are transitioning to side-mounted sticks from center sticks. Almost all of the US combat aircraft have side-sticks. In Europe, it's somewhat of a mix- Typhoon (and Gripen) have center sticks, while Rafale has a side stick. The Russians are sticking to a center stick in the T-50.

The location of the control stick depends on a number of factors:

Ergonomics- Side stick is better in this regard, limiting the display area in the center; this might be a problem in smaller cockpits.
One of the reasons for using side stick in F-16 was that it (the hand-rest) supported the Pilot's hand during high 'g' maneuvers, along with the inclined seat (which is also being used in a number of aircraft).
There is also the personal choice- some pilots may prefer using a center stick, and others, the other way around (there seems to be little problem in transitioning from one to another).

Historically, combat aircraft have had center sticks. Side sticks have come about after fly-by-wire controls. Eurofighter has the following to say in this regard:

  Every military aircraft cockpit design has to be something of a compromise, but in a small cockpit, a center stick can cause particular ergonomic difficulties, perhaps limiting the size of the display area for the instruments and/or screens ... That is when options are considered, and one of those would be the sidestick as used in the F-16. However, if the cockpit is sufficiently large and the geometry of the seating position is correct, there is no reason to have anything other than a center stick—and the pilots of the four partner nations agreed. The choice between center sticks and sidesticks is 40% personal—the balance being on technical grounds. We find that F-16 pilots convert easily and naturally to a center stick.


Commercial airliners generally fly IFR as such they do what ATC tells them to do. They are not free to chose their own path. The airway system in the USA and elsewhere on the globe stems from the pre-GPS era when VOR's and other ground based nav aids were the predominant method of navigation. Airways tend to be either between VOR's or between points you can triangulate with a VOR. The system was built around this idea of traffic routing and in the GPS days it has simply stuck. As the FAA shuts down VOR's slowly some of the airways remain and seem a bit arbitrary but GPS allows us to follow them easily.

The FAA has a brief on it here.

Broadly speaking aircraft can request a deviation for weather or other inflight issues like turbulence but aside from that courses are followed as they are assigned.

The FAA has discussed routing directly but until the nextGen system is in place its likely not going to be a widespread reality.

  What differences (if any) are there between traffic over sea and over land?

At least over the Atlantic there is the North Atlantic track which is a set of predefined routes. Since there is no ATC over the ocean aircraft do a certain amount of self regulating.


Could D.B. Cooper know where he was landing, when he parachuted?

We have no idea how much he did know, but it's possible he had a basic idea. The fact he came up with the plan at all suggests some kind of aviation background (the air stair, for example, wouldn't occur to many people): so we can probably assume at least some basic navigation ability, especially with how thought out the plan appears to be.

He knew where the aircraft was starting from. He knew the airspeed it would travel at (100 knots, just above stall speed), which would give him a rough idea of how far it had travelled – particularly if he had information about the wind at that level.

Distance = Time x Speed... so all we're missing is the Time. If he knows how long they've been travelling, that's sufficient to allow him to have a reasonable idea of where they are along a known track, particularly if he carried a compass.

It's hard to be sure without the precise weather conditions available, but even in rain then as long as you're below the cloud layer, I wouldn't be surprised if Cooper could see the lights of either Portland or, more likely (considering the angle) Ariel: towns are pretty visible from the air. For example this photo is London from 10,000ft. Admittedly London is quite a lot bigger than Ariel, and the weather is clear, but considering the "closer" areas of the picture are a couple of miles, and the "distant" areas are more like 20 miles away, it's fairly easy to see how even a small town can be quite visible.

(source: https://www.flickr.com/photos/andygocher/12836895204)

Note that at 100 knots ground speed the aircraft is travelling at ~2 miles per minute across the ground. I can't find any wind data, but considering the 8:13 jump time was only approximate, it's more than possible that he saw the town below and a mile or two below and behind the aircraft, took a minute or so to prepare, and then jumped when 4-5 miles from the town.

He probably didn't know precisely where he was, but there's a reasonable chance that he saw a town below and that was sufficient for him to attempt the jump. He had the equivalent of over a million dollars in his pocket, he only had to find a town and get on a train in order to escape.

We also have no idea whether he had a chart or similar in his bag, nor whether there was any potential visibility of Mount St Helens (possible, considering the 1/4 moon that night). It's unlikely at 10 miles, but they'd passed it earlier and he may have used it as a point of reference.

In short: he probably had a rough idea of the region (to within 50 miles or so) and quite possibly had just seen a small town.

Did he know what town it was? Probably not, but he likely didn't care either: he had enough money that as long as he survived the jump and walk into the town, it probably didn't matter what the town was.


How much lift do planes produce before rotation?

Check out this plot (taken from wikipedia), of the lift coefficient against angle of attack. During takeoff roll, lift will be increasing due to the speed increasing, however not enough (in general) for the plane to achieve takeoff without rotation.

Once rotation starts, the lift increases dramatically, for (a very rough) example, consider a plane on takeoff roll, then rotating to 15 degrees angle of attack. By looking at the lift/AoA plot, we can see that the lift coefficient increases from roughly 0.55 to 1.7, giving roughly 3 times as much lift as before rotation.


How much data do FDR and CVR generate on commercial jets?

The requirements for Flight Data Recorders (FDRs) on airliners like the A320 is in 14 CFR 121. For a good summary of the requirements for flight recorders in various types of aircraft, see the NTSB FDR Handbook.

Depending on the certification date of the aircraft (14 CFR 121.343), the FDR must record between 6 and 17 parameters. Since engine parameters are included, this could be up to 23 different parameters on 4-engine aircraft. Newer Digital FDRs (DFDRs) must record at least 88 items (14 CFR 121.344). Some of these parameters are conditional, and some require multiple parameters, so the total will probably be more than 88 parameters. The NTSB report on the US Airways 1549 accident notes that the FDR recorded 178 parameters during the flight.

According to this page, tape-based DFDRs can record at 64 12-bit data words per second. Newer DFDRs using solid state recording can store up to 256 12-bit data words per second. This 256 words per second is also specified in the information on this FDR. It says it can record >100 hours at 64 words/second, which translates to about 35 MB total. The parameters are per DO-160C standard, which you have to pay to access.

Accident reports will generally include readouts from relevant parameters, to give an idea of the items that are recorded. There is a table here showing some parameters, the measurement range, and recording intervals. The interval is anywhere from 0.125 seconds to 4 seconds depending on the parameter.

Aircraft may also have a Quick Access Recorder (QAR), which records many parameters (over 2000). However, aircraft are not required to have one and the QAR is not required to survive an accident. Airlines use them for data to improve operations, and they can also be used in less severe crashes for analysis.


It comes down to a matter of philosophy. Or, if you are less generous, it’s a matter of fashion. Note that the Russian equivalent to the C-141, the Il-76, uses a T-tail, too, while the An-124 uses a conventional tail.

T-tails were in fashion in the early jet age. They were widespread, first in fighters and later in transport aircraft, too, because they:

Have a longer lever arm when placed on top of a swept vertical, reducing tail size and mass,
Allow for a smaller vertical tail due to the endplate effect, which lowers the requirement for hangar doors and ceilings. In case of the A400M, the T-tail allowed for a height reduction of approximately 2 meters.
Stay in undisturbed flow during stall. With wing-mounted engines resulting in a more forward wing position, the danger of a deep stall is reduced when compared to a configuration with tail-mounted engines.
Allow for less interference drag, especially when combined with a Küchemann body in the intersection between vertical and horizontal tail.

Therefore, if your conceptual design team is dominated by aerodynamicists, it will tend to favor the T-tail version. Only when second-order effects are considered will the conventional tail win. And those second-order effects will only become known by experience.

What are those effects?

Flutter! The rear loading ramp requires cutting the rear fuselage open, reducing its torsional stiffness. Add to that a mass on a long lever arm (the horizontal tail) and you get a structure with a very low eigenfrequency. In order to get that frequency out of the usable speed range, you need to stiffen the fuselage,
which adds structural mass. Both tail surfaces might be smaller for a T-tail, but the whole aircraft will be quite a bit heavier. During flight test of the C-141 it was found that the antimetric wing bending mode would nicely couple with the torsional Eigenmode of the the tail, resulting in some scary footage. Also, a coupling between elevator rotation and horizontal tail torsion required to generously add mass balance to the tail.
And then there still is this lingering suspicion that you might get into a deep stall. However, with the long distance between wing and tail of a configuration with wing mounted engines this is less of a threat than it might seem.
A real problem, however, is the tendency of a T-tail to produce a strong nose-down pitching moment in a sideslip. A low tail will eliminate this effect.


Can quadcopters save energy by flying in formation?

Yes, drag can be reduced by formation flying.

Geese using a low drag "V" formation. The authors of a 2001 Nature article stated that pelicans that fly alone beat their wings more frequently and have higher heart rates than those that fly in formation. It follows that birds that fly in formation glide more often and reduce energy expenditure (Weimerskirch, 2001).

