What is the purpose of the inlet doors of the Pratt & Whitney JT3D?

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

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

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

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


How to get from IAS in MPH to KCAS using POH graph of best TAS/GPH?

Several questions here that need answers;
V(L/D max) is best glide speed.  Look it up under emergency procedures in the POH.
Airspeeds:
TAS is EAS corrected for temperature.
EAS is CAS corrected for compressibility.
CAS is IAS corrected for installation errors.

IAS/CAS correction is available from the POH.
Pilots can use an E6-B flight computer to convert between CAS, EAS, and TAS.

For more on this topic; here's a good link


Can a wing generate lift in excess of its aircraft's weight?

Yes, otherwise airplanes would be unable to go upwards into the sky.


Why does the airplane not 'feel' the crosswind?

When dealing with crosswinds and drift, I find the easiest way to understand the concept is to imagine a fast flowing river of water. If you tried to swim straight across a fast river you will end up downstream. You don't "feel" the current. You just keep swimming and the current carries you.

Any time you have wind it is like a river of air flowing over the earth.

Traveling through a river of air is exactly like traveling through a river of water.  You don't go where you are pointed, you go where the current carries you, plus the direction you are swimming.


Two reasons:

Because they don't have a very long moment arm.
Because their movement works in opposite directions to each other, any pitching moment one side might cause is cancelled out by a pitching moment going the other way from the aileron on the other wing.


A "preferable" solution depends on many factors, both in aerodynamics and in design/construction. There is no single best solution for all situations.

On slow(er) subsonic aircraft with reversible control, the main concern is to provide convenient force on the yoke/stick. For symmetric airfoils, which are common for tails, there is a point at 0.25 chord where the hinge moment will be zero for all the operational range of deflections. To provide some positive force, you'll want to put the axis slightly ahead of this point.

However, there is another problem: stability with relaxed controls. If you move the axis too much forward, making the surface 'weathervane' with the stream, you reduce the control-free stability. This is an important consideration because when you are flying 'hands off' on a well-trimmed airplane, this is exactly the situation you are in. With classic hinged elevators, this effect is already quite noticeable, but with all-moving tails, it can be extreme, up to the total loss of stability. On the other hand, obviously, if you put the axis aft of this 25% chord, you'll have an outright unstable configuration.

So the ideal situation for such airplanes is to have the axis at 25% chord, but to create positive control force with springs or with an anti-servo tab, like on PA-28:

$\hskip 6cm$
Source

For supersonic aircraft, the situation is somewhat different.

First, their control is typically irreversible: the actuators take all the load. This means that the control-free stability problem doesn't exist; and the actuators can potentially control high (or even negative) hinge moments.

Still, you don't want to oversize the actuators unnecessarily. In addition, many (older) supersonic aircraft allowed for fully manual or assisted control in case of hydraulics failure, so the aircraft still had to be flyable by hand.

This again puts the best hinge location near the 25% chord point.

However, the second difference is the fact that at supersonic speeds, this magic 25% chord point (called aerodynamic centre) moves to 50%. On top of that, the relative efficiency of control (per degree of deflection) reduces, and this is actually one of the main reasons of using all-moving surfaces on supersonic aircraft. This, obviously, creates enormous hinge moments.

To reduce them, we could move the axis a bit back. This may create negative moment at subsonic speeds, but if we have a full-authority hydraulic control, this may be the optimal solution. Some 4th-generation fighters seem to follow it.

The third difference is that supersonic tails are often highly swept back. How do you align the axis then?

$\hskip 1cm$

Again, there is no best solution for all cases. Aerodynamically and kinematically, the straight axis (on the left) is a bit better: it creates direct linear control. Structurally, it depends on how the stabiliser and the tail section are designed.

If there is a possibility to use a common shaft for both halves of the horizontal tail (which is typically true for T- and cruciform tails), this is a enormous advantage and it naturally leads to the straight axis. On the other hand, it is usually easier to integrate the shaft (or hinge points) along the spar of the stabiliser, which favours the swept axis for most swept tails.


What stops an international private airplane from landing outside of airports with a customs zone?

It is usually required to file a flight plan for international flights (with very few exceptions), and I'll assume this is the case for Canada/US as well, but I'm not 100% sure of this. In this case you have two options

You close your flight plan and let ATC or FSS know where you are, they'll note it's an international flight that landed somewhere without customs, and you're likely to end up in trouble.
You don't close your flight plan, and you'll trigger a search-and-rescue operation as you'll eventually be overdue, drawing attention to yourself, and you're equally likely to end up in trouble. Not to mention that you'll probably be liable for the cost of the search-and-rescue.

You could of course file a plan with a drastically reduced TAS, do a very quick stop, and arrive at the original destination within the grace period. I don't know if that will trigger any surveillance systems though.

If you don't file a flight plan and cross the border anyway, I'm guessing that you will indeed trigger some alerts though.


Can we replace an attitude indicator with a ball half full of liquid?

Nope.  In a perfectly flown turn, the ball of liquid would appear to indicate level flight.

You might argue that it could replace the "ball" of a turn & bank indicator, but that's about it.

It would also be tough to read in turbulence.

See also this YouTube video showing Bob Hoover pouring iced tea while flying the plane through a roll, even while inverted.  Your liquid would behave similarly.


Where can I find a canonical list of sources for worldwide digital GA VFR maps?

This answer is a Community Wiki. Please edit as appropriate.

Australia

OzRunways.com

United States

VFR Charts from the U.S. Federal Aviation Administration


Where is the fuel vent on a Piper Archer TX?

There are four main tank vents on all PA28 (Cherokee/Warrior/Archer/Dakota/Arrow) aircraft, two in each tank.

The most visible vent is under the wing at the outboard aft side of the fuel tank (near the jack points):

This vent is a small tube that protrudes from the bottom of the wing and is cut at an angle facing forward to provide ram air pressure into the tank. It is the vent you typically check during preflight to ensure mud wasps or similar haven't built a nest in the conveniently bug-sized tube.

The second vent is located in the fuel cap: It is an orange rubber "umbrella" in the center of the fuel cap which covers breather holes.

If pressure in the tank becomes negative (because the under-wing vent is obstructed) the umbrella will be forced open by air pressure and allow air into the tank through the fuel cap.
This vent is not normally "checked" during preflight except to ensure that the rubber umbrella is in fact there. A failure of the rubber "umbrella" is one common way that water finds its way into the fuel tank (the other being a failure of the O-ring around the perimeter of the cap).


I'm oversimplifying a bit, but more or less the DAL level of a piece of software is decided by asking the question, "What is the worst that could happen if this software fails to operate as designed?"  Note that "failure" in this case could mean that it simply stops working, but it could also mean that it does the wrong thing, like pitch up when it should pitch down. So for example,

The software that runs the in flight entertainment system.  Worst thing that could happen has no safety effect whatsoever, therefore level E.
The software that runs certain cockpit indicators, like the engine vibration display or engine oil temperature display: Usually considered a "major" safety effect, but not more than that, so level C.
The software that controls the engine thrust: if that fails to set the thrust according to the pilot demand, that could result in the plane crashing, so "catastrophic effect", thus level A.

It may be a little subjective, but I'd say that if we consider the pilot to be "software", if the pilot fails, the worst thing that could happen would be "Catastrophic" condition, and thus I would label the pilot as level A software.

Edits: clarified that DO178 applies specifically to software


What causes the adverse pressure gradient that leads to flow separation?

Friction plus pressure rise.

No, the effect certainly isn't independent of surface friction, otherwise the location and angle of attack of separation wouldn't change with Reynolds number.

First, friction will prevent the surface layer to become as fast as the outer flow in the suction peak, and once pressure rises, all air will slow down by the same amount. Now the already slower part near the wall will actually reverse because it slows down to a standstill and is attracted by the low pressure ahead.

Pressure has to rise past the suction peak in order for the air to get back to ambient pressure. The upper side suction is caused by the airfoil's curvature, and curvature  over the rear part of the airfoil is very low or even negative - that is what makes the air assume ambient pressure again.

I think you know already my slightly longer answer on that topic, but I will link to it nevertheless. Please let me know if I need to explain more!


The top picture is from Otto Lilienthal's book "Der Vogelflug als Grundlage der Fliegekunst", published in 1889. The two diagrams are a polar plot of the resulting aero forces for several angles of attack, and the polar angle is measured relative to a line which stands orthogonal on the airfoil chord. This plot type is the reason why we still call a plot of an airfoil's lift and drag a polar plot even when using Cartesian coordinates.
Fig.1 shows the angle of attack on its outer arc and the resulting force vector hits this arc at a lower angle for AoA < 40°, showing that there is some forward component of the resulting force relative to the orthogonal on the chord line of cambered surfaces. This angular difference is singled out in Fig. 2 where the arc of Fig. 1 is collapsed into one point such that the fan of vectors allows a direct comparison of their direction. Between 15° and 20° AoA this forward component reaches its maximum.
The results were obtained by rotating wing models around a vertical axis. Those models had a low aspect ratio, so their stall angle is quite a bit higher than what you read today from a wind tunnel test where an infinite aspect ratio is simulated.
Today's plots don't show a resulting forward force component anymore because we define lift as the component orthogonal to the direction of flow and drag as the parallel component, plotting one on the Y-axis and the other on the X-axis of a Cartesian system. Lilienthal only plotted the resulting force and used the orthogonal to the chord line as the reference for the polar angle, so you would need to split the forces in the diagram by use of the cosine of the angle of attack for lift and the sine for drag. But converting a given table of lift and drag coefficients is easy with a spread sheet, so you can generate as many of those plots by yourself, if you like.
The lower plot shows the lengthwise drift of the center of pressure over angle of attack, with resultant aerodynamic force vectors added. A similar plot can be found here, only that the angle of attack varies over span so the local center of pressure for each section shifts back with increasing wing station as long as lift is positive and for negative lift approaches the wing from infinitely far forward.
This kind of plot shows the resulting aero force acting at a point where it is free from any moment. The direction of the vector is again orthogonal to the chord line and shows how much in-plane force component exists at high angle of attack before the airfoil stalls.


