What materials are used for the gas bags in hydrogen airships?

Have you ever heard of goldbeater's skin? This was the preferred material before synthetic materials became available.

What is it? Wikipedia says:

  Goldbeater's skin—the outer membrane of a calf's intestine—is a
  parchment traditionally used in the process of making gold leaf by
  beating, reducing gold into mere 1 μm-thick leaves. […] To manufacture goldbeater's skin, the gut of oxen (or other cattle) is soaked in a dilute solution of potassium hydroxide, washed, stretched, beaten flat and thin, and treated chemically to prevent putrefaction.

From the 1880's on it became the preferred material for gas bags, displacing rubberized cloth or silk which would become brittle quickly. The first man-carrying hydrogen ballon used already silk which had been rubberized by varnishing it with rubber that had been dissolved in turpentine. Again Wikipedia:

  Large quantities of goldbeater's skin were used to make the gas bags
  of early balloons created by the Royal Engineers at Chatham, Kent
  starting in 1881–82 culminating in 1883 with "The Heron", of 10,000 cu
  ft capacity. The method of preparing and making gas-tight joins in the
  skins was known only to a family from Alsatia called Weinling who were
  employed by the RE for many years. The British had a monopoly on the
  technique until around 1912 when the Germans adopted the material for
  the internal gas bags of the "Zeppelin" rigid airships, exhausting the
  available supply: about 200,000 sheets were used for a typical World
  War I Zeppelin, while the USS Shenandoah needed 750,000 sheets. The
  sheets were joined together and folded into impermeable layers.

There is a more detailed description in the Wikipedia article on the ZR-1 which reveals that the goldbeater's skin was reinforced with cotton cloth:

  The gas cells were made of goldbeater's skins, one of the most
  gas-impervious materials known at the time. Named for their use in
  beating and separating gold leaf, goldbeater's skins were made from
  the outer membrane of the large intestines of cattle. The membranes
  were washed and scraped to remove fat and dirt, and then placed in a
  solution of water and glycerine in preparation for application to the
  rubberized cotton fabric providing the strength of the gas cells.
  The membranes were wrung out by hand to remove the water-glycerine
  storage solution and then rubber-cemented to the cotton fabric and
  finally given a light coat of varnish.

Today the preferred material is polyethylene therephtalate, a polyester, which is also used for blister packages or soda bottles. By itself, it would have far too high a rate of permeation, therefore it is sealed with a metallic layer of a thickness of only a few atoms. Such metallized films are also used for thermal blankets, in spacesuits, on satellites and for toy helium balloons. To increase its tensile strength, the film is stretched in two perpendicular directions such that the molecule chains become elongated and the film becomes thinner. The result is called biaxially oriented PET (BoPET) and metallized by vapor deposition of a thin layer of aluminium.


What is the difference between a spin and an uncontrolled roll in an high speed dive?

In the beginning of the article on Wikipedia it is stated that the plane experienced "high G forces and high speed". Not knowing the story of the flight but with this statement we can suppose that it wasn't a spin but rather a spiral dive. But when reading the full story it is stated that the lose of engine 4 leads to a decrease of the speed, that the pilot didn't had any rudder control, applied full left yoke and yet the airplane was still rolling right. Those are clear sign of a spin and not a spiral dive!!
A spin begin in situation where asymmetric stall occurs. In this situation the airplane start spinning on the yaw axis, the external wing creates more lift wherase the inner wing is stalled and drops, thus driving the nose down. But the main characteristic in a spin is that your airspeed is very small (which doesn't mean that the sink rate is not high) but prevent extreme aerodynamic loads on the aircraft and the g loading remain somewhat close to one during the spin. In one word the airplane is falling vertically from the sky spinning on itself, wing inclination can be close to 0 up to 90° depending on the aircraft.
On the other hand a spiral dive can occurs without any stall. When starting a turn if you let the nose drop, the inclination will increase and your speed will increase. At one point pulling on the yoke will only  tighten the turn and increase g forces but will not move the nose up. This situation can happen when you're focused on something on the ground and start turning to see it better. If you don't pay attention the nose will drop and it's a one way trip as you are focused on the ground and not your instruments. You reach VNE and loose control of the aircraft. In this situation the aircraft is flying a tight spiral to the ground with high wing inclination always increasing which can be seen as a slow roll.
In a spin you mostly use your rudder pedal to stop the rotation and then pull on the yoke wherase in a spiral dive you need to use the yoke to reduce inclination first and then pull. Adding some yoke to stop rotation in a spin will not help and worse may even flatten it and increase rotation speed. Using rudder pedals in a spiral dive won't have much effect, you need to bring down the inclination, level the wings and this is mostly done using the yoke while keeping your flight coordinated with rudder input if needed. Therefore it is important to differentiate both situations if you want to come out of one of them alive.


How unusual is the 737 MAX 8's crash history?

Historically, it's not unusual at all to have many accidents of an aircraft type in relatively short order, especially when the type is still new and hasn't had a lot of operating hours across the fleet to detect and work out teething problems.

BUT, and that's a big BUT, in recent history that's been happening less and less as testing and certification programs have gotten ever longer and stricter (in large part as a result of those earlier crash series).

As for the 737Max, according to Wikipedia some 350 have so far been delivered (out of over 5000 on the order book) over the space of only 2 years (yes, they're that new). Of those 2 have gone down. That's not a particularly high number historically, but as said in recent history people have come to expect nothing bad to ever happen because there have been so few accidents overall and most of those easily explained by external factors (MH17 being shot down, for example).

So this number is both highly unusual, in that it's not something that's been seen in recent memory, and rather normal, in that in the history of aviation it's not unusual at all.


How to programmatically access AIP data, METAR, TAF, NOTAMs?

ICAO is offering a very nice API for retrieval of, for example, aerodrome names/ICAO codes and NOTAM's. It is currently in public beta, which means that it is free to use. Once they finish testing it, there will be some kind of cost involved, but they have not published any pricing details yet. You can find it here: https://www.icao.int/safety/iStars/Pages/API-Data-Service.aspx

NOAA offers API access to weather information such as METAR's and TAF's: https://aviationweather.gov/adds/dataserver

You can find waypoints and navaids, including names and locations, via EUROCONTROL EAD: https://eadbasic.ead-it.com/fwf-eadbasic/restricted/reporting/reporting.faces (register first, then return to that link). However, what they seem to offer is a one time search/database dump, more than an actual API.


Why do small jets usually have one low flap setting?

The provision of lots of flaps settings is motivated in part by the desire to provide as many profile optimizations as possible. Generally, the less the flap extension for takeoff, the more efficiency you have in a departure plus better engine failure performance, but you need the longest runway, so you would use the least extension allowable for a given runway.

The CRJ was originally certified without an 8 degree flaps setting, and it was added as an option a year or so into production.  In general, you will always use Flaps 8, unless conditions require Flaps 20 (shorter runway).

It can create a trap for the CRJ because if you are departing an airport where 20 is required to meet departure performace requirements, and you set Flaps 8 by mistake, there is no Takeoff Configuration Warning mode to warn you you're at the wrong takeoff flap setting (only a warning for 0, 30 and 45... it can't know which 8/20 TO setting you actually want) and you can take off with insufficient flap extension and get into a bit of trouble when you rotate at the Flaps 20 Vr and you don't lift off.  When departing a short field that required Flaps 20, you had to be extra careful to make sure Flaps 20 was selected.


Gliding: why pulling the stick back in a spiral dive would tighten the turn?

The effects of rudder and elevator get mixed when the aircraft flies at a roll angle. See it this way: When you pull the elevator at a roll angle of 60°, it will mostly (86.6%) work like a rudder would for a level airplane.
Why those 86.6%? When looking at the aircraft in an earth-fixed reference system, the additional tail force from an elevator deflection will produce a side load component proportional to the sine of the roll angle (which is 86.6% at 60°). The vertical component which would help to get the nose up is only 50% at a roll angle of 60° because it is proportional to the cosine of the roll angle.
Coordinated flight at higher roll angles is almost like flying a loop sideways. You will only lift your glider's nose in the reference system of the glider and tighten the loop. The reference system that counts more here is the one in which gravity acts, and you should lift the glider's nose up in this system.
Not knowing this subtlety ended the life of several early aviators until it was found out in 1912 that the rudder is what lifts the nose in a spiral dive. Of course, it is even more efficient to first roll wings level and make the elevator work properly again.
The horizontal-loop thing works in reverse, too: Rüdiger Kunz, the designer of the Standard Austria, was always fond of telling the story that happened on occasion of the first flight: At some point, the pilot started to fly loops. After the third, he asked Herr Kunz over the radio: "Notice anything?" - "Yes, you're climbing" was his answer. Thereupon the pilot replied: "That's how you can circle in thermals, too!"


What is a Visual Climb Over Airport (VCOA) and how is it properly executed?

It's defined in the pilot/controller glossary:

  VISUAL CLIMB OVER AIRPORT (VCOA)− A departure option for an IFR
  aircraft, operating in visual meteorological conditions equal to or
  greater than the specified visibility and ceiling, to visually conduct
  climbing turns over the airport to the published “climb−to” altitude
  from which to proceed with the instrument portion of the departure.
  VCOA procedures are developed to avoid obstacles greater than 3
  statute miles from the departure end of the runway as an alternative
  to complying with climb gradients greater than 200 feet per nautical
  mile. These procedures are published in the ‘Take−Off Minimums and
  (Obstacle) Departure Procedures’ section of the Terminal Procedures
  Publications.

The AIM 5-2-8 says that pilots must request it:

  Pilots are responsible to advise ATC as early as possible of the
  intent to fly the VCOA option prior to departure

And the ATC orders 4-3-2 say that ATC may not offer it:

  Do not solicit use of the Visual Climb over Airport (VCOA) option.

The TERPS 14-4 has all the technical details and explains the purpose, which is to allow aircraft that can't maintain a 200fpnm climb to climb safely within a limited area over the airport:

  VCOA is an alternative method for pilots to depart the airport where
  aircraft performance does not meet the specified climb gradient.