Any flying object (plane, bird, helicopter, etc) that creates a down-wash to create lift must also produce an opposite and equal up-wash. A trailing aircraft positioned properly can ride this "wave".

Research aircraft flying a test point (low drag) for the Autonomous Formation Flight project over California's Mojave Desert. From the NASA Dryden Flight Research Center Web site.

The goal of the Autonomous Formation Flight project was to demonstrate 10 percent fuel savings of the trailing aircraft. The project extended the symbiotic relationship of migrating formations of birds. In December 2001, an F/A-18 flying in the wingtip vortex behind another F/A-18 exhibited a 14-percent fuel savings. The project ended in late 2001


Why high air speed at flat surface don't always produce low pressure?

TL;DR
Speed does not produce low pressure, increasing it does!
Bernoulli's equation says that for a fluid flow, in absence of elements doing work on the stream or having work done on them by the stream¹, $p + \frac12\rho v^2$ stays constant². But it does not tell us what constant. You can have flows with any combination of $p$ and $v$, and all will still satisfy the Bernoulli's equation.
Straight box in the first video
The blowers increase the energy of the air inside the hose. The energy can manifest as either static pressure, $p$, or dynamic pressure, $\frac12\rho v^2$. How it chooses? Well, if the static pressure $p$ inside the hose is higher than outside, the pressure gradient accelerates the air out, converting the static pressure to dynamic, and if the static pressure in the hose is lower, the gradient decelerates the air. So the flow will quickly stabilize so that the static pressure matches the ambient³, and the added energy determines the velocity.
Therefore there is the same static pressure everywhere and the box between the two nozzles has no reason to move.
Curved surfaces in the first video
The other experiments in the first video are not explainable by Bernoulli's equation alone, but instead effects of inertia and viscosity have to be taken into account. As the air flows around the curvature:

If the curvature is towards the stream, the air starts hitting the wall and needs to turn around it. Since it has mass, it requires force to turn it, which creates additional pressure on the wall, and a pressure gradient forms near it turning the air molecules further away. Bernoulli's principle tells us the velocity will, somewhat counter-intuitively⁴, decrease, but that is just a side-effect; we already know the pressure is increased.

It the curvature is away from the stream, the air would want to just continue straight. However there can't remain a wedge of a stagnant air behind the receding surface, because it would have higher pressure that forces it to move, and viscosity prevents forming another flow in a wedge shape to fill in, so the original stream sticks to the surface. And this means a pressure gradient again establishes that overcomes its inertia, this time with low pressure near the surface.
Bernoulli's principle tells us the velocity, again somewhat counter-intuitively⁵, increases, but that is again just a side-effect; we already know the pressure is reduced.

Second video
Last, the experiment in the second video relies on the fact that the air is expanding in multiple directions, so the cross-section available to it increases⁶.
According to the above, the velocity on the outer edge will be such that the static pressure matches ambient. But before it gets there, it has to squeeze through the still much smaller area near where the hose is connected. Since the same volume⁷ of air needs to get through both per second (due to conservation of mass), and to do that, it must move faster. And since it moves faster, its static pressure is lower, and this creates the pressure differential that pulls the plate towards the nozzle.
This is the only of the experiments that is actually demonstrating Bernoulli's principle.
Curvature v.s. speed
You see some of the cases are governed by one and some by the other. Fluid dynamics is extremely complex and difficult to calculate – the “easy solvability” or Navier-Stokes equations (that describe fluid dynamics; Bernoulli's equation is included in the set) is famous.
Generally, the curvature works in most situations, but watch out for aerodynamic stall (when the angle is too steep for viscosity to keep the fluid flowing ‘orderly’), while velocity only works when you have control over it, which is generally just in closed pipes.

¹ Bernoulli's equation simply expresses the internal energy of a fluid. If you do work on it you are adding energy, so that energy has to be added in the equation.
² Potential energy term omitted for brevity.
³ Dynamics of jet and rocket nozzles is much more complicated, but at the low velocities involved it holds pretty well.
⁴ It is counter-intuitive because it looks like the air has to move through less space, but since there is no opposite wall, it is free to expand as it suits it.
⁵ This is counter-intuitive because it looks like the air needs to expand to more space, but again there is no other wall, so it can pull more air from far and wide to fill that space.
⁶ I am not completely sure this case does not involve some additional effects.
⁷ Actually the same mass, and to be accurate we should include the adiabatic expansion of air, but for the speeds involved air can be approximated as incompressible.


What is this white-tail, short upper deck 747 I just saw?

All factory freighter 747s have short upper-decks, that includes the -400F and -8F. Some don't, but those are converted from passenger 747s that had the stretched upper-deck.

It could have been a 747-400F contracted by the military, plenty of those come in all-white, such as N404KZ:

source

I don't know on what basis the military prefers contractors over C-5s, etc., but an example of a civilian 747 contracted by the Air Mobility Command that crashed is National Airlines Flight 102.


What can be approach of balancing canard glider model?

Static longitudinal stability is defined as the tendency of the airplane to pitch down when it slows down and vice versa. In other words, when the lift coefficient goes up, the pitch moment coefficient must go down. This is expressed in equations like $$-\frac{c_{M\alpha}}{c_{L\alpha}} = \frac{x_N - x_{CG}}{l_\mu} = X\%$$ with X a positive number between 0.1 and 0.2. $x_N$ is the lengthwise coordinate of the neutral point, counted positive backwards, and $x_{CG}$ is the lengthwise coordinate of the center of gravity. The neutral point of both surfaces is at their respective quarter point and the combined neutral point sits between both, proportionally closer to the larger surface. Place the center of gravity X% of the mean aerodynamic chord $l_\mu$ ahead of this common neutral point and your stability is X. 0.2 is a rather high value and 0.1 makes for better performance but also less inherent stability.

Now all that is missing is that elusive neutral point of a canard. To explain what follows, it is the combination of the neutral point of both surfaces scaled by their area and corrected for the downwash of the forward wing which reduces angle of attack changes on the rear wing. Since only the inner wing will be hit by this downwash, we need to get a grip on the fraction of the wing which is exposed. This involves the span ratio of both wings $\frac{b_c}{b}$ and the taper ratio $\lambda = \frac{c_{outer}}{c_{inner}}$ of the main wing.
$$x_{N_{canard}} = \frac{x_{N_{canard}}\cdot c_{l\alpha_{c}}\cdot S_{c}+x_{N_{wing+fuselage}}\cdot c_{l\alpha_{w+f}}\cdot S_{w+f}\cdot\left(1-\frac{\delta\alpha}{\delta\alpha_W}\right)\cdot\frac{2+(\lambda-1)\cdot\frac{b_c}{b}}{(1+\lambda)}}{c_{l\alpha_{c}}\cdot S_{c}+c_{l\alpha_{w+f}}\cdot S_{w+f}\cdot\left(1-\frac{\delta\alpha}{\delta\alpha_W}\right)\cdot\frac{2+(\lambda-1)\cdot\frac{b_c}{b}}{(1+\lambda)}}$$
In absence of a precise value, use 0.7 for the downwash factor $\left(1-\frac{\delta\alpha}{\delta\alpha_W}\right)$.


How can coefficient of lift on the ground with flaps in the takeoff position be calculated using airfoil data?

There is quite a lot written on the topic, especially as part of "ground effect" vehicles research.
For example, Cui and Zhang cite two different types of lift coefficient. The first one (simpler and given here) is based on flat plate airfoils and is a modification to the well-known
$\frac{dC_l}{d\alpha}=2\pi$:

$$C_L=2\pi\alpha(1+\delta^2)(1-2\zeta)$$

with

$$\zeta=\frac{\sin\alpha}{4(h/c)}$$

and

$$\delta=\frac{\cos\alpha}{4(h/c)}$$

In the above expressions, $h$ is the height of the wing above the surface (ground, water, etc.), $c$ is the chord and $\alpha$ is the angle of attack.

I strongly recommend you check out "ground effect"/"Ekranoplan" literature - should be useful.


ATA100 was introduced as a publications standard in the late 50s for manuals and was in wide use by the late 60s.  Some manufacturers organized their engineering drawing numbering systems around ATA100 (ex.: flight control systems drawings numbered with a "27" or landing gear with a "32" prefix), but not all.


What do "LC/P" and "PTD" stand for in reference to airport communications?

LC/P: Local control, primary frequency, from LC, local control, which is a usual control position in the tower.

"The Local Controller is primarily responsible for separating and providing safety alerts to aircraft in the airspace surrounding the airport and aircraft on the active runways" (source)

Source

PTD: As @Riccati commented, it means Pilot to Dispatcher, according to the Airport / Facility Directory Legend.

Airlines are required by regulation to provide direct communication means for aircrew.

Aircraft dispatcher training (source)

See also: Can airline pilots communicate with dispatchers?


