Is the sweep angle of an inverted wing zero degrees?

Sweep is normally given as the quarter-chord sweep. If not, it must be made clear by some qualifier, like leading-edge sweep.

Your previous questions let me suspect that you are into a special version of ground-effect vehicles, the Lippisch aerofoil. The picture below shows the X-114.

The wing is a delta with negative sweep, so it has a straight leading edge and a forward-swept trailing edge. The idea is to have a positive angle of attack and very little space between the trailing edge and the ground for maximum ground effect.

Such a wing has a negative sweep angle. If you take the weighed average of leading ($\varphi_{le}$) and trailing edge sweep angle ($\varphi_{te}$), you get the quarter chord sweep angle ($\varphi_{25}$): $$\varphi_{25} = \frac{\varphi_{te} + 3\cdot\varphi_{le}}{4}$$


Why does pitching moment coefficient decrease with an increase in the angle of attack?

There are several, equivalent explanations:

The airplane has positive static longitudinal stability.
As the angle of attack increases, wing lift grows more slowly relative to tail lift because the coefficient of lift on the wing is larger than that on the tail.
The airplane exhibits a growing nose-down pitching moment as the angle of attack increases in order to return to its trimmed angle of attack of -3°.

The increasing slope of the moment curve at 12° angle of attack indicates stall onset on the inner wing. Here, the flow starts to separate on the rear upper wing root which reduces local circulation and, consequently, the downwash angle at the tail so the lift increase on the tail becomes disproportionally large. This is typical for a straight wing with a moderately high aspect ratio.

The two lines show the behavior at two different Reynolds numbers which seem to belong to a model airplane or a low-speed tunnel measurement. They also show the behavior of a full airplane, not an isolated airfoil.

  If we see pitching moment value then I think it should increases with increase in angle of attack as centre of pressure shifts away from center of gravity with increase in angle of attack.

It does grow, but in the negative direction. This is because a nose-up pitching moment is defined as positive and what we see here is an increasing nose-down pitching moment. The center of pressure of the wing-tail combination is shifting backwards with increasing angle of attack and the pilot would need to trim a negative elevator deflection to bring it back to the center of gravity.

  My question is different from suggested question Should the pitching moment be up or down? as my question is talking about pitching moment coffecient which depend on pitching moment, dynamic pressure, and other parameter.

No, it's not. As the constant Reynolds number indicates, this has been measured at constant speed and it does not matter if it is a coefficient or the "real" moment because both only differ by a constant factor.


What level of damage to an aircraft is acceptable to let it depart?

Aircraft Maintenance Engineer here.

To evaluate if an aircraft is safe to fly we use three main reference documents:

1. MEL (minimum equipment list).

This document contains the list of elements that are allowed to be inoperative.

For example a pneumatic valve, a computer, a seat, etc...

All unserviceable elements will have to be fixed within a predetermined duration/limit.
(Usually 1, 3, 10 or 120 days)

If the failure is not fixed by the due date, the whole aircraft will not be released to fly. (No matter how small the fault is)

2. CDL (components deviation list).

This document contains the list of components that may be removed from the aircraft.

For example a damaged refueling service door can be removed, if the maintenance team is unable to fix it before departure.

These will usually be associated with performance penalties.

CDL defects will also come with a deadline.

3. SRM (structural repair manual).

This manual contains all information needed to evaluate structural damage such as dents, scratch, lightning strike.

This manual will also give you the information needed to evaluate if a damage is "no-go" (meaning the aircraft is not safe for flight) or if the damage is within tolerance.

If it is within tolerance you will also get time or aircraft cycles limitations (a cycle is a takeoff and landing)

Regarding your picture:

The plane will not be allowed to fly with a blown tyre. A blown/damaged/worn out tyre is a no-go.

However, airliners are designed to land safely with one blown tyre. Aircraft incorporate lots of redundancy so as to eliminate single points of failure — for example, most commercial jetliners are designed to be able to land safely even if they lose an engine. Efficiency, comfort, and ease of operation may be reduced but the situation can be kept under control and is still within the aircraft's design specifications.

I can see damage on the landing gear door. It is possible that an aircraft would be allowed to fly with the landing gear door removed if a fix cannot be performed before departure, however; the aircraft may experience increased drag. This would be covered in the CDL.

The aircraft should also receive extensive inspection before departure and after landing.


I would say that it is a reasonably safe aircraft, but, like most aircraft of that class, often operated in unsafe conditions.
The Wikipedia page does not have description for many of the listed accidents, but you can turn to the Aviation Safety Network which does.
There are 41 total hull losses listed there. From them:

Most common type of accident is controlled flight into terrain. That has nothing to do with the type and everything with operating to/from remote airports in mountainous terrain with limited choice of navigational aids around.
Then there are quite a few collisions, both midair and on the ground. That also has nothing to do with the type and much with operating at smaller fields often under VFR and without any radar.
Then there were several accidents where pilots mishandled engine failure. The aircraft is capable of flying on one engine and it had to be demonstrated during certification, but the pilots have to follow the correct procedure and may easily lose control if they don't. This kind of accident is more common in turboprops than in jets, but it does not seem to be more common in B1900 compared to other turboprops.
Only the three accidents that were contributed by improper installation of elevator or elevator trim control cables really have something to do with the specific type. They still have a lot to do with operating by smaller airlines and maintaining in smaller facilities with less experienced staff.

So no, the relatively long list of accident does not suggest any serious problem with the type.
Also, the accident rate and types are similar to the other types used in similar operating conditions like DHC-6, EMB-120, L410 or Do-228.


Identify an USAF transport aircraft in Greenland around 1960

That is the Douglas DC-6 / C-118/R6D Liftmaster.

This specific plane was built in June 1947 with modifications for presidential use. It is well known for flying US President Truman. It was the first presidential aircraft that received a distinctive exterior.

It actively served USAF until 1965 when it was transfered to an USAF museum. It is now on display at USAF museum in Dayton, Ohio, USA.

[Source]

[Image Source]


What is the crank on the ceiling of this aircraft?

The pilot is operating the overhead stabilator trim crank in an early model Piper Cherokee. Here's a close up the trim crank:

Piper Cherokee overhead trim crank; image from tnwings.com

In later models, it was replaced with airliner type trim wheels


Do passengers notice if the aircraft is struck by lightning?

In my experience: Yes, the passengers notice, at least for some lightning strikes.

I was flying from San Francisco to Chicago, this was sometime in the 90s. There was a big snowstorm coming in to Chicago, and all you could see out the windows was snow illuminated by the landing lights. There was the usual murmur of conversations going on in the cabin, and all of a sudden there was an almighty BANG and the cabin went completely silent - everybody had stopped talking and was probably holding their breath. Several seconds later (seemed like a long time) the captain came on the PA and said

  Nothing to worry about folks, we were just hit by lightning. We will be on the ground in Chicago shortly.

I am 100% certain that every single passenger on that flight "noticed".


What are these engine gauges in the Comet 4 for?

They were used on takeoff to monitor engine performance. Takeoff power should result in the needles moving to the vertical marker and all engines should be registering the same reading.

The indicators monitor the pressure in the exhaust cone of each engine and will give a warning of failure which might in certain cases only become apparent by watching a combination of other instruments.


As a generic answer, the only setting the crew inputs in most pressurization systems is the destination field elevation. This determines the pressure schedule on descent, and the system will use a cabin altitude descent rate that will put the pressure differential to near zero, usually slightly positive (that is, a hundred feet or so below field elevation) upon landing before the outflow valves are opened fully to vent the cabin to atmosphere.

So if the crew sets the field elevation at 3000 ft, the system will use a pressure differential reduction schedule on descent that gets the pressure hull close to zero differential when at 3000 ft, so there is minimum effect on people when the system goes into dump mode to vent the cabin.


Can ADS-B DF-17 messages with a non-zero remainder after CRC check be used?

Messages with Downlink Format 17 are, as you know, ADS-B messages that do not require interrogation. As such the interrogator ID will be zero.

Frequently you will see that, after applying the CRC check, the CRC remainder is non-zero. This is caused by corruption of the ADS-B message between the transmitter (see note) and the receiver. This is partly because transmissions of multiple aircraft are unsynchronised so that they may overlap an become difficult to untangle. This overlapping is called garbling.

Since you have IQ information available, you may be able to de-garble overlapping message with some success. Also, you could try to do error correction using the CRC algorithm. Up to two rogue bits can usually be corrected, more than that becomes difficult.

To improve the performance of the CRC error correction you can again use information from the IQ sampling, by giving each bit a confidence value based on the shape of the received radio data.

If you are not able to correct the error bits (i.e. you have a non-zero remainder after correction), you should consider that ADS-B message as useless.

Note: there are a couple of aircraft flying with rogue transponders that transmit garbage DF-17 messages with non-zero remainder. They will be phased out in the next couple of years.


What is the difference between ground speed and airspeed?

The ground speed is the speed the aircraft is making over ground below.

The indicated airspeed (IAS) is the speed shown on the airspeed indicator. This will be different than the ground speed for various reasons, such as wind and the attitude of the aircraft. For example, if the aircraft is diving then the IAS will be greater than the ground speed. In fact, if the aircraft were diving straight down, then the ground speed could be 0 while the IAS was hundreds of knots per hour.

The indicated airspeed is the usual measure used in flight.

The ground speed can be calculated to estimate the time of arrival of the aircraft at some destination.


Can anybody give an example about Clearance Expiry Time?

The current version of the AIM says:

  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.

  EXAMPLE− Clearance void if not off by (clearance void time) and, if
  required, if not off by (clearance void time) advise (facility) not
  later than (time) of intentions.

Let's say you're using a GCO or telephone to contact an ARTCC for your clearance out in a low-population area with little infrastructure making it impossible to contact ATC from the ground via standard radio equipment.  After you receive your clearance, you will hang up the call or change frequencies and break contact with ATC.

