When not cleared straight in for a VOR approach, how is this type of procedure executed?

When talking about approaches, you should limit your terminology to mean the same thing for you as it does for ATC. In other words, for a Category A aircraft:

ATC would not clear you for a Straight-in Approach unless you were inbound from the South for Runway 36 or inbound from the North for Runway 18. There are no Straight-in minimums for this approach, only circling minimums. To use this approach, you are required to fly a modified Traffic Pattern no lower than 1760 feet MSL until you reach a point where you can descend safely to the runway while maintaining cloud clearance. ATC will give you instructions on which direction to circle. In the absence of instructions, circle in the direction prescribed for Traffic Patterns in the US Chart Supplement and Sectional.
There is no Back-Course for a VOR. A Back-Course implies an ILS or Localizer transmitter. VORs transmit their navigation signals differently than ILSs and Localizers. There are Radials for each VOR dependent on your bearing from it. There are To-indications and From-indications for each Radial depending on your location on the extended Radial relative to the VOR. There is normal/direct-sensing and reverse-sensing for each Indication dependent on your Heading.
If you were flying inbound direct to the GDM VOR on the 097° Radial, you are East of the VOR flying Westbound on a heading of 277° Magnetic. The safe altitude at which to remain above is the Minimum Sector Altitude of 3300 feet MSL unless assigned otherwise.
You may request from ATC a Hold at the VOR to buy time to prepare for the approach or to lose altitude. Although, it is not part of the official Instrument Approach Procedure until the Missed Approach Procedure, you may use it to get established inbound to the airport or VOR on the 277° Radial instead with ATC clearance.
If you are not cleared for the hold, or accept clearance for the full procedure, once you pass the VOR, you may descend down to an altitude at or above 3000 feet MSL as long as you remain on the 277° Radial.
You may perform a Procedure Turn (starting with a left turn to a Heading of 232° Magnetic for 1 minute, then a right turn to 052° until reestablished back on the original Radial) anywhere along the 277° Radial as long as your aircraft remains within a 10 Nautical Mile distance from the VOR during the entire procedure. The general convention is to travel at least 2 miles or 2 minutes before starting the Procedure Turn.
Once you are established inbound on the 277° Radial with a Course and Ground Track of 097°, you may descend down to, but no lower than, 2400 feet MSL until you reach the Final Approach Fix which is the GDM VOR. Please be cognizant of the fact that when ATC Radar services are unavailable, other aircraft may be holding at the GDM VOR at or above 3000 feet MSL. It would behoove you to cross GDM inbound below 3000 feet MSL.
Once you have passed the Final Approach Fix, you may descend down to, but no lower than, 1760 feet MSL to start your Circling Approach. The Circling Approach May only be started if you have established visual contact with the runway.
If you never establish or are unable to maintain visual contact with the runway environment during the entire Circling Approach, remain on or return to the 097-From/277-To Radial until you reach the Missed Approach Point of 1.9 miles DME from the VOR (1 minute, 54 seconds at 60 knots ground speed). If you still have not established visual contact, perform the Missed Approach Procedure (a climbing left turn to circle back to the GDM VOR, climbing to hold at GDM at an altitude of 3000 feet MSL).
If, at any time during your descent past the Missed Approach Point, you can not maintain cloud clearance, circle back over the runway to join the Missed Approach Procedure.

—————————————————————————————————
Per the OP’s edit:

Remain at or above the MSA for the inbound heading from which you are coming or your assigned altitude.
Do not descend until you have crossed GDM on an outbound leg.
Once you have crossed GDM, utilize the hold to descend down to but not below 3000 feet MSL.
Notify ATC on whether you will proceed in on the inbound leg directly from the hold, or do the full procedure outbound to the Procedure Turn.
If no altitude loss is necessary, notify ATC of your request to proceed inbound on the procedure. You will have more than 1.9 miles to descend down to 1760 feet MSL (depending on where you get established on the Initial Approach course/leg).
Alternatively, you can intercept the procedure anywhere on the Initial Approach course/leg if cleared by ATC. Provided you do not start your descent to 3000 feet MSL until you are established on the procedure outbound or to 2400 feet MSL until you are established on the procedure inbound. Either inbound or outbound legs must be within 10 NM of GDM.