What is the difference between Cruise and Service ceilings?

Ceiling refers to the density altitude the aircraft is capable of reaching under a set conditions (configutaion etc.). The main difference between service and cruise ceilings is the aircraft rate of climb at those altitudes. At service ceiling, the rate of climb is 100 fpm. From FAA pilot handbook:

  The service ceiling is the altitude at which the aircraft is unable to climb at a rate greater than 100 feet per minute (fpm).

The cruise ceiling, on the other hand, is the altitude at which the maximum climb rate is 300 fpm, though I've not seen it used much. The absolute ceiling is the maximum altitude where the aircraft can sustain level flight. From the same document:

  At the absolute ceiling, there is no excess of power and only one speed will allow steady, level flight.

The image below shows the data for A-10.

Image from forums.eagle.ru

Note that these values depend on the aircraft configuration.


Is it possible to obtain a CPL without a PPL?

You've tagged your question with 3 different sets of regulations, I can't answer all of them but here are the FAA ones.

The answer depends on what you mean by "exam". If you mean the written test, then anyone can take it if they have a graduation certificate or a logbook endorsement:

  An airman applicant may present one or more of the following
  acceptable forms of authorization for all tests listed above [...]

  Certificate of graduation or a statement of
  accomplishment certifying the satisfactory completion of the ground
  school portion of a course for the certificate or rating sought. The
  certificate or statement may be issued by an FAA certificated pilot
  school or an agency, such as a high school, college, adult education
  program, Civil Air Patrol, or Reserve Officers Training Corps (ROTC)
  flight training school.
  Written statement or logbook endorsement
  from an authorized ground or flight instructor certifying that the
  applicant completed an applicable ground training or home study course
  and is prepared for the knowledge test. (AC 61-65E) (14 CFR § 61.35)

At least in theory, you could get a certificate or endorsement to take the commercial written test without ever having taken any other FAA test. But in practice I suspect that it's very unlikely: why would an organization or CFI endorse you as ready for the commercial written if you haven't already done the private one?

However, if "exam" means the actual checkride then the situation is much clearer: it isn't possible to do it unless you already have a private certificate, at least for civilian pilots. 14 CFR 61.123 says:

  §61.123   Eligibility requirements: General.

  To be eligible for a commercial pilot certificate, a person must:
  [...]
  (h) Hold at least a private pilot certificate issued under this
  part or meet the requirements of §61.73

61.73 lets you use military experience to apply for a civilian certificate.

Finally, some people use the term "commercial pilot" when "ATP" would be more accurate. I'm not saying that you're using it incorrectly, but since I have no idea how EASA and CASA define a CPL it may be worth mentioning. In the US, ATP multi-engine candidates need a certificate for the written exam, and a commercial certificate with instrument rating (or military equivalent) for the checkride. See 61.153.


Why was the "Dispatch Reliability" of the A320 "low" during its first years?

The A320 saw entry into service on 18 April 1988 with Air France.
About a year later Flight International covered the dispatch topic. For British Airways – whose initial acquisition of the A320 was through the British Caledonian takeover, and not an order – the problems weren't with the plane, but with the Boeing culture that had to adapt.
Otherwise there was nothing notable.

(...) The "culture difference", says Bracken, materialised in apparently trifling (but actually problematic) ways, such as line mechanics having trouble with Airbus literature. There was nothing wrong with the literature, it was just different.
November 1 came. "It wasn't a very good start," says Bracken, "but the reasons were outside the aircraft." By January, BA's A320 technical dispatch rate had not climbed out of the 96 per cent range, reveals Barry Gosnold, chief engineer technical services.
That is a revealing figure. BA had expected much more, even though the performance is not unusual for a completely new aircraft.The A320's performance with Air Inter at the same time was between 2 and 3 percentage points better, although Air Inter had received its first A320 months later than BA.

Source: AIRBUS A320 AT WORK, Flight International, 17 June 1989


In the traffic pattern, what is the appropriate distance from the runway for the downwind leg?

Most instructors say that (in a low wing plane) you should see the runway on the tip of the wing or between the tip and 1/4 in the wing. AOPA has a nice over view of non towered airports here. This FAA brief states it should be flown 1/2 to 1 mile out (page 7-4 of FAA Airplane Flying Handbook, FAA-H-8083-3B).

  This leg should be
  approximately 1/2 to 1 mile out from the landing runway,
  and at the specified traffic pattern altitude.


Inside some jet engines there is a powerful electric motor that starts the engine by spinning the compressor up to speed, at which point the fuel flow and the ignition spark are turned on, and the resulting combustion then forces the turbine to spin. The turbine's power output is sent to the compressor stage to spin it, and the electric start motor can then be turned off.
On other jet engines, a small gas turbine is used to blow hot combustion exhaust through the main turbine, which spins the main compressor up to speed, then the fuel and spark come on, etc. The start turbine is then shut down once the main engine starts.
Note that whenever the engine is running, the compressor stage at the entrance to the engine is actively sucking in tremendous quantities of outside air and jamming that air into the combustors in the center of the engine, where it is burned with fuel. The power of the exhaust from the combustors then spins the turbine at the outlet end of the engine, and that power is fed back to the compressor on a spinning shaft to keep it sucking in more air.
Some military jet engines can be started by connecting a huge blower to the engine inlet and forcing compressed air through the engine, to spin the turbine and get the compressor going. This is called an air start. Some military jets can also be started by burning a solid-fuel cartridge made of black powder and shooting the hot exhaust into the turbine stage of the engine to spin it, and thereby spin the compressor stage up to speed and get things going.
Other military jets are started by a start cart which contains a powerful gasoline engine that spins a shaft geared onto the main shaft of the jet engine. Once the jet is up and running, the start shaft is disconnected from the jet and the start cart is rolled out of the way.


The biggest reason is the negative (or spoiling) effect of having a body "in the way" and disrupting the flow field above the wing.  A wing makes lift by redirecting a very large package of air downward (actually, up a bit, then down a lot) as the wing moves along, and most of the package of redirected air (which ends up as downwash) is above the wing.  (Otherwise, the wing would just be an air deflector and a flat sheet of plywood would do the job.)  Disruptions to this upper flow have to be minimized.

Look at any tactical fighter, like a Skyraider, festooned with rockets and bombs and tanks under the wing almost across the entire span. The disruptive effects of all those shapes on the underside have minimal effect other than parasitic drag. Put them on top and they act like spoilers and the airplane would never get off the ground.

On airplanes that do put the engines on top, either the engine is so far aft that it minimizes the effect and it might was well be mounted on tail pylons (like the Honda Jet), or the engines are mounted way forward to minimize the effect, plus gain from having the high velocity fan stream across the top, or it's propeller driven and the propeller flow field mitigates the effect, or, it was done anyway and the design was unsuccessful.


Starting on the left magneto only - why?

The reasons seems to be that only the left magneto is equipped with an impulse coupling which delays and intensifies the spark during engine starting:

Many opposed reciprocating engines are equipped with an impulse coupling as the auxiliary starting system. An impulse coupling gives one of the magnetos attached to the engine, generally the left, a brief acceleration, that produces an intense spark for starting. This device consists of a cam and flyweight assembly, spring, and a body assembly.
[...]
The impulse coupling has performed two functions: rotating the magneto fast enough to produce a good spark and retarding the timing of the spark during the start cycle. After the engine is started and the magneto reaches a speed at which it furnishes sufficient current, the flyweights in the impulse coupling fly outward due to centrifugal force or rapid rotation. This action prevents the two flyweight coupling members from contact with the stop pin. That makes it a solid unit, returning the magneto to a normal timing position relative to the engine.

(flight-mechanic.com on Impulse Coupling, emphasis mine)
On an aircraft with key operated ignition (where the start position comes after both), the right magneto is grounded while you hold the key in start:

Most airplanes have an impulse coupling on the left mag only. During engine start, the ignition key grounds out the right mag, so only the left mag provides spark. The timing of the spark from the left mag is retarded (later than usual) during engine start so the engine will start easier. Once the ignition key is released from the start position, the timing of the spark is restored to normal, and the right mag starts firing.

(askacfi.com)


What is the physical scale of the turbulence that affects large passenger planes?

Summary

There are in effect no minimum levels of displacement that will not have noticeable effects. Even very small displacements, of an appropriate period and spectrum, can affect a large aircraft. Scale alone is not a limiting factor.

In more detail

Question 2: Are there any minimum limits, so that even fiercely turbulent air below a certain displacement is not noticeable?

The title addresses "large passenger planes" and not passengers, or passengers with vestibular anomalies, etc.

Generally mountain wave is considered "fiercely turbulent" as the updrafts and downdrafts can exceed the climb and descent capability of an aircraft.  However, the "frequency component" of the mountain wave can be very low, at times creating no human perception of motion. Yet the turbulence causes loss of control by virtue of exceeding the aircraft's capabilities of counteracting an upsetting force.

So in this instance, it is possible to have an extreme situation, and actually loss of control, by turbulence which is imperceptible by a human without instrumentation.

Question #1: What magnitude of displacement of air in turbulence conditions is large enough to affect (i.e. be noticeable in) an airliner?

The aircraft, and it's contents (including people) act as a system. The system can dampen the effects of some forces and cause amplification of other forces.

A tiny, repeated push to a child on a swing, can result in very large displacements of the swing, and is a good example of a form of resonance or amplification of the effects of a small push.  The pushes have to have the right periodicity, matching the resonance of the swing to be effective.