Due to the break in communication, they will end your IFR clearance by assigning a void time in case you don't get airborne as soon as expected.  If you delay beyond your void time, you would need to call and get a new clearance that you would expect to have issued with a new void time.  The void time is short enough that the implied intention is for the aircraft to receive the clearance while stopped at the runway's hold short line already prepared for takeoff.


Do I need 35 hours of ground school for knowledge test for PPL?

For 141 you need a graduation certificate to take the test. For part 61 you need an instructor endorsement. 141 is for complete courses start to end. 61 is for getting a piece here and a piece there mix and match style. in either case there is a minimum amount of ground training required before you can get your private certificate.

Any either case you don't need the knowledge test to start flying, you only need the knowledge test before your final FAA check ride.


It's all about making sure everybody knows what's going on. While you are on an instrument approach and thus flying IFR, ATC is responsible for spacing you, giving instructions on heading and speed in order to fit you in with other inbound aircraft. In that situation they expect you to confirm and follow those instructions, and if you don't it makes for a lot of headaches.

If you're on a visual approach, such as when you've cancelled your IFR flight plan after descending below sparse clouds with the runway now in plain sight, then the tower knows not to give you spacing instructions; they instead space IFR traffic around you, while other VFR traffic just has to maintain their separation minima from you. Knowing that you're on a visual approach means ATC won't waste their breath trying to get you to adjust your airspeed, altitude or course to follow a traffic pattern; they'll just give those instructions to other aircraft in the pattern to maintain a steady traffic flow.

... unless you're in Class B space. If you are flying into a Bravo airport, you can still request a visual approach, but approach control has to OK that change (especially for airliners), and ATC is still required to provide separation services to you whether you're VFR or IFR, so they can still give you instructions regarding your airspeed to make sure you fit into the IFR traffic pattern. This may, at times, require an uncomfortably fast landing for GA pilots being spaced in between commercial airliners at a busy time. Some Class Bs like Phoenix or DFW will have a runway more or less reserved for smaller aircraft so there's less disturbance to commercial airliners with higher approach speeds, but not all airports have that luxury; if you want to land your small single at O'Hare or Dulles, you'll probably find yourself spaced in-between airliners and forced to very precisely time your approach so you can touch down and clear the runway in time for the plane behind you to take off. If traffic is heavy, you may not get in if you greet them with "Hello O'Hare Approach, Piper 354 Alpha...". As soon as the O'Hare controllers hear your GA small-prop callsign, if the pattern's too busy they'll deny you Bravo entry and recommend you head for a smaller airport like Chicago Exec or Gary.


Do we have a unit to measure turbulence?

Turbulence can be quantitatively expressed as Eddy Dissipation Rate (EDR). EDR can be calculated from available onboard measurements, however it is not yet common for aircraft to have software to calculate the EDR.

Originally it was thought to have EDR reported over ADS-B, but it did not become a part of the current specifications. It might be in the next ADS-B version.

NASA Langley paper

World Meteorological Organization (WMO) presentation


Why are First/Business Class seats at the front of aircraft where it is the least safe?

There are several reasons:

First In, First Out
They are given precedence to board the airplane before others, and precedence to disembark the airplane before others too.
Quieter Environment
On commercial airplanes, engines are on the wings which are in the aft of first class. Hence first class is quieter, which is a better experience.
Low Turbulence
Turbulence at the front of the plane is lower than the back. Aircraft are extremely flexible and will bounce around more in the back than the because the bump hits the front and causes the back to vibrate.
Seating Arrangement
Important and prominent passengers feel that they should be in the front or on the upper deck (747 or A380), when available. They also feel that they do not need to see other parts of the airplane which are not of their concern.
Marketing Strategy
When economy class people walk through first and/or business class, they will notice the perks and benefits of traveling in these higher classes. They may upgrade their seats on future travels for a higher price.


Why are winglets swept back and not front?

A winglet makes lift like a wing. That's why it's called a wing...let.  It's basically a sail (originally they were called "tip sails") designed to exploit the otherwise wasted circulation of air from bottom to top around the tip, placed so that the local flow curling around the tip strikes it at a positive angle of attack.  It makes lift/thrust and, being a wing, creates a downwash while doing so, which means some of the circulation is being redirected back the other way (an "outwash" in other words).

So the overall effect is the vortex field is weakened, and some its energy is used to make lift and/or thrust.  If the winglet is straight up, its lift component is straight inboard and forward, and is purely thrust and a (wasted) bending moment on the winglet root (which leads to angled winglets).

If it's angled, its lift component is a combination of vertical lift that contributes to holding up the plane, and thrust.   So the dihedral angle is a function of how much the designer wanted the winglet's lift force to be extra lift and how much was to be thrust, the resulting angle being a compromise sweet spot between inboard lift and vertical lift in that particular wing's tip flow pattern (nobody seems to use straight up winglets any more).

The sweep angle is for the same reason the main wing is swept, to reduce high Mach# drag (remember it's not just a fin "blocking" airflow;  it's a wing making lift like the main wing), and also to place the winglet in the optimal location in the circulation for exploiting the energy in the flow.


For general aviation small aircraft flying VFR under 10,000 feet, can I use my phone GPS for navigation in the US?

Can you do it? Sure. It is recommendable? That depends.

If you are planning to use it as your primary source of navigation, then I would say definitely no. Aviation GPS units are designed to be used on aircraft, they are reliable and have airspace maps. There's a reason they are expensive. The same cannot be said for your phone GPS.

But, if you remain aware of your position by other means (visually recognising landmarks and cross referencing them with aeronautical charts) then sure, you can use your phone GPS as an additional source of navigational information.

Just keep in mind that if the GPS suddenly fails, you should be able to continue the flight normally. If you can't, then it means you are trusting the GPS more than you should.


Do any UK universities include PPL/Commercial flight training as part of an aviation degree?

One UK list is here, it mentions several universities that include PPL and/or ATPL training in the degree course, but they appear to cover only the theory part of the courses with actual flight training being optional. Where a full PPL is included, they mention extra costs of around GBP 7000 on top of regular tuition.

If you look directly at the university sites they seem to confirm this: Leeds includes only 10 flight hours for the PPL; Brunel and Salford both include a UK NPPL but at extra cost.

You might be able to get a scholarship of some kind to cover all or part of your tuition, but that depends very heavily on your personal situation and what the individual university offers. Finally, you could consider the RAF as an alternative route to paid flight training but I have no idea if that's of any interest to you and I imagine that competition for cockpit places is very tough.


Has anyone successfully solo'd a helicopter with no actual flight experience?

Yes, Eric "Winkle" Brown took delivery of a Sikorsky R-4 and flew it after reading the flight manual and getting a briefing from a USAF Sergeant. He had no experience or training in helicopters. He wrote about it in "Wings of the Weird and Wonderful".


The concept of 'density altitude' is kind of like the concept of 'wind chill'.
Stick with me here, I'm going somewhere with this.
Cold weather is dangerous for the human body, and wind (because of increased heat loss on human skin) makes it worse. But how much worse? Is it worse to be outside in -10C temperatures with a 20 knot wind, or -15C temperatures with a 12 knot wind? The concept of 'wind chill' resolves those two values into a single easy number.
Density altitude works the same way. It's difficult and tedious to compare and contrast how an airplane will perform on a 25C day with a pressure of 29.80 at an elevation of 600 MSL, versus a 20C day with a pressure of 30.17 at an elevation of 1250 MSL. We need a way to mash all of these variables into one easy-to-use number. That number is density altitude.
So just as I can say "The wind chill is -10C" and it doesn't matter whether it's warm but windy or cold and calm, I can say "The density altitude is 2,000" and everyone will have the same idea of the expected performance of the airplane, no matter what combination of factors led to that result.
Once you start thinking of (and using!) density altitude as a simplification tool, its value becomes a lot more obvious.


Is this type of wing design structurally sound?

The simple answer is yes. There are many examples of working tube and fabric aircraft, both ultralights and larger designs, that make use of wing designs similar to your example.

The more complex answer is that it depends on a lot of things:

Did the designer anticipate flight loads correctly?
Did the designer model flight loads correctly when validating the wing design?
Do the materials used to construct the wing have physical properties close enough to the model used during design?
Was the aircraft built correctly according to the design?
Was the aircraft damaged such that the structure was compromised?

I don't want to discourage you from designing and building your own aircraft--one of the most exciting things about ultralights is how accessible they are to people with little or no formal training. Before you build and fly though, you should either learn how to determine the safety of a wing design using analytical methods, or stick to copying proven designs exactly.


How much lift do supercritical airfoils produce?

Supercritical airfoils produce comparably more lift than conventional cambered airfoils but less than optimised high-lift airfoils. This is because

they have blunter noses which work over a wider angle of attack range,
the blunt nose also allows to use effective slats or Krüger flaps,
they have plenty of camber in the aft section of the airfoil and
the high rear loading allows for thin, highly cambered flaps.

From history.nasa.gov:

  In the course of developing the wings for these flight programs, it was learned that the supercritical airfoils had excellent high lift characteristics because of their large leading-edge radii. This important benefit tended to offset the fact that their subcritical profile drag is higher than for comparable 6-series sections.

However, the forward part contributes relatively little to lift because it has negative camber. Also, the rear loading causes a high pitching moment.

In the supercritical speed region their upper side produces more suction since local speeds can reach up to Mach 1.3 while conventional airfoils must struggle to keep speeds subsonic. Part of that advantage is used up by making the airfoil thicker so suction on the lower side is also higher than on conventional airfoils, but an advantage remains. More recent airfoils add some more lift at the nose by designing a slow speed rise on the lower side while the upper side accelerates the local flow more quickly. Their main lift, however, comes from the thin, highly cambered aft section.