Is the Cirrus SR22T a turbo-prop? Or does in just have a turbocharger?

It is a turbo charged piston aircraft (much like turbo charged car, the engine is somewhat similar to that found in the older air cooled Porsche 911's (think 930 turbo)).

Turboprop is generally short for Turbine-Propeller which is a propeller aircraft powered by a turbine engine.

Turbo charged piston aircraft are piston driven engines with a turbo charger, many often may carry the T or TC designation after the model number. Such as the Piper Saratoga TC or SR22T

In this case we can reference the SR22T's Specs from the website

  Airframe & Powerplant

  Continental TSIO-550-K 315 HP Turbocharged Engine


How is the ALSF-II approach lighting system built and powered?

The specifications for an ALSF-2 (and other approach lighting systems) can be found in FAA JO 6850.2B - Conveniently titled "Visual Guidance Lighting Systems."
As an FAA order these are obviously US-specific, but other jurisdictions' guidance is probably similar.

The ALSF-2 is a High Intensity lighting system, so lights used in the system are defined in the FAA order as follows:

  High Intensity Systems.

  Specification FAA-E-2408b, 300-watt, 20A, PAR-56 lamps for steady-burning, aboveground, white lights with Specification FAA-E-982 lampholder.
  Specification FAA-E-2408b, 500-watt, 20A, PAR-56 lamps for steady-burning, aboveground, colored lights with Specification FAA-E-982 lampholder.
  Specifications FAA-E-2952 or FAA-E-2491 approach lights, 500-watt, for all semiflush lights.
  Specifications FAA-E-2628b or FAA-E-2689a for all sequenced flashers.

From the number of required lights we can then derive the total system power used for an FAA-Specified ALSF-2 lighting system -- As I'm not fond of doing math it's fortunate that some manufacturers have already done this for us, and a full system (with all the flashers) requires approximately 104kW (varying slightly depending on input voltage). Actual utility power consumption will vary based on the efficiency of certain components but will likely shake out to something around 104kVA since the incandescent lamps have a power factor close to 1.0.


Aerodynamic advantage of blunt noses and WHY (subsonic)

Nose shape on fuselages

Fuselage noses of airliners hold the antenna of the weather radar, and a blunter nose will put the least amount of material in the way of the electromagnetic waves. On a three-dimensional body the shape of the nose is of less importance and sometimes used to express a brand image. In subsonic flow, sharp changes in the contour must be avoided: Here, air would have to change direction abruptly which requires strong pressure gradients. Therefore, curvature along any flow path should change gradually. This favors an elliptic body shape. If you want to have no jump even in the second derivative, use a lemniscatic function.

A hemispherical nose would have constant curvature followed by no curvature along the cylindrical portion of the fuselage. The sudden change in curvature at the transition between the rounded nose and the cylinder would require a sudden jump in pressure which causes more drag than the gradual reduction of curvature (causing a gradual increase of pressure) of an elliptic nose.

  Edit 2: basically, why does No 2 in the following have less drag than No 5?

Because the sharp corner at the base of the cone in No. 5 will cause a suction peak which in turn will cause a jump in boundary layer thickness. The hemispherical nose of No. 2 is better, but best is the elliptical nose in No. 1.

A blunter ellipse will have lower overall surface but a steeper change in pressure - here, as so often, the optimum is a compromise which needs to include the Mach and Reynolds numbers as well as structural considerations. Since the optimum is flat, there is considerable space for individual solutions.

  What if No 5 had a smoother “ridge” (cone to cylinder transition) but was still pointy? Say, if No 1 ended in a point?

The pointy tip would not "fix" the stagnation point to that point - rather, it would still move with the angle of attack, but the part of the flow negotiating this tip would exhibit a strong suction peak and "smooth out" the contour by means of a local separation. This would place more strain on the boundary layer and increase its thickness downstream. Any hope for laminar flow over this nose past such a tip would be destroyed by this, of course.