The departure designer creates a vertical cylinder (called a Visual Climb Area in the TERPS) of airspace over the airport that's safe to climb in, based on airport elevation, runway positions and a default airspeed of 250kt (although other airspeeds can be used). Even if the VCA dimensions aren't charted or stated in the DP, for an airport at 2000msl (KAVL is at 2162msl) the radius of the cylinder is at least 2nm at 90kts, or 2.8nm at 250kts, which seems to be plenty for a spiral climb (in the aircraft I fly, anyway). And the climb has to be in visual conditions, so terrain clearance shouldn't be an issue.

I don't know of any detailed guidance on how execute it, but "climbing turns over the airport" seems fairly clear. I've never done it myself, but I'd simply fly a continuous spiral climb after takeoff, up to the en route altitude mentioned in the procedures and go from there.


Why is the alternator connected only to one electrical bus in the C152?

There's a direct connection from the primary bus to the other bus, so they are essentially one. If you have a wire connecting the two, the current will flow from one to the other.


Is it possible to build a paper airplane that will enter a fully developed spin?

Besides the location of the center of gravity, two things determine how an aircraft spins, and both are not well shaped for spinning in paper airplanes:

The wings (obviously): They should ideally be straight with an aspect ratio between 5 and 10. Swept wings will also spin, but straight wings make spinning and recovery easier.
The forward fuselage. Here a rounded shape which is higher than wide is ideal and will promote flat spins. The two-dimensional shape of paper airplanes will not produce the vortices which drive the yaw rate in spins.

Also, for spinning it needs a stalled wing, some asymmetry and some time until the spin fully develops. To create a strong pitch-up moment, masses at the tip and the end of the fuselage are also needed. A pure paper airplane will have too much yaw damping and too little inertial pitch up to spin.

Size should be less of an issue because separated flow changes less with Reynolds Number, and spin tunnels regularly use scales of 1:20 or 1:30 with good results. But I wonder whether the low wing loading of a paper airplane will prevent spins - the models used in spin tunnels are carefully sized not only for geometric agreement, but also for their masses and mass distribution.

Maybe it will be better to show your students videos from spin tunnel tests.

Spin tunnel footage of fighter aircraft
Spin tunnel footage of GA aircraft
Description of a spin tunnel


What's the difference between a "runaway prop" and a "feathered prop"?

A "run away prop" is caused by a failure of the pitch control mechanism of a constant speed propeller.  Feathering turns the prop blade as close to 90 degrees to the wind as possible, minimizing the effects of the airstream.  This obviously reduces drag and is used when an engine is shut down.

A run away condition is created when the propeller malfunction causes the blade to move to the "fine" setting, which is used at slower air speeds.  It is the opposite of being feathered in that now the prop blades are much more angled to the wind as in a windmill.  The force of the airstream literally now turns the engine to excessive rpm.  The prop is at the wrong pitch for its airspeed, much like a truck dropping into low gear at highway speed.  The engine rpm redlines and causes overheating, while the drag of the improperly set prop slows the plane down.

Thankfully, the pilot was aware of the danger and was able to get away.


What temperature are Pitot Tubes heated to?

This page talks about the Pitot probes that operate at temperature range -65°C to +70°C. They are FAA TSO-C16 and Military Approved. It is probably unwise to heat them more during normal operation. This is not very hot but +70°C would already burn the skin. The probes are used at Pilatus PC-12, Bell 212, Bell UH-1 Special, Cessna Caravan, Diamond DA42, IAI Heron and Quest Kodiak.

This contradicts another source where heater temperatures of 200 °C are reported, and also this industrial Pitot device (while not used on the aircraft) is rated up till the 200 °C, so the design itself allows the temperatures that high. However the heating device is only part of the probe and should be hotter than the tube it is heating up.

This answer says there are significant differences between Pitot heat systems, and some on the light aircraft are rather "anemic". Hence it may be that some models are heated and operate at much higher temperatures. There is also a phrase "plastic cover could melt" there. Plastic melts well over 150 °C.


Do airplanes have any kind of fire insulation?

Yes, aircraft have fire-protection systems as well with many of the materials being of such a sort.

From boeing.com about the fire-resistant/fire-suppression systems on their aircraft. You can see that most of the aircraft is covered in these materials. Emphasis mine.

  Interior components of Boeing airplanes meet flammability requirements prescribed in Title 14 Code of Federal Regulations (CFR) Part 25. These components include:

- Interior ceiling.
- Interior sidewall panels.
- Partitions.
- Galley surfaces and structure.
- Exposed surfaces of stowed galley carts and standard galley containers.
- Large cabinets and cabin stowage compartments.
- Passenger seat material.

  For materials in areas not covered by the CFR requirements, Boeing design guidelines are used to identify additional flammability, smoke, and toxicity requirements.
  The standards for flammability of insulation blankets have improved over time. A recent requirement change calls for the enhancement of the fire-protective features of insulation blankets in the event of an in-flight or post-crash fire. The latest standard increases protection by minimizing the contribution of the insulation blankets to the propagation of a fire. Thermal/acoustic insulation installed behind cabin interior panels with the appropriate fire-resistant properties can delay the onset of fire into the cabin in the event of a crash (see fig. 1). The insulation blankets, along with the airplane skin, must be capable of resisting burn-through from a fuel-fed post-crash fire next to the bottom half of the fuselage for a minimum of four minutes to allow passengers to evacuate the airplane before burn-through can occur.

You can also find out much more in that webpage about the various active and passive fire-suppression systems in aircraft.

The FAA Regulation about these systems can be found here in PDF format.


When airplanes first got windows, what material was used?

Polymethyl methacrylate (Plexiglas)

The material was developed in 1928 in several different laboratories by many chemists.
Historically, PMMA was an important improvement in the design of aircraft windows, making possible such iconic designs as the bombardier's transparent nose compartment in the Boeing B-17 Flying Fortress.

From early on Plexiglas found a market in aviation, as the alternatives were either flammable or too heavy:
History (albeit short) of aircraft windows

The largest impediment to having closed cabins was the material the windows were to be made of. Prior to [Plexiglas] becoming available in 1933, windows were either safety glass, which was heavy, or cellulose nitrate (i.e.: guncotton), which yellowed quickly and was extremely flammable.

(wikimedia.org) B-17 nose Plexiglas.


Where can I find full wing polar data to validate an aerodynamic code?

"Theory of Wing Sections" by Ira H Abbott and A. Von Doenhoff has extensive graphed and tabulated data for Cl and Cd vs alpha of a wide variety of wing sections.

Airfoiltools.com has also got a lot of the data you're looking for, but from a quick scan it doesnt appear to cite their sources. The data is readily available in tabulated format, however.

In either case, its not explicitly mentioned but I'd assume that the data provided is for wings of straight planform. I'd guess that you can find some kind of formula to account for the difference either in the Abbott book mentioned or elsewhere, possibly applying a variation of the blade element method.


Would USAF ATC experience help in getting a civilian ATC job?

Yes, military air traffic controllers often transition to the civilian system after their military retirement. But if your end goal is the commercial world it might not be for you. The military want people fully committed to the job - committed enough to serve in a war zone where people would love to drop a bomb on your head. It is not something you can do for just a year or two and then move on, the minimum service commitment is, I think, 8 years. If you are motivated enough to accept all that then you can sign up after high school.

In the US the civilian ATC is run by the Federal Aviation Administration. Their requirements are on their website, and it looks like you'll need a degree or three years of suitable work experience. Once you meet those requirements you just wait for an advertisement and apply.


What is "scintillation" and are "qualified pilots" aware of it?

"Scintillation" is the flickering of starlight due to the turbulence in the Earth's atmosphere through which the light passes. It is also known as stars "twinkling."

The current version of the PHAK discusses a number of nighttime optical illusions, but not scintillation. It does discuss, however, autokinesis, which is a less well-known but similar illusion. I don't think it would be outrageous for a pilot who knows about scintillation to connect the two. After all, a pilot can discern the difference between a ground-based light source and a star because the star will scintillate and the ground-based light source will not. Maybe the PHAK should include it, given its interest in identifying false horizons created by lines of ground lights.

Someone knowledgable who observed the visual flicker of the Crab Pulsar would be able to easily tell that it wasn't scintillating because scintillation is random, whereas pulsar flickers are more precisely timed than atomic clocks.


What type of visa is required to be allowed to earn a PPL in the US?

For a foreigner to come to USA to learn to fly, one would need an F-1 or M-1 visa.

This article in Flying magazine provides some information. You can get more information on from this school1. Also, you can look at this meta question: Can I ask about details of coming to the US to get a PPL?

Update:

It appears that you are already in the US on H-1b visa (it wasn't mentioned in the question), you need to go through the security clearances mentioned in Alien Flight Student Program (official version on TSA website).

AOPA has put together the related information (website). I also found about this flight school1 which has outlined the process.

If you search on Google about Alien Flight Student Program, you will find a ton of information.

1: Disclaimer I am not advertising any flight school.


Under 3rd Class Medical Reform, When Do I Need To See My Doctor?

The FAA hasn't written the regs yet so who knows. All that has happened is congress has told the FAA to write the refs

Personally I think it's foolish to use your personal doc. You're waving all your HIPPA rights and obligating your personal doc to report anything to the FAA without your consent.

-Robert


Why do helicopters typically have blades at the top?

Yes the answer is that simple, that mincer needs to be out of the way so it doesn't chop peoples' head or feet off, or slashes into a bit of concrete and shatters into 100 pieces scattered around like bullets. That is the main reason why the light 2-seater R22 has the rotor mounted on a pylon, at 2.7 m it is out of the way of most people.

Image source

Flight dynamics wise, a rotor underneath the helicopter has speed stability in the hover, which makes the helicopter easier to control. The rotor acting as a pusher makes it more efficient; being closer to the ground means more lift in ground effect and softer take-off and landing. Manufacturing:

The rotorhead of an underslung rotor would look pretty similar to the top mounted one, a hinge offset head would line up the fuselage with the rotor disk, top or bottom.
The rotor would have to be situated away from the fuselage to allow for the upwards coning and tilting under flight conditions - for instance mounted on an upside down pylon quite similar to the R22.