When R. Whitcomb researched supercritical airfoils, he noticed their high subsonic lift and modified the general shape by adding more forward camber into the GA(W)-1 and -2 (General Aviation Whitcomb) airfoils which became popular choices for modern GA aircraft due to their high lift and benign stall characteristics (witness 93 hits on Dave Lednicer's incomplete guide to airfoil usage, among them the Piper PA-38, the Glasair and the Edgley Optica).


Is there any specific reason why Delta Air Lines doesn't have its own callsign?

Delta Air Lines uses the ICAO three-letter designator DAL and the ICAO telephony designator (also known as callsign) DELTA.

In general, callsigns should be similar or equal to the name of the airline according to the following ICAO rules:

  3.2 In the registration of telephony designators the following rules will apply:

  a) the chosen telephony designator should attempt to resemble the name of the aircraft operating agency or its function
  and be distinct and dissimilar from any other telephony designators in Doc 8585. Ideally it should reflect correlation
  between the three-letter designator, the telephony designator and the name of the aircraft operating agency or its
  function (examples: ARO – ARROW – Arrow Aviation; RAJ – RAJI – Raji Airlines);

  b) in order to reduce the length of transmission the telephony designator should be brief, comprising if possible one
  word of two or three syllables. It should not exceed two words;

  c) three-letter designators may not be used in phonetic form as telephony designators. However, telephony designators
  of long standing (such as KLM or TWA) may be retained, provided that an acceptable alphabetic representation is
  used (example: KAY-ELL-EMM); and

  d) the telephony designator should be easily and phonetically pronounceable in at least one of the following languages:
  English, French, Russian, Spanish.

(ICAO Doc 8585 3. Telephony designators, emphasis mine)

Different callsigns can exist for two reasons:

Old callsigns: Several old airlines use callsigns that existed before these rules were made, like e.g. KLM as mentioned in c). British Airways inherited the callsign SPEEDBIRD from BOAC in 1974, which in turn inherited it from Imperial Airways in 1939. It is named after the design of the Speedbird logo.
Conflicting callsigns: When the airline name is too similar (or equal) to an existing callsign, a different one must be chosen, like e.g. Norwegian mentioned in J. Hougaard's answer. US Airways inherited CACTUS from America West in 2008:

  Early in its history, the airline used the call sign “America West” but it often caused confusion with other airlines ending in “west” (Southwest, Northwest, Skywest). The FAA suggested that America West come up with a new call sign. The company held an employee contest to come up with the new name. Cactus was chosen.

(aerosavvy.com)

While Delta Air Lines is quite old ("Passenger operations began on June 17, 1929", Wikipedia), it did not chose a different callsign. I can only speculate on why, but note that the ICAO phonetic alphabet is only officially used for civilian aviation since 1 April 1952. Before than, other alphabets were common, including the Allied military phonetic spelling alphabets, which used DOG instead of DELTA, so there was no conflict at the time.

Using a different word when confusion is likely (like DIXIE in Atlanta), is explicitly allowed in the US:

  ATC facilities may also request pilots to use
  phonetic letter equivalents when aircraft with similar
  sounding identifications are receiving communications on the same frequency.

(AIM 4−2−7. Phonetic Alphabet)


Are these pressure and velocity distributions calculated by CFD simulation correct?

Qualitatively they seem to be correct:

Highest pressure at stagnation point and maximal pressure in diagram close to total pressure:
$$101843.84\ Pa \approx P_{tot}= P_{\infty}+\frac{\rho}{2}\cdot U_\infty^2 =101853\ Pa$$
Speed drop at stagnation point.
The slight flow deflection downwards behind the airfoil shows that your airfoil is generating lift (I am assuming your free stream flow is in x-direction).
Low pressure (dark blue) region above airfoil is bigger than below.

As @ROIMaison wrote the correct way to validate the results is looking at the aerodynamic coefficients and pressure ditribution $c_p$. In a quick search I found some polars calculated with xfoil you should be able to replicate. With xfoil you can calculate the pressure distribution too.

If you are only interested in lift at small angles of attack laminar flow simulation should be enough. If you also want to calculate drag I would set turbulent air. To validate your setup (model, boundary conditions,...) for turbulent flow there is a great NASA webpage.


Is it safe to fly if the flaps won't retract on ground?

The main thing to be concerned with, is that the spring tension is important for keeping the bicycle chain segment secure on the eccentric sprocket (item 17 below) that drives the torque tube.  If there are issues with spring tension in the return spring (item 32), I'd be concerned that slack could result in the chain potentially coming off, but that would require the tension spring on the bottom (item 22) to be not doing its job.
The top return spring should be a fair bit stronger than the tension spring, which is only there to keep the chain from going slack when the cable from the cockpit lever is slack.  There is likely a combination of a weakened return spring and some binding in the bushing blocks for the torque tube.
There isn't any risk of flap asymmetry unless the torque tube itself breaks or a control rod lets go, so a problem with the chain arrangement where it jams up or disconnects will, in a worst case scenario, just leave you stuck with flaps partially extended or at zero.
So what to do about it?  Depends.
I'm assuming they went almost fully up and you had to push them a bit to get them to move the last little bit.  If I knew the plane well, and if the panels came up almost all of the way and just needed a little nudge to overcome some friction, and they stayed up after the nudge, and every thing otherwise felt smooth and solid (when you do a walkaround, you shouldn't just be looking at stuff; you should be gently tugging and moving bits as part of your inspection to detect unusual play), it should be ok and air loads will do the job fine at retracting them as soon as you are moving with some speed, although I would snag it and expect it to be fixed the next time I saw it.
On the other hand, technically it shouldn't be doing that and if there is significant friction, or slop, or you are just uncomfortable with it, you should refuse it and shouldn't be charged for it.  A weak return spring and/or binding in the mechanism that prevents retraction on the ground is still a "defect" and needs to be rectified.
If you have access to the A&P that maintains the airplane it might be a good idea to mention it.  They may not be aware of the problem.


Does an heavy propeller aircraft (CS-25) have the Blu Line (Vyse) displayed on ASI?

No, larger aircraft typically don't have such speeds shown on the ASI because they vary significantly with weight. See also Why don't Transport Category aircraft have a listed Vy?.

For the Dash-8 Q400 for example, the $V_{\text{y}_\text{SE}}$ speed is called $V_\text{CLIMB}$:

  VCLIMB – Single-engine best rate-of-climb speed.

The following table shows the values for $V_\text{CLIMB}$ as a function of weight:

+-----------+--------+
|  Weight   | VCLIMB |
+-----------+--------+
| 18.000 kg | 130 kt |
| 20.000 kg | 131 kt |
| 22.000 kg | 137 kt |
| 24.000 kg | 143 kt |
| 26.000 kg | 148 kt |
| 28.000 kg | 154 kt |
| 29.000 kg | 158 kt |
+-----------+--------+

As you can see, the values vary too much to show a single blue line on the ASI.

This does not mean that the speed cannot be displayed at all. The Dash-8 Q400 has a Primary Flight Display (PFD) and can show up to 5 speed markers on the speed tape ($V_1$, $V_\text{R}$, $V_2$ and two more called #1 and #2, typically used for $V_\text{FRI}$ and $V_\text{CLIMB}$ respectively during takeoff and initial climb).

(Dash-8 Q400 FCOM - Indicating and Recording Systems)

The resulting markers on the PFD look like this:

(Dash-8 Q400 FCOM - Indicating and Recording Systems)

The electronic standby instrument does not show these markers.


There is no need to attach the wing to the rotor head - simply attach it to the top of the fuselage. You could run the rotor through a hollow tube and attach the wings to that - the inner uncovered section of the wing is an interesting idea.

If you are looking for VTOL, a tilt rotor with two electric motors driven by the gas turbine may be a better option.


CO-meter indicates 10ppm of Carbon Monoxide when I turn on Cabin heat - should I be concerned?

The short answer is you should be concerned.

Although 10PPM of carbon monoxide doesn't pose an immediate health risk for short term exposure the fact that the level is rising when you turn on the cabin heat & falling when you ventilate the cabin (by opening the door) indicates you may have an exhaust leak, and those tend to get worse over time.
You should have your shop open the heat muff & inspect the exhaust system for cracks/leaks before your next flight (or if that isn't possible avoid (or limit) use of cabin heat & defrost & monitor the CO meter as part of your scan until the exhaust system can be checked).

You should also check your CO meter: electronic CO meters have a finite life, just like chemical spot detectors (though they generally last several years) - your detector's manual will specify a replacement interval. As the sensors age they may give inaccurate readings.

A note about ground operations - Electronic CO detectors are sensitive enough to occasionally generate transient alarms during ground operations due to exhaust gasses entering the cabin. This can happen during summer or winter.
As long as the levels are low and the alarm clears when you're moving (taxi or during climb-out) this is probably not a problem, but you should know what is "normal" for your aircraft.


How do airlines ensure each passenger has a life jacket before a flight?

It's essentially a manual check for many airlines, performed as part of the pre-flight checks.

However, the airline I have worked for recently upgraded from manual checks to RFID tags on each lifejacket (a sticky tag kind of like this). The RFID tags can be read from the aisle by a handheld reader. So the person who does the checks simply walks down the aisle with the reader and is able to tell if a seat is missing a jacket and also if the jacket needs to be replaced due to the expiry date fast approaching.

In addition to a tag on the actual life jacket there are also security RFID tags across the life jacket holster under the seat. So the scanner is able to tell if the life jacket has been tampered with as the security tag would have been broken.

As for the frequency of checks, different regions have different requirements. Usually life jackets are only needed at all if flying over a large body of water. I believe the USA requires a check before any flight which passes over the USA (as well as water). Europe requires less frequent checks.


Where to get spatial data for ATC airspace?

Data about ATC sectors in easy to use format, such as shapefiles, is generally not publically available. Take it from someone who has spent hundreds of hours manually importing such data from PDF text files to various GIS applications. You can find a few individual data sources here and there via Google, but they are limited geographically and more often than not, outdated.