What is the difference between point-to-point and low-cost airlines?

Point to point describes an airline that focuses on delivering services without a connecting hub. This cannot be said to imply low cost.

A independent airline flying between some islands and not offering connections would fall into this category as much as Ryanair, Easyjet or Spirit Airlines.

Low Cost generally implies a point-to-point operation. They structure their business model around filling aircraft on high-demand routes.

Reasons for this: The cost and complexity of handling and organising hubs and connecting flights violates their carefully controlled profit margins and structure.

Hence IMHO you can normally assume a low-cost operation will be a point-to-point operation, but not the other way around.


What is the definining line between a helicopter and a drone a person can ride in?

A helicopter is an aircraft in which thrust and lift are provided by rotors.

A drone is an unmanned, self-piloted or remotely-controlled aircraft, which can use rotors to provide thrust and lift, but can also use other means such as propellers or turbines.

To put it in simpler terms, a helicopter can only be a helicopter, regardless of whether it has a human inside or not. A drone can be any kind of aircraft.


How do airliners facilitate in flight calls for mobile phones?

Cribbing liberally from Manfred's answer on a related question, it basically works like this:

The airline contracts with a communication company and installs satellite transceivers and "small cell" cellular equipment on their aircraft. Your phone talks to the on-board cellular equipment just like it would to a cell site on the ground, but the information is relayed to the satellite and sent back to earth to join the regular telephone/data networks.

It's not generally feasible to use these datalinks to track the location of the aircraft -- getting your basic position (latitude/longitude) requires triangulation from at least 3 satellites, while data communication only requires one (satellites are expensive, so providers aren't likely to launch their own constellation of satellites to duplicate the functionality of GPS).

This same type of data uplink could theoretically be used to transmit flight data (including GPS position that the aircraft can get from the existing GPS equipment on board) back to the airline's headquarters, ATC (to fill gaps in RADAR coverage), etc., but I'm not aware of any system or plan to implement that currently.


How does wake turbulence affect parachutists?

Qualification: I am a master parachute rigger and I taught sport parachuting for 10 years.

The back door of a 747 is actually in a pretty good position - the tail is above the door and the fuselage is narrowing at that point so it's not a complete side exit.

A freefall jumper will drop out of the aircraft's wake in under a second so that's not really an issue. If we pick a theoretical worst-case where a jumper goes straight through a huge vortex they might get flipped over. Remember that a human does not have large surfaces like wings to interact with. Any jumper that can't recover from that won't be doing freefall jumps anyway. Seriously, this is in the Jump #1 stability lesson, and if you haven't got it sorted by jump #6 any reputable school will send you home.

Exit speed is a consideration - sport gear is meant for < 200km/h. If we are going out at 747 cruise speeds I would tighten everything right up1, wrap some tape around the riser covers2 and tuck the pilot chute handle completely into the pouch. Maybe also tack the closing flap cover down. Yes guys, it will still open if you sew it to the proper flap. If the plane is at approach speeds with flaps and gear down then you're fine - not much different than a King Air.

So in conclusion, yes, you can jump out of a 747. Rent one and you will have every jumper in the state lining up for tickets.

To reply to the comment of the pilot chute going over the tail, that happened at our place too (and to me once, but mine went under the tail). It's got nothing to do with turbulence - the pilot chute got pulled mostly out of the pouch, most likely snagging something, and the air picked it up from there. Our guy actually hit the tail. He had nasty bruises and needed a few lines changed, the pilot needed new underwear, and the Cessna needed a bunch of rivets and sheet metal work. When it happened to me I had a surprise deployment at 9,500 feet and a longer than usual ride down.

1 to the point where you will need my tools to close it.
2 it will break away after opening, and won't be more than an annoyance if it doesn't


Why does the Falcon 900 have anhedral wings?

For the same reason why the Lockheed F-104 Starfighter had such a pronounced anhedral: It is the size and location of the vertical tail.

Lockheed F-104 Starfighter (picture source).

In both cases, the vertical tail is large relative to the wing and fully above the roll axis. In a sideslip it produces a high rolling moment, so any more rolling moment from the wing would reduce the handling qualities of the aircraft.

Especially in case of the F-104, the raised position of the horizontal tail shifts the center of pressure on the vertical tail upwards, exacerbating the offset location of the vertical tail.

The rear-mounted engines on the Falcon 900 move the center of gravity backwards, so the wing is more backwards than usual for aircraft with wing-mounted engines. Therefore, the part of the fuselage ahead of the center of gravity is large, requiring a larger vertical tail to overcome the destabilizing yawing moment of the fuselage.

The same is true for the Tupolev 134 and 154, and - surprise! - they also have no dihedral, even though both are low-wing configurations.

Tupolev 154M of the Polish Air Force (picture source)


What are the exact meanings of roll, pitch and yaw?

The key thing missing in your interpretation is that the order is important. Applying the same roll, pitch and yaw angles in a different order will result in a different orientation (mathematically this is because rotation matrices do not commute).

The correct order is:

Yaw (for an airplane this is typically called the heading)
Pitch
Roll

For any given attitude, there is a unique combination of yaw, pitch and roll angles that will (if applied in this order) give you the correct attitude.

These angles are only meaningful in a given coordinate system. We typically use a system where all angles zero means you are pointing North and wings and body are parallel to the ground. Based on this starting point you would apply the given angles.

(source: Wikimedia from this Wikipedia article: Aircraft principal axes)

See also the answer to this question:
What is the relation between roll angle and pitch angle?

Let me expand a bit on your example:

We start will all angles equal to zero (level, pointing North) and pitch up 90 degrees such that our nose points straight into the sky. Yaw and roll have not changed. Now we start rolling. The roll angle would now tell us in which direction the belly of the aircraft is pointing (0 degrees means belly pointing North, 90 degrees means belly pointing West and right wing pointing North, and so on).


What does performance speed refers to? Aircraft performance in general

Read the POH - it will show the conditions the speed was measured under.

  Cruise, 75% Power at 8500 Feet. . . . . . . . . . . . . . . . . . 122 KNOTS

  NOTE

  *Speed performance is shown for an airplane equipped with
  speed fairings which increase the speeds by
  approximately 2 knots. There is a corresponding difference
  in range, while all other performance figures are
  unchanged when speed fairings are installed.

  The above performance figures are based on airplane weights at 2450
  pounds, standard atmospheric conditions, level, hard-surfaced dry
  runways and no wind. They are calculated values derived from flight
  tests conducted by Cessna Aircraft Company under carefully
  documented conditions and will vary with individual airplanes and
  numerous factors affecting flight performance.

(http://www.airilm.com/wp-content/uploads/172R-POH.pdf)


What would happen if I bank a helicopter then pull hard on the cyclic?

I can't provide references specific to this manoeuvre. The main texts on helicopter dynamics only generalise since such a manoeuvre isn't flown in real life or tested during certifications.

When you raise the collective, you increase the pitch of the all of the blades together.  In most flight regimes, this means increased angle of attack and therefore lift.

The cost of producing lift is drag which always increases.  So, there is a direct relationship between the collective lever and power required from the engine.

As you continue to raise the collective, AoA and lift will increase until you reach a point where the power required to overcome drag exceeds the rated power of the engine.  Most helicopters permit operation beyond the maximum for a short time, e.g. 2 or 5 minutes, typically during take-off.  Continue to increase AoA beyond this limit and drag will exceed power available and the rotor RPM will drop (a reduction in rotor RPM caused by drag exceeding power through too much collective is known as "droop").

As RRPM decreases, so does lift and the helicopter will descend.  The relative airflow into the disk now has a greater horizontal component and the angle of attack will increase, either causing yet more drag or stalling the rotor.

So there is some point, before the collective is pulled fully up where maximum lift is attained.  Beyond that, lift reduces until recovery or a stall.  The only time I have ever raised the collective fully is during "full and free" movement checks with the engine off as part of pre-flight.

Your hypothetical manoeuvre:

From VNE in level flight, you roll into a 70 degree bank.

The TRT( total rotor thrust) is always acting perpendicular to the plane of the rotor disk.  Since you are in a bank, and I assume it is at constant altitude, there is a large horizontal component of the TRT to maintain the acceleration in the turn and a large vertical component to oppose gravity and maintain altitude.

This requires a lot of power which means a lot of collective and a lot of drag.  I don't know for the AH-6, maybe there's a chart relating bank angle and speed to power but let's just say, your collective is going to be somewhere near the max lift point with a manoeuvre like this.

Since you are now accelerating with max power, your speed will decrease.  Therefore lift will decrease and since you are at or near max power, you will begin to descend.

So far, this is probably recoverable simply by rolling level and reducing collective until back inside the performance envelope.

The second part is where it definitely will go wrong. In all helicopters with a non-rigid head, including the AH-6, the disk will adopt a new plane of rotation rapidly.  There is then a significant delay before the fuselage adopts a new attitude which puts the TRT perpendicular with the aircraft Z axis.