So all in all an underslung rotor poses no particularly vexing engineering problems and provides improved stability and better performance - it would just be an immensely impractical machine.


Alpha floor is a thrust function which provides TOGA thrust regardless of thrust lever position in Airbus. From Airbus Technical Digest FAST 20:

  Alpha Floor is a low speed protection (in normal law) which is purely an
  autothrust mode. When activated, it provides TOGA thrust. As the aircraft
  decelerates into the alpha protection range, the Alpha Floor is activated,
  even if the autothrust is disengaged. Activation is roughly proportional to
  the rate of deceleration.

  ... at low speeds, if a rapid avoidance manoeuvre is required to avoid terrain, windshear or another aircraft, it is safe to rapidly pull the sidestick fully aft and/or bank and hold it there. The aircraft will pitch up
  to max Alpha, engage TOGA thrust and climb away.

Note that this protection is only available under normal law and is lost under alternate law.


Lift is created by accelerating air downwards. The same goes for wake turbulence: The vortices form at the boundary between the downward-moving strip of air (wake) from surrounding air that tries to fill the void left by the wake.

Friction will eventually dissipate the kinetic energy of the wake, but in case of heavy aircraft the motion will continue for several minutes. By that time it will sink down hundreds of feet or hit the ground. This is driven by inertia; density differences do not exist unless the atmospheric lapse rate is above the adiabatic rate and the wake warms less quickly from increasing atmospheric pressure than the surrounding air.

In the end, the weight of any heavier-than-air vehicle is supported by an area of higher pressure on the ground. Some mechanism has to transfer the weight force from the flying aircraft to the ground, and that is the downward-moving wake. When it slowly dissipates, it will leave a slightly higher air pressure below it. When that happens at supersonic speed, the more focussed pressure change is high enough to be audible on the ground.


Why do Boeing planes have more flap settings than Airbus planes?

And some older Mooneys have infinite flap settings depending on how much you pump them up...

The reality is that there are no regulations on how many flap settings an airplane must have and on an airliner that may also have things like speed breaks or slats as well, flap settings may vary by design or simply by engineering choice.

Flaps are used for improved short field take off as well as to make a steeper approach at a slower airspeed. If Airbus can achieve desired performance with 4 settings there is no need to complicate matters and add to pilots work load.


What steps can pilots take to mitigate the risk of controlled flight into terrain (CFIT)?

There are a few things you can do to help mitigate it.

Get An Instrument Rating: Some CFIT accidents happen during a nasty mix of mountain flying, scud running and an inevitable flight into IMC, which ends with hitting the side of a mountain, an instrument rating here can be a truly valuable thing in these situations.

Plan Your Routes and Minimums Carefully: This takes place in the preflight and you should always check your charts for minimum altitudes as well as man made obstructions that may be in your flight path. Once you have your minimum altitude selected, stick to it. If weather does not permit that altitude either alter your course or scrub the flight. There is some good info from AOPA on it here.

Synthetic Vision: Apps like ForeFlight are quickly becoming common place in aviation as are glass cockpits, and these have proven invaluable in terms of terrain awareness and avoidance as many of them now include synthetic vision with terrain maps. Good preflighting is still key and you should always inform your self of all potential terrain before a flight, while technology can aid in avoidance you should always have a backup plan.

Avoid The Mountains: This is sometimes not an option but if you as the PIC feel uncomfortable you should always choose a route around terrain if possible.

Practice Mountain Flying: Just as you would with any other aspect of aviation you can always get an instructor and practice mountain flying. The FAA has some tips here but mountain flying carries its own risks and unique cases that you should be ready for. Mountains can have turbulent airflow and you should make an effort to understand how wind flows over them, you can find a discussion on it here.

Get a FIS-B Weather Receiver: As mentioned above winds can be important when flying. It's worth it to have an FIS-B weather in receiver so that you always have the most up to date information.


What does Lockheed have to do to mark the end of the F-35's SDD?

The clearest explanation I have found, comes from the Congressional Research Service (www.crs.gov), from RL30563.pdf:

  The F-35 is currently in low-rate initial production, with 280
  aircraft delivered as of April 2018. At least 250 of those were in
  U.S. service. Four to five aircraft are currently delivered each
  month, with the production rate scheduled to increase to 120 per year
  by 2019. In keeping with the acquisition plan that overlapped
  development and production (known as “concurrency”), the F-35 was also
  in system development and demonstration (SDD), with testing and
  software development ongoing, from October 2001 until April 11, 2018.
  The SDD phase will formally continue until the end of Initial
  Operational Test and Evaluation, when a “Milestone C” full-rate
  production decision will be made.

A copy of the full PDF is available here:

https://fas.org/sgp/crs/weapons/RL30563.pdf

We know from other sources that Initial Operational Test and Evaluation (IOT&E), will formally begin later this year, 2018.


What is area in equation of lift?Does it remain constant during flight?

The área $A$ in lift equation $L=\frac{1}{2} \rho AV^{2}C_{L}$ is a reference area, so you can choose the one you prefer, but some election can be more apropiate than others depending on what you want to compute. For example if you are going to relate lift coefficient with drag one, the reference area should be the same, in this case a good selection will be the projected area in lift direction (aka planform area). It has the drawback that it depends on angle of attack/incidence since the true area is the same but lift vector changes. Another well selected area could be the true area (aka wetted) when working with drag, specially when friction drag is dominant considering that viscous stresses are proporcional to this area.

I give you some references when this is better explained:

  There are several different areas from which to choose when developing the reference area used in the drag equation. If we think of drag as being caused by friction between the air and the body, a logical choice would be the total surface area (As) of the body. If we think of drag as being a resistance to the flow, a more logical choice would be the frontal area (Af) of the body which is perpendicular to the flow direction. This is the area shown in blue on the figure. Finally, if we want to compare with the lift coefficient, we should use the same area used to derive the lift coefficient, the wing area, (Aw). Each of the various areas are proportional to the other areas, as designated by the "~" sign on the figure. Since the drag coefficient is determined experimentally, by measuring the drag and measuring the area and performing the necessary math to produce the coefficient, we are free to use any area which can be easily measured. If we choose the wing area, the computed coefficient has a different value than if we choose the cross-sectional area, but the drag is the same, and the coefficients are related by the ratio of the areas. In practice, drag coefficients are reported based on a wide variety of object areas.

Size effects on drag - NASA

  A comment is in order regarding the reference area Sin Eqs. (2.3) to (2.5). This
  is nothing other than just a reference area, suitably chosen for the definition of the
  force and moment coefficients. Beginning students in aerodynamics frequently want
  to think that S should be the total wetted area of the airplane. (Wetted area is the
  actual surface area of the material making up the skin of the airplane-it is the total
  surface area that is in actual contact with, i.e., wetted by, the fluid in which the body
  is immersed.) Indeed, the wetted surface area is the surface on which the pressure
  and shear stress distributions are acting; hence it is a meaningful geometric quantity
  when one is discussing aerodynamic force. However, the wetted surface area is not
  easily calculated, especially for complex body shapes. In contrast, it is much easier
  to calculate the planform area of a wing, that is, the projected area that we see when
  we look down on the wing.

Aircraft performance and design - John D. Anderson


What is the maximum distance for class C service?

There is no such thing as "Class C radar service".  "Class C airspace" and "radar service" are mostly orthogonal.  "Flight Following" is just shorthand for getting radar service when you're not required to.

Terminal areas around a Class C airport generally comprise a 5-mile "inner ring", 10-mile "outer ring" and 20-mile "outer area".  Approach is guaranteed to control (at least) that much airspace, regardless of class.  Within the airspace they control, they provide (roughly) the same radar services regardless of whether you're in Class C or Class E at any given moment.  The only practical difference is that in Class E, you're not required to talk to them, whereas in Class C, you are.

The reason the box says "CTC KNOXVILLE APP WITHIN 20NM" is that 20 miles guarantees you're within the "outer area" and thus Approach's airspace.  If you contacted them at 30 miles out, you may be Center's airspace, in which case Approach would need to redirect you to Center, and then Center would hand you back to Approach a few minutes later.  That's a waste of both your time and the controllers' time, so the chart reminds you of the minimum range to help you avoid that.

There is no theoretical maximum, and many Approach facilities do in fact control more (sometimes far more) airspace than the minimum.  However, the FAA does not publish the actual sector boundaries, so the best practice is to assume no more than the minimums—or get Flight Following for your entire trip, in which case you don't have to worry about it at all.


You can model the fan disc driving the bypass air flow as a well-shrouded propeller inside a short duct. It does indeed generate thrust which, as in the general case of a propeller, is transmitted to the engine case via a thrust bearing that supports the fan disc and thence to the airframe via the engine mounts.


How far should commercial jets be from each other when on the runway or while departing?

The FAA's Federal Aviation Regulations (FAR's) are all publicly listed here.

There are a wide variety of cases to consider on the ground and during departure but you can find a lot of the regulations/guidelines in this document.

For example here is the case for wake turbulance seperation,

  Separate a small aircraft weighing 12,500 lbs. or less taking off from an intersection on the same runway (same or opposite direction
  takeoff) behind a departing small aircraft weighing more than 12,500
  lbs. by ensuring that the aircraft does not start takeoff roll until
  at least 3 minutes after the preceding aircraft has taken off.
  Separate a small aircraft taking off from an intersection on the same runway (same or opposite direction takeoff) behind a departing
  large aircraft (except B757) by ensuring that the aircraft does not
  start takeoff roll until at least 3 minutes after the preceding
  aircraft has taken off.

That is just one example, the document is very lengthy and covers a lot of cases, you should be able to find what you are looking for. You should also keep in mind that things like weather and local airport policies will cause the regulations to vary a bit. For example a local noise abatement may cause the seperation/interval timing to be larger than required to adhear to the noise abatement.

That covers mostly what ATC will see and do, you can take a look at this document for how pilots look at departure procedures.


Does the nearest function on a G1000 allow for listing of anything other than nearest airports?