The above is an example of a resonant system, but we also have around us systems which dampen energy, such as the suspension on a car.  Driving a car over a bump in the road might create a very mild sensation in the car, but if we were riding on a solid wheel hitting the same bump at the same speed the resulting bump could be injurious.

So to only ask about displacement is akin to only asking about sound pressure of music. The frequency (notes) help create the music, and the periodic occurrence of the frequency energy also helps create the music (like beat). A drum strike can be very loud or a triangle very soft, but they are both noticeable in music, in part due to our perceptual training, but also due to the human auditory system.

So for an aircraft, a displacement at one frequency may have essentially no impact on the flight path, due to the systemic response of the aircraft. Furthermore that displacement may or may not be detectable by a human riding in that aircraft.
 To answer your broad, but limited to displacement question, becomes impossible to do so accurately if you choose to limit the dimensionality of the system you are seeking analysis of. (reference your comment)


Why do aircraft models end their life as freighters?

Passenger operations are overwhelmingly scheduled whereas cargo is on demand and thus less easy to plan. The upside: A cargo plane crew will fly quite diverse routes. The downside: Capacity utilisation is worse. This shifts the balance of cost such that fixed cost must be lower than what is possible with scheduled services. As a consequence, older aircraft with lower depreciation cost are preferred. With them it is less costly if they don't move for some hours or even days.

Caveat: Integrators (freight companies which ship door-to-door like FedEx or UPS) run their freighter fleet like a scheduled service, but still their utilisation is lower. Please see @TomMcW's answer for a detailed analysis.

Operating cost are more important when the aircraft flies as much as technically possible. Especially low cost airlines need to operate hardware with the lowest operating cost possible so they survive the next price war. Premium airlines also have a need for the newest planes: This makes them look premium and more safe. Cargo operators have no such incentive to prefer the newest airplanes; they profit from the need of passenger airlines to sell their older hardware off.

Next caveat: Older airplanes are at least as reliable as new ones when maintained well. The need of premium airlines for newer planes is only to impress their ignorant customers, not to increase real reliability numbers.

The Economist wrote in April 2013 about Norwegian Air Shuttle:

  As fuel prices soared in the mid-2000s, and competition from the likes
  of Ryanair and easyJet intensified, Mr Kjos realised that the only
  route to survival was to buy lots of new, fuel-efficient planes to
  achieve economies of scale, and to base some of them in cheaper
  countries. Boeing and Airbus are straining to keep up with booming
  demand for new aircraft, so by reserving a large chunk of their
  delivery slots NAS is denying potential rivals the chance to refresh
  their fleets with better planes.

If an airline is large enough, ordering new aircraft has the added benefit of saturating supply and denying the same aircraft to its competitors. If oil prices are high, this airline will have won the price war before it started in earnest.


How can pressure pattern navigation be applied to light airplanes?

Here is an interesting slide show that may be of some help.

The FAA still does (although extremely rarely from my understanding), issue a flight navigator rating. This rating dates back to the early days before avionics were as sophisticated as they are today. The FAA publishes this handbook (for the rating), Chapter 15 covers pressure pattern navigation and may be what you are looking for.

This document on pressure patterns from a weather standpoint may also be of interest.


How long is a clearance void time valid?

Since you didn't state jurisdiction, I'm assuming US.

From the AIM:

  1. Clearance Void Times. A pilot may receive a clearance, when operating from an airport without a control tower, which contains a
  provision for the clearance to be void if not airborne by a specific
  time. A pilot who does not depart prior to the clearance void time
  must advise ATC as soon as possible of their intentions. ATC will
  normally advise the pilot of the time allotted to notify ATC that the
  aircraft did not depart prior to the clearance void time. This time
  cannot exceed 30 minutes. Failure of an aircraft to contact ATC within
  30 minutes after the clearance void time will result in the aircraft
  being considered overdue and search and rescue procedures initiated.

The requirement is to be airborne prior to the clearance void time. You need to contact ATC within 30 minutes of the void time either in the air or to notify them that you did not depart.


Why does Turkish Airlines avoid Israel when going to Amman, but not on the return leg?

There are no issues overflying Israel to Jordan:

From the Israeli AIP:

Prior permission is not required for commercial scheduled
flights by aircraft registered in countries that are parties to the
International Air Services Transit Agreement (IASTA) or
where the relevant Israeli bilateral Air Services Agreement
allows overflying the State of Israel or making stops for non-
traffic purposes.

The prohibited airspaces on the shorter route allow flights above 6000 feet (per the AIP).

There are two airways that are used. Each is one-way.

(skyvector)

Amman arrival procedures allow flights coming via waypoint SALAM on the Israeli side (the OSAMA arrival). See SALAM and OSAMA above.

The AIP confirms airway P51 (eastbound to Jordan) is used.

There is a whole section titled 'Procedure for IFR flights to and from Amman (Jordan)', nothing stands out, just to have two-way communication established with both ends.

As far back as flightaware allows, it's consistent. So that rules out airspace closure for military exercises, etc., and also similar threads exist on airliners.net as far back as 2012.

Conclusion:
What all that hints at, is that the reason is in the fine print of the non-publicly-available air service agreement between the nations and carriers involved.
Note: the site that hosts the eAIP of Jordan is not loading, so I couldn't check the Jordanian side.
Also, it's not a diplomatic thing: Pegasus Airlines 1710 (Turkish) and Royal Wings 6558 (Jordanian) when flying from Turkey to Amman, overfly Israel (shown below).


Why didn't the U-2 spyplane have any chaff?

There are two main reasons for this:

USAF and CIA believed that the Russians (or anyone else) wouldn't be able to shoot down aircraft operating at (>60000ft).
Most of the aircraft operating during that time did not have chaff dispensers to deploy chaff as needed. For that to work, the pilot has to either visually acquire the missile or have a RWR, which became widely used only in the later stages of Vietnam War.

Another important reason is tactics. The chaff was not used in WWII in a tactical capacity i.e. to protect individual fighters, but to provide blanket coverage to bomber fleets. This practice continued well into the Vietnam War. For example, specialized aircraft (usually fighters) will dump chaff to provide a 'corridor' for following bombers. For an aircraft on reconnaissance mission, this is as good as advertising its presence.

A lot of aircraft in that era went without any countermeasures. For example, the English Electric Lightning carried no chaffs till its retirement in 80's.


Does aircraft allways rotate about center of gravity like weather vane about pivot point?

It's purely a mathematical trick, but a very good one at that. In reality, pure rotations around the center of gravity are rare.
The key is that it's often convenient to split movement of a body into translations and rotations. So we need some definition to distinguish between a rotation and a translation. For example, consider lifting a pencil from a table on one end. Intuitively, there's definitely some rotation as the pencil is now at an angle, but is there also a translation? The end of the pencil resting on the table has not translated at all, while the end you picked up has translated a lot. So we can describe the same situation by pure rotation around the point resting on the table, or a combination of a translation of the point you're holding and a rotation around that point, or anything in between.
By defining rotations and translations with respect to the center of gravity, you simplify a lot of equations, and you can analyse the system with a free body diagram. If all forces in one direction sum to exactly zero, you will have no translational acceleration, and only rotational acceleration. In reality of course, forces rarely sum to exactly zero, so there's always some translational acceleration of the center of gravity.


How to arrive and depart surface class E airspace

Class E airspace has no requirements for communication when flying VFR. You only have to maintain two-way communication in Class E airspace when flying IFR. All airports where there is an operating control tower requires two-way communication within 4 Nautical Miles of the airport housing the control tower at and below 2500 feet AGL at a minimum, regardless of airspace. You basically treat all airports with an operating control tower like a Class D airport. A, B and C airspace rules are even more strict. You maintain that communication either through radio or light gun signals.

§91.127 Operating on or in the vicinity of an airport in Class E airspace.
(a) Unless otherwise required by part 93 of this chapter or unless otherwise authorized or required by the ATC facility having jurisdiction over the Class E airspace area, each person operating an aircraft on or in the vicinity of an airport in a Class E airspace area must comply with the requirements of §91.126.
(b) Departures. Each pilot of an aircraft must comply with any traffic patterns established for that airport in part 93 of this chapter.
(c) Communications with control towers. Unless otherwise authorized or required by ATC, no person may operate an aircraft to, from, through, or on an airport having an operational control tower unless two-way radio communications are maintained between that aircraft and the control tower. Communications must be established prior to 4 nautical miles from the airport, up to and including 2,500 feet AGL. However, if the aircraft radio fails in flight, the pilot in command may operate that aircraft and land if weather conditions are at or above basic VFR weather minimums, visual contact with the tower is maintained, and a clearance to land is received. If the aircraft radio fails while in flight under IFR, the pilot must comply with §91.185.

§91.126 Operating on or in the vicinity of an airport in Class G airspace.
(a) General. Unless otherwise authorized or required, each person operating an aircraft on or in the vicinity of an airport in a Class G airspace area must comply with the requirements of this section.
(b) Direction of turns. When approaching to land at an airport without an operating control tower in Class G airspace—
(1) Each pilot of an airplane must make all turns of that airplane to the left unless the airport displays approved light signals or visual markings indicating that turns should be made to the right, in which case the pilot must make all turns to the right; and
(2) Each pilot of a helicopter or a powered parachute must avoid the flow of fixed-wing aircraft.
(c) Flap settings. Except when necessary for training or certification, the pilot in command of a civil turbojet-powered aircraft must use, as a final flap setting, the minimum certificated landing flap setting set forth in the approved performance information in the Airplane Flight Manual for the applicable conditions. However, each pilot in command has the final authority and responsibility for the safe operation of the pilot’s airplane, and may use a different flap setting for that airplane if the pilot determines that it is necessary in the interest of safety.
(d) Communications with control towers. Unless otherwise authorized or required by ATC, no person may operate an aircraft to, from, through, or on an airport having an operational control tower unless two-way radio communications are maintained between that aircraft and the control tower. Communications must be established prior to 4 nautical miles from the airport, up to and including 2,500 feet AGL. However, if the aircraft radio fails in flight, the pilot in command may operate that aircraft and land if weather conditions are at or above basic VFR weather minimums, visual contact with the tower is maintained, and a clearance to land is received. If the aircraft radio fails while in flight under IFR, the pilot must comply with §91.185.