Pulling hard on the cyclic will very rapidly put the disk into a new plane of rotation which has a large angle between the plane of rotation and the fuselage.  Additionally, the disk loading will increase dramatically and the blades will be forced downwards.  On an articulated head such as that on the AH-6, droop stops will prevent the blades from bending down so much that they fold.

Hitting the droop stops a little is uncomfortable with a lot of vibration. Pulling hard on the cyclic at VNE in a 70 degree bank at or near max power may cause failure of the droop stops or blades or cause the static droop stops to contact the  mast possibly leading to rotor separation.

Let's assume that there is no failure.  The plane of rotation will be at the maximum permitted by the physical limits of the flapping hinges which is greater than the angle between the plane of rotation and the fuselage.

Therefore, I would expect a hard pull on the cyclic to result in a blade hitting the tail or the top of the cockpit.  Either results in destruction of the aircraft, either as it breaks up at that point or when it hits the ground 10 seconds later.


D.B Cooper incident - What caused the tail to move after a jump?

The "sudden upward tail movement" requires nothing more than a sudden unloading of the tail by removing that 90+ kg of man, parachute, and money.

Even with an aircraft as large as a 727, suddenly removing 90+ kg from the tailcone airstair would result in a pretty noticeable shift in trim requirement, with additional tail downforce required to restore level flight.  This is especially true when flying at the low speed Cooper had demanded (to allow a more-or-less safe jump).


Why does supersonic flight detach airflow from a wing?

Flow separation happens when the pressure gradient of the airflow along the flow path becomes too steep. In subsonic flow, the oncoming air is first decelerated ahead of the wing, then swiftly accelerated when it flows around the strongly curved nose section of a wing. This acceleration is the consequence of the wing's curvature.
After this acceleration, how will the airflow behave? Will the airflow keep moving in a straight line without further acceleration or deceleration? (which implies that airflow will separate from the wing surface, creating a local vacuum) Or will it faithfully stick to the wing surface? In reality, the air settles at a compromise between the straight path and following the contour, creating decreasing pressure along a surface with increasing curvature and increasing pressure along surfaces with decreasing curvature. More precisely, it is always in an equilibrium between inertial, viscous and pressure forces.
This suction not only bends the airflow into following the wing's contour, but also accelerates the air ahead of it. The lower the pressure, the more the air speeds up, such that the total energy of air (the sum of pressure and kinetic energy) stays constant. Therefore, pressure and local speed change in sync.
When the curvature decreases further downstream, the flow path becomes straighter and pressure rises again. However, the air particles close to the wing slow down because of friction. The layer of air where this slowing is noticeable is called boundary layer. In it, the deceleration effects due to pressure rise and due to friction add up, and at some point the air comes to a standstill relative to the wing. Where this happens, static air will collect and build up, causing flow separation. Thankfully, the exchange of air across a turbulent boundary layer kicks the slowest air particles downstream, so at moderate angles of attack the air still moves along until it reaches the trailing edge. Only when the suction peak around the nose becomes very high at high angle of attack, the consequent steep pressure rise along the remaining flow path overpowers the possibilities of the turbulent boundary layer, the air decelerates completely and the flow separates. This is a wholly subsonic affair.

If the wing moves at high subsonic speed, the curvature-created suction accelerates the flow such that it reaches supersonic speed. Now something odd happens: Supersonic flow accelerates further when subsonic flow would decelerate. This is caused by the change in density which is dominant at supersonic speed. Incompressible (= very slow) flow has constant density, and all speed changes affect pressure. At Mach 1, the pressure and density changes are of equal magnitude, and in supersonic flow the density changes dominate. Now we have a supersonic pocket of air on the upper surface of the wing where speed increases and density decreases downstream, and the surrounding subsonic air sees little change in density. This picture should give you some idea how it looks:
![Lambda shock in high subsonic flow][1]
The whole wing moves at Mach 0.68. Compare the green color at some distance from the wing with the scale on the left side, which gives the Mach number for each color shade. At the airfoil nose, you see a blue area. This is where the air decelerates - it gets pushed together by the approaching wing. Now follow the colors along the upper side - they quickly turn green, yellow and red as the air is accelerated into the low pressure area (remember, low pressure equals high speed, so the reddest area has the highest local flow speed and the lowest pressure). In subsonic flow, the suction peak would be somewhere between 20% and 30% of chord, and the colors would slowly change back to yellow and green if you move further downstream. Now we have local supersonic flow (everything redder than light orange is supersonic here), and instead of slowly decelerating, the air is speeding up to a maximum Mach number of 1.23 at almost 60% of chord length.
This cannot last, and at some point this supersonic pocket collapses. This happens instantly in a shock, and as you know, in a straight shock density increases suddenly and speed decreases such that the Mach number after the shock is the inverse of the Mach number ahead of the shock. In the picture above, boundary layer effects create a lambda shock, which has its name from the Greek letter which looks like the shock pattern here. Aft of the shock, you have subsonic flow again, and a much thicker boundary layer which moves very slowly (blue shade). This is due to the energy conversion through the shock, which converts kinetic energy into heat. But the flow is still attached - even this shock did not cause separation.

If this pressure rise is big enough, the boundary layer will come to an instantaneous standstill, and then the flow separates. This is the shock induced separation you asked about. Unfortunately, the picture above is the best I could find, and I have none with separated flow aft of the shock. But it helps to show that the center of pressure moves aft. This causes a strong nose-down moment. Also, with higher subsonic Mach numbers directional stability decreases. Now even more nasty things can happen: The location of the shock might move forward and back. This changes the size of the supersonic area, causing lift changes. On a horizontal tail, this will also cause pitch changes. If you change elevator position slightly with such a shock on the horizontal tail, the lift change could be severe and in the opposite direction of what you would expect. This causes total loss of control, just when you need the control surfaces to counteract the Mach effects mentioned above. Also, the shock position could oscillate, causing a buzzing sound and, if you are really unlucky, coupling into an elastic eigenfrequency of your structure, resulting in flutter. Not only on the tail surfaces, but also on the wing, affecting the ailerons as well. Now you can start to see what scared the early pioneers about flying near Mach 1 and why they spoke of a "sound barrier".

If you fly fully supersonic, this effect goes away because now the shock moves to the trailing edge and stays there. Now everything will be calm again because the shock location stays fixed. This effect was first experienced and survived on April 9, 1945 by [Hans Guido Mutke in a Me-262][3], which briefly flew fully supersonic in a dive. However, even in fully supersonic flow separation is possible, but then because the air flow will not bend more than what can be caused by total vacuum. In hypersonic flow the density changes become so severe that pockets of "air" are possible which do not contain any air, but a vacuum. But this is more an academic case, except for reentry vehicles with a blunt, rear-facing base.


After hernia surgery, what do I need to do to be cleared to exercise the privileges of my PPL?

What makes you think you need to inform the FAA of your status? I know of no rule that requires notifying the FAA until you go for your next medical. The FAA has a list of disqualifying medical conditions, and hernias aren’t among them. They do recommend that you not fly for 5-times the maximal hour dose interval after taking certain medications, but presumably you are off the pain meds by now.

If your doctor clears you for normal activity (including driving) then you would have no reason to know of any medical condition that would make you unable to meed the requirements of the medical certificate.

  14 CFR §61.53   Prohibition on operations during medical deficiency.

  (a) Operations that require a medical certificate. Except as provided
  for in paragraph (b) of this section, no person who holds a medical
  certificate issued under part 67 of this chapter may act as pilot in
  command, or in any other capacity as a required pilot flight
  crewmember, while that person:

  (1) Knows or has reason to know of any medical condition that would
  make the person unable to meet the requirements for the medical
  certificate necessary for the pilot operation; or

  (2) Is taking medication or receiving other treatment for a medical
  condition that results in the person being unable to meet the
  requirements for the medical certificate necessary for the pilot
  operation.


What’s this vortex generator like thing below the wing?

The manufacturer, Big Ass Fans, calls those devices “air fences.” Based on this naming I assume they are most likely there to redirect spanwise flow on the longer airfoils, not to prevent separation. Such a narrow airfoil built to move at a steady speed and loading would probably not suffer from separation issues. They are basically the same as wing fences.

source:Big Ass Fans website

Wing fences are usually found on swept wings, because that’s what’s responsible for the spanwise flow, and the fan blades don’t appear to be swept. But in the case of a fan there will probably be a certain amount of radial flow due to the spinning motion. After a significant distance this could build up to be significant enough that the fence might help to redirect it.


What is that device on an Su-27 pilot's helmet?

That is the mounting point for night vision goggles. The following image shows a helmet with NVG mounted.

Helmet mounted NVG; image from lemt.by

The USAF has helmets with different mounting and some of the modern pilot helmets (like BAE Striker II, for example) have an integrated night vision device.


How to keep a Garmin GTX330 turned off when powering on avionics?