The G1000 and G500 do indeed have pages for the nearest VOR, NDB, Intersections, and user defined waypoints. The  G1000 Integrated Flight Deck Pilot’s Guide gives a list and flowchart / interface map for these.


Jet-A is mostly for turbine engines in general. Turbine engines can be fueled with nearly anything that you can spray over an open flame and will burn.

It is not a substitute for avgas, for the same reasons that diesel is not a substitute for petrol. You can fuel a diesel engine with Jet-A under some circumstances.


What is the reason why dihedrals are used when building a plane?

Because when an airplane rolls to a bank angle, the airplane begins to slip toward the lowered wing.  If the wings are mounted with a dihedral, this causes two things:  the inside wing will experience an increased angle of attack, and the outside wing experiences a decrease in angle of attack.  This combination causes an imbalance in the lift forces on the wings resulting in a rolling moment about the center of mass and returning the airplane to a wings level condition.
Dihedral angle is much more common on low wing airplanes, since the location of the CG above the CL creates a static unstable roll condition when the airplane is banked.  Dihedral angle can be used on high wing aircraft as well to enhance roll stability along with the pendulum effect of the CG below the CL e.g. a Cessna 172 still employs a slight dihedral angle to its wings.
In the age of digital flight control systems dihedral angles to wings are not absolutely necessary, though for light airplanes and gliders it’s far more elegant to design the airplane with a slight natural static and dynamic stability in all three axes.  Aircraft designers also work to ensure that airplanes have more authoritative directional stability than lateral stability from a dihedral angle to prevent unwanted Dutch rolling tendencies.


Which is safer? The Piper Tomahawk or the Cessna 152?

From the Wikipedia article on the Tomahawk:

  Safety record:
  According to the Aircraft Owners and Pilots Association Air Safety Foundation, which published a Safety Highlight report on the Piper Tomahawk, the Piper Tomahawk has a one-third lower accident rate per flying hour than the comparable Cessna 150/152 series of two-place benchmark trainers. The Tomahawk has a higher rate of fatal spin accidents per flying hour. The National Transportation Safety Board (NTSB) estimated that the Tomahawk's stall/spin accident rate was three to five times that of the Cessna 150/152.[2]

Earlier in the article it was stated:

  Before designing the aircraft, Piper widely surveyed flight instructors for their input into the design. Instructors requested a more spinnable aircraft for training purposes, since other two-place trainers such as the Cessna 150 and 152 were designed to spontaneously fly out of a spin. The Tomahawk's NASA[1] GA(W)-1 Whitcomb airfoil addresses this requirement by making specific pilot input necessary in recovering from spins, thus allowing pilots to develop proficiency in dealing with spin recovery.

So what this means is that the Tomahawk was intentionally designed to be easy to spin so that instructors could teach students how to recover from a spin.  Other aircraft, like the Piper Cherokee, are almost impossible to spin so instructors cannot really teach spin recovery in those aircraft.

To answer your question on how this relates to safety that depends on how you look at it.  I would argue that a pilot who knows how to recover from a spin is a safer pilot than one who doesn't.  However since the Tomahawk is actually made to spin, there is the potential for increased spin/stall accidents if the pilot messes up.  Aircraft like the Cessna 152 or Piper Cherokee have design features that can "cover up" a pilots mistakes because they are difficult to spin or stall, but if you plan on flying aircraft that are not so hard to stall then not knowing the proper recovery procedures can get you killed.

The main factor in safety is not usually the safety of the plane but the safety of the pilot.  The vast majority of accidents are caused by pilot error.  I would worry more about evaluating yourself as a pilot and understanding if you have the right mindset to be a safe pilot.


Which aircraft have fought against its own type in active combat?

Probably not the only situation, but in 1948 Israeli Spitfires faced Egyptian Spitfires, and just before the truce also faced a few RAF Spitfires that were too close to the combat area (reference)


How dangerous is flying in a single-engine plane?

I get this question a lot from people who are apprehensive about flying with a private pilot.  I'm afraid I won't be reducing these fears in any way.  Let's review some general statistics during 2008.  Note - these stats aren't specific to light or single engine aircraft:

NTSB reported there were 1.21 fatalities per 100,000 flight hours for private aircraft (Part 91 operators).
NHTSA reported there were 1.26 fatalities per 100 million miles travelled by automobile

We can equate that to about 2 million hours (estimating an average speed of 50mph). This gives us 0.063 fatalities per 100,000 driving hours.

Private aircraft have a fatality rate about 19 times greater than driving.  It is also true that a majority of the accidents that occur are pilot error (71%) and could have been prevented.

There are risks involved when taking to the sky as a private pilot and understanding these risks is part of the continual learning process.  The key to safety is performing careful planning, keeping current and proficient, knowing when to cancel flights or turn around and not to exceed your capabilities or the capabilities of your aircraft.


What triggers the trim horn and the "stabilizer motion" aural callout on the MD80?

There are two aural sounds, the horn, and the voice callout "stabilizer motion".

The horn sounds every 0.5° of trim change after the initial 0.5–1°.
"Stabilizer motion" sounds when the autopilot or runaway trim motor make a >2° in 30 seconds.

So your second source is the correct one. See text below from an MD-80 flight manual:

  Momentary horn sound starts after 0.5º to 1.0º of stabilizer movement and repeats every 0.5º of stabilizer movement . "STABILIZER MOTION" voice warnings will sound when either the autopilot or a runaway trim motor moves the stabilizer more than 2º in 30 seconds and will continue to sound as long as stabilizer moves. Cycle continues until stabilizer movement ceases. (Manual page)

Note that it also says:

  Vocal warning on some aircraft, other aircraft no vocal warning.

So not all MD-80s have the voice saying "stabilizer motion".


If I'm on a IFR flight plan with appropriate IFR reserves, and I'm given a lower altitude which will consume more fuel, what should I do?

As I said in my comment, I don't think there's any single answer to this question because there are just too many variables and the scenario itself seems a little artificial.

But, the first thing you can do is negotiate with ATC. Accept the descent instruction but request a return to FL250 as soon as possible. Offer to fly at another altitude (including above FL250 if possible), or on an alternative route, or with a 10-degree course deviation or anything else that gives ATC options for keeping you at the altitude you want. Of course, there's still no guarantee that the controller will be able to allow it.

The next thing would be fuel management: depending on the aircraft and your current power settings, you may be able to sacrifice speed in order to gain range. But that might not help either if the end result is still that you won't have the reserves you need.

If none of those those things are possible then there's no good option except to plan a fuel stop. Declaring an emergency would be an abuse of the system and could lead to an investigation: you have plenty of fuel, just not enough to get where you want to go, and there's no immediate danger to the aircraft. (I read an NTSB ruling where a pilot had repeatedly said "unable" and eventually declared an emergency simply in order to keep on his preferred route. The FAA grounded him, he appealed to the NTSB and lost.)

In fact, you hopefully already planned for a possible fuel stop because if the winds are so different at FL210 and FL250 that it makes the difference between making it to your destination or not, that's something you should have found out during flight planning. But even if you didn't, or if the wind just changed suddenly, a diversion for fuel shouldn't be a big deal.


Do two stroke engines for small aircraft suffer from degradation between overhauls?

Don't overthink this stuff.

If the rings are good and there are no other abnormalities, there's no reason a 2 stroke shouldn't put out rated power over its service life.  And you would confirm this with a compression test periodically and a quick run at full throttle, while tied down, to check the static RPM.  With a fixed pitch prop, if the power output is down, the static RPM at full throttle will be down, assuming temperature and altitude are more or less the same each time.

Engines like the Rotax 582 were typically overhauled frequently by 4 stroke standards, something like 300 hours, due to potential fatigue on the crankshaft, not from performance degradation.  Possibly the newer versions are being run longer than that.  I've heard that some guys are running them to 500 or more.

So if you maintain the engine, don't allow it to overheat, do a compression test periodically, say every 25 or 50 hours along with a static RPM check at full throttle, you can assume the engine is making rated power and overhaul it only if it fails those tests or has reached the recommended rebuild interval.

And in your flying, don't put too much trust in those engines. It's not the power degradation from wear that'll get you, it'll be the thrown rod or seized piston at the worst possible time.  They are much more highly stressed than 4 stroke aircraft engines, which live a much easier direct-drive life and have really beefy components.  I would NEVER fly a 2 stroke beyond gliding distance of a landing spot.  The saving grace of ultralights is the fact that you can put them into very small spaces when the unreliable 2 stroke lets you down.


What does "Clear Of Clouds" mean in reference to Class B Airspace in the United States?

That specific phrasing means that you can't touch a cloud. See the VFR minimums in 14 CFR 91.155, where it says:

  no person may operate an aircraft under VFR when the flight visibility
  is less, or at a distance from clouds that is less, than that
  prescribed for the corresponding altitude and class of airspace in the
  following table [...]

The table lists the "distance from clouds" in class B as "clear of clouds".

You can also find it in the AIM section 3-2-3 on class B airspace, and in other sections about SVFR, visual approaches etc. where a requirement for those procedures is to remain clear of clouds.

The actual cloud cover in the class B airspace is reported by METARs and ATIS.


Is a safety pilot required to share costs?

This actually came up in chat recently -- the answer (if only the two pilots are on board) is a qualified "No", not because the time isn't "compensation" but because the safety pilot is a required crewmember for simulated instrument flight.
Since both pilots are required crewmembers the safety pilot is not acting as a PIC of a flight carrying passengers as long as they are only acting as the Legal PIC while the other pilot is under the hood.
(Note however that while neither pilot is in simulated instrument conditions the safety pilot is not required crew, and therefore someone is PIC of a flight carrying at least one passenger - so log your time carefully and try to wrap your head around a "pro rata share" of that mess.)

This is further addressed (more directly) in the interpretation issued to Roberts, July 11, 2012.

Note that if we're talking about a flight with more than the two pilots on board (carrying someone who is NOT a required crewmember, like a friend in the back seat) then the whole thing goes out the window: No matter who is acting as the Legal PIC for the flight they're "PIC of a flight carrying passengers", and you're back to pro rata share territory.