Operating a floatplane/seaplane on water through a drawspan?

When I sail we use simple air horns to signal the local drawbridges. Something like this one is usually kept on the boat. I see no reason not to keep one in your float plane if you anticipate need to hail a draw bridge or just for general marine hailing. If memory serves, it's one long blast (4-6 seconds) followed by a quick one. Similarly you could opt to carry a hand held marine radio in your plane.

Now all of that is assuming you knew you were going to encounter a bridge. If you did not plan for it then you have a few options. If memory serves some (if not all) of the bridges have land lines and if you have a cell phone you may be able to call them (provided you can find the number). If you are flying a float plane you may (should?) have a PFD on board. Many PFDs have whistles attached. This, in theory should suffice to hail the bridge. The bridge attendant may see you floating there and open the bridge as well.

You can always try, um, yelling at the guy...

Now what if the bridge is broken, there is a long train stuck on it and the attendant stepped out for a cigar break and has no intention of returning for a while? You are in the end of the day in a float plane and you have a unique ability that most ships don't have, you can go over the bridge...


Is there employment opportunities of becoming a pilot

By the time you get your Commercial checkout (or ATP) or what ever your local jurisdiction requires you should have no issues finding a job just based on the numbers. There is a growing shortage of pilots and its no secret there are jobs available:

Pilot Shortage Is Real and Getting Worse
The pilot shortage
Cargo Airlines face pilot shortages
A 'Perfect Storm' Pilot Shortage Threatens Global Aviation


Can ATC decide whether or not to provide tower enroute services?

According to the AIM, 4-1-19-c:

  There are no unique requirements upon pilots to use the TEC program. Normal flight plan filing procedures will ensure proper flight plan processing. Pilots should include the acronym “TEC” in the remarks section of the flight plan when requesting tower en route control.

Based on that, it would seem that the TEC request was essentially equivalent to that of getting a pop-up IFR clearance. If you pre-file, the controller has all the information necessary to enter you into the system rather than asking for it over the radio. It wouldn't be legally required, but neither would the controller be required to issue a clearance if workload doesn't permit.

When using a TEC route, there is still the benefit of only talking to local controllers, and using a known routing that you're less likely to have amended.


Can you report a crash landing on an emergency frequency even if you are not at the airport?

If you are in a car, you probably don't have the full picture. Chances are, if the accident happened "near the airport", emergency services are probably already on the way. This is because the pilot would have been communicating with ATC, and would probably have indicated that he was in trouble before the actual accident. Even if the pilot did not have time to notify ATC, if a plane suddenly "dissapears", search and rescus will be initiated.

In any case, if will do no harm to call the normal emergency services (dial 112 or 911 in some countries). Don't overcomplicate things and try to get in touch with ATC - just dial the emergency number directly, and they will know what to do.


After a crash, for how long is the airport closed?

It completely depends on the accident and airport.

For example if I crash land on 10R at San Francisco, and it's a fairly quiet time of day (eg below the capacity of one runway), the airport could just use 10L and continue operations as normal. Similarly if there was no wind they could just switch to 1/19 L/R instead of 10/28 L/R. I'll discuss this toward the end of this answer.

Even if the crash didn't damage the runway, if the accident didn't damage the runway and the aircraft could be safely moved the airport would be back to normal as soon as the aircraft was moved and the runway had been checked. This could be within an hour.

On the other hand if at a single runway airport, with severe damage to the runway and a need to leave the aircraft in situ to be inspected, it could take several weeks.

In the cases you mentioned, BA 38 landed short of the runway. The airport was closed to arrivals for a few hours, however, and hundreds of flights were delayed, cancelled, or diverted. In the meantime, the runway in question (27L) was closed entirely for arrivals until 3 days later when the aircraft was recovered. However the runway re-opened on the day of the accident for departures only. As Heathrow typically operates with one runway for Departures and one for Arrivals, this had minimal impact on operations: ref

In Turkish flight 1878, the aircraft impacted Runway 5, then again on Runway 35L, but as far as I'm aware no significant damage was done. The other runways were inspected and used until the aircraft was moved. Ataturk is a busy airport, but not so busy that it can't cope with the loss of a single runway. Again a few hundred flights were delayed, cancelled, or diverted.

In neither incident was wreckage left on the actual runway, so once inspected they could be used as normal. There was some disruption in both cases - although no more than that caused by bad weather. If the aircraft had remained on the runway in either case, however, and had the accidents been at a single airport runway (eg London Stansted rather than Heathrow), the airport would likely have been closed for several days.

Another interesting case is Asiana Airlines flight 214 - an accident at San Francisco which closed the Airport for a few hours, and the runway for around a day. The other two runways (1/19 L/R which are perpendicular, as San Francisco has 4 runways arranged as 2 pairs) re-opened after around an hour and meant there was minimal impact, along with the parallel runway (10L/28R) which re-opened 24 hours later. The accident runway (10R/27L) re-opened around a week later.

One thing to note is that in most cases, an airport only needs runways in one direction - so for example at San Francisco there is a lot of resilience in the runways. As long as the accident doesn't cause damage too close to the "crossover" of the runways, San Francisco should almost always be able to operate at near full capacity shortly after an accident, even with a runway closed, due to always having a pair of runways available. Airports such as Heathrow with a single pair of runways are less resilient


How come Boeing invested \$32 billion on the 787 program and Airbus only \$15 billion on the A350?

I don't think the breakdown is public, but there are two main areas that are obvious once you dig into the details.

Part 1 relates to the 787-8 and -9/-10 commonality. Surprisingly, the stretched derivatives only share 50% of the -8 parts. As Wikipedia puts it:

  The 787-8 and 787-9 have 50% commonality: the wing, fuselage and systems of the 787-8 had required radical revision to achieve the payload-range goals of the 787-9.

So you could say Boeing developed 1½ planes, while Airbus developed 1. I tried to find details on the differences before, but it was a futile attempt. All the official statements were as vague as they come. (Boeing will implement at least some of the new -9/-10 developments into the future -8's.)

Part 2, Boeing went with an all-electric design,* introducing many new technologies in one go. The A350 still uses pneumatic engine start and bleed air for pressurization. They were improved on the A350, but still they were existing technologies.

* Lithium-ion batteries are not really part of the differences as they were introduced quietly on the A350.


Why are the minima for Cat A higher than for other categories at KSEA ILS 16L?

The minima's are usually decided based on the Missed Approach procedure, which one of the criteria being obstacle clearance. I believe the answer lies in your question, its due to the heading course and altitude restriction on the MAP point. RWY 16L needs to climb straight 165 deg which puts you through the obstacle , whereas 16R Missed approach procedure heading can clear the '464' obstacle. . In either case Obstacle driving the change, Having CAT A with the lower speed in the heading of 165 deg it won't get you clear the obstacle. The PBN approaches are even marked higher for all aircraft type which can be inferred from other terminal charts.


Is medical evacuation aircraft D-AIGZ "Robert Koch" protected against decompression?

I'll try to answer in a general sense. It appears that the Lufthansa engineers have changed only the interiors- basically they've changed the passenger fittings with the containment unit. As such, the effect of decompression will be the same as in commercial aircraft- in case of negative pressure containment unit, the air will tend to leak from it. In general, there are some safeguards in such cases:

By sucking air out through the filters, a negative pressure is created within the unit, so that while fresh air may leak in, contaminated air will not leak out. An alarm sounds if negative pressure is lost.
...  in case the negative pressure protection failed, the air inlet at the front (crew-facing) side of the tent is also fitted with a filter to prevent airborne contamination of the main cabin. As a further protection for the crew, Phoenix Air’s mechanics have reversed the airflow in the whole aircraft, so that now it flows fore to aft (instead of aft to fore as it comes from the factory); the pilots and medical crew are in effect located ‘upwind’ of the patient.

However, as @abelenky already noted, in flight decompression is quite rare and the people on board have to worry more about the patient they are transporting than decompression.
Negative Pressure tents with HEPA filters are usually used only in medivac cases where the diseases are highly infectious. In other civil medivacs, the patients are usually exposed to the cabin.
There is a FAA approved Aeromedical Biological Containment System (ABCS) devised by CDC and used in US, which uses the same negative pressure principle- so I'd say that it is safe to operate.
For medical aircraft, the cruise altitude is usually set lower- FL200 or so to enable having the cabin pressure altitude at a lower level (for military aircraft, it is maintained at 4000', though it can vary depending on the circumstances). One thing to note is that most medivac aircraft are business jets like Gulfstream and Learjet, which can maintain sea level cabin pressure upto FL200.
Also, any interior modification will have to be approved by the corresponding regulatory body (here its EASA, though I'm not able to find anything about this particular aircraft). Other manufacturers have obtained EASA certification for negative pressure tents and as already seen, FAA certification too.
European Air Ambulance explains the EASA certification process for medical aircraft, which basically involves:

Type certificate for the original aircraft

Supplement Type Certificate for the medical configuration and

Air Operator Certificate for the aircraft equipped with the approved medical configuration.

Image from European Air Ambulance


Do carrier-less militaries retain the equipment for carrier operations?

Taking the RAAF F/A-18's for example:

  The Australian Hornets were very similar to the standard US Navy variants, but incorporated a number of minor modifications [including] the deletion of all equipment used to launch the aircraft using catapults.

Even though they retain the tailhook, if they landed on a carrier, they won't be able to takeoff.

The tailhook serves the same function as on the land-based F-15 and F-16, provide arrested landing capability for emergency situations.