If pin number 1 on the back of the GTX330 unit is connected to power (+), the unit will turn on. If it is not, you will have to manually press the on button. If you remove power from pin 1, it will not turn it off.
See the installation manual, which is findable by a simple Google/Bing search, page D-1, figure D-1, Note 3.


What rotors would be best for atmospheric reentry?

You couldn't do it to control reentry, but you certainly could do it for a vertical landing system that might be a bit less technically demanding than a rocket that can descend under control with its engine running as SpaceX does.

You are limited by the mach# of the tips of the rotor, which have to be subsonic while spinning.  You would want a rotor that can fold alongside the rocket and deploy and accelerate to operating RPM once it's approaching the surface, and reduce the descent rate to an acceptable level where the craft can land with its gear absorbing the landing shock in a zero forward airspeed autorotation.

This is not hard to do with a well designed gear and there are gyrocopters with gear that is designed to handle zero-forward speed autorotation landings (basically, they can cope with high descent rates by using long-stroke shock absorbers).

The rotor would simply be the simplest, lightest, cheapest system that you can come up with, that the landing gear can handle.  If you could do it with two blades you'd go with that, but maybe packaging would require more blades, and you'd have to find some sweet spot with the best overall landing performance between the rotor and landing gear, and your physical packaging limitations.


What the preparations and procedures to consider for circumnavigation?

Circumnavigating the planet Earth very popular hobby and a courageous endeavor. It is not an easy task and requires extensive preparation, planning and training.

What is circumnavigation?
Travelling around an island, a continent, or the entire world is called circumnavigation.

What is considered global circumnavigation?
Crossing all meridians is not considered global circumnavigation. If you  walk completely around either Pole, you have crossed all meridians, but this is not generally considered a circumnavigation.

A basic definition is a route which covers at least one great circle, and it passes through at least one pair of points of antipodal to each other.

What are requirements for circumnavigation by airplane?

The route should start and finish at the same point.
It should cross all meridians.
The course must be at least 22,858.729 miles (36,787.559 kilometers) long. This is length of Tropic of Cancer.
It must include set control points at latitudes outside the Arctic and Antarctic circles.

Do people do it?
Yes, a lot of people do it. Earth Rounders shares experiences of pilots who have flown around the world. It is categorized by engine types and solos etc.

What should I need to know?
A lot. Typically, people who flew around the world typically break their journey in 80-100 flights, or even more. It takes several months. Each flight of circumnavigation is significantly longer than a typical cross-country flight. It is much harder on many levels than an ordinary cross-country flight.

This website takes about the preparations thoroughly. It covers these section:

The Pilot
The Aircraft
Preparation
Survival equipment
The flight

Because of the longevity of this article, I am not duplicating it here as it contains all very useful information.

What are other people's experience?
There are many people who have thoroughly documented their journeys, the preparations, the hazards and difficulties they faced, and many pictures and videos. I am listing a few, which certainly are not the best ones, but give a decent idea about their trip.

Robert Gannon
Carol Ann Garratt
Flight Simulator X
Although this is in a simulation, but this trip is very thoroughly planned and it contains detailed path descriptions.


Do experienced pilots look at the inclinometers during flight?

Yes to some degree but it depends a lot on your seating position.  If your torso is fairly upright and not too well supported, slips and skids become easy to sense, beyond a certain point, as your waist muscles have to work to keep up upright as you "lean into the ball".

With experience it's possible to stay within, say, half a ball width of centre, without looking at it.  It's tougher if the airplane has reclined seats or has seats with strong side bolsters that hold you in well.

That being said, you still need to glance at the ball it to keep it really centred while maneuvering and I tend to use both my leaning sense and glances at the skid ball at the same time, leaning sense for large yaw excursions and the skid ball for fine tuning.

It's a good skill to master when VFR flying because you can keep your head outside the cockpit where it belongs (in VFR) a lot more.

Gliders use a yaw string for skid control and because you are steeply reclining the leaning sense is fairly useless and you rely mostly on the string, which is easy because it's right in front of your face and you don't have to put your head down.

In any case, you still "step on the ball" or "step toward the lean".  It helps to develop the automatic foot action so you squeeze the pedal with your foot instinctively whenever you move the ailerons.  Pilots who learn on gliders have this action fully automated.


What are the engine-out implications of giant turbofans?

Both-engine-out glide performance isn't accounted for and isn't a certification requirement (of any airplane that I'm aware of).  Airliners have good glide performance as a happy side effect of design optimization for cruising at high altitudes.
What does have to be accounted for is single engine performance issues; things like departure performance with an engine failure at V1, single engine ceilings, etc.  Gliding ability comes into it in a secondary way, in terms of what is called "driftdown".  Driftdown is when you are forced to descend from a both engines cruising altitude to the single engine ceiling following an engine failure or shutdown when your power is cut in half, and  could be thought of as "semi-gliding" or "power assisted" gliding performance.
Driftdown speeds are speeds to use to get the flattest descent with maximum continuous thrust on the good engine, and you may use these speeds where terrain clearance means you need to stay at high as possible for as long as possible. Driftdown performance is measured during certification for the purpose of providing data for the pilots in the QRH, but there is no minimum driftdown "flatness" or glide ratio requirement per se.
As far as designing around geared TFs, well, the effects are what they are, and since nobody makes feathering fan blades, you are stuck with whatever the drag penalty is. A windmilling GTF is going to have more windmilling drag than a direct drive TF. But a direct drive TF has more windmilling drag than a traditional turbojet.  So what did the manufacturers do when TFs came out?
They made allowance for the differences, and maybe an airliner would need somewhat more thrust from a given engine in TF form, all else being equal, to accommodate the higher windmilling drag of a failed engine on the other side, compared to the same airplane with pure turbojets. The drag penalties are not as bad as windmilling propellers, and don't necessitate radical solutions like feathering fan blades.
Geared TFs just take the same problem a little farther, but still get away without needing things like feathering fans (which would have killed the entire GTF concept) by making sure the thrust reserve is there to deal with a windmilling fan.  But if a GTF equipped airplane ends up with a both-engines-out glide ratio of 12:1 instead or 14:1 because of the drag of its super-size fans, that's neither here nor there.


Why does the cabin air sometimes smell of kerosene?

You might have been smelling cooked turbo oil.  Leaking seals can potentially allow oil traces into the compressor bleed discharge, especially on auxiliary power units.  The other possibility is cooked deicing fluid, which is quite common.

Turbo oil and deicing fluid are the two major sources of petrochemical cabin stink.

Another possibility would be an APU with stalling/surging problems where there can be backflow into the compressor end of the motor from the burner can, which will admit burner can gasses into the air conditioning system.  If it is really kerosene you are smelling, that is the probable source.

It could be coming from the main engine but there would likely have been audible rumbling or thumping sounds if it was stalling/surging while on main engine bleed.


What type of BITE is involved after installing a new display on an aircraft?

First, let's clarify the terms.
BITE (Built-In Test Equipment) is the hardware and software within a product that performs BIT (Built-In Test).
There are three common forms of BIT:

Power-on BIT (PBIT).  This is run every time the unit powers up. Depending on the unit function, some units have two variations of PBIT; A comprehensive check performed after a cold start, and an abbreviated check performed after a warm start.  Warm starts typically occur in-air after short power interruptions. The abbreviated PBIT is intended to get the unit back online as quickly as possible. PBIT only reports faults.
Continuous BIT (CBIT).  Generally implemented only in critical functions, CBIT provides a continuous (or near continuous, periodic) test. CBIT is a background test and like PBIT, only reports faults.
Initiated  BIT (IBIT).  This BIT is only run when it is initiated by the crew or maintenance personnel. It will typically take the longest time and be the most thorough BIT that can be performed. IBIT will, upon completion, provide a status report.

So to your question; when a new display is installed (or any other unit is replaced) and power is restored the unit will perform a PBIT and potentially begin CBIT (if it exists). After the unit is in normal operation, the technician that replaced it will normally perform an IBIT and verify the unit passed all tests.
It's important to remember that BIT is intended to identify hardware faults. If you are concerned with 'glitches' due to flipped bits (caused by single event upsets) or similar, that is typically addressed using integrity monitoring. This is more common than CBIT and it will only flag 'bad data' as it can't specifically identify what caused a fault. It only identifies that data is suspect as the primary computation and the monitor disagree. A hardware failure will generally result in the integrity monitor providing a continuous fault condition, but it's primary purpose is to identify 'soft' or intermittent faults.


Can an issued landing clearance be replaced with "continue approach"?

The example is correct, except that the landing clearance for stack 1 must be cancelled before the takeoff clearance is given to stack 88. The logic is that only one aircraft can ever be cleared to land on or take off from a given runway at any one time. So in your example, you just have to swap the order of instructions 3 and 4.