The smoother ridge would be better, so the elliptical nose with a pointy tip would improve things like No. 4 does over No. 5. On No. 4 the sharp ridge will most likely produce a separation on the ridge which will make the whole body look larger to the oncoming flow. The bisection of the drag coefficient in No. 4 indicates to me that the separation is much smaller. Note that this is all only valid in subsonic flow! In supersonic flow No. 4 would look best.

Nose shape on wings

On wings, however, the nose shape is extremely important - witness the jumps im performance that sometimes were achieved by optimizing the nose shape. To get to the bottom of the wing nose shape, we need to talk about boundary layers, Mach effects and much more, so be prepared for a long answer.

Glider airfoils can afford to use small nose radii because they fly at low Mach numbers. Airliners, on the other hand, need to keep local Mach numbers low which favors a larger nose radius. If you are happy with this answer, better stop reading now.

At the Symposium Transsonicum of the International Union of Theoretical and Applied Mechanics (IUTAM) in 1964, E.V. Laitone presented something like a magic number for transsonic flow:
$$Ma = \sqrt{\frac{1}{\gamma - 1}} = 1.581$$
with $\gamma$ the ratio of specific heats of a fluid. Once the local suction peak at the nose of an airfoil reaches such a speed, lift stops growing any further. With stall speeds of around 120 knots, the early jet airliners routinely hit this limit using the 6-series NACA airfoils of that time.

Below you see the pressure distribution on a flapped airfoil near maximum lift (picture source). The specifics of the airfoil don't matter much; important is the suction peak at the nose which is made possible by a large angle of attack and a flap with a ventilated gap between wing and flap.

In such a case, a small nose radius creates a very strong but narrow suction peak because a very strong pressure gradient is needed to force the flow around the tight nose contour. If the nose is blunter, the suction peak can spread out lengthwise and becomes less peaky, simply because the trajectory change of the flow around the nose is becoming more gradual. Since there is a direct relationship between local suction and local Mach number, those suction peaks must be spread out and flatter in order to allow higher angles of attack. That is the main reason why supercritical airfoils have blunter noses - they will tolerate higher lift coefficients made possible with powerful flaps.

Initial glider airfoils used rather blunt noses and high camber. With composite technology the smoothness of wings could be improved and early numerical codes helped to shape the pressure distribution such that a fairly wide angle of attack range can be covered without incurring sharp suction peaks on either surface. The Eppler code of Richard Eppler was the first such tool and by prescribing pressure levels over sections of the airfoil at specified angles of attack it made laminar airfoils easy to design on the low-power computers of that time. The result are contours with a small nose radius which would produce a sharp suction peak once the specified angle of attack range was exceeded. By suppressing the suction peak over that specified angle of attack range, the laminar bucket could be maximized in a way that a blunter nose would not allow.

Now for that promised excursion into boundary layer theory: A laminar boundary layer is stabilized by a positive speed gradient (accelerating flow) but a negative speed gradient will trip the transition to turbulent flow rather quickly, especially at higher Reynolds numbers. The back side of a suction peak has just such a negative gradient; therefore suction peaks must be avoided in order to keep the flow laminar over much of the wing chord of gliders. In addition, using camber flaps allows to shift the laminar bucket up and down on the lift coefficient scale, so the combination of a small nose radius and a camber flap will allow laminar flow over a large lift coefficient range.

Airliners cannot do the same: The Reynolds number on their wings is so large that turbulent transition happens near the nose regardless of the local pressure distribution. Therefore, they are not concerned with laminar buckets and will happily tolerate the suction peaks that come with higher angles of attack. In cruise, when managing the pressure distribution is essential to reduce shocks, the angle of attack range is extremely narrow, so the blunt nose produces no disadvantage. However, this transsonic pressure distribution demands very little camber in the forward part of the airfoil, so in order to tolerate high angles of attack during approach and landing, they need a blunt nose. The larger nose radius also helps to integrate leading edge devices like slats and Krüger flaps, which is another advantage at low speed.