(Source) F-16 arrested landing.

As for the Harrier, all the foreign operators were navies. So, export Harriers were used on carriers. (Rhyming unintentional.)

The carrier-borne Rafale fighter jet will retain its carrier capability when sold to the Indian Navy.


At what distance between two aircraft will the air traffic controller get an alarm?

The STCA (Short Term Conflict Alert) or pre-alert settings depend on the radar client in use and the ACC's or ARTCC's own standard operation procedures. Some ATC systems give a warning before a loss of separation can occur if criteria are met, informing the controller of a possible conflict. Factors can be clearing aircraft on the same altitude / flight level, aircraft on converging flight profiles, lateral or vertical separation reducing below threshold values, e.g. where 5nm lateral separation is required, a STCA could be set to occur at 7nm lateral separation.

The minimum radar separation between aircraft depends on airspace (TMA area vs enroute area), aircraft equipment and airspace separation requirements (RVSM vs CVSM), flight rules (e.g. IFR to VFR is not separated outside class C airspace in Germany) or local rules imposed by the Civil Aviation Authority.

The general rule of thumb for IFR separation in most countries (as most countries follow RVSM rules):

5nm lateral separation enroute (upper airspace)
3nm lateral separation in TMA area (lower airspace)
1.000ft vertical separation in lower and upper airspace where RVSM is observed and aircraft equipped
2.000ft vertical separation in non-RVSM airspace or between aircraft not equipped for RVSM

Oceanic separation is achieved differently, as non-radar procedures are in place. The separation in non-radar environments is achieved procedurally and through position reports and timing.


The name of the condition is nyctalopia. I found it mentioned by the UK's CAA (sorry it doesn't match your array of jurisdictions, but maybe this information will lead to more):

If the pilot is found to have a demonstrable nyctalopia, a medical flight or simulator test may be required, depending on the degree of severity.  For pilots with demonstrated nyctalopia enough to cause concerns for night flying, a VCL limitation will be required.
— caa.co.uk

"VCL" being a daytime-only limitation, which would typically not allow a Class 1.


Who decides how fast commercial aircraft fly: the pilots or ATC?

ATC can assign speeds en-route for separation or in the TMA area for sequencing aircraft onto the approach. Speeds en-route in upper sectors are assigned using Mach numbers:

R: "DLH123, maintain speed Mach .78"

In the TMA area, aircraft are sequenced for an approach and reduced to similar speeds to maintain lateral separation. These restrictions can be paired with conditions, until when the speed needs to be maintained:

R: "BER456, reduce speed 210 knots"
R: "BER456, cleared ILS approach runway 24,
    maintain 170 knots indicated until outer marker / 5 DME"

Certain approach or arrival charts list maximum speeds for certain turns, marked with MAX 210 KIAS, indicating that this turn should not be flown with more than 210 KIAS to make the radius.

Speed restrictions imposed by ATC need to be maintained until cancelled by the ATC unit:

R: "DLH123, no speed restrictions / resume normal speed"

The pilot however is responsible for ensuring a safe flight and can report that he is unable to follow the instructions due to performance limitations.

A: "DLH123, unable to comply, indicated airspeed will be 150 knots"

There is also legal considerations when it comes to aircraft speeds, such as general rules for traffic below certain altitudes or in some jurisdictions, only for certain airspace classes (related: What is the speed limit in European Airspace).

The FAA CFR 91.117 says:

  §91.117   Aircraft speed.

  (a) Unless otherwise authorized by the Administrator, no person may operate an aircraft below 10,000 feet MSL at an indicated airspeed of more than 250 knots (288 m.p.h.).

The German LuftVO however rules that a speed limit of 250 KIAS below FL100 only applies to VFR and IFR aircraft in airspaces class D,E,F,G and only to VFR aircraft in airspace class C. IFR aircraft in airspace class C do not have a speed limitation.

Regarding the general authority over a flight, see the related question:
Who has the higher authority, the pilot in command or ATC?


Will the ATC be in constant communication with aircraft?

Generally speaking airliners are always able to be in contact with ATC, either by VHF (regular radio), HF (usable over longer distances, e.g. on oceanic crossings), or satellite communication (possibly relayed through their company dispatch). It is also possible for one aircraft to relay messages for another (for example if one aircraft's HF radio has failed they may contact a nearby aircraft on VHF to relay messages to ATC).

Because they can usually talk to ATC even when they are out of radar contact airliners can and will provide position reports, and ATC updates their position information the old fashioned way (which we talked about over here). The controllers can then assign altitudes, speeds, and headings/routes in such a way that no two aircraft attempt to occupy the same piece of sky at the same time.  (This is generally the way oceanic routes are handled. There are also air-to-air frequencies for aircraft flying these routes so they can coordinate with each other if necessary.)

In the event an aircraft is out of contact with ATC - whether through a radio blind spot, an equipment failure, or for some other reason - they rely on a combination of lost communication procedures (here's a quick reference from the FAA) and "see-and-avoid" (if you see another aircraft avoid hitting it) until contact can be re-established.
The use of standard lost communication procedures also allow the controllers to continue updating the aircraft's position in a non-radar environment (they can estimate progress based on the standard procedures and the aircraft's prior position reports/speed/wind data). A larger block of airspace will be reserved in such situations to account for any error.


It's possible to land by touching the tail first, but this is undesirable as it puts a lot of stress on the tail, so it's not trained for and there are no circumstances where you would do this deliberately. There are two common methods of landing a tail-dragger: a three-point landing (where the tail and mainwheels touch together) and a wheel landing (where the mainwheels touch first).

Aircraft design

First, be aware that the aircraft is not level when it's on the ground, like a tricycle aircraft is. The tail position is set so that the aircraft is quite nose-up on the ground - in fact, the angle is about the same as the stalling angle, and the reason for this will become apparent. If your nose is on the horizon, the tail is off the ground (about five feet up in a Tiger Moth). Also, the prop is mounted higher up on a tail-dragger than on a tricycle, for exactly the reason you say. In the Tiger Moth, there is two or three feet of clearance between the ground and the prop if the nose is on the horizon.

The clearance itself isn't the direct problem. If you do go nose-down on the ground, the airplane is not stable. If you're moving forwards, the chief component of drag is acting through the mainwheels, so the engine thrust will tend to tip you further nose-forward (especially if you run over a hump or rabbit hole). In addition, the CG of the aircraft is closer to the mainwheels, and eventually goes in front of the mainwheels. If this happens, a prop strike is inevitable.

Three-point landings

Three-point landings are the default because the aircraft is fully controllable as soon as it touches down. The approach to the runway is done at low or idle power, and set you up to be flying level at rooftop height above the runway. As you lose speed (because you have idle power), you progressively pitch up to maintain height, which in turn causes you to lose more speed. This is called the flare Soon enough you are pitched up at the three-point attitude, and you can no longer maintain height - if you pitch up any further the wings will stall. At this point, you will drop down onto the ground and all three points will touch at once.

Wheel landings

Wheel landings are also used, as they offer more control in windy conditions, but they typically have a longer landing roll because you have more speed when you touch down. The approach is flown with some power on, but less than cruise power. It's a gradual descent towards the runway, and there's no flare, so you touch down in the level attitude. You then close the throttle to idle, and let the speed decay on the ground. As your speed decays and less lift is produced, you need to progressively push the stick forward to keep the tail from dropping, keeping the aircraft level as long as possible. Once the tail starts to drop, pull the stick back to drop the tail firmly onto the ground.

When do tail-first landings happen?

To reiterate, you never land tail-first deliberately. It offers very little control; the aircraft would stall at a higher speed (and therefore the landing roll is harder and longer); it puts undesirable load on the tail. They do sometimes happen by accident, if a student pilot has pitched up too aggressively when trying to do a three-point landing. Typically, too much nose-up input results in "ballooning" as the aircraft gains a little more height, but this results in the speed decreasing rapidly. If left uncorrected, the aircraft stalls, resulting in the tail touching down hard.


How close are planes allowed to fly next to each other?

Your plane was at least 300 meters above the aircraft you saw.

The limits are determined by the altitude and capabilities of the aircraft.

Up to 29,000 ft - 300 meters vertical separation
29,000 ft and above - 600 meters
29,000 - 41,000 ft (where there is RVSM (Reduced Vertical Spacing Minima)), the minimum separation is 300 meters, but the aircraft have to be equipped with specific instruments. Most modern airliners are equipped to fly in RVSM approved airspace.
41,000 feet - 60,000 ft, 300 meter RVSM
60,000 feet and above, 1.5 KM irrespective of RVSM

You won't feel turbulence unless you are flying behind an airplane, or you fly into an airplane's wake.  Turbulence happens in disturbed air and this is usually behind the airplane's wingtips.

All modern airliners are equipped with TCAS (Traffic Collision Avoidance System); which consist of visual indicators on the relative position of nearby aircraft.

Here is an image from flightgear wiki that shows how this looks on some modern airliners:


No, it is torsion which reduces wave drag.

The sweptback wing will twist as well as bend, and this twist will reduce the local angle of attack, and with it the suction peak on the top side of the wing. The same mechanism twists the outer part of a forward swept wing to higher angles of attack, necessitating a stiffer structure to avoid aeroelastic instability.

Air will not "see" any difference in wing thickness, regardless of the dihedral or bending. For air, there is no difference between a wing and a vertical surface (which would have almost infinite thickness following your assumption, but obviously doesn't).


Why does pressure remain constant in the boundary layer?