What’s the purpose of the small “hole” in front of wing root on A320-214?

It leads to a vent in the cargo compartment that Airbus calls "Outlet-Extraction". It must be an option that isn't installed very often.


Do stealth airplanes need to limit contrail generation?

Not just stealth aircraft, most military aircraft are required to avoid contrails. Contrails form due to moisture in the aircraft's exhaust. A tried and tested method by NASA is NOT to fly in regions of air that support contrail formation.

Ophir's Pilot Alert System which is used by the B-2 stealth bomber uses LIDAR (light detection and ranging) to differentiate contrails from clouds and tells the pilot to change his altitude as/when necessary.

That being said, it is essential to remember that though contrails are visible to the naked human eye they are NOT visible to Radars. Ergo, until the stealth aircraft is really close (assuming it is on a SEAD/DEAD mission) one cannot see the contrails and by the time they do, it is too late. Moreover, stealth aircraft generally operate during night when contrails are less visible.

  Stealth aircraft have low visibility on radar.

Not necessarily. VHF radar should be able to detect them at long range. Moreover, RCS of aircraft is a complex phenomenon, depending on many stuff (frequency, aspect angle, polarization of signal etc). You will never find a "static" figure for this.


What is the cause of unusually strong wake turbulence from the 757?

My understanding, primarily based on this archived NASA document (also linked to on the B757 Wikipedia article), is that there were several issues leading to the decision to make the 757 a special case.

First, there were some notable accidents where witnesses seemed to indicate the cause was 757 wake turbulence.

Cessna Citation - December 18, 1992 - Billings, Montana

  Witnesses reported that the airplane suddenly and rapidly rolled left and then contacted the ground while in a near-vertical dive. Recorded ATC radar data showed that, at the point of upset, the Citation was about 2.8 nmi behind a Boeing 757 and on a flight path that was about 300 ft below the flight path of the 757.

IAI Westwind - December 15, 1993 - Santa Ana, California

  witnesses reported that the airplane rolled abruptly and that the onset of the event was sudden. The Westwind was about 2.1 nmi behind a Boeing 757 and on a flight path that was about 400 ft below the flight path of the 757.

Additional accidents were noted with a Cessna 182, MD-88, and a Boeing 737.

Second, there were some controversial NOAA flyby tests which showed:

  for a peculiar set of weather conditions that lasted about 0.5 hour, the vortex velocity of the 757 was approximately 50 percent higher than that of the 767 at similar vortex ages (younger than 60 sec) measured in less favorable weather conditions. However, the results also showed that overall the wake of the 757 decayed faster than that of the 767; in fact, the wake behaved as would be expected for an aircraft of the size and weight of the 757.

Third, they felt the slower approach speed of the 757 (vs. similar aircraft) made it more likely that trailing aircraft would inadvertently close the separation gap during approach.

Ultimately, I believe the correct answer to your question is actually that there is nothing particularly special about the B757 in regards to wake turbulence. This Air&Space Magazine Article indicates that

  Boeing maintains that there is nothing about the wing design that would cause wake turbulence.

and

  "The wake from the 757 is no stronger or weaker than you would predict from the weight, span, and speed of the airplane," agrees NASA's David Hinton, who has taken wake measurements at airports around the country and is principal investigator of NASA's NextGen Airportal Project.

The decision to put the 757 in its own category was controversial at the time and not based on particularly strong evidence. It was done out of precaution due to anecdotal evidence and possibly flawed test results. On the other hand, you could argue that ATC erring on the side of caution is best if there is any doubt. To be fair, a 757 is on the larger side of the small category.


Under what circumstances can Part 91 allow compensation for air transportation?

The only way you can carry passengers for hire as a common carrier without an air carrier cert (either 121 or 135) is the sightseeing exemption -- 25 SM, landing at point of takeoff. However you will still need an LOA and to participate in an approved drug enforcement program.

If there is a 'commonality of purpose' e.g. all the passengers are employees of the company that owns the airplane, then it isn't common carriage, and you don't need the air carrier cert. This is how corporate flight depts operate, they're Part 91. (But they can only fly employees etc., they cannot fly the general public.)


Can anyone identify the aircraft in the background of this photo?

It looks like a 1921 Blackburn Dart, British carrier-based torpedo bomber biplane.

(source)


What's the difference between FMS and FMC?

(Own work)

The FMC is part of the FMS.

  The FMC is a computer system that uses a large data base to allow routes to be pre-programmed and fed into the system by means of a data loader.

The FMS encompasses:

  The Flight Management Computer (FMC)
  The Automatic Flight Control or Automatic Flight Guidance System (AFCS or AFGS)
  The Aircraft Navigation System
  An Electronic Flight Instrument System (EFIS) or equivalent electromechanical instrumentation.

Further reading: Flight Management System (SKYbrary)


Do modern aircraft still have mechanical sensors as backup to GPS?

In light aircraft they typically do: Glass-Panel aircraft will also have traditional "steam gauge" backup instruments of some kind.

Note the airspeed indicator, attitude indicator, and altimeter to the left of the pilot's PFD.

This is however no longer guaranteed: A completely independent electronic backup system with its own power source may be used in place of the traditional "steam gauges".
This can be seen on new Cirrus aircraft, which have digital standby instruments (in the green box):

Similarly transport category jets may have traditional backup instruments:

Or they may have an electronic backup system:

Note that the electronic instruments in these cases aren't GPS-referenced: They are gyroscopic instruments with pitot and static input for airspeed and altitude/vertical speed calculation.


What happens if a pilot uses left rudder during a turn to the right?

what mechanical measures are in place to avoid this?

The short answer is none.

Cross-controlling (the usual term for applying opposing aileron and rudder) is sometimes desirable.  Its greatest use in large aircraft is to align an airplane with the runway just before touchdown from a crabbed approach in a crosswind while keeping the upwind wing from rising. In other words, if you had a crosswind from the right and you were crabbed to the right to counteract it, just before touchdown you would apply left rudder to swing the nose left while applying right aileron to keep the right wing from coming up (or maybe put the right wing down a little).

  What happens when the pilot makes a right aileron turn but then gives a left rudder turn simultaneously

I'm making the assumption that you're talking about a turn not as part of a crosswind landing, and I suspect that your assumption is that pilots of large aircraft normally use coordinated rudder input when they use the ailerons. However, that is not the case. Once the aircraft is off the ground and absent any need to counter asymmetrical thrust from a failed engine, pilots generally take their feet off the rudder pedals and do not put them back on until short final. Yaw dampers or the equivalent for fly-by-wire systems take care of any adverse yaw ailerons may produce. Thus a cross-control scenario for en route flying would mean the pilot would have to make not one, but two errors. They shouldn't have had their feet on the rudders in the first place. Then they shouldn't have cross controlled.

But let's say that, for whatever reason, during en route flight, a pilot deliberately cross-controls once and then releases the controls. Generally speaking, that would not be disastrous. However, if they held a cross-control condition or kept repeating it in sequence, that's another story and would depend on a number of other conditions, and you could certainly come up with scenarios that ended in the destruction of the aircraft. Although I'm not sure the fly-by-wire airplanes would permit you to do that, but I've never flown any.

If you were in a 747-100/200 en route, you could if you wanted give it left aileron and right rudder in proportions that matched. In other words, the turning force of the ailerons would be cancelled out by the turning force of the rudder. The airplane would keep going straight ahead. However, you would be in a sideslip, and you'd have to increase the wing's angle of attack to keep from descending, which would mean more drag. Plus, there would be all that additional drag from the rudder. In other words, you'd be burning a lot more fuel than you should. Plus, you'll get a call from the purser asking what the hell is going on because gravity is no longer perpendicular to the cabin floor.


Could a modified J58 jet engine be used commercially?

According to Wikipedia the J58 is comparable to the Rolls-Royce/Snecma Olympus 593 that powered the Concorde, so yes it's possible.

Will it be efficient? The Olympus 593 was as efficient as can be, even by today's standards:

  The Concorde cruised at Mach 2.05 with its engines giving a SFC of 1.195 lb/(lbf·h); this is equivalent to a SFC of 0.51 lb/(lbf·h) for an aircraft flying at Mach 0.85, which would be better than even modern engines; the Olympus 593 was the world's most efficient jet engine. However, Concorde ultimately has a heavier airframe and, due to being supersonic, is less aerodynamically efficient, i.e., the lift to drag ratio is far lower. In general the total fuel burn of a complete aircraft is of far more importance to the customer.

It's not just the engine that makes an aircraft efficient, as the paragraph above highlights. The amount of fuel the Concorde burnt for 100 passengers is the same for 700 passengers on two Jumbo Jets.

As I highlighted here, even if fuel is cheap, or the ticket price covers the fuel cost, propulsive efficiency isn't the whole story. The Concorde wasn't retired because of fuel. I'm mentioning the Concorde because of its very efficient engines and supersonic regime.


What was the lowest ground speed landing?

Considering groundspeed as a vector, 0.

You can land vertically with strong headwind: Could a plane land vertically in a strong headwind?

As the answers to the question, previously linked in Flexo's comment -  Is it possible to fly backward if you have really strong headwind? - detail, the answer is "yes". It's even possible to have one's groundspeed vector directed tailwise.

The Antonov An-2, can be flown at as little as 25 knots of airspeed, and safely landed below that speed. When turned into decent headwind, it can be kept in hover relative to the ground. I haven't found a video of a hover landing, but it's been reported to be done. With a 12-passenger capacity, it's no 737, but it can count as a passenger airplane, and it's designed to do this.

Vertical landings of smaller planes have been filmed:

             (thanks bukwyrm).

P.S. Vector representation is most appropriate for groundspeed, as it's not normally directed along the aircraft's longitudinal axis due to side winds. So landing "backwards" can be considered positive groundspeed once again.


When is an aircraft considered dispatched?