Stack 1 cancel landing clearance, continue approach
Continue approach, Stack 1
Stack 88 runway 12 cleared for takeoff
Runway 12 cleared for takeoff, Stack 88
...
Stack 1 runway 12 cleared to land


Why is ram air not used for air conditioning in airplanes?

The problem is pressure. In order to use outside air for the cabin, it needs to be pressurized. Otherwise you could not reach higher pressures inside the cabin than outside.
The advantage of using bleed air is that this air is already pressurized, so you don't need to do any additional work. It is however too hot to be used as cabin air directly, so a PACK (Pneumatic Air Cycle Kit) is used to reach lower temperatures using ram air for cooling.
Ram air cannot be directly used without increasing its pressure. On the Boeing 787, which is equipped with bleed-less engines, the outside air therefore needs to be pressurized with electrically driven compressors:

In the no-bleed architecture, electrically driven
compressors provide the cabin pressurization
function, with fresh air brought onboard via
dedicated cabin air inlets. This approach is
significantly more efficient than the traditional
bleed system because it avoids excessive energy
extraction from engines with the associated
energy waste by pre-coolers and modulating
valves. There is no need to regulate down the
supplied compressed air. Instead, the compressed
air is produced by adjustable speed motor compressors at the required pressure without significant
energy waste. That results in significant improvements in engine fuel consumption.

(Boeing Aero Quarterly - 787 No-Bleed Systems, emphasis mine)
Note however that this compression also adds heat to the air, so the air still needs to go through a PACK before it can be used as cabin air:

In the 787 electrical architecture, the output of the
cabin pressurization compressors flows through
low-pressure air-conditioning packs for improved
efficiency. The adjustable speed feature of electrical motors will allow further optimization of
airplane energy usage by not requiring excessive
energy from the supplied compressed air and
later regulating it down through modulating valves
resulting in energy loss.
Avoiding the energy waste associated with
down regulation results in improvements in engine
fuel consumption, and the environmental-control
system air inflow can be adjusted in accordance
with the number of airplane occupants to achieve
the lowest energy waste while meeting the air-flow
requirements.

(Boeing Aero Quarterly - 787 No-Bleed Systems, emphasis mine)
You can see the inlet for the compressors and the ram air inlet in the following picture:

(image source)


How long can the stop in a stop-and-go be?

The AIM covers this briefly (bolded the important part) but effectively you cant sit there very long:

  4−3−22. Option Approach

  The “Cleared for the Option” procedure will permit an instructor,
  flight examiner or pilot the option to make a touch−and−go, low
  approach, missed approach, stop−and−go, or full stop landing. This
  procedure can be very beneficial in a training situation in that
  neither the student pilot nor examinee would know what maneuver would
  be accomplished. The pilot should make a request for this procedure
  passing the final approach fix inbound on an instrument approach or
  entering downwind for a VFR traffic pattern. After ATC approval of
  the option, the pilot should inform ATC as soon as possible of any
  delay on the runway during their stop-and-go or full stop landing.
  The advantages of this procedure as a training aid are that it enables
  an instructor or examiner to obtain the reaction of a trainee or
  examinee under changing conditions, the pilot would not have to
  discontinue an approach in the middle of the procedure due to student
  error or pilot proficiency requirements, and finally it allows more
  flexibility and economy in training programs. This procedure will only
  be used at those locations with an operational control tower and will
  be subject to ATC approval.

The way I read that is effectively you come to a stop and get going as soon as possible. If you intend on doing anything else you should inform ATC. I take this to be a time limit of, as soon as you stop, get going again.


Is it bad practice to say my callsign first in a transmission to ATC?

It depends on the country where you are flying, and also what the message is.

For example in the UK according to CAP 413 Chapter 2, section 2.48:

  2.48

  Ground to Air: Aircraft callsign – message or reply.

  Air to Ground:

  Initiation of new information/request etc. – Aircraft callsign then message;
  Reply – Repeat of pertinent information/readback/acknowledgement then aircraft callsign.

In Germany the normal procedure is to start a message with the call sign, a reply may have the call sign at the end.  From the AIP, GEN section 3.4-9:

  6.1 Procedures for Radio Communications

  ...

  (4) The call sign shall be transmitted at the beginning of a message.
  A direct answer to a message may be terminated by the call sign
  instead.


Is LHV or HHV used for calculating the thrust of a jet engine?

The difference between both values is the energy set free when the water created by the combustion process condenses. Use the lower heat value for jet engine calculations. The water in a jet engine leaves it as vapor, and the vaporisation heat is lost for propulsion.


What is the "tennis ball between cylinders" trick used by flight instructors?

From aircraft flying handbook (page 2-9),

  The pilot should also check for loose or foreign objects inside the cowling, such as bird nests, shop rags, and/or tools.

This tennis ball is a tool for instructor to see if a pilot does a proper walkaround of the airplane. A pilot should be able to detect this strange object inside the cowling easily if he/she does the walkaround properly.


Why aren't aircraft cabins pressurized to sea level pressure?

Two reasons: Longevity and weight. Which really come down to just weight.

Airframes have a limited fatigue life, measured in flight cycles.
The main driving factor for airliner airframe wear is pressurizing and depressurizing them. Each millibar of difference between cabin pressure and outside pressure effectively consumes some percentage of the airframe's fatigue life.

Reducing the cabin altitude means increasing this pressure difference, and thus consuming more of the airframe's life. This could be compensated for with sturdier construction, which adds weight. It would also consume a little more bleed air, requiring slightly heavier packs, which, as well as weight, means a loss of efficiency.

Luxury business jets often maintain a lower cabin altitude, such as 4,000 ft. This eats into their flight cycles, so they can still be switched to the usual 8,000 ft for flights without the owner/VIP inside.
Carbon fiber has a much longer fatigue life, so CFRP fuselages can afford to lower the cabin altitude to 6,000 ft. This pressure altitude can also be maintained in other airliners at flight levels well below their ceiling.

The optimum compromise point is subject to a lot of debate. The highest cabin altitude that could be permitted is 15,000 ft, above which hypoxia-induced loss of consciousness can occur. The regulatory bodies have settled at 8,000 ft, so that's what the manufacturers targeted with most of their aluminum airliners, and that's where airlines prefer to run them even if they have a choice, to get more life out of their planes.


How do we get Required RVR or Actual RVR for an instrument approach?

Current RVR is obtained from the METAR, yes, which in turn is obtained from transmissometer(s) on the airport site.

Minimum allowable RVR is obtained from charts (chart legend from Jeppesen, PDF), more specifically the instrument approach plate for approaching an airport. And on the SID chart for departing.

Example:

R is RVR in hundreds of feet, V is Visibility in miles, in this example. Units can change and will be denoted, check full PDF for all examples.


Is the "glass nose" Tu-134 design for easier conversion to military use?

Converting a Tu-104, a Tu-124 or a Tu-134 to a bomber only would had worked if there had also been provisions for a bomb bay. Including this in a pressurized fuselage would had incurred a severe weight penalty, so there was no easy way for a bomber conversion. This should make the speculation about dual use as a bomber moot.

However, there are several good reasons to use the glazed nose:

Easier conversion of trained bomber crews (trained on the Il-28 and Tu-16) for civilian duties. Remember that the Tu-104 was the second jet airliner in service and the only operational jet airliner worldwide between 1956 and 1958.
Operation as a military transport on improvised flights (where the better view of the navigator in the nose would help in visual navigation).
Civilian operation on flights on routes without navigation aids.

The last point is added by @JanHudec as a comment to this answer, but would exclude the operation on a scheduled service, because the aircraft would not be able to navigate in fog or at night.

Since the glazed nose was removed quite early in the production run of the Tu-134 (from the A-2 version on), it seems that the first reason (dual use of Tu-16-trained crews) is the most likely. While more than 1,500 Tu-16s were built, the Tu-104 (201 built from 1956 to 1960) and Tu-124 (164 built from 1960 to 1965) were relatively rare. From the Wikipedia page on the Tu-104:

  Pilots with previous Tu-16 experience transitioned into the Tu-104
  with relative ease


Is it possible for a civilian to use a VORTAC station?

Totally legal to tune it up & use it. If it is a VORTAC, it will work just like every other VORTAC out there. If it is a TACAN, you may be able to get DME from it, but you won”t get bearing from it without a TACAN receiver.

Many, perhaps most VOR’s out there are in fact VORTAC’s — the TACAN station is colocated with the VOR. It is possible to have a VOR-DME station, which gives a civillian user everything a VORTAC does, but no bearing info to TACAN-only military aircraft. And you can have a VOR station with no associated DME at all.

But you won’t find any rule prohibiting you from tuning up any of the above - doesn’t exist (at least in the U.S., nor anywhere else I’ve ever heard of). If the military wanted a NAVAID they could use that you couldn’t, they could put up a TACAN using a channel that civillian DME receivers can’t pair with (ex ch 16X “pairs” to 135.9 - not a frequency in the range civil VOR receivers can tune to) and not publish it. But realistically, they’d probably just use a GPS coordinate & call it good.