There does exist a velocity gradient within the boundary layer.  However, the boundary layer, being a viscous phenomenon, does not obey the Bernoulli equation, which is what trips up our usual intuition
So Where Does Bernoulli Come From?
Bernoulli is a simplification of the Euler equation (which is a simplification of the Navier-Stokes equations).  The Euler equation is an inviscid formulation of the Navier-Stokes equations, and Bernoulli further neglects rotationality and states that the total pressure of a fluid is constant (in steady flow, along a streamline), giving the following expression that we're familiar with:
$\frac{1}{2}\rho v^2+P=const$
This works great outside of the boundary layer, in incompressible (typ. Mach < 0.3), irrotational, inviscid flow...which, conveniently, helps us to understand a good many things that we're interested in (e.g., model airplanes, sailplanes, some GA aircraft, etc.).  However, it has its limits, and one of the many tricks of aerodynamics is knowing the assumptions of your models and when they can and cannot be applied.  This is why panel methods, relatively low-fidelity fluid modeling techniques, are still being used in hydrodynamic modeling.  So long as you don't violate their assumptions, you have a body-fitted model that is faster than more modern finite element or finite volume methods and still gives an accurate result that can be used to predict body loads (and, thus, do performance analysis, etc.).
OK, but why is the pressure still constant?
To the point, Anderson states that pressure is constant through the boundary layer along a line normal to the surface of the body (or airfoil, in this case).  The normal part is important: it means that pressure is not necessarily equal along the length of the airfoil.  Let's look at a small element of fluid moving along the body surface (below) to play with this:

So, pressure gradients drive flow.  If there was no pressure gradient along the length of the airfoil ($P_{right}=P_{left}$), the flow velocity would be zero.  This is not the case, and flow is moving from left to right, therefore, $P_{left}>P_{right}$.  Furthermore, as a result of viscosity, we have a velocity gradient as a move away from the body (e.g., low/zero flow velocity close to the body and increasing flow velocity as we move away from the body through the boundary layer).  This implies shear along the top and bottom of the fluid element in the picture, which corresponds to the pressure difference we mentioned.  However, there is no vertical motion in the boundary layer (there can be in a turbulent boundary layer, but I'm assuming a laminar one for the purposes of this explanation) and no vertical shear.  As a result, $P_{down}$ must equal $P_{up}$ on this element.  If I think with a Newton's 3rd law kind of mindset, the fluid directly above and below this element would feel the same pressure and so on and so forth until you get to either the edge of the boundary layer or the surface of the airfoil.  The assumptions for this fall apart outside of the boundary layer, since there is non-negligible fluid motion towards/away from the airfoil, so that's why I'm stopping the analogy at the edge of the boundary layer.
Hence, along a line normal to the surface, since this does not imply that $P_{up}$ on this element is equal to $P_{up}$ on the element to its left or right, the pressure is constant throughout the boundary layer.
Other references: https://web.mit.edu/hml/ncfmf/10FBL.pdf


Can the cracks found on the A380 wings tear the wing apart?

The issue has been fixed, for both the old and new A380's. Before the fix, the first 120 A380's did not face any danger according to Airbus.
From begadistrictnews.com.au:

"The aircraft is absolutely safe because there are so many ways for the loads to travel within the structure of the wing," [Airbus's head of engineering, Charles Champion] said.
"As it really is not a safety issue, we will inspect them over time ... within the next four years – some of them before."

From Wikipedia:

High-strength aluminum (type 7449) reinforced with carbon fiber was used in the wing brackets of the first 120 A380s to reduce weight, but cracks have been discovered and new sets of the more critical brackets will be made of standard aluminum 7010, increasing weight by 90 kg (198 lb).

From aviationweek.com:

Airbus has developed two fixes it says will permanently deal with the cracking of some rib-feet, which has already resulted in an airworthiness directive requiring enhanced inspection intervals and fixes where component cracking is found. One solution addresses the retrofit fix while the other alters the production process so the problem never occurs. The fixes should restore the aircraft to 19,000 flight cycles and regular inspection intervals, says [Tom Williams, Airbus’s executive VP-programs].

From flightglobal.com:

[21 March, 2014] LHT and Ameco to complete A380 wing-rib fixes next year.

Related: How strong is the A380 fuselage-wing joint?


Is it possible to use normal headphones for passengers in a light aircraft?

There are some solutions to convert a normal set of headphones into an aircraft headset, but in total they are going to cost you more than a base level headset would.

One popular (available in TSO and non-TSO options) option is UFlyMike which is an adapter for Bose noise cancelling headphones that adds a microphone and connects to a standard 2 jack airplane audio panel.  But back to your issue at hand, this is not cheap.  The UFlyMike will set you back around \$200 and the Bose headphones are another \$300, making this adapted headset cost a total of \$500.  This is a bargain compared to an actual Bose aviation headset, which runs around \$1000, but is well above an entry level non-ANR headset.

Your best bet is to ebay some old clamp style headsets (e.g. DC) to get some cheap headsets and then keep around your current headset when you eventually upgrade (unless those trade-up deals from e.g. Lightspeed are too hard to resist) to build up a stock.  Alternatively, if you are renting, you may be able to rent a few headsets with the plane so everyone can be equipped for a trip.


What is the difference between centripetal and centrifugal force?

Two of the given answers - choices (2) and (3) - are correct in different reference systems. It depends on the reference system of the observer:

In a non-accelerated reference system, centrifugal forces do not exist. Increasing angle of attack increases the centripetal force, that increases the turn rate. The reaction force of the centripetal force is the innate force of the plane.
A passenger in the plane observes the scenario from within an accelerated reference system. The plane is turning without obvious reason. The passenger's body wants to move in a straight line and seems to exert a force on the seat. This virtual force is called centrifugal. The seat seems to react by supporting the body with the same amount of force. When the pilot increases the angle of attack of the balanced turn, the centrifugal force will increase. The observer inside the rotating system is not aware of the centripetal force. The centripetal force does not exist for him.

Centripetal and centrifugal forces never co-exist in the same reference system! Even though they might appear like counter forces to each other, they should not be understood like that.

Centrifugal force is a construct to explain our perception and sometimes to simplify calculations in rotating reference systems.

Between the two, centripetal force is the more fundamental notion. It really describes what is going on, therefore (2) is the better answer from a scientific point of view. Nevertheless, (3) centrifugal force is a valid answer if the reference to a rotating system is established. That seems to be the case in the question. The centrifugal force describes what an occupant feels who is not aware of the rotation.


Why are some taxiways named with two characters?

Coding the taxiways this way makes it easy to create mnemonics. Note that from northwest to southeast, the first letters are alphabetically increasing. From southwest to northeast the numbers are increasing.

The combination of a letter and a number gives a good indication of the location of the taxiway.

Every taxiway close to the northern runway (5L-23R) is named Nx, everything close the southern runway (5R-23L) is called Sx. Numbering is from southwest to northeast, so N1 is close to the northern runway, at the west end (5L); S10 is the southern runway close to the eastern end (23L).

Letters can also be combined:

The apron side of the N taxiway is where you park, and called NP.

You cross on the east of the terminal on the eastern crossing or EC, or you cross on the west side of the terminal on WC or W2.


In wind tunnel testing "same Mach number" is not "same airspeed"?

Why identical Mach number in a wind tunnel and around the actual object is -not- equal to having identical airspeeds?

The speed of sound depends on temperature, humidity, and pressure. It is highly unlikely that temperature, humidity, and pressure in the wind tunnel are exactly the same as the temperature, humidity, and pressure at the altitude the aircraft will be flying at.

If plane fly at 0.5 Mach(617km/h) that mean airflow speed for test model must also be 617km/h ,to has same compresibility effects? Isnt it?

Which altitude are you flying at? From the looks of it, you are talking about a car, not a plane!
At typical cruising altitudes for jets, 0.5 Mach is not 617 km∕h. At 10000 m, 0.5 Mach is 540 km∕h.


What are the main factors that make an aircraft have good STOL performance?

Good STOL performance is directly linked to how slowly an aircraft can fly and how quickly it can accelerate and decelerate when on the ground.

Low wing loading (a lot of wing for a given aircraft weight) with a lot of camber is the primary means for attaining slow flight. It is not used for every aircraft because a big, cambered wing creates more drag than a small wing, which limits top speed. Flaps and slats can help by modifying the shape of the wing to provide more lift when flying slowly while keeping drag low when retracted for top speed. These add complexity and weight so they are used by aircraft needing speed.

Rapid acceleration requires power, which costs money and is heavy. You pick how much you want. Rapid deceleration usually depends on brakes. It is limited by friction between tires and ground, and sometimes by aircraft configuration. Most every aircraft has brakes strong enough to make a tire skid.

Google "valdez stol competition" to see the world's shortest takeoffs and landings. These guys practice a lot, and you can see some of them nearly flip onto their noses by how hard they brake.


It seems to be the symbol "M" for "missing cloud information". Note that CWIJ has a circle for cloud coverage that is slightly bigger than the standard circle size used for most other airports.
Southeast of CWIJ, there is CWIC which has the symbol M as well, but there the edge of the circle coincides with the lower part of the symbol shown in the CWIJ circle.

Another example can be found with CXDK (normal size circle) and CXLL (big circle size)

There is another station towards the east (CYBK) that has a bigger circle as well, and it has a 6/8 cloud coverage symbol that does not fully fill the circle. For comparison, PAUM has a standard circle size with the same 6/8 cloud coverage symbol filling all the way to the circle's edge.

This leads me to believe that the software behind aviationweather.gov has a symbol library which includes cloud coverage symbols designed for a certain standard circle size. Some airport are represent by a circle which is bigger than the standard, and this leads to the strange representation of the symbol 'M' seen at CWIJ.


What are the landing stall speeds of a Cessna 172?

Short answer

At low speed, near the stall, 40 KIAS (the low end of the white arc) and 48 KCAS (Vs0 in the POH) refer to the same actual speed. While there is a limited divergence (0 to 2%) between CAS and IAS at medium and high speed, this difference can be as much as 20% at low speed, when the aircraft pitch is large.

Details

In your question, you mention:

The POH states a Vs0 of 48 KCAS.
The low end of the airspeed indicator white arc is 40 KIAS.