What you need to do is locate the aeronautical information publication (AIP) of your country of interest, and navigate to the En-route section. Airspace data is usually contained in section ENR 2.1. What you will find is a more or less well formatted PDF file containing textual airspace definitions (WGS-84 based coordinates). You will need to manually transfer this text data into which ever format you need, using your preferred text editor or spreadsheet application.


Normally, those masts are located on the upper fuselage and sometimes on the wings. They are used to string antennas between them and other airplane parts, or are antennas themselves.
In case of the Boulton-Paul Defiant, those antenna masts could not be located in the upper hemisphere in order to keep the field of fire of its gun turret free. Together with the Blackburn Roc and the Hawker Hotspur, those airplanes had a unique layout for a fighter with a single pilot and a gun turret placed behind the cockpit. How well this idea turned out in practice can be read in this answer.
In case of the Defiant, those antenna masts were made retractable to allow take-off and landing. The antennas were needed for communication and navigation.

The different antennas on a Grumman TBF Avenger (picture source).


I don't know if such a structure actually exists, but I find it difficult to see why anybody would build one.

Firstly, placing the horizontal stabilisers at the top of two separate tail fins will require that each fin is strengthened to support the extra weight and the aerodynamic loads,

Secondly, there will probably be any number of aerodynamic issues around the joints between the components, each of which will be doubled.

Thirdly, in a real aeroplane you will have introduced substantial complexity into the control systems that have to manage all of this.

There may be other issues, but I see very little benefit aerodynamically or structurally to such a layout.

A conventional V-tail involves some control complexity, but does away with the weight of the vertical stabiliser. A T-tail retains the vertical stabiliser, but avoids control complexity, aerodynamic issues that may occur close to the engines, and can use the extra length to improve the leverage available to the horizontal stabiliser.

All that said, aircraft design is about compromise and utility. There may exist some powerful reason for adopting a sub-optimal layout to fulfil some specific purpose. You can see such an example in the design of the AN-225 Mriya which was originally built to transport the Soviet Buran spacecraft. The vertical stabilisers were moved to the ends of the horizontal stabilisers to keep the clear of the wake from the spacecraft riding piggy back. The 747 used to transport the US Space Shuttle had a similar modification, but retained the original vertical stabiliser.

Soviet Buran spacecraft during transport:

US Space Shuttle Atlantis during transport (image source: Wikipedia/NASA):


How often is a passenger jet aircraft painted?

As always, it depends. From what I've seen and read, it's usually redone every 5-10 years. This can vary on various factors from paintshop availability, to new color scheme rollout, to deterioration of the previous one due to bad painting, to quality of the paint used. Sometimes, special liveries are just an aviation specific sticker that's put onto an aircraft, other times it's fully painted onto the aircraft, just depending on the airline, the planned length of time for it to be special, and any other considerations.


Why is an accident referred to by flight number instead of registration number of the airframe?

Most accidents are referred to by flight number

It depends who it is that is referring to the accident, and on who they are addressing.

News media and the general public

Newpapers and other media use flight numbers because that is the number known to friends and relatives of the individuals concerned and because most members of the public who have flown as passengers are very familiar with  the concept of flight numbers but not really familiar with aircraft registration numbers.

Investigators

Investigating authorities have their own numbering schemes for accidents they investigate.

Aircraft registration numbers change when an aircraft is sold to a purchaser based in another country.

The only number that stays with an aircraft for its entire life is the manufacturer's serial number.

All these numbers can appear in official accident investigations reports. The registration number is usually prominent.

AAIB

NTSB

BEA


How many minutes between jetway (air bridge) detachment and take-off?

while I don't think such a regulatory minimum would make sense, I'm still curious about whether it exists.

No, there is no such regulatory minimum.

On average, it's between 10 and 20 minutes from pushback to takeoff, although there are a couple of places I can think of (very short taxi from gate to runway) that I've seen it done in under five minutes.  That's moving pretty fast, though -- almost nonstop checklists, other than during the time the engines are starting.  But whatever you can do safely, is fine as far as regulations are concerned.  If you're ready to go, no regulation makes you wait.

Depending on the aircraft, there may be an engine warmup requirement or recommendation.  That's typically much more restrictive for the first flight of the day, and that would be a limiting factor in the very short taxi cases.


Is a firm landing an indication of the pilot inexperience?

First of all, the landing should be in the touchdown zone. Often I see pilots try to achieve a very smooth landing but floating far out of the touchdown zone before touching terra firma. Then they have to brake hard and taxi of at the far end. There are no points for landing outside the touchdown zone, even if it is the smoothest landing imaginable. In many overruns, landing late is a contributing factor.

A good landing is in the touchdown zone, firm enough to quickly spin up the wheels (and prevent wear by smearing rubber over hundreds of feet), and not so hard that it bounces up again. In calm winds, that can be very smooth, cross wind and wet / contaminated runways require a bit more of firm landing.

Hard landings1 can occur due to turbulence (wind shifts), upslope of the runway or when the air density above the runway is lower due to heating (typically in calm winds, with sun heating the runway surface)

So a hard landing is not necessarily an indication of pilot inexperience. However, some of the hardest landing I have ever experienced were when I joined in a training flight where new airline pilots were practicing landings in a Boeing 737 for the first time after their simulator training. I wouldn't be surprised if the runway was dented :-)

1Hard landing is used as a colloquial term here, not in the context of EASA CS 25 §473. See this answer for that context.


What operation is this aircraft performing?

The flight in question is most probably an evaluation/calibration flight for radio navigation systems at Ust-Kut. The aircraft is a Yak-40 special version designated for such tasks:
Aviamuseum - Yak-40 "Calibrator"


How does the location of a propeller affect an aircraft's stability?

There is a misconception that putting the prop at the back is destabilizing in yaw/pitch, like you were pushing a car as opposed to pulling it.  That only applies if the thrust element has a flexible connection to the body it's pushing/pulling.
When rigidly connected, the thrust axis is the same at any location along the plane's length and has no effect on stability.  All that matters is how far the thrust axis is from the center of mass where it passes by.  If the thrust axis is above the center of mass, power will make it pitch down whether the prop is forward or aft, and vice versa for a low thrust line.
Beyond that, the location of the engine/prop can have inertial effects related to the mass of the engine relative to center of mass, and gyroscopic propeller effects that vary because of the different location of the precession source, and the tail pusher can actually have enhanced stability due to various aerodynamic factors like an effective fin area increase due to the extra surface area of the engine/prop aft, but the actual placement of the thrust element on the longitudinal axis is not part of it.
Engine/propellers are usually on the front because it resolves so many design compromises much more readily.  Pushers are tail heavy when unloaded, which is a big problem (tail pusher jets have to have their main gears farther aft than optimal for takeoff and landing so they don't tip back when unloaded, which requires a larger horizontal tail to provide downforce for rotation).  The prop is subjected a lot more to junk thrown up by the wheels.  Aluminum propeller blades can suffer from corrosion from carbon in the exhaust. All sorts of little problems present themselves, that go away if the engine is at the front.


How does the relative wind on a propeller, strike the blade from behind (the non camber side)?

Plane moving forward plus blade moving down gives us a combined velocity vector, shown above in red.

Relative air(speed) is still coming from ahead—but this now no longer concerns the propeller as it has its own velocity vector.

Tilt your head (or check right image to avoid neck injury) and you'll see the blade's angle-of-attack. Remember, air (shown above in light blue) comes in from the direction opposite to the velocity vector.

Increase forward speed, velocity vector changes, angle-of-attack is now smaller, but the airfoil is also faster.


This is a broad question. The Wikipedia page on vehicle speed records gives a nice breakdown by various categories. Some criteria are manned vs. unmanned, air-breathing vs. rocket, civilian vs. military.

The fastest manned, air-breathing aircraft was the Lockheed SR-71 Blackbird with an (unclassified) record of 2,193.2 mph on July 28, 1976.

However, some would consider the rocket-powered North American X-15 the fastest manned aircraft at 4,510 mph.

If you consider the Space Shuttle an "aircraft" during its re-entry phase, it passed Entry Interface at around 17,500 mph. This was unpowered flight though, and its speed dropped off as it entered the atmosphere.

I believe the rocket-boosted HTV-2 Falcon currently qualifies as the fastest unmanned aircraft at 13,201 mph.

The record for the fastest human-made craft of any kind is currently held by the Helios probes, at 157,078 mph. The fastest manned human-made craft was the Apollo 10 spacecraft at 24,790 mph. (Note that Apollo 10 holds the record not because it was any more or less capable than any other Apollo mission, but that its chosen moon fly-by trajectory resulted in a higher top speed relative to Earth. It could also have achieved a much higher top speed on a different trajectory had that been a goal for mission planners, but it wasn't.)

Finally, every single one of these is either military or NASA (not civilian). The fastest civilian, air-breathing aircraft were the Tupolev Tu-144 supersonic transport at 1,518 mph, and its Western counterpart, the legendary Aérospatiale-BAC Concorde, at 1,488 mph. Both are now retired. Several private organizations are developing civilian suborbital and orbital rocket-powered spacecraft; I'm not sure which is currently fastest.

And though these aren't crafts or vehicles, honorable mentions go to a 150,000 mph steel plate launched by a nuclear weapon, and to particle accelerators for propelling subatomic particles and "man-made" particle collision byproducts at ridiculously close to (99.99something percent of) the speed of light :-)


How is an out-of-trim condition (in relation to weight distribution) detected on the ground?

The most likely scenario, where you have an unforeseen deviation from plan at the last minute, is pax loading where due to a statistical fluke there is an uneven distribution of pax in the cabin, say because a lot of them didn't show up and the no-shows tended to concentrate toward one end.