Technically, before the aircraft is dispatched / released, everything can be inoperative.
However, most everything needs to be in working order to be dispatched / released, with exceptions as per the MEL / CDL.
MEL / CDL procedures are followed until the takeoff is commenced.
If an item is discovered to be missing or inoperative,
A) prior to door close
B) after door closed but prior to block out
C) after block out but prior to flight
there are specific MEL procedures to follow in each of those three scenarios.
Taking the runway and setting takeoff thrust / power is the act of commencing a takeoff.
After that point, equipment failures are treated as an in flight emergency, and flight crew follow QRH / AFM guidance. At that point the MEL no longer applies.
Applicable US FARs for Airline ops:
121.687 Dispatch Release - Flag or Domestic Operations
121.689 Flight Release - Supplemental Operations
135 has a similar regulation.
91k (fractional) does as well.
Plain Jane 91 owners/operators/pilots don't normally operate with an MEL. If they do, they have a letter of authorization with the FAA.


How do I interpret electrical system indications in a G1000 C-182?

You can find this in the C182T Operating Handbook. Section 3 has information about what values should be considered a malfunction:

  HIGH VOLTS ANNUNCIATOR COMES ON OR M BAT AMPS MORE THAN 40

It also says that the M bus should be at least 20 volts in normal operation:

  The Main Battery supplies electrical power to the Main and Essential
  Buses until M BUS VOLTS decreases below 20 volts. When M BUS VOLTS
  falls below 20 volts, the Standby Battery System will automatically
  supply electrical power to the Essential Bus for at least 30 minutes.

Section 4 has the expected values when starting the engine:

  BUS E Volts

  CHECK (Verify 24 VOLTS minimum shown)

  M BUS Volts

  CHECK (Verify
  1.5 VOLTS or less shown)

  BATT S Amps

  CHECK (Verify discharge shown (negative)

Section 7-54 describes the electrical system in detail, including wiring diagrams. It says:

  Normal bus voltages with the alternator operating shall be about 28
  volts.

And:

  Indicated voltages between
  24.5 and 28 volts may occur during low engine RPM conditions

As for the ammeter:

  A positive current value (shown in white) indicates that the battery
  is charging. A negative current value (shown in amber) indicates that
  the battery is discharging. In the event the alternator is not
  functioning or the electrical load exceeds the output of the
  alternator, the main battery ammeter indicates the main battery
  discharge rate. In the event that Standby battery discharge is
  required, normal steady state discharge should be less than 4 amps.

So long story short, all the information you need is in the POH and the best way to get all the details is to read it carefully. Your POH may be slightly different from the one I've quoted here, so if there are any differences then you should follow it, not this answer.


What are "ground-based navigation systems which provide vertical guidance" and how do they compare to altimeters?

They mean an ILS - Instrument Landing System.

An ILS consist of:

A localizer component, which tells the angle between the aircraft's position and the extended runway centerline. In short, it tells the pilot whether they are left or right.
A glideslope component, which tells the angle between the aircraft's position and the extended runway surface on the ground. Typical glide slope angle is 3 degrees. In short, it tells the pilot whether they are high or low.

An ILS is usually coupled with an DME (Distance Measure Equipment), which tells the distance between the aircraft and the runway threshold. Using the glideslope angle together with the horizontal distance, one can compute the height above runway level.

From page 12 of the report:

  AC624 was then cleared for the straight-in LOC approach ...

Which means only the lateral (left or right) component is available, there is no signal which tells the pilot whether they are high or low relative to the ideal descent path.

A radio altimeter is not very useful when the approach path leading to the runway has uneven terrain. It tells the height above the ground directly under the aircraft.

A barometric altimeter needs to be calibrated by a reading at ground level. It tells the height above sea level.

Both of these altimeters cannot tell the height above runway level. Therefore the pilots must rely on other data to monitor the descent profile, which increases the pilots' workload. In a situation such as low visibility and high crosswinds, this presents a very challenging situation.

If both vertical and horizontal guidance is available, then the pilots can simply follow this guidance all the way to the runway. They can also program the autopilot to follow this guidance, if suitable equipment is installed on the ground as well as onboard the aircraft.

Note the green bars: one is horizontal and one is vertical. They form a cross on the display. If the horizontal bar is up a little bit, the pilot would pull up a bit. If the vertical bar is to the right, the pilot would turn right a bit. These green bars only possible when ILS is used.

It is possible to compute the relative height by using a DME and a barometric altimeter only. For example, I am now 5.5 nautical miles from the runway threshold, and according to the approach chart, I should be at 1,800 feet. Right now my altimeter reads 1,600 feet. That means I am too low and I should reduce my vertical speed to slow down my descent. This calculation has to be done by the pilot himself and not by a computer. Furthermore the pilot must fly the plane, i.e. adjust the control surfaces, run landing checklists, visually verify the alignment with the runway, talk to ATC, set the flaps etc. A pilot is trained to do all this, but increased workload also increases the chance of making an error.


Is there an equivalent of cash transportation in air cargo?

Decades ago, when we moved checks at night for Federal Reserve clearing, we would sometimes have sealed bags which were cash.  On my flights, I never had an armed guard on board, but on taking off and landing, we were met on the ramp by an armored truck with armed guards on a routine basis.

The impression I had is that checks and other paper were the main cargo, and that cash and bullion were minor incidentals.

Related, items of national security are routinely carried by scheduled airliners, and are placed into the hold by cleared individuals, and then the hold is opened upon arrival, in the presence of cleared individuals who retrieve the packages.  Contents are logged, in the event of compromise.  Similar parcels are also handled by the major parcel and courier enterprises, with slightly different processes.


Do hours flying "advanced ultralight" aircraft contribute towards your license in Canada?

It looks like you can count the flight hours towards a recreational license if you logged the hours in an advanced ultralight with an aeroplane certificate of airworthiness.
For the private pilot license only 10 hours in three-axis ultralights can be counted.
For a commercial ticket a maximum of 25 hours pilot-in-command flight time in three axis ultra-light aeroplanes can be counted towards the 200 hours total flight time requirement but this time does not count towards the 100 hour pilot-in-command flight time.
This is based on my cursory reading of the requirements. I would suggest you talk to a reputable flight training organization or Canadian aviation authorities for confirmation.


What are the requirements for foreigners to join the RAAF as a fighter pilot?

Only citizens of a country can apply in the government jobs (including armed forces) of that country. There can be some exceptions but I hardly doubt that a foreigner can be hired or trained for any position in military which has access to classified information. Being a fighter pilot even has more restrictions.

Royal Australian Air Force does not hire any foreigner:

  Can I apply if I am not an Australia Citizen?

  To be eligible to join the Air Force, you must be an Australian
  Citizen.

According to Recruitment Centre of Australian Defence Force, this page lists the proofs required for citizenship.


Why are there no Mach 4+ fighter planes?

Today's fighter are designed with a max speed around mach 2 (max speed is generally calculated with no emport and 50% fuel, the operational top speed is way lower). They keep this ability to pass Mach 1 only to be able to respond fast enough to intercept / air policing missions.

Countries do not design very fast fighters (and planes in general) because :

It's extremely hard to achieve.
It does not give any advantage.
It has no use anymore.

Back in the cold war period, radars and SAMs where not very sophisticated yet, they did not see very far and Ground to Air missiles were not able to get very high. So to protect their airspace from potential intruders / enemy bombers, countries were relying on interceptor aircraft, a fast jet able to catch up the target before it gets to its objective.
At this time, going very fast, very high was an advantage : SAMs where not threatening. The US made a very successful reconnaissance plane exploiting that: the SR-71 Blackbird (max speed : mach 3.3). Many missiles were fired towards the SR-71, none reached its target.

Speed was used for intercept and reconnaissance / intel.

But radars and SAMs improved, getting more range, becoming able to threaten very fast planes. Also satellite images got better and better. The need for very fast interceptors decreased as radar range increased, and the need for very fast recon aircraft decreased as satellite images got better.

Now why does going very very fast not give any advantage to a fighter? Because you cannot turn.
The thing to keep in mind is G force: force the same turn rate, the faster you go, the more g you have to take. And a human can sustain a very limited amount of g. This means a plane going at Mach 4 will barely be able to turn without blacking out its pilot. And if you can't turn, you won't evade enemy missiles, making useless the bit more range gained on the missiles.

And then there is the technical challenge, dealing with a little drag as possible (that means no missile / bomb / fuel under the wings), heat, airframe deformation and wear ... for an advantage which isn't there anymore.

Despite all that, the US are suspected to work on a Mach 6+ aircraft, probably unpiloted.

(Please keep in mind that I simplified a lot of things to not make this post too complex)


Can trimmer and throttle be used for controlling large aircraft

To some extent, yes. There was United Airlines Flight 232 in which catastrophic failure in the tail engine destroyed all the hydraulic lines. The plane was crash-landed in Sioux City, Iowa after the pilots controlled the aircraft using only the two intact wing engines. The trimmer might not produce as large as an effect on larger aircraft as on smaller ones; control of the engines would be enough because they affect the aircraft more than the trimmer due to their thrust.

However, if the damage is so severe that a control surface is destroyed or locked into a certain position, the engines might not be able to counteract that.


In the US, those are called either "Access Panels" or "Inspection Covers", depending on their function, but many people combine the two terms and call them "Inspection Panels". Based on the comments, other parts of the world apparently refer to them as "Inspection Ports".

We remove both panels and covers to perform routine inspections, but access panels cover system components that may be accessed more often for servicing. Both panels and covers are designed for quick removal, typically fastened with screws, as is the case in the aircraft depicted in the photos above.

Here is an inspection hole with the cover removed. This is on a Cessna 170 that I am in process of rigging. Note the aileron cable turnbuckle visible inside the wing. Through this access panel I will adjust the cable tension, among other things, and once the cable is properly tensioned I will safety wire this turnbuckle. Here the panel is removed for maintenance, but during an inspection I would remove this panel to inspect these same systems.


What is the influence of drag on an airliner's block fuel consumption?

I would suspect that reducing drag in this case would be result of reducing airspeed. If your drag coefficient 1 is for a certain speed, slowing down would cause smaller fuel flow but increase flight time and hence increase total consumption.


Why does Turkish airlines still fly over Iraq, is it safe?

Flying over conflict zones has been a common practice at all times. However after MH17 was destroyed over Ukraine some changes occurred.