The end of the white arc is also Vs0. That's a bit confusing indeed as two types of measures are used, but in fact they refer in practical to the same actual airspeed.

Indicated airspeed

Unless computations are done, airspeed values displayed to the pilot IAS (indicated airspeed).

(Source)

Errors affecting IAS

IAS reflects directly the difference between the total air pressure in the pitot tube, and the (mean) static air pressure at different static air pressure ports (on the side of the aircraft and in the pitot). The difference (the dynamic pressure) is subject to possible errors:

Total air pressure is wrong it the probe axis is not parallel to the airflow. Obviously the pitot tube angle changes between low and high speeds. For a given speed the pitot pitch also varies with mass and balance, and the status of drag devices such as flaps and landing gear.
Static air pressure at a pressure port is likely affected by pressure variations created by the aircraft fuselage. This error greatly depends on the port location and its design, but also on the yaw angle.
Airspeed indicator itself.

Obtaining CAS from IAS

All V-speeds are defined in term of "calibrated" airspeed (CAS). The aircraft manufacturer provides tables to find the CAS from the IAS read on the airspeed indicator. These tables defined for different drag devices status (e.g. flaps values) are valid at sea level and 15°C. They are included in the POH. I found this C172N one online:

(Source: POH C172N 1978)

For a 40° full flaps the table says that 40 KIAS are actually 47 KCAS. This is close to the 48 KCAS in your POH.

As you realized, the difference between KCAS and KIAS becomes larger as the air speed decreases, because the angle of attack and the errors in pressure measurement increase.

Multiple ways to express airspeed

TAS depends on parameters and need to be computed. However it can be approximated by the indicated airspeed. IAS is close to the TAS in favorable atmospheric conditions (sea level air density), and at low angles of attack.

The IAS is easy to display to the pilot using the an aneroid cell. For higher angles of attack, the calibrated airspeed (CAS) corrects for instrument and probe placement errors. CAS can be obtained using manufacturer tables. CAS, as IAS, is valid at sea level and 15°C.

When the air temperature or pressure are not close to the ideal ones (15° and 1013 hPa), then CAS differs from TAS.

The ASI usually has a window (see above) for reading the TAS after the outside temperature and altitude pressure have been selected.

(At speed higher than 200 KCAS or altitude higher than 10,000 ft, the compressibility of air becomes significant, and the equivalent airspeed (EAS) is better suited to describe the aerodynamic effects.)

Vs0 is determined assuming the maximum landing weight and the most unfavorable CG location. It is expressed in KCAS. If the Vs0 CAS were unknown, the pilot wouldn't be able to correct it to include the effect of weight and balance changes.

The ASI, because no correction is done, can display only IAS. So the IAS value for the lower speed of the white arc (Vs0) is in KIAS.


How large are the banners that are trailed behind small planes for advertising?

Pre-911 I towed banners, and towed them with a C172 and a C182.  Parasitic drag of the banner is the limiting factor.  We had different letters, and for the C-172, the letters were about 1.5 meter block letters, during the spring and fall, when the temps were below 20C about 18 to 22 letters was the max. Climb performance in the 172 was about 150 FPM.  In hot weather, up to about 40C, the max was about 12 letters with the C172.  The C182 could handle longer banners, and was used the most during the summer.

The weight of a 20 character banner was about 135 pounds.  The banners are made up by a crew on the ground, and then hooked on a low pass over an area adjacent to the runway.  Similarly, a low pass would be made to drop the banner.

The letters we used the most were red synthetic rip-stop fabric, and they were repaired with a sail / parachute sewing machine.  Later, just before 9-11, we got White / Black letters which were out a spun fabric like Tyvek. The banner business died in 2001 due to 9-11, with TFRs (Temporary Flight Restrictions) over major sporting events. NFL did not like banners being towed near games, and the emergence of TFRs effectively blocked banner towing.

FWIW, banner towing is more risky than glider towing, even though the gliders weigh more, they have less drag. (Although I used different aircraft for each.)  Also if a letter tears, or gets loose, the flapping and increased drag can substantially impact the drag, and climb performance.


It is a relatively early Mirage IIIc. You can tell it from other common modifications by the longer rear fuselage, which accommodates the longer afterburner of the older Snecma Atar 9B engine.

The exhaust nozzle on your model is closed indicating military power. On most photos you will see it wide open (it has two buckets on a hinge, and a horizontal gap is clearly visible).
I know all this because not too long ago I built a similar model, but with a smaller 1:100 scale.


Why use plywood to hold the bearings for the airbrakes on a fibreglass glider?

This use of plywood is not ideal, but it works. The wood soaks up the epoxy and becomes an integral part of the fiberglass construction, and is easy to mold, so laying up the same geometry from glass fibers alone would be much more laborious.

Also, plywood is reasonably isotropic, at least in the two directions of the plywood layers. If you want to achieve similar isotropy with glass fibers, you need to interweave them - just wrapping one roving around the bearing will not do.

You could use other materials - aluminum would do, but doesn't bond as well to the epoxy resin. That Schleicher uses plywood has historic reasons. They used to build gliders with wood, some had steel tube fuselages, but wood was their primary construction material. When they started to build glass fiber gliders (the first was the ASW-12, which was a commercial version of the D-36, on which Gerhard Waibel cut his teeth as a glider designer and won the German gliding championship in 1964) they used what they knew best. And it worked.


Does SDAC display data for both the PFD, ND and the ECAM display?

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

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

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

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


Is 1 propeller always more efficient than 2?

Ducts are only used for highly loaded propellers of a very small radius, namely the fan stage of turbofan engines. For lower disk loadings a shroud or duct would create more friction drag than it saves in efficiency.

If the tip Mach number of the propeller is low enough (say below Mach 0.9), then the single propeller of maximum diameter will be the most efficient way to create thrust. This condition is met when both flight speed and thrust per propeller disk area (disk loading) are low.

As the aircraft gets faster, the tip Mach number can best be limited by reducing the propeller diameter. Now more blades will be needed in order to create the desired thrust, and interference between them will reduce efficiency. Also, the higher disk loading will result in more swirl losses, and now it becomes attractive to run a second, counter-rotating propeller behind the first. The two can be either close together or - even better - on opposite ends of the aircraft.


Are strategic bombers still useful, or are fighters taking over this role?

The reason that small aircraft are used in the bombing role is flexibility and cost. If you load up a B-52, it can only be in one place at one time, travel at subsonic speeds, and be expensive to operate, risking more crew and equipment. Using smaller attack jets allows reaching targets across a wider geographical area faster. And while past conflicts saw massive bombing campaigns that favored the use of large bombers, more recent conflicts have seen much less of this tactic. Smaller, more targeted strikes are more typical, where a large bomber would be overkill.

Take a look at the cost per flight hour for various aircraft. A B-52 will cost \$70,000 per hour to operate, and a B-1B \$60,000. Compare this to an F-16C for \$23,000 or an F-15C for \$42,000. The A-10 costs \$18,000. For stealth, the costs go up to \$170,000 for a B-2 and \$70,000 for an F-22. So why send a B-52, when a few F-16's would cost the same amount? The fighters will also be able to defend themselves, and still be able to carry out a mission if an aircraft drops out. The same argument can be made for the stealthy class.

Strategic bombers still have their uses, but the bomber role is much less demanding than the fighter role. With proper maintenance and multiple equipment upgrades, old bombers can still fly bombs to their targets just fine. Fighter jets have much more complicated requirements, such as stealth, speed, and maneuverability, where advances in technology will justify the costs of continued development of new designs.

Aircraft are becoming more generalized because it is more efficient. In some areas such as naval aviation, this is even more critical. Why have separate planes for electronic warfare, attack, and air defense? You would be limited to a certain number of each type, with little or no cross capability. But with more generalized aircraft, you can have more aircraft with a certain capability across the fleet. If a certain capability becomes less important, you don't end up with a whole group of aircraft becoming useless. Aircraft can fulfill multiple roles in a single mission or deployment.


Your website source from Iowa State University may or may not be open for commercial use. As Freeman has said in his comment, it would be difficult for anyone here on the internet not affiliated with the university to know. Your best opportunity to use their data is to contact the university directly. Their contact information is at the bottom of the link you provided.
Otherwise, your best option is to contact NOAA, the NCEP or the National Weather Service (basically, one and the same organization). They collect, consolidate, and archive the data at their source. That is where many commercial applications get their data.


What are the design considerations for variable-pitch folding-propellers?

Yes, although contest rules to date haven't yet motivated doing both at once.  In F1D international competition, propellers are commonly variable pitch (Indoor Model Airplanes: The Best of Indoor News and Views, ed. Tim Goldstein, pp. 138-144) to better exploit the gradually diminishing power during a flight, and sometimes variable diameter (ibid., pp. 119-126) to reduce drag like a folding propeller would.  Although not a contest winner, R/N Models' "Maverick" kit included a folding propeller purely for drag reduction.

No: if you need variable pitch for efficiency from a variable-power engine, and folding for drag reduction, it's simpler to reduce drag by feathering (changing pitch so far that the propeller won't windmill while unpowered) instead of by folding.

No, for a special case:  For propellers of the size you show, 8 to 16 inches diameter, 300 to 2500 watts electric power, a typical flight profile alternates full power climb with zero power glide (hopefully finding thermals to stay aloft).  Partial throttle only wastes energy.  Proving that mathematically is hard, but it has been verified with numerical simulation.  "Cruising" is best done with a burst of climb (what the propeller is best at) followed by a long glide (what the wing is best at).  At any rate, for a full on / full off power source, a single pitch is optimal.


What makes the Antonov An-2 have no stall speed?