(Edit: as @UnrecognizedFallingObject mentioned, it is not the “Y” TACAN channels that conventional VOR receivers can’t pair with, but there are some channels you can’t get DME from using normal VOR tuning — see this chart.)

Flying over that navaid on a military base may entail issues with the airspace involved, but that is its own issue, unrelated to using or not using the navaid.


How does an engine driven vacuum pump used in gyroscopic instrument work?

It uses a system of vanes to draw air through the inlet port and expel it through the outlet port. The engine's lubricating system provides oil for lubrication as well as sealing the internals. The inlet is connected to the instrument and this provides the vacuum the instrument needs. To answer your last question, if the instruments are connected in series and then to the vacuum pump, and one of the instruments obstructs the air flow in the system, you may loose vacuum and instruments can become unreliable or stop working.


What are the mechanical deflection angles for Airbus side-stick controllers?

From this site:

So max deflection in pitch is +/- 16 deg, in roll it is +/- 20 deg


When/where must a flight go missed on the JFK VOR/GPS 13 L/R approach?

This approach is different than most others, in that it has a fly visual section of the approach, as noted in the lower portion of the plan view:

  When visual reference established, fly visual to airport via lead-in lights to Rwy 13L or 13R.

When flying the approach, the pilot must have visual contact with the ground, and be able to maintain visual flight by reference to the ground. Arriving at DMYHL, the flight visibility must be at least two miles, and there must be at least an 800-foot ceiling. Considering that DMYHL is either 2.6 or 3.6 NM from the landing runway, it's entirely legal to continue flight without the runway in sight. Without ground contact and the required visibility and ceiling, a missed approach must be executed at DMYHL.

After passing DMYHL, any time the pilot loses visual contact with the ground, or is unable to fly the Fly Visual path as charted (if, for instance, they could see the ground near them but not the lead-in lights) they must execute the missed approach at that time. Note that continuous ground contact must exist - the lead-in lights could disappear behind a small cloud for 3-5 seconds if the pilots still had the approach path in sight, but the aircraft would not be permitted to enter a cloud after the MAP.

For reference, the AIM contains the following information about Fly Visual segments (AIM 5-4-5(l), page 5-4-19 or 345):

  l. In  isolated  cases,  an  IAP  may  contain  a  published  visual  flight  path.  These  procedures  are
  annotated “Fly Visual to Airport” or “Fly Visual.” A
  dashed arrow
  indicating the visual flight path will be
  included in
  the profile and plan views with an approximate heading and distance to the end of the runway.

  1.
  The depicted ground track associated with the
  “Fly Visual to Airport” segment should be flown as
  a  “Dead  Reckoning”  course.  When  executing  the
  “Fly Visual to Airport” segment, the flight visibility
  must not be less than that prescribed in the IAP; the
  pilot must remain clear of clouds and proceed to the
  airport  maintaining  visual  contact  with  the  ground.
  Altitude on the visual flight path is at the discretion
  of the pilot, and it is the responsibility of the pilot to
  visually  acquire  and  avoid  obstacles  in  the  “Fly
  Visual to Airport” segment.

  2.
  Missed  approach  obstacle  clearance  is
  assured only if the missed approach is commenced at
  the  published  MAP.  Before  initiating  an  IAP  that
  contains a “Fly Visual to Airport” segment, the pilot
  should have preplanned climb out options based on
  aircraft  performance  and  terrain  features.  Obstacle
  clearance is the responsibility of the pilot when the
  approach is continued beyond the MAP.

  NOTE−
  The  FAA  Administrator  retains  the  authority  to  approve
  instrument approach procedures where the pilot may not
  necessarily have one of the visual references specified in
  14 CFR
  §91.175 and related rules. It is not a function of
  procedure
  design to ensure compliance with
  §91.175. The
  annotation  “Fly  Visual  to  Airport”  provides  relief  from
  §91.175 requirements that the pilot have distinctly visible
  and identifiable visual references prior to descent below
  MDA/DA.


May I fly with passengers on a homebuilt experimental airplane with only a simple PPL?

Yes, you can, you don't need anything other than the words EXPERIMENTAL to be visible by the passenger. Your PPL is fine for flying experimentals, although like any other aircraft change, I would get instruction from an experienced instructor in that type/model before going solo. Experimental aircraft can have some pretty different handling characteristics.

So, from a regulations standpoint, you are looking for CFR 91.319: Aircraft having experimental certificates: Operating limitations.. According to (j) of that:

  No person may operate an aircraft that has an experimental certificate under § 61.113(i) of this chapter unless the aircraft is carrying not more than 6 occupants.

So you can have up to 6 occupants (including the pilot).

  Do I need something special if I wanted to buy one of these planes?

No, your PPL allows you to fly experimentals, but as I said above, please get training from an instructor familiar with the aircraft and its handling characteristics. Many dear departed pilots probably wished in the last moments that they had some formal training in the experimentals they bought. You should also get somebody to go over the aircraft mechanically that is familiar with that model. Also look into your insurance costs, the airplane may be cheaper, but your insurance will probably be more expensive.


What is the difference between COM and Auxilary COM signals?

That frequency range is not for navigation equipment at all, so I would speculate that it isn't related to the navigation system.

They don't receive two types of signals; they do the exact same thing, but the reason someone may want two radios is to make things easier during flight.

What I think you are seeing is your primary communications radio system, and then a switch for an additional, optional "Com 2" unit that may or may not be installed in the aircraft.

Let's say you are talking to air traffic control, and you want to listen to an automated weather station without having to leave the ATC frequency; most systems provide you with an option to listen to both Com 1 and Com 2 at the same time, while allowing you to transmit on only one of them.  That way, you could monitor and talk on ATC's frequency on Com 1 while getting a weather update on Com 2.

Another advantage is that you can transition between frequencies faster by simply changing from Com 1 to Com 2 (ie. "AUX COM") after having already set the frequency you want to switch to in the other Com.

Yet another advantage is that one com will act as a backup to the other if either one stops working for some reason.


What is this second smaller runway next to London City Airport?

Good Spot! That was a temporary runway for the Red Bull Air Race:

https://www.forgottenairfields.com/airfield-london-redbull-airstrip-1429.html

You can see "RB" at the eastern end on your satellite imagery


What exactly is an FBO and what services are provided?

A Fixed Base Operator (FBO) is an entity - usually a commercial company - which provides "aviation services" at an airport. One of the main services they provide is fueling, either full service or a self service pump.  They don't all provide both 100LL and JetA, but most do.

Larger ones provide other services, such as catering, lavatory service, pre-flight briefing computers and wifi access, ground power units, and tie down or hangar space for parking.  Charges vary from place to place, most large FBOs will charge a fee for aircraft parking, but sometimes waive a day or so of that if you purchase fuel.  They can often assist in arranging ground transport or hotels as well.

Most are open hours like 7AM - 8PM, but that varies as well.  Most self-service pumps can be used 24 hours a day.

Some have a "crew car" available to borrow for a short trip to get lunch or run an errand, though calling ahead to check on that is important.


What is the role of the flight dispatcher? How do the dispatcher and the flight crew interact?

Have you ever been waiting for your flight at an airport and noticed that big roll of paper being printed up at the counter? That is called the "Flight Release". That roll of paper has all the information that the pilots need to fly their flight. It has their fuel, their route, the weather information, probable payload, and everything else they need. A Flight Dispatcher put that together for them.

The pilot looks it over and calls the Dispatcher if there are any questions. The aircraft cannot push back until both the Captain and the Dispatcher agree that it is safe to do so.

Once the flight is airborne the Flight dispatcher keeps an eye on that flight and makes any recommendations to the Captain regarding any change in route, altitude, speed, destination or anything else relating to flight safety and the efficient operation of the flight. This is called "flight following". If ATC gives the Captain a route change, the Dispatcher will calculate the new fuel burn. If the flight has to hold, the dispatcher will calculate how long the flight can hold before it has to land somewhere.

The Captain and Dispatcher communicate with each other while the aircraft is in flight primarily by ACARS. Think of it as an instant messaging system between the Flight Deck and the Dispatcher's computer. They type messages back and forth to each other and each sees the message on a screen.

The Flight Dispatcher is also there to help if there is a problem with the flight, i.e., a sick passenger, a mechanical issue, a change in the weather, or any other problem that the Captain may be facing.

The Captain and Dispatcher have joint authority to initiate, delay, continue, deviate, or cancel a flight. Nothing can happen until they both agree on a plan of action. Because of this the FAA considers the dispatcher to be a "crew member" and the Dispatcher's base of knowledge is very close to that of the pilots. Because of this, Dispatchers are allowed to ride up in the Flight Deck if they want to and are required to do it at least five hours a year.

No, the Dispather is not an Air Traffic Controller. However, the Dispatcher's plan for the route is given to ATC. ATC then takes all the flight plans filed by all the Flight Dispatchers who work for the airlines and put them all together for the big picture and then make it happen.