When the FAs inspect the cabin after boarding, they do a pax count and one thing they are looking for is a minimum number of pax in each cabin weight and balance zone.  If the FA sees something that is out of whack, like the front half of the cabin near empty with a crowd at the back, they will let the Capt know, and if necessary will start going around to pax to request them to change seats to address the under filled zones.

For cargo, the crew has to trust the people that loaded the cargo weight and balance wise.  For fuel, it's mostly about making sure the fuel load indicated in the cockpit matches the fuel load paperwork and the weight and balance load sheets and trip requirements.

In a line operation, the crew doesn't have to do any of the actual dirty work, they are just presented with the information prepared by specialists and review it to make sure it jives with expectations and requirements.


Is a releasable locator beacon (ELT) required for aircraft carrying passengers over a water area?

Is an ELT required on any type of aircraft carrying passengers over water?

Generally, commercial transport aircraft are not required to carry an ELT.

  how are aircraft planned to be located after an accident over the ocean?

I believe pilots are expected to use radio to inform controllers of any emergency.
If ATC are unable to contact an aircraft for an extended time, they instigate S&R based on the last known location of the aircraft.
The FDR and CVR are required to include an underwater location beacon (ULB), though this is only short range and has limited duration.
It is expected that, sometime in the future, commercial transport aircraft will be required to have automated systems that report their position frequently (perhaps every 15 minutes)

References:

IATA - Aircraft Tracking Task Force (ATTF)
ATTF FAQ
ICAO Normal Aircraft Tracking Implementation Initiative


What were those two military helicopters that I recorded?

It is hard to tell from such a low resolution picture. Due to the shape of the narrow body, they look like USMC Bell AH-1 SuperCobra. It is probably the AH-1Z Viper version based on the four blade rotor.


What is this star shaped symbol on the tarmac?

That is a Compass Rose, painted on a Compass Calibration Pad. It's used to mark a location on the airport surface that is suitable for calibrating the compass of an aircraft.

Here's one in a slightly different style:

and a more basic one from an FAA diagram:

More information on the requirements for the compass rose / compass pad can be found in AC 150/5300-13, in Appendix 6.


Was the 727 the last airliner whose doors could be opened in flight?

The door of an airplane can be opened in any of the aircraft only when the airplane is depressurized. When an airplane is pressurized, the extra pressure inside the hull of the plane forces the door outwards which in turn presses on a seal and hence, to open a door in such a configuration, where 6 p.s.i. (pounds per square inch) of pressure is acting against several square feet of the door, one would need a tremendous amount of force, and even if the door was to be opened electronically or hydraulically, one would still expect rapid decompression of the cabin after opening the door.

Hence it is categorically incorrect to claim that modern (new) airplanes can't open their doors mid flight. It is quite achievable, but after depressurizing the cabin. Here is a source for your further reading.

Edit

The following airplanes have Air-Stair installed. I'm not sure which of them has an over-ride to open this Air-Stair mid-air, but any airplane which does not have a cooper vane installed (Or any other external barrier) can open the Air-Stair midair (technically):

Lockheed L1011

Boeing VC 25

Ilyushin Il-86

DC-9/MD-80/MD-90/B-717

Boeing 737

Some modified Airbus and Boeing planes have Air-Stairs too.


How can some stretched derivative airplanes have longer range while most have less?

When aircraft are stretched, they usually just receive extra fuselage plugs on both sides of the wing: wing redesigns are incredibly expensive and take a long time. So the longer aircraft has a higher empty weight, and due to the unchanged wing design the fuel capacity remains the same - this reduces range. The aircraft is fuel limited. Conversely, if fuselage plugs are removed the same amount of fuel can take the aircraft further, as the B707-138 demonstrated, Qantas' first jetliner.

The A321NeoLR fixes the fuel limitation by adding extra fuel tanks in the cargo bays as this article explains, so that the aircraft now becomes weight limited. The extra fuel means that there can be less extra revenue from cargo. The B737-9 cannot follow suit, because it is has a shorter landing gear and has rotation limitations: from this article:

  737 MAX9 is not suitable for stretch to an international version, not because the wing is not good enough but because the MAX9 cannot bring the wing to an angle at take-off where it can work efficiently; the landing gear is too short.

The longer fuselage limits the rotation angle before a tailstrike occurs, so max. TO weight has a hard limit that cannot be managed by trading cargo weight for fuel weight. Above photo is from this site, which also contains a table with geometric angle limitations for the different 737 versions: the -9 has a tail strike pitch attitude of 10 deg, and has increased flap deflection of 25 deg to limit lift-off attitude to 6.8 degrees. A small variation in centre of gravity at a high TO weight can cause a tail strike.

Boeing has launched the B737-10 which will address the rotation limit by a redesigned landing gear.


How to convert a PPL (Gyrocopter) to a PPL (Helicopter)?

In the US, Gyroplane and Helicopter are class ratings within the Rotorcraft category. FAR 61.63(c) specifies that the addition of a class rating does not require any specific amount of flight training time, and no more total experience than would be required for the gyroplane rating you already have, so the question is moot. All you need is an instructor willing to sign you off and to pass the abbreviated practical test.


What's the relation between cabin pressure and altitude?

Here is a plot of cabin pressure on one flight. You can see a small increase in pressure right before takeoff. The decrease and increase in pressure is fairly constant during climb and descent. The pressure will change based on the aircraft's altitude, and will not reach a minimum limit until closer to the aircraft's service ceiling. This keeps pressure changes as slow as possible while giving a lower cabin altitude when below the service ceiling.

Source


Why don't aircraft fly direct via oceanic airspace from UK to Iceland?

I don't think they are specifically avoiding Shanwick Oceanic airspace. The reason seems to be related to the airways in the Scottish airspace. In general, you have to file a route using airways when operating in this airspace and there simply isn't one available that provides a more direct route. I recreated what looks like the route you show on skyvector.com:

This route would use UL612 between DCS (Dean Cross) and STN (Stornoway) and then continues via the N610 to RATSU, which marks the border between Scottish, Shanwick Oceanic and Reykjavik airspace. There are no airways from RATSU in the Reykjavik airspace, so they probably filed a direct route from there. The total length of this route would 1049 NM.

You can see alternative airways to use in this zoom (or go to skyvector.com, select "World Hi" for high altitude airways and zoom in):

There are two waypoints South of RATSU where one could enter Shanwick Oceanic airspace: ATSIX and BALIX. The shortest routes I could find with skyvector were 1042 NM and 1035 NM long respectively. This is basically the same distance and the optimal route would likely depend on the current winds.


How does a PFD handle unphysically abrupt changes in the parameters it's displaying?

Generally, PFD will smoothen the indication for most of the indicated data.

I don't know any 'legal' requirements for it, but as you mention, basic ergonomics require it.

I have first hand experience with Garmin G1000, so I'll try to explain its PFD behaviour as an example. (It's a bit hard to capture and post a video).

Most (if not all) 'graphical' indications are filtered with the rate (characteristic frequency) consistent with the expected rate of change of the relevant parameter: pitch/roll quickly (but still noticeably, about 0.3 s), airspeed moderately, engine temperature slowly (3-5 s). Digital only indications (say, battery voltage) are also filtered, and in addition their precision is limited such that the updates didn't happen too frequently (e.g. round 0.5 V).

The filter seems to be a second-order low-pass in most cases. For some parameters, notably the indicated airspeed, it is underdamped, which means the indication overshoots slightly for abrupt changes, and generally tends to oscillate to and fro for normal changes. This provides faster response while still being smooth.

An abrupt, unrealistic change of parameters per se doesn't cause a fault for PFD. It's just filtered as usual. However, significantly out-of-range values will fail the indication, but only temporarily, until sensible values resume. Generally, it's the job of the peripheral computers (ADC, AHRS, etc.), as well as of the integration unit (GIA), to detect faults and convey them in special messages. PFD is, after all, just a display.


Why do gases in the combustion chamber only flow one direction to the gas turbine in a jet engine?

It is actually not that simple to ensure the proper gas speed in a gas turbine. In the compressor you want to limit the flow speed over the compressor vanes to the high subsonic range, so the inlet has to decelerate the flow down to approx. Mach 0.4 - 0.5. Less would mean less throughput and consequently less thrust.

This speed, however, is far too high for ignition. The fuel needs some time to mix with the compressed air, and if flow speed is high, your combustion chamber becomes very long and the engine becomes heavier than necessary. Therefore, the cross section leading from the compressor to the combustion chamber is carefully widened to slow down the airflow without separation (see the section below named "diffusor"). Around the fuel injectors you will find the lowest gas speed in the whole engine. Now the combustion heats the gas up, and makes it expand. The highest pressure in the whole engine is right at the last compressor stage - from there on pressure only drops the farther you progress. This ensures that no backflow into the compressor is possible. However, when the compressor stalls (this is quite like a wing stalling - the compressor vanes are little wings and have the same limitations), it cannot maintain the high pressure and you get reverse flow. This is called a surge.

The graph below shows typical values of flow speed, temperature and pressure in a jet engine. Getting these right is the task of the engine designer.

Plot of engine flow parameters over the length of a turbojet (picture taken from this publication)

The rear part of the engine must block the flow of the expanding gas less than the forward part to make sure it continues to flow in the right direction. By keeping the cross section of the combustor constant, the engine designer ensures that the expanding gas will accelerate, converting thermal energy to kinetic energy, without losing its pressure (the small pressure drop in the combustor is caused by friction and the Rayleigh effect). Now the accelerated flow hits the turbine, and the pressure of the gas drops in each of its stages, which again makes sure that no backflow occurs. The turbine has to take as much energy from the flow as needed to run the compressor and the attached pumps and generators without blocking the flow too much.

The remaining pressure is again converted to speed in the nozzle. Now the gas is still much hotter than ambient air, and even though the flow at the end of the nozzle is still subsonic in modern airliner engines, the actual flow speed is much higher than the flight speed. The speed difference between flight speed and the exit speed of the gas in the nozzle is what produces thrust.