Why are there many pilots who don't say "mayday, mayday, mayday" to declare an emergency?

I'm a controller, not a pilot, so I can only speak from my own perspective.
What we are taught in ATC school is that many pilots are reluctant to use the word mayday because they feel it might escalate a situation unnecessarily and potentially create a lot of paperwork. I guess, mentally, it seems like calling mayday is a significant, irreversible step which can be daunting to take. They might be afraid, assuming the situation goes well, that they have to explain why they decided to escalate the situation, get emergency services involved etc.
That's not really a resonable fear, though. In fact, as a controller, I would much prefer a pilot to call mayday as opposed to trying to make the situation seem less critical than it is. In an emergency, clear communication is absolutely essential. I've had several situations where pilots have had "minor technical issues" or an engine that is "a bit jumpy" or something like that, and it puts me in a really uncomfortable situation because I can't really tell how serious it is. And if I'm in doubt, I will just call the emergency services anyway. On the other hand, the one time I actually had someone call mayday, within 30 seconds, the firetrucks were on the move, all the runways were cleared and the aircraft in question was cleared to land. It's much easier to help you if you are clear about what you need!

Pilots believing themselves to be facing an emergency situation should declare an emergency as soon as possible and cancel it later if the situation allows.
The correct method of communicating this information to ATC is by using the prefix “MAYDAY, MAYDAY, MAYDAY” or “PAN PAN, PAN PAN, PAN PAN” as appropriate. This procedure, which is an international standard, is the single most effective means of alerting the controller to the need to give priority to the message that will follow.

Emergency Communications on SKYbrary
In the end, it is always up to the pilot(s) to decide what is the right thing to do. Every emergency situation is unique, and there isn't (and can't be) rules describing what exactly must be done in every single situation.


Can I legally taxi a seaplane in a canal or channel?

I would advise checking out the FAA's seaplane handbook for more information. In general local ordinances tend to be the issue here and not the FAA's regulations.

You can find the right of way rules in 14 CFR 91.115

  § 91.115 Right-of-way rules: Water operations.

  (a) General. Each
  person operating an aircraft on the water shall, insofar as possible,
  keep clear of all vessels and avoid impeding their navigation, and
  shall give way to any vessel or other aircraft that is given the
  right-of-way by any rule of this section.

  (b) Crossing. When aircraft, or an aircraft and a vessel, are on
  crossing courses, the aircraft or vessel to the other's right has the
  right-of-way.

  (c) Approaching head-on. When aircraft, or an aircraft and a vessel,
  are approaching head-on, or nearly so, each shall alter its course to
  the right to keep well clear.

  (d) Overtaking. Each aircraft or vessel that is being overtaken has
  the right-of-way, and the one overtaking shall alter course to keep
  well clear.

  (e) Special circumstances. When aircraft, or an aircraft and a vessel,
  approach so as to involve risk of collision, each aircraft or vessel
  shall proceed with careful regard to existing circumstances, including
  the limitations of the respective craft.

There is also a note from the hand book,

  Pilots are encouraged to obtain the USCG Navigation Rules,
  International-Inland, M16672.2D, available from the U.S. Government
  Printing Office. These rules apply to all public or private vessels
  navigating upon the high seas and certain inland waters.

Although you should not expect your average boater to understand or even be aware of the FAA regulations on this. As such caution should always be used. Keep in mind that in most if not all states you don't even need a license to operate most personal sized boats (under 65 ft. generally).

You will need to check with your local municipality (who ultimately has jurisdiction over the water way most likely). Unfortunately they may have local ordinances that are more intended for land based aircraft but may incidentally apply to you. Things like noise abatement may also stand in your way, keep in mind a plane is significantly louder than most boats.

This FAQ covers a lot of really good questions about where you can and can't land and other seaplane related topics.

Of interest to you may be Question 2

  2. Where can I keep my seaplane?

  A seaplane can be kept at a certified seaplane base with permission, or at an  airport if it is equipped with
  amphibious floats. If you want to keep your seaplane "off airport",
  