In short, the Flight Dispatcher works for an airline to plan flights and give that information to both the flight crew and air traffic control, monitor the flights while they are airborne, and work with the Captain to solve any problems that may arise. Their main concerns are, in order, flight safety and flight efficiency.


What is the difference between HFEC and LFEC inspections?

In general, higher frequency gives better resolution of potential defects but with lower penetration into the material (depth). So HFEC is used to detect surface defects and near-surface defects, but LFEC will be required to detect deeper defects.

From this page on the NDT Resource Center website:

  The depth that eddy currents penetrate into a material is affected by the frequency of the excitation current and the electrical conductivity and magnetic permeability of the specimen. The depth of penetration decreases with increasing frequency and increasing conductivity and magnetic permeability. The depth at which eddy current density has decreased to 1/e, or about 37% of the surface density, is called the standard depth of penetration (d). The word 'standard' denotes plane wave electromagnetic field excitation within the test sample (conditions which are rarely achieved in practice). Although eddy currents penetrate deeper than one standard depth of penetration, they decrease rapidly with depth. At two standard depths of penetration (2d), eddy current density has decreased to 1/e squared or 13.5% of the surface density. At three depths (3d), the eddy current density is down to only 5% of the surface density.
  Since the sensitivity of an eddy current inspection depends on the eddy current density at the defect location, it is important to know the strength of the eddy currents at this location. When attempting to locate flaws, a frequency is often selected which places the expected flaw depth within one standard depth of penetration. This helps to assure that the strength of the eddy currents will be sufficient to produce a flaw indication.


What are the advantages of the angled flight deck layout used by some aircraft carriers?

This kind of layout is called the angled flight deck (also called 'skewed' or 'canted' deck) and was first used by Royal Navy. There are a number of advantages to this layout compared to using the same direction of landing and takeoffs:

It allows concurrent landings and takeoffs.
By having an angled deck, the aircraft which fail to connect (with the arrestor cables) during landing can go around and try again without risk of damage to other (parked and taking off) aircraft).

The main reason for the emergence of angled deck was the introduction of jet aircraft in naval operations, which due to their weight and high speeds required most of the deck available to complete an arrested landing. Also, in case of missed wires, it was possible to use a barrier in case of propeller aircraft, which was not possible in case of jet aircraft. According to Rear Admiral Dennis Campbell, the inventor of angled deck,:

  But with the advent of the heavier faster jets, it soon became obvious that the dimensions of the carrier's deck would impose a limit to the standard of aircraft that could be operated. The inevitable increase in weight and landing speed of the jets would make it impracticable to retain the existing combination of arrester wires/barriers/and deck park without a drastic reallocation of the deck space required to absorb the much longer pull-out of the wires.

Prior to the application of the angled deck, this caused significant problems and requirement of complex machinery to overcome.

  Ark sister ship and predecessor, the Eagle, was equipped with sixteen arrester wires and three different types of barrier (together with all their several hydraulic machines), and all this gear could only just cope with the first generation for (straight wing) jets.  No way would such a layout be good enough for the newer types.

The solution was to skew the flight deck by about 10 degrees (the HMS Ark Royal first had the flight deck angled by 5 degrees, which was then increased to 10) and use it for landing. This lead to:

  The saving in arrester gear units and barriers – Ark needed only four wires and one (emergency only) barrier.  The reduction in weight and the extra space that this conferred enabled more mess-decks to be fitted in, thus reducing congestion in living spaces.
  Since landing accidents would obviously be far less frequent, fewer aircraft and spare parts would be needed so costs would be greatly reduced.
  Similarly, death and injuries to aircrew and deck handlers could be kept to a minimum – with a big boost to confidence and morale, not to mention the saving in training costs.
  As experience was gained of the new scheme, less obvious advantages came to light.  New areas of parking, (abaft the island for instance); an added flexibility in the relative wind speed and direction, easier and faster deck handling, less wear and tear on the command, etc.

Perhaps surprisingly, though the USN has adopted this system in its supercarriers, the RN dropped it later on.


Why are turbine-powered aircraft "considered ready for takeoff"?

Piston aircraft require a run up, in which the pilot sets the power to a specified RPM, in order to check proper functioning of a variety of systems, and to ensure that the oil is warmed up to a specified temperature. Among the systems to be tested are carborator heat (for normally-aspirated engines), magneto operation, alternate air (for fuel-injected engines), hydraulic systems, and propeller governor (for variable-pitch props). In an aircraft with a hydraulically-activated variable pitch prop, the prop is "exercised" by varying the pitch, which allows the cold motor oil in the prop governor to mix with the warm oil in the engine sump, to bring the entire lubrication system's oil temperature up into the green arc. During the runup, the vacuum system is checked to ensure that enough vacuum pressure is provided to critical instruments. Lastly, a grounding check is made with the magnetos, to make sure that the magneto electrical system is properly grounded.

Performing a run up at the ramp would prove dangerous to people and aircraft if not tied down and due to FOD.

Generally turbine warmup occurs pretty quickly after startup and during taxi. They are usually ready to go at the ramp.


Does flying an airplane upside down imply negative AoA? Why?

Level flight (no change in altitude) upside down, with an airfoil with a zero-lift AoA of less than or equal to zero degrees, requires a negative AoA to get upwards lift.

This is assuming the AoA is measured in the body frame of the aircraft, aligned with the chord line of the airfoil.

Also this assumes there is no thrust/drag or thrust/drag do not meaningfully contribute to vertical net forces.


Are military aircraft required to have black boxes?

Military aircraft need not adhere to FAA regulations. Modern military aircraft with digital controls have black boxes, older ones may not.


Your Question:

  Why is the Departure leg of the traffic pattern being mistaught as Upwind?

Probably because it is quite common to use the term "Upwind" when flying the "Departure Leg" because when doing so, you are flying "upwind."  If you look at the Pilot/Controller Glossary  in both the AIM and ATC Handbook (same P/C glossary), the term "Departure" leg is not even mentioned.  It only shows in the body of the AIM.

Here is what the Pilot/Controller Glossary defines for a "Traffic Pattern:"

(emphasis mine)

  TRAFFIC  PATTERN
  −
    The  traffic  flow  that  is
  prescribed
  for aircraft landing at, taxiing on, or taking
  off  from  an  airport.  The  components  of  a  typical
  traffic  pattern  are  upwind  leg,  crosswind  leg,
  downwind leg, base leg, and final approach.

I'm not suggesting that the segment in the pattern you are referring to is not the "Departure leg," just that it is quite common to refer to this segment of the pattern while you are flying it as "upwind" (without the use of the word "leg").  The actual "Upwind leg" (as I'm certain you know) in a traffic pattern is flown parallel to the runway on the opposite side of the "Downwind leg".


Why do entertainment system headphones on aircraft have two pins?

The three main reasons are:

Prevent theft of headphones due to incompatibility with home systems
Redundancy, if one connector breaks, the other will still provide audio
Before the 3.5mm jack was used, a pneumatic system was in use, which required two connectors.

This question has been asked on different websites already, but here is a detailed explanation that sums it up nicely:

There are a couple main reasons:
First, the airlines were trying to prevent theft of headphones. The two-pronged plugs have been around on airplanes since way before the airlines started charging for headsets. Back then, headphones were not usually easily carried around as the portable earbud style had not become popular yet. By using a two-pronged setup that is not used in most other audio systems, the motivation for passengers to steal the headsets went down immensely.
The second reason is redundancy. When a standard stereo headphone jack with three internal connections (right, left, and ground) breaks, you will often lose audio in both the left and right channels. However, with the two-pronged design, if one of the jacks breaks then there will still be audio going through the other channel. While this is annoying for the user, it is an easy way for the airline to defer maintenance on that individual seat's audio system until both the left and right channel are non-functional.
The two-prong design is a relic of the past, before headphones were bundled with so many of the mobile devices we now buy. Planes with updated inflight entertainment (IFE) systems usually have the standard stereo jack as most passengers prefer using their own headsets now. Slowly these old connectors will be completely phased out.
Another less obvious (and probably less likely, but interesting nonetheless) reason, though, is based on airline complacency. Airlines, especially US based airlines, are notoriously slow in changing their tactics. Back before the 3.5mm jack became the standard for headphone audio connections for IFEs, airplanes used a pneumatic headphone design. These headphones were similar to stethoscopes where hollow tubes would be plugged into the armrest which had actual soundwaves coming out (instead of analog electrical signals). These systems were much more reliable than the standard electrical system and the headsets themselves were very cheap to replace. Since the pneumatic system required two separate plugs for the left and right channel (since you cannot mix physical sound waves into a single hole), all armrests were designed with two holes for audio. Airlines were likely so used to having two jacks for audio that they continued with the same design when they switched to electrical audio signals.
(Source: www.quora.com - Author: Raj Misra)