Fighter engines usually have supersonic flow at the end of the nozzle, which requires careful shaping and adjustment of the nozzle contour. Read all about it here.


Why does my wind tunnel experiment give two different stall angles of attack?

This is called stall hysteresis. You have two different situations and the flow reacts differently in each of them.
When increasing the AOA
The flow is attached to the wing and the boundary layer is resisting the adverse pressure gradient as much as possible. At some point the flow detach from your profile and you have stalled let's say at 18°. At this point a huge recirculation bubble appears on the succion side of the airfoil.
Decreasing AOA / recovery from the stall
This recirculation bubble represents an area where the average flow speed is close to 0. Looking from the outside, for the free stream flow coming at your wing it looks like you have a new airfoil profile which is made from the old airfoil and the recirculation bubble. Most of the time this recirculation zone extend far beyond the trailing edge of the actual profile, reducing thus the overall aspect ratio (thickness/chord) of the profile. Slimmer profiles have less tolerance to stall and lower stall AOA thus explaining why your recovery only comes when the AOA goes below 13°.
Once the flow is reattached, you are back to the first configuration and you can go back up to the initial stall angle 18°.
Your stall angle is thus 18° but if you are in an airplane it means you'll have to lower the nose below 13° of AOA to recover.
There's an excelent thesis on this subject here from which this picture is taken showing the hysteresis phenomenon.


What is this propeller found underwater in the English Channel?

My best guess is that propeller is related to the 19 June 1954 – Swissair Convair CV-240 flight which was ditched in the English Channel near Folkestone due to fuel exhaustion. As @AEhere mentioned earlier it looks like the propeller wasn't under power.
the propeller shape look similar but still the size of the propeller being provided in the photo relatively small compared to the actual aircraft propeller.

Sources :

Wikipedia:https://en.wikipedia.org/wiki/List_of_accidents_and_incidents_involving_airliners_in_the_United_Kingdom

http://www.sr692.com/fleet/15_cv240/index.html


Why isn't the arresting hook always used on land?

There is a lot of stress placed on the airframe when you use the tailhoook. You can see the skin buckling in the picture below behind the landing gear. I'll admit, you probably wouldn't land intentionally with this sort of technique on land but if you catch the wire during the flare it could probably pull you down pretty hard. For these types of landing and it would be difficult to get it right every time.
(Like  I wrote in the comments, I do realize that the aircraft below has not yet caught the cable. When you flare, and you might catch the cable mid-air if you have a nose-high attitude, and slam down hard as it pulls you backwards and you loose lift, or you do a non-flare like carrier pilots, also very hard. IMO I think either would end up like the landing below.)

You have no ability to control the aircraft and you end up stopping and rolling backwards on the runway. You can only be one aircraft at a time and you cannot roll off straight away, wasting time.

You'd have to have an arresting gear system and along with that more staff and maintenance.

At least on aircraft carriers, I believe the arresting cable has a limit of only 250 catches for safety reasons. Even if it the limit was greater on land, you'd have to have people changing the cables on a regular basis.

You'd still have to have conventional brakes which you could just as well use.


As far as your question on sectional lift given a wing geometry and total lift coefficient is concerned, there is no one single formula that estimates this. The sectional lift is affected by downwash and spanwise flow, which are in turn dictated by the wing geometry and airfoil themselves.

For straight tapered wing, you can use the lifting-line, which computes the downwash angle at the discrete spanwise locations, which you can use to easily back-out local $C_l$. For swept wing, your easiest solution is to use a vortex-lattice method, such as AVL. AVL directly outputs the section lift coefficient at the corresponding control points.

As far as selecting/designing an airfoil is concerned, it's much more involved than just looking at the cruise lift coefficient. You should consider:

Takeoff and landing requirements. What kind of performance do you need? What kind of trailing-edge and/or leading-edge devices can you fit with the airfoil?
Operating conditions. Is it a point-design? Or does it need to cater for large range of operating conditions?
Structural concerns. Can you actually manufacture a super thin trailing-edge, even if it gives the best stall characteristics? Can you actually fit a spar tall enough (and at the right location), even if it gives the best drag characteristics?
Space concerns. Can you actually fit fuel tanks in the wing if it's super thin?


Do thrust reversers only have idle and full modes or are there intermediate settings?

My experience is on the CRJs but most airliners are similar.  Normally you can work reverse thrust variably between idle and max reverse.  The reverser levers will have a stowed and locked position (all the way down), and there will be a latching device you operate with your fingers to pull them up from stowed.

From there you end up at an idle reverse position (which may have a physical detent you can feel), and there is a range of movement above idle reverse going up to maximum reverse.  You can vary the reverse thrust between idle and max as you wish, although most pilots will just go from stowed to idle to max, then back to idle then stowed on a landing.

One technique issue on using reversers is if you slam the reverser levers back down to fully stowed really quickly, the reverser devices on the engine stow while the fan is still running at high rpm and on some engines you will get a little shot of forward thrust at the end of the landing roll.  It feels like you let off the brakes at bit for a couple seconds.  Once you get a bit of experience, you learn to ease them off to idle reverse and wait at idle a few seconds before stowing.

Reverse thrust is not taken credit for in landing distance calculations, so their use is optional by the landing pilot.  Some airlines may prefer a bit faster brake and tire wear to wear and tear on the reverser system, based on a relative cost analysis, and encourage crews to avoid reverser use unless they need them.

Some pilots will plan not to use reverse but will want it ready just in case, so they will unstow on landing but keep the reverser levers at idle.  This practice is frowned upon by the maintenance types because it counts as an operating cycle of the reverser system (impacting long term maintenance) even though reverse wasn't used to help slow down.


Why would a Blue Angels pilot leave their gear down?

What possible reasons would justify leaving the aircraft in this state?

Because it's part of their show routine:

  Solos Take-off (Blue Angel #5: Dirty Roll on Take-Off; Blue Angel #6: Low Transition to High Performance Climb).

This dirty roll is not easy (it killed a Blue Angel back in 1982 in a Skyhawk during winter training), so it shows off skill and plane capability.

And later four planes fly gear down:

  Diamond Dirty Loop: The diamond flies a loop with all 4 jets in the carrier landing configuration.

The dirty loop formation started back in 1986 to celebrate the 40th anniversary and the new F/A-18:

  On 8 November 1986, the Blue Angels completed their 40th anniversary year during ceremonies unveiling their present aircraft, the McDonnell Douglas F/A-18 Hornet. The power and aerodynamics of the Hornet allows them to perform a slow, high angle of attack "tail sitting" maneuver, and to fly a "dirty" (landing gear down) formation loop.

Dirty is plane-speak for gear and/or flaps out, as opposed to a clean configuration.

Source: pinterest.com


What profile will a Boeing 737 NG follow after pushing the ALT INTV button?

What happens when pushing the ALT INTV button depends on the currently selected mode. You can find a full description from the FCOM in this answer.

VNAV Climb: The aircraft will remain in VNAV Climb mode and will follow the profile as programmed in the CLB page on the CDU.
Cruise: The aircraft will start an early descent:

  Early Descent

  To start an early descent, use DES NOW prompt on the DES page. An early descent can also be started by pushing the altitude intervention switch.

  (1) DES NOW (VNAV PTH)

  With a VNAV path descent planned, VNAV starts an early descent at 1000 fpm
  and captures the idle descent path. VNAV uses FMC SPD for the autothrottle
  mode and VNAV PTH for the pitch mode.

  (2) DES NOW (VNAV SPD)

  With a VNAV speed descent planned, VNAV starts an idle thrust early descent.
  VNAV does not attempt to capture the VNAV descent path. VNAV uses VNAV
  SPD for the pitch mode and the autothrottle commands IDLE, followed by ARM.

(Boeing 737 NG FCOM 11.31.36 - Flight Management, Navigation - Flight Management System Operation)

So in this case it depends on the VNAV descent that is planned in the FMC. If you have an altitude constraint selected, the aircraft will start a descent at 1000 feet per minute and intercept the idle descent path. Otherwise it will immediately start the speed descent as programmed in the DES page of the CDU.
VNAV Descent: The aircraft will remain in VNAV Descent mode, but the idle descent path can change due to the deletion of an altitude constraint. Should this result in the aircraft no longer being on the idle descent path, the mode will change from VNAV PATH to VNAV SPD and the aircraft follows the speed programmed in the DES page of the CDU. The same thing happens when all altitude constraints are removed.
Any other mode: Nothing happens. The ALT INTV button has no effect on other modes like V/S for example.


What is the best VFR route to CYKZ (ButtonVille) when approaching from the southeast?

I fly out of CYTZ and know the area fairly well. A few things to keep in mind. If you'd like to transition through CYTZ airspace to Buttonville, you'll find controllers there pretty accommodating. Getting in touch with Toronto Terminal is not a problem, you're not asking to cut through Pearson, but you'll be sticking along the shoreline so I wouldn't worry too much.

I would recommend against flying at 1700 feet. That's awfully low. Here's what I would recommend.

Get a VTA sectional for Toronto. This will have much greater detail for the area as well as recommended VFR routes.

Hug the coastline at 2500'. Contact Toronto Approach at 133.4 before entering the zone. Ask for clearance through the zone to Buttonville via the shoreline and city centre. Follow the coast line until you're about 10 miles outside of City Centre (YTZ). There Approach will transition you to Tower if you're at 2500 or below. They will clear you to Buttonville by staying east of the DVP/404. That's the major highway cutting N/S through downtown, east of the airport. Once you're clear of City Centre, you'll get passed over to Buttonville for your landing. As long as you follow DVP/404, you'll find the airport easily. I believe there's a landing fee as well.


What are these devices on or adjacent to an airport taxi-way?

It is not a road that you are looking at, it is the edge of the runway / taxiway.

The double yellow lines are used to define the taxiway
edge from the shoulder or some other abutting paved
surface not intended for use by aircraft.

The blue lights in the yellow boxes are taxiway edge lights.

The poles next to the lights are blue reflective markers that will mark the taxiway in case the light fails. Systems for light control often work for groups of lights (e.g. sections of taxiways) or, if individually controlled, through a control signal on the power line powering the light. In the latter case, there is a small piece of electronics in each light housing controlling the intensity of that individual light and reporting back if the light fails.

The yellow objects in the blue boxes are signs. They indicate names of taxiways, runways, holding points etc. You see the backside of light casings, not the signs themselves. A bit further to the right you can see similar signs.

I am not sure what the item in the red box is.


Is c.g rear or fore of wing leading edge?

For longitudinal stability in a conventional wing-and-tail layout, the CG needs to be ahead of the wing's AC.  This causes a nose-down moment that is countered by downward lift at the stabilizer, so that the stabilizer will raise the nose as speed builds up.  Thus, if the aircraft is disturbed into a dive, the nose will automatically rise as speed builds; conversely, if disturbed nose up, as speed falls off the nose will tend to drop.

Aircraft with active control (especially computer control), as well as some alternative layouts (such as a classic canard) can alter this conventional stability setup -- for instance, in a canard, the stabilizer at the nose lifts, and the CG will be ahead of the leading edge of the wing in most cases.


Autorotation in helicopters is rarely ideal, with net rotor torque = 0, because some energy is drained from the turning blades to feed the tail rotor.
There's, however, a continuum between the ideal 'autorotation' and the 'windmill brake state/moulinet frein'. The 'point of ideal autorotation' should be taken as the 'point of minimum'. For any increase of the load (the weight of the gyro, for instance), from that point on, there's a multitude of possible equilibrium points in autorotation along that curve of 'windmill brake state'. Real-word autorotation is always in one of those points, and never at the 'point of ideal autorotation'...  You may find useful this illustration and text. Source included.


Why does exhaust flow inwards through a radial turbocharger?

This has to fight against centrifugal force instead of leveraging it.

In fact, the opposite is true. The arrangement shown actually helps to extract energy, perhaps counterintuitively. A typical example of this arrangement is the Francis turbine.
A high flow speed plus high tangential velocity input, fed through to a low flow speed/low tangential velocity exit, will create the maximum energy differential for extraction. And that describes inwards flow.
There are a few ways of looking at this. One is to look at the exit velocity of the fluid. In a compressor, it's okay if the fluid has some leftover velocity as it will by converted to static pressure when slowed down by Bernoulli's law. For a turbine, the exit velocity should be as low as possible, as kinetic energy of the exhaust flow is 'wasted' energy. So it makes sense to place the exit at the inner radius of the turbine, where the radial velocity component is lowest.
Another way of looking at it is pressure. Centrifugal force creates a pressure gradient across the turbine in radial direction. As a 'packet' of air moves inwards, its pressure becomes lower. By conservation of energy, this must mean that it is expending work on its surroundings. The turbine extracts this work done from the packet of air moving inside.
Another intuitive explanation involves an ice skater retracting their arms in a pirouette (or a bored office worker on a swivelling chair). By conservation of angular momentum, their angular velocity increases. A packet of air would 'like' to increase its angular velocity, but this is kept fixed by the turbine rotating at a fixed velocity. The packet of air must thus be slowed down moving inwards, and as a result exerts a pressure on the turbine blade in front of it.
The choice between axial, centrifugal or mixed-flow is done based on the flow conditions, with high-flow low pressure differential favouring axial design and lower flow or higher pressure differential favouring the radial setup as shown. A more compact design means fewer turbine or compressor stages, which results in a higher pressure differential across the single stage. Thus, a radial setup is preferred.


Could bypass air be used to cool a rocket engine like in turbofans?

Air cooling is not going to contribute much here.

The Shuttle engines were cooled by liquid hydrogen (-253°C) from the fuel supply being pumped up cooling ducts inside the nozzle. Typical nozzle temperature in operation was around 54°C. Yes, fifty-four Celsius. (BBC Engineering Connections - see this Youtube clip)

The top speed of the SR-71 (about Mach 3.2) was limited by the temperature of the air arriving at the engines which, after being compressed in the inlet ducts, reached temperatures around 400°C. (Wikipedia: SR-71)

The shuttle passes Mach 4 about 2 minutes after launch.


It means that both Flight Directors are engaged:

Flight Director (FD) Engagement
The FDs are engaged automatically whenever the FMGC powers up.
Ground Engagement

The symbol "1FD2" appears on both PFDs.
No FD bars appear on the PFDs. The PFD displays FD orders when a mode is active on the corresponding axis.
The FCU windows display dashes.

(A320 FCOM Autoflight - Flight Guidance)
It will look something like this:

The pilots can disengage one or both FDs. In this case -FD2, 1FD- or nothing will be displayed.

Manual Flight Director Disengagement
One FD off:

The FD bars no longer appear on the associated PFD.
The corresponding FD is disengaged.

Both FDs off:

The FD bars disappear on both PFDs.
If no AP was engaged, lateral and vertical modes disengage. The A/THR, if active, automatically reverts to (or remains in) SPEED/MACH mode.
If one AP was engaged when the FDs are switched OFF, this AP remains engaged in the active modes but the FDs are no longer displayed.

If one FD fails, the other one will be used to provide guidance on both PFDs. In this case 1FD1 or 2FD2 will be displayed.

Automatic Flight Director Disengagement
If one FD fails or one FMGC is not valid, both PFDs display the remaining FD.


What is the difference between Tilt-rotor aircraft and Tilt-wing aircraft?

I would say that the complexities of tilting the entire wing are what compelled Bell to choose just the nacelles.

By complexities, I am thinking 2 major systems. The first is the flights controls. Based on the how old that image looks, I suspect that aircraft want designed with a fly-by-wire flight controls system if it was even an option. That would be, in my head, the only simple way of managing the flight control sister's. Otherwise the designers would need to get VERY creative with the pushrods or cables & pulley systems. That would be an engineering nightmare!

The other system would be fuel. Engineers are going want to put fuel in the wings, as opposed to under the floor in the fuselage. That way cargo space is not used, bit more importantly, that much weight centered on the shortest works reduce the ability of the aircraft top perform missions for to the Zero Fuel Weight considerations.

Again a tilting wing creates some engineering challenges they may just want to avoid. The wing tanks would shift weight as they tilted back, and they would need to ensure fuel could still get to the engines. That's not even the hard part either. Think about trying to run the cross-feed lines through a rotating wing root! No thanks!!

Hope that helps, that's all I've got!!


What type of computers are in a flight system?

Historically, avionics manufacturers have used off-the-shelf processors on custom circuit cards.  Processor selection has been based on getting a reliable processor (established history) at a reasonable price and an expectation that it will be available for a long time.  The high turnover (obsolescence) in processor design along with a lack of design integrity in commercial products is causing avionics companies to move towards custom chips.  Though some products meet the integrity by using two dissimilar processors (though it doesn't address the obsolescence issue).  I am aware of a number of older designs using 486 and PowerPC processors.

Operating systems (just like all the a/c software) have to comply with the appropriate level of DO-178 (currently rev C).  In practical terms, that means the OS has to meet DO-178 Level A.  There are a number of commercial solutions available.  LynxOS-178 is one.  Integrity-178 is available from Green Hills.  Wind River also has a product available.


There's no mention that I can find of "landing long" ("landing deep" is sometimes used outside the US) in the FAA's P/CG, AIM or ATC procedures. So it's fairly safe to say that it's an unofficial instruction (assuming I didn't miss it somewhere).

The main reason for doing it is to avoid a long taxi after landing, but I suppose there could be other reasons too, like coming in too high on a long runway or avoiding wake turbulence. The point of requesting it - and not just doing it - is that it may mess up sequencing at a busy airport because you take more time to clear the runway, with the result that the tower has to instruct the aircraft following you to go around. As my first instructor said, "once you're cleared to land it's your runway".

I've never been asked by ATC to do it but I have been asked by the tower to "land short", to minimize my time on the runway and let an airline flight take off ASAP. However, it was clearly phrased as an informal request, not an instruction; something like this:

  N12345 Clear to land 36L, appreciate a short landing if you can manage it

So I think the conclusion is that landing "short" or "long" is simply an informal way for pilots and ATC to make things work a bit more smoothly.


What were the safety recommendations following the Aeroperú Flight 603 crash?

who was responsible for ATC relaying false altitude information to the pilots

Not an easy question mostly because it asks (even if not directly) to put the blame on someone. And although there is not an explicit rule for not doing this, I prefer avoiding it in respect to the dead and the ATCOs doing their job as good as they can.

I will focus on the reason behind the "false reports" and not the responsibilities. The RADAR system the ATCOs use relies on aircraft systems for two things: one is to identify the flight (Mode A) and the other is to get its altitude in hundreds of feet (Mode C).

Now if you recall what happened, the static ports were duct taped for some routine cleaning of the fuselage and were not removed. This affected the airplane's altitude indicator and consequently the altitude transmitted by Mode C.

So the ATCO was reporting back to the plane the same erroneous altitude the pilots were seeing in the cockpit.

  has anything changed since then to prevent that from happening again?

I don't know if there was any action taken after the accident but I think that there wasn't anyone needed: there is regulation (need to make some research to find it) which specifies that all "foreign" parts attached to a plane for protecting its vital parts should have a prominent red "REMOVE BEFORE FLIGHT" ribbon.

The duct tape was of silver non reflective color (if I recall correctly) and had no ribbons. When the crew inspected the plane at night with flashlights they couldn't spot them.


How many wing attachment nuts are there on a DC-3/C-47?