The horizontal tail can't make sufficient down force to slow the plane to get the slatted wings to stalling AOA.  With the slat system, the stalling AOA will be perhaps 10 degrees higher than without.  With the AN2's large wing area, to get the slatted wings to stalling AOA might require the speed to drop to, say, 20 kt.  But the tail simply runs out of down-push before that angle is reached, in this case at 25 kt, so no stall can occur and the airplane settles into a near vertical mush as long as you hold the full up elevator.

Because there is no actual stall break or departure, the published "stall speed" is the lowest speed that airplane can be flown and maintain altitude.

Note that a "parachute descent rate" is about 20-25 fps, or 1200-1500 fpm.  That's still coming down pretty fast (you would probably collapse the gear landing normally at that descent rate) and the airplane will be wrecked when it hits the ground, but you'll live.

The interesting thing is that if you do get a slatted wing to actually stall, the departure can be very violent.  An acquaintance who flew a Pegazair he built, which has automatic slats, was horsing it around and was able to get an aerodynamic stall by using large elevator pulsations as he was approaching minimum speed, reported that it flipped over violently and his flight bag flew from the baggage well behind him up to around the windshield.  It recovered fine however.


How does an inexperienced pilot handle G-Forces?

The  G-limit for the Extra 300 (the parent airplane to the 330):

  The Extra 300 is stressed for ±10 G with one person on board and ±8 G with two

So it is certainly possible from an airplane perspective that you pulled that many Gs - in an aerobatic airplane, it's not unheard of to pull 1G less than the aircraft limit (usually there is a large margin of safety built into that number before a catastrophic structural failure).

Now did you pull 7Gs? As others have noted, 7Gs for 10-20 seconds seems very unlikely. But it is not out of the question your peak instantaneous G load was 7.

From a human factor standpoint, G-suits typically don't increase the max G-load, but rather increase the sustained G-load and the time period it can be held for. G-suits do very little to actually decrease blood flow (pooling in the legs) - they primarily serve as a reminder to the pilot to perform an AGSM.

  A g-suit does not so much increase the g-threshold, but makes it possible to sustain high g longer without excessive physical fatigue. The resting g-tolerance of a typical person is anywhere from 3-5 g depending on the person. A g-suit will typically add 1 g of tolerance to that limit.

Now there are a vast array of factors affecting this resting G-tolerance. I flew  a military high-performance trainer, and my resting G-tolerance was around 3, or the lower end of the spectrum. One of the other pilots I knew had a resting G-tolerance of about 7.5. The primary difference is blood pressure. I am 5'10" and only 145 pounds, with a resting blood pressure of about 90/60-100/70. He was 6'0" and about 215 pounds, with a much higher blood pressure. Tall, skinny, nonsmoking runners have actually the worst resting G-tolerance while short, stocky, sedentary or heavily muscled smokers have much higher tolerance. Other factors, such as how hydrated and rested you are and how much stress you are feeling, can vary vastly your daily G-tolerance.

All of those numbers I just listed do not take into account the pilot performing an AGSM (Anti-G Straining Maneuver). This can add several Gs to the pilot's tolerance, and is one of the biggest things taught in physiological training for high-G maneuvers.

So TL; DR: could you have pulled 7Gs without prior exposure? It is possible, depending on varying personal factors and body type, but it is not likely. However, with a little training, 7Gs is perfectly attainable.


Is it possible to 'derate' a plane as a safety measure?

The answer is yes, if there is an operational procedural restriction limitation that effectively mitigates the risk, without adding its own risks, and is something that is operationally feasible, pending a design change.

Examples would be an AD that:

Imposes a speed, altitude, weight or temperature limitation.

Imposes additional reserve fuel requirements (this was done for CRJ200s that had a problem with the flaps getting stuck while extended following a missed approach).

Limits operation in icing conditions (although for most airline operations this effectively grounds you, because "legal" icing conditions exist in any cloud above the freezing level).

All sorts of other system restrictions.


Is a clearance needed to depart a helipad in class B airspace?

The "Wedding Cake" literally defines the airspace, so for a the sectional part you listed above, the floor of the Class-B is at 3000 feet in the outer ring and 2000 feet in the inner ring, these heights are MSL.

Anything outside of the wedding cake is not Class B airspace. You may be confusing the Class B airspace with the Mode-C Veil that surrounds 30nm outside of most Class B airports, which only requires that you have an operating transponder, you are still allowed to fly around VFR squawking 1200 below the Class-B without any clearance.

The airspace under the wedding cake, provided you are not within a Class-E airport, is most likely Class-G. So no, you are not required to contact Tower until you want to enter Class-B airspace or if you need a departure clearance for IFR operations.


Do stall strips mean that wing washout is not required?

Rectangular wings can get away without washout because that planform tends to have favourable root to tip stall progression as a characteristic of the rectangular shape.

The stall strips aren't really to change the root to tip stall behaviour as a substitute for washout; their job is to generate a local flow separation, just prior to the natural stall, who's turbulent flow can be felt through the airframe and at the elevator while the rest of the wing is still unstalled.  That's why they are just short sections in front of the outer parts of the horizontal tail, so that the burble they produce will be felt through the controls well before the rest of the wing lets go.

The Yankee had a NACA 64-416 laminar airfoil which had an abrupt stall and very little natural pre-stall buffet - helped by the strips.  Later variants got a revised airfoil with a drooped nose to make the stall itself more gentle, but they still retained the strips to provide the pre-stall buffet.

Many years ago I flew a Yankee that was in a flying club I was in, and it was considered a demanding airplane.  The stall strips made it a bit less dangerous than without, but it still took careful speed control when landing.  If you got too slow it would start to sink and pulling a bit more pitch had no effect.  It had a reputation as a very "hot" airplane for something with only 108 HP.

On a tapered wing, you would be adding stall strips for the same reason as the Yankee's wing, near the root forward of the outer end of the horizontal tail, to provide a distinct pre-stall warning buffet.  If your tapered wing had no washout but had the strips, all that would happen is you would get a pre-stall buffet that wouldn't be there without them, but then when you get to the wing's stalling AOA the whole thing goes at once because it's still a tapered wing with no washout.

Some designers get some of the benefit of washout without twisting the wing by varying the airfoil section itself from root to tip.


What is a straight wing on an airplane?

A straight wing is a wing that is not swept (see the images).

Straight and swept wings

Unswept, straight wings

Swept wings


What is the minimum number of engines required for taxiing an airliner?

Whether it's a twin-, tri-, or quad-jet, it very much depends on the aircraft design, airline policy, and conditions such as taxiway slope, contamination, weight, etc.
Read the below example for the Airbus A320 to get an idea:

An example of what can go wrong when taxiing on two out of four engines (or 1 out of 2) is having the aircraft in the wrong configuration. Like what happened on August 23, 2001 when the ground engineer had no steering or braking and they ended up in a ditch:

For the Airbus A380:

The minimum is a symmetrical two on the A380 (likewise for the 747).
777
In the 777 it is not allowed. That's an example of how it depends on the aircraft design.

Taxi – One Engine
Because of additional operational procedural requirements and crew workload, taxiing with an engine shut down is not allowed. High bypass engines require warm up prior to applying takeoff thrust and cool down prior to shutting down. If the engine has been shut down for several hours, it is desirable to operate at as low a thrust setting as practicable for several minutes prior to takeoff.
Note: Single engine taxi with an inoperative engine is authorized consistent with good judgement.
(Boeing 777 FCOM)

Fuel saving
According to this Eurocontrol document, the saving is about 4-8 kg of fuel per minute.


Did British aircraft fly at a specific speed while returning from missions during WWII?

Ground speed
Real time wind components were not available to the pilots. They wouldn't have been able to tell the ground speed. Especially over the English Channel on a calm or foggy day where a drift meter is not very useful (if equipped).

[A drift meter] indicates the aircraft's drift angle due to winds aloft, and can be used to calculate the ground speed.

If they flew at an agreed true airspeed, then the controllers wouldn't have been able to tell that true airspeed.
Early IFF
Looking into the history of IFF and radar in WWII, it's apparent from early on radio IFF was used. It started with the pip-squeak system.
Mistaken identity
In the Battle of Barking Creek a radar issue caused a friendly fire incident where returning British fighters were attacked. According to Wikipedia, as of 2010 the final report has not been declassified yet.
Another one was the air attack on the fortress of Koepenick, the Germans thought there was an air raid en-route, they launched fighters, only to mistake their fighters for the raid, only to send more fighters, etc. And there was no raid, just the Americans were returning from a bombing mission and judging their direction of flight aurally led to this confusion. The German fighters started to run out of fuel and landed.

Göring freely admitted the laugh was on him, that he had sent the Luftwaffe on a mammoth tour of their own air space.

Navigation
Wind and ground speed aside, if we say it was a certain route combined with speed, that would require a level of area navigation that just wasn't there in WWII.
Logistics
Logistically, when an operation is launched, the air command has an idea of when to expect its return.
Speeds
Comparing the Focke-Wulf Fw 190 with the Supermarine Spitfire, the German Fw 190 was faster. 426 mph compared to 370 mph.
As for slow speeds, slow flying on the return leg is just calling for the surviving planes to be caught up with and shot down.

I'm calling it myth busted.


High altitude turbine efficiency question

This isn't true. Less dense air at higher altitudes means less oxygen to burn with the fuel. Less burn means less thrust. In the limit of no oxygen, there is no burn, and no thrust at all.


Do you need to have a raft if you're in a seaplane?

Here is an answer I found after a few minutes of looking up regulations online.

Federal Aviation regulations require carriage of FAA approved floatation gear when operating under Part 91 for hire (FAR 91.205(b)(12)), but Part 91 flights not conducted for hire are not required by the FAA to carry floatation equipment.
It is worth noting that local authorities, including state, county and municipal authorities, may require that all vessels, including seaplanes, carry U.S. Coast Guard approved floatation gear on board.

Found on the https://www.seaplanepilotsassociation.org/resources/faq/operations/ website.