From an overall standpoint, EASA, the safety agency for the European Union, has an active information bulletin dated March 31, 2017, which is the extension of one issued on December 23, 2015:

  Iraq airspace, all altitudes.
  This CZIB applies to operators.
  This CZIB is valid until 30 September 2017

  Due to the hazardous security situation, with the presence of
  terrorist organisations and ongoing high intensity military
  operations, there is a risk of both intentional targeting and
  misidentification of civil aircraft. The presence of a wide range of
  ground-to-ground and dedicated anti-aviation weaponry poses a HIGH
  risk to operations at all flight altitudes.

  Recommendation(s):

  Operators should take this information and any other relevant
  information into account in their own risk assessments, alongside any
  available guidance or directions from their national authority as
  appropriate.

It must be noted that Turkey while not a member of the EU, is still a partner of EASA and Eurocontrol.

On their side, the Iraqi Civil Aviation Authority states the sky over Iraq is safe.

Bottom line, operators have been informed of a high risk, and must evaluate their individual situation based on this warning and all other elements they can get on their own.

As mentioned by EASA, operators must comply with national directives if any.

For French operators, the situation is clear. DGAC, the national aviation agency has purely and simply forbidden Iraqi airspace (FIR ORBB) since July 2014, in an AIC valid for several conflict zones.
A similar order for Turkey would be issued by DGCA, but I was
not able to find such document.

In addition of Air France, many airlines have stopped flying in Iraqi airspace: press articles mention e.g. KLM and Qantas, Lufthansa and Royal Jordanian, and many other similar information can be found online.

The situation of Turkey is a little bit complex. Their have a southern border with Iraq and Syria, and the Sinai peninsula is also a conflict zone.

This may push operators to take more risks than others with southward routes.

About conflict zones, see also: Apart Kenya, what are the current dangerous / unsafe airspaces?


Strange facility out west near California; what is this place?

In case anyone's wondering, it's the Crescent Dunes Solar Energy Project, credit to Trish Ling at Sci-Fi.


Is landing the 'wrong way' possible on an aircraft carrier?

You're going to run into a few issues.

Below is a diagram of a typical aircraft carrier layout. Normally aircraft would land from right to left. The four cables are located closer to the right end than the left. The extra length after the cables is both to allow a "bolter" (YouTube) time to accelerate in case of missing the arresting hook; and to allow an aircraft that caught a cable time to decelerate.

Notice how far an aircraft can continue down the deck (YouTube) after catching the cable. Even if you catch the first cable, you might go off the aft end of the ship.

Another issue is the way that the arrestor cables work. The housing where the cable enters the flight deck is curved to allow the landing aircraft to pull on the cable as it decelerates. The cable is not designed to be pulled in the opposite direction, which could cause it to break (YouTube).

Aside from these issues, you would also lack the FLOLS system that provides physical guidance during the landing. And as voretaq7 also mentioned, you have to either touch down late, or catch the cable late, both of which could pose problems.


What should a pilot do if they feel sleepy?

If a pilot is not rested enough to safely operate the aircraft, they should not fly. If a pilot is flying and falling asleep, they should switch out with another pilot or land.

Current FAA regulations for domestic flights generally limit pilots to eight hours of flight time during a 24-hour period.

For the Air Force, actual flight duty periods vary depending on the aircraft type and mission but we are required to get 12 hours of crew rest with the opportunity for 8 hours of uninterrupted rest prior to flying.

You can read the FAA's Fact Sheet on Pilot Flight Time, Rest, and Fatigue here.


Can APU start in-flight using RAT or battery power?

On the 737, the APU is typically started from battery power; this is normally done on the ground, but it could be started this way in flight as well.  (The 737 doesn't have a RAT.)  The APU is typically operated only on the ground for short-haul flights, although there are cases such as a generator that failed in flight when it's started & used in the air.  It's also used in cases of a "bleeds off takeoff" to provide air conditioning and pressurization when the engines aren't supplying bleed air (to increase power for a hot, high, and/or heavy takeoff).

But yes, it's more common than not for modern passenger jets to fly with the APU off on moderately short legs.

The exception to this is that some jets are required to run the APU during long over-water (ETOPS) legs, so that if one generator fails, the APU's generator is available without the question of "will the APU start after having been cold-soaked at altitude for so long?"  So in that case running it for much of the flight is common.

As far as the APU vs a RAT, the APU is almost always going to be a bigger generator than a RAT, so it can power more electrical systems on the aircraft than a RAT.  These would probably include electric-motor driven hydraulic pumps, which again will probably have more capacity to provide hydraulic power to flight controls & other things than a RAT would alone.  Plus, an APU can provide pneumatic power (bleed air, i.e. pressurization) than a RAT can't.  The other side of that coin is, a RAT is typically able to produce power more immediately after it's called on than an APU, which takes some tens of seconds to start up.


Why do some IAPs lack an Intermediate Fix and/or include other fixes between Final Approach Fix (FAF) and Intermediate Fix (IF)?

The rules are quite lengthy -- a 509 page PDF.  They're all spelled out in excruciating detail in the FAA Order 8260.3D, United States Standard for Terminal Instrument Procedures commonly referred to as the TERPS.

It's available for download here.

I would suggest researching your questions there, and returning here if you have a specific question regarding the application of the TERPS.


Where can I ask questions (career advice) about jobs in Aviation Management?

There is a huge demand for aviation professionals. However, how that demand is looks like is very difficult to answer, since it's a pretty fragmented market and getting an overview is very difficult.

As such, you're best chances of understanding it are:

Going to recruitment fairs that major airlines attend.
Attending events that airlines hold themselves.
Trying to get in touch with human resources in an appropriate way directly.

From this you should be to gauge how interesting a candidate you will be. As with most things, this is hardly an exact science. Getting a program with a placement/internship as part of the course for instance might be quite beneficial. As for the no previous experience, I think this is the case for most people heading into this area.


Does sex occur in aircraft cockpits, and is this safe?

As jwenting said, it's unlikely to happen for many reasons, and if it does, the participants are even less likely to admit it since doing so could get them in trouble both professionally, and with a government regulatory agency for endangering safety. I seem to remember the FAA taking action against a pilot who posted photographs of himself flying with either naked or bikini clad women at his side, but I can't seem to find a link right now.

There is one NTSB report which is somewhat famous in the aviation community for describing a general aviation crash which was likely caused by sexual activity. However, the summary is very delicately worded, to say the least:

  The private pilot and a pilot rated passenger were going to practice simulated instrument flight. Witnesses observed the airplane's right wing fail in a dive and crash. Examination of the wreckage and bodies revealed that both occupants were partially clothed and the front right seat was in the full aft reclining position. Neither body showed evidence of seatbelts or shoulder harnesses being worn. Examination of the individuals' clothing revealed no evidence of ripping or distress to the zippers and belts.


Why are the flaperons on the 787 located so far in-board?

Quoting the answer @fooot linked to:

  At high speed, the outboard ailerons are locked and all roll control is achieved by deflecting the inboard ailerons (and spoilers, if needed). This helps to reduce wing torsion and twist.

Yes, roll authority will be larger once the force is applied further away from the fuselage (/ roll axis), but the cost at which this comes (wing twist) is larger than that benefit.


The number will vary depending on the aircraft type & installed engines (how much fuel it holds, and how much fuel it burns per hour at taxi speed), but the ballpark figure is "Much less than the range in flight", because the specific fuel consumption figures (how much power, and thus distance, you get per unit of fuel burned) are less favorable during taxi than at cruise.

To work an example I used the numbers for an A320 taken from here and not checked it burns 270-330kg/hr (depending on the engines installed) so let's split the difference and say 300kg/hr as an average.

The A320 can hold around 15,000kg of fuel, so doing the division it could run its engines at taxi power for around 45 hours.

If we assume a taxi speed of 20kts for 45 hours the aircraft could theoretically taxi 900 nautical miles (1036 statute miles, or 1667km), a little less than one-third of the range in flight.


Is there a standard definition of the "stall" phenomena?

The scientific definition of a stall is as you state in the first line, however in the cockpit that definition isn't actually that useful. Practically there's 2 parts of stalls a pilot needs to be aware of because they are things a pilot experiences, and impact the flying characteristics of the airplane:

Stall buffet: this isn't a stall, but it happens when you're very close to one. Recognizing a buffet may lead to a pilot decreasing AoA before entering a fully developed stall
Flow separation: when the flow separates from the top of the wings it may do so on one wing before the other, causing a wing drop, aileron input may work opposite of intent, and you of course have a high rate of descent. This can feel mild or violent depending on the airplane's characteristics and the speed at which the stall happens

Whether the definition of a stall is when the airflow separates or a change of the center of lift is academic, what a pilots needs to know is how to recognize the condition and recover.


How does the "pendulum effect" affect biplanes?

It will affect a biplane just as much as a monoplane.

Not at all.

The pendulum effect does not exist in airplanes. It does in airships, but not in heavier than air craft.

For a proper discussion, we should first define what a pendulum is. Only then can be established if such an effect can exist in airplanes.

Let's base the definition on Wikipedia. It says that

  A pendulum is a weight suspended from a pivot so that it can swing freely.

Maybe it is also worth to look closer what a pivot is: A thing on which something turns.

So the pendulum is fixed to a fulcrum which keeps it suspended and allows it to swing freely. The ideal pendulum has all its mass in its massive bob and, therefore, pivot and center of gravity are not in the same place. If the center of gravity and the pivot would fall together, a pendulum could only rotate but not swing. And that swinging motion is what the pendulum is all about.

Now for airplanes: Here we have no pivot. All rotation can only happen around the center of gravity. This is equivalent to the pendulum with no length which is no pendulum any more.

Lift is the sum of all pressure acting perpendicular to the direction of movement. Lift on a banked wing will also bank with it. The lift vector will still be in the plane of symmetry of the banked aircraft and will have no lever arm with the center of gravity, thus causing no uprighting rolling moment. The Figure 34 of your copied book page is simply wrong. The author did not know what he was talking about.