Information on PPRUNE (without a source) says:

  328 per side (102 x AN4-7A & 226 x AN410A bolts to be exact).

I was able to verify the 328 figure from various books.

  Each wing of a DC-3 transport plane is fastened on with 328 nuts and bolts. Unless the stress and strain are distributed equally, some of the bolts shear off. The answer was found in Elastic Stop Nuts. These nuts can be given precisely the right tension—then lock fast. This is one of the important structural fastening jobs which Elastic Stop Nuts have solved.

(American Aviation - Volume 7, 1943, p.82)

I tried to find more info, to no avail.


When do the LNAV and VNAV NPS pointers appear in the PFD of a B737NG?

The lateral NPS will be present if there is a defined LNAV track active, and the APPROACH mode (i.e. to track an ILS) is not active. The vertical NPS is inhibited until the FMC-computed Top-Of-Descent point, so it won't be present during an intermediate level-off during an RNAV departure. It's also inhibited after GS Capture, since the ILS glideslope is then primary & having the NPS would be distracting & unhelpful.


Is using flaps the same as increasing the AOA?

The ultimate goal is to reduce the landing speed and takeoff run. Not to get the higher angle of attack. The speed without flaps would not be low enough even at higher critical AoA. The key is to generate the most lift as possible, with all means available.


As noted in the linked question to the question you linked to (this one), runways are almost always bidirectional. In your example, the other runway would be simply numbered 18 higher or lower, for 180 degrees off. Thus, the runways would be 26R and 26L - the "L" and "R" would swap, because you are pointed at them the opposite way. You can see this in the diagram of LAX below (src):

Yes, generally they flip based on prevailing winds, though I can't imagine they flip based on the "whims" of ATC often. Most airports will have a "calm wind runway", which is the preferred runway for operations in calm or near-calm winds.

Due to terrain or similar factors, some runways may be only usable one direction, but both directions will still be numbered, as you will takeoff and land pointing different directions (for example, Lukla airport in Nepal). Alternatively, as Jan Hudec noted, some will operate one-way, such as TNCM, though again, both runways are numbered. Occasionally, the opposite direction is not numbered, though this is rare.


Can all airborne aircraft be stored at their origin or destination airport?

Short answer: Yes.

You need to distinguish between short-haul and long-haul fleets.

For example, when it is 4 a.m. in Central Europe, almost all the aircraft which fly short-haul routes throughout Europe during the day will be on the ground somewhere in Europe simultaneously and there must, therefore, be ample parking for all those aircraft (though not necessarily at gates). Similarly for other parts of the world: when it is 4 a.m. in the central U.S., all the short-haulers will be parked-up somewhere in the U.S.; when it is 4 a.m. in South East Asia, all the short-haulers will be parked-up somewhere in South East Asia, and so on. There must, therefore, be ample parking for all short-haulers globally.

For long-haul fleets, it is a different matter. Such aircraft spend up to 16 hours a day in the air, and therefore well over half the aircraft will usually be airborne at any one time, round the clock. It is highly abnormal to have all long-haulers on the ground simultaneously. If all those long-haulers need to suddenly be grounded, it can cause serious problems, as happened after 9/11 when the entire transatlantic and transpacific fleets suddenly stopped flying. However, even then, the airport operators did manage to find sufficient parking.


What are these small propellers in the A330 MRTT's wings?

Those two locations you circled out are the under-wing refuelling pods, mounted on pylons (where, I suspect, the A340 outboard engines would be).

You can find a picture of them on the A330 MRTT Wikipedia entry, under the Design section.

And those propellers on the pods are likely the ram air turbines that power the refuelling pods, according to the manufacturer:

  Wing-mounted aerial refuelling pods feature a drogue refuelling system and an integral fuel boost pump which provides a fuel transfer capability up to 450USGPM (1703 Litres per minute) at 45 to 55 PSI (310 to 379 kPa). Pods operate with aircraft power or are self-powered by a Ram Air Turbine.


Exemption 3585 is an exemption to 14 CFR 121 dispatch requirements for weather originally requested by Peoples Express.  Lots of airlines have requested similar approval, and here is an example on regulations.gov for Air Transportation International.

The exemption that they asked and were approved for was:

  ...an exemption from §§ 121.613, 121.619(a), and 121.625 of Title
  14, Code of Federal Regulations (14 CFR) to the extent necessary to
  dispatch airplanes under instrument flight rules when conditional
  language in a one-time increment of the weather forecast states
  that the weather at the destination, alternate airport, or both
  airports could be below the authorized weather minimums when other
  time increments of the weather forecast state the weather conditions
  will be at or above the authorized weather minimums.

In other words, if there is a conditional or temporary weather condition that is below minimums at the destination or the alternate airport at the flight's ETA1,2, where the flight could not normally be released, they can depart under the 3585 exemption as long as a second alternate that is above required minimums3 is filed in the flight plan and enough fuel is carried to reach the furthest alternate plus the required 45 minute reserve.  There are a few other requirements too (training, manual and OpSpec revisions, etc.) but you can take a look at the letter to see them.

The exemption must be requested of and approved by the FAA for each air carrier that wants to use it.

1 The forecast weather conditions at the destination airport must not be less
than one-half of the lowest weather minimum visibility value established
for the instrument approach procedure expected to be used at the
destination

2 The forecast weather conditions at the first alternate airport must not be less than one-half of the alternate weather minimum ceiling and visibility values
specified in ATI’s operations specifications for that airport

3 With respect to the second alternate airport listed in the flight release
pursuant to this exemption, the METAR, the appropriate time increment of
the TAF, or any combination thereof for that airport must indicate in the
main body and remarks section of the METAR or TAF that the forecast
weather conditions will be at or above the alternate airport weather
minimum ceiling and visibility values specified in ATI’s operations
specifications for that airport


Why are these rivets on the 777 so visible?

At the risk of sounding ridiculous, the purpose of the rivets is to hold the plane together. More specifically, they hold the outer aluminum skin to the structural skeleton of the aircraft:

(That skeleton happens to be made of manilla folders, but the real one is pretty much identical except that it's bigger and made of metal.)

There are also additional rivets around things like the AOA vane, holding doublers in place to stiffen the skin where these instruments penetrate and create additional load.
The radome has no rivets as it is a single molded piece of plastic. It does however have structural stiffeners, which you can see running fore-to-aft in the photo.

You can see the rivets clearly on this particular aircraft because of the paint scheme (all white) and lighting, however if you look closely you can spot them on any 777 nose.
Here's one from an aircraft during assembly, and as the interior has not been installed you can also see the skeleton through the windscreen openings:


Does the FAA generally consider a door opening in an unpressurized cabin during flight to be an emergency?

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

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


Why do older students take longer to learn to fly?

This question is more related to human aging and learning than it is to aviation. When people age, there is a toll on several parts of body and the acquired skills may not stay as sharp as they used to be.

As we age, learning new things become harder, not because we are getting stupid, but because learning new things is intrinsically hard. Almost all people get into a profession in their early 20s, but it becomes harder to change their career later in life. The reason is that they have more things on their mind to worry about as opposed to the early phases of life.

One does not stay young forever. And you cannot point out one or two or ten things which makes learning new things hard. It is a combination of many factors. That being said, becoming a pilot in your 50s or 60s is harder than in your 20s or 30s, but there are many old pilots who can do and have done things which younger ones cannot do. Example: Captain Sully

You can ask why most athletes are in their 20s, not in their 60s, and will get a similar answer.


Under what circumstances does the F-16 and possibly similar fighters deploy leading edge slats?

Technically, F-16 and "similar fighters" have leading edge flaps (LEF) (or droop flaps), rather than slats. The difference is that they don't form a gap between themselves and the main surface when deflected. This makes them usable at high speeds, whereas slats are typically used for low-speed takeoff/landing.
For fighters, the most important use for them is to extend the range of usable angles of attack (AoA), which is important for manoeuvring as well as for takeoff/landing.
For F-16, @Mr R posted some details. I have some info for Soviet fighters of the same generation.
MiG-29
LEFs have two positions, up and down (20°). They are deployed:

When landing gear is down (including takeoff and landing).
Automatically in flight when AoA > 9° and M < 0.8 (Mach number).
On manual override by the pilot.

Su-27
LEF on Su-27 don't have fixed positions and constantly track AoA in flight. Maximum deflection 30°.
They are automatically deployed to 3/4 of travel when landing gear is down for takeoff or landing.
The pilot has a manual override (down-auto-up selector).
For both aircraft, if LEF fail, the allowed AoA is limited to 10°, which is roughly half the normal (depending on speed).


How do flaps help an aircraft take off at a lower speed, yet cause drag at the same time?

Increasing the flaps does increase the drag, but not by that much initially. For the first stages of flaps you gain more by reducing required takeoff speed. If you would increase the flaps more and more, eventually the drag would become too much and you would lose takeoff distance again.

Flap setting has an affect on the wing’s lift coefficient and on the aerodynamic drag. Increasing flap angle increases the lift coefficient, and therefore reduces stalling speed and the required takeoff speed (the same lift will be created at smaller air speed due to greater lift coefficient). This reduces the takeoff distance. In the same time increased flap angle increases drag, reduces acceleration, and increases the takeoff distance.
The net effect is that takeoff distance will decrease with increase of flap angle initially, but above a certain flap angle the takeoff distance will increase again. An optimum takeoff setting can be determined for each type of aircraft and any deviation from this setting will give an increase in the takeoff distance.
The flap setting also affects the climb gradient. Increasing the flap angle increases the drag, and so reduces the climb gradient for a given aircraft mass. If there are obstacles to be considered in the takeoff flight path, the flap setting that gives the shortest takeoff distance may not give the required climb gradient for obstacle clearance.

(skybrary.aero)
Note that the climb gradient is also affected:

More flaps allows taking off from shorter runways, but reduces obstacle clearance capabilities.