Edit especially for @JohnK:

I added an answer here about the rolling maneuver of paragliders. The description should make obvious that no pendulum effect is involved. Rather, the whole is quite similar to roll control in hang gliders where the lift is shifted sideways in order to create an imbalance, but then again different in wonderful ways. The discussion below would not allow me to explain my thoughts in such detail.


What was the first aircraft that could fly inverted?

Deliberate momentarily inverted flight
The first reported deliberately inverted flight was probably in a Blériot model XI.

Inverted flight was also being considered. Several pilots had inadvertently found themselves upside down as a result of wind gusts, but no one had yet attempted it intentionally. Adolphe Pegoud decided he would be the one to try.
Pegoud was one of the first true aerobatic pilots. An accomplished test pilot with the Bleriot team, he was also the first flyer to jump from a plane with a parachute. Both Pegoud and the aircraft landed, unharmed. Having decided to try upside-down flying, Pegoud first practiced in a hanger [sic] in a plane hung upside down from the ceiling. He correctly assumed the controls would have to be operated in a reverse manner and he wanted to get the feel of it before actually flying.
It must have worked. Shortly thereafter, Pegoud made the first public demonstration of inverted flight in September of 1913. During this flight, he was the first to perform a half roll to an inverted position--another aerobatic maneuver was born. The flight did point out one potential problem: While Pegoud was prepared to fly inverted, the aircraft was not. It proceeded to drench the flyer with fuel!

From The history of aerobatics
Deliberate sustained inverted flight
This would have to have been an aerobatic aircraft specially modified for inverted flight

flop tubes in the fuel tank?
fuel-injection not carburettor?
strengthened and/or braced to allow wings to support fuselage in inverted position?

1925
June 1925, No 32 Squadron did an air display demonstrating Flight-converging bombing at the R.A.F Display, Hendon. Scott was selected to do individual aerobatics in a brand-new Snipe which he was allowed to paint red, this pleased Scott greatly as it meant that he was also allowed to practice his aerobatics at a low altitude, rather than above 2000 feet which was R.A.F regulations at that time. In Scott's book he tells of how for his solo display he was allotted exactly seven minutes during the luncheon break, to complete his show but after just two minutes, a flying wire broke in the near edge of the port side, anxious not to cut his allotted seven minutes down too much he began to fly the aircraft upside down in an effort to reduce the strain on the flying wires, he continued flying in an inverted position for some time until he noticed a worrying quiver in the top plane and promptly landed slightly short of his seven minutes.
From Wikipedia
1933 record
In 1933, NC872H, a Boeing 100, was flown inverted for 4 hours 5 minutes and 22 seconds.

After its career with Pratt & Whitney, the Museum of Flight's aircraft was sold to Milo Burcham on September 27, 1933. He modified it for skywriting and air-show work, painted it blood red, added fuel and oil systems for inverted flight, and flew it upside down from Los Angeles to San Diego.

From Seattle Museum of Flight


Why is tangential velocity a loss of kinetic energy in a turbine section?

We take axial flow turbines as our example here.
Tangental flow of air (flow that circulates around the engine's central axis) is imparted to the air by viscous drag between the air and the rotating fan and compressor blades. Between those fan discs are stator blades that act to straighten out the tangental flow and help convert the kinetic energy of the axial flow into pressure rise in the compressor stage. Viscous drag losses occur in the stator stages too as they perform this function. These losses dissipate some of the compression work being performed on the air by the compressor as a whole, and that amount of work then cannot contribute to thrust generation in the engine.


Is there an FAA-published handbook for Aviation Physiology?

I found this Introduction to Aviation Physiology on the FAA web site.


Why do airlines have geographical names in them?

In a lot of cases it's because they started serving a specific region.

Lucky tells the story of Cathay Pacific. Virgin Atlantic was started specifically to fly from Britain across the Atlantic. Lots of US airlines started in a specific region, like Southwest. Westjet started serving the west of Canada.

It doesn't make sense to change a well-known name just because the reach of the airline is extended.


Why local(section) lift coefficient(CLL) is higher near the wing tips compared to the root on untwisted tapered wings and untwisted swept(aft) wings?

For the swept wing, as you go out along the wing, each section of the wing is effectively flying in the upwash of the tip vortex of the section immediately next to, and inboard of it. So, each section has its angle of attack increased by all of the sections inboard of it. The root section, of course, does not have anything inboard of it, and the tip section is affected (to at least some extent) by all the sections inboard (and ahead) of it.

Birds fly in V-formations to take advantage of this effect. Each bird is flying in the upwash of the tip vortex of the bird ahead and toward the center of the V, and is thus "flying downhill" aerodynamically.

But, that upwash increases the angle of attack, and the effects add up as you move outboard, until they reach a peak effect near the tip.

You can really see this on hang glider wings: if you stand next to one as it's about to take off, you can see the tips appear to be at a negative incidence relative to the wind, and yet they are lifting, because the wing sweep increases their local AoA.


How can a designer maximize the glide ratio of an aircraft with low aspect ratio wings, other than by increasing the aspect ratio?

With a given wing span, everything that reduces drag will increase the gliding range. Aspect ratio is less of a factor here because induced drag depends on wing span, not aspect ratio. But increasing the aspect ratio with a given wing span will reduce the wetted surface and, consequently, friction drag.

What can be done to increase gliding range outside of increasing span?

retract the landing gear,
avoid struts or bracing wires,
minimize friction drag by allowing for more laminar flow and minimum surface area.
minimize pressure drag by streamlining everything

Generally, a low aspect ratio shifts the polar point of best gliding range to lower lift coefficients, so reducing all that parasitic drag becomes more important the stubbier the wings are.

The most effective way, however, to increase gliding range is indeed a larger wing span if all other parameters are held constant. Increasing span while keeping wing area constant will:

increase aspect ratio and reduce wing chord,
reduce induced drag at the same lift coefficient,
and, as a consequence, lower the optimum gliding speed, so the parasitic drag is lower as well.


Are there any 100% renewable energy aircraft in development?

Contrary to one clause of your question, there are a number of battery-electric light aircraft (or what the USA calls "light sport" class) in development, with prototypes flying.  This includes both fixed-wing conventional types with single or multiple electric powered propellers, and "drone" style craft using what amounts to thrust hover with electric powered propellers.  Flight time is limited, but better with the fixed wing types -- enough so that I'd call an electric motor glider a very feasible design.

Hydrogen has limited usefulness, for the same reasons it doesn't work well for cars: it's technologically difficult to store enough for reasonable range.  Either extreme pressure, extreme cold, or a chemical storage method that limits the rate at which you can draw fuel are reqired.

A submarine type nuclear reactor was operated aboard a modified B36 in the 1950s -- the project, intended to lead to a strategic bomber with a flight time of weeks or months, was dropped due to lack of need as well as cost and safety concerns (most of the safety concerns, such as "roll-up" explosions, were later resolved or found to be overblown).  This kind of power isn't "fully renewable" anyway -- fission fuel must be mined, it isn't grown.

Algae fuels, as drop-in replacements for kerosene, may have some usefulness, but there are storage issues (hard to stockpile something that can mold or rot) -- as you note, however, testing is in progress, and we'll know in a few years.


Which type of fan is better for wind tunnel testing?

You could make either one work, especially for a "straight  through" tunnel.  The key is to smooth out the turbulence after the fans to as straight and undisturbed a line as possible.

Smoke trails help check your wind stream before the model is put into place for testing,
and remain valuable to see changes in the wind flow pattern as it passes over the forms.

There is an excellent article on the internet:  howstuffworks wind tunnel, and a very good diagram showing post fan stages such as stators, vanes, diffusers, and settling chamber that help create a desirable flow.

For testing of lower subsonic designs, a straight through tunnel, such as the one the Wright brothers built, may be easiest to construct.


Instrument landing: why do all electronic devices need to be shut off?

Short answer: ILS is rather sensitive to interference and not all electronic devices take much precaution in avoiding the generation of interference. The pilot wants to be sure that the readings he's getting on the localizer and glideslope are accurate, since he can't actually see the runway to verify the final approach path visually.

Longer answer: ILS is actually relatively simplistic as RF signals go. Both the localizer and the glideslope just consist of a pair of AM signals that are broadcast directionally. For both systems, the modulated frequencies are 90 Hz and 150 Hz. For the localizer, 90 Hz is broadcast to the left of the runway centerline and 150 Hz is broadcast to the right of the centerline. When the aircraft is aligned with the runway, it will receive equal strength signals of both frequencies. When it's to the right, it will receive more power from the 150 Hz signal and when it's to the left, it will receive more power from the 90 Hz signal. The glideslope works pretty much the same way, just with the 150 Hz signal transmitted below the glide path and the 90 Hz signal transmitted above it. [1] [2]

Now, suppose some passenger's electronic device is emitting a frequency that it shouldn't be. This is not exactly unheard of among consumer electronic devices. This is more common among devices manufactured in countries with less-strict emissions testing requirements/enforcement, but it's possible due to slight defects in almost any device. [Source: I design RF receivers for a living.] Now suppose that this noise happens to be on the localizer or glideslope frequency for the runway to which the aircraft is flying an approach. Hopefully you're starting to see a problem here... Whatever amplitude changes are happening on this unintended emission are now being received by the AM demodulator of the localizer or glideslope, potentially confusing it and, in low visibility conditions, causing the aircraft to be directed somewhere other than the runway threshold. Needless to say, that's bad.

Boeing recommends the following actions for flight crews flying ILS approaches in order to detect erroneous ILS readings:

Crosschecking altitude and DME distance periodically.
Crosschecking altitude and flight management system (FMS) threshold distance.
Crosschecking altitude and the crossing altitude of the outer marker (or locator, very-high-frequency omnirange [VOR] navigation equipment, or FMS).
Crosschecking radio altitude and barometric altitude.
Crosschecking ground speed and rate of descent.
Questioning air traffic controllers when indications do not appear to be correct.

However, even when following those instructions, slightly erroneous ILS signals may be hard to detect until too late. If a DME is showing that you're half a mile off, you can probably notice that, but a couple hundred feet off might not be noticeable (but is still plenty to be a big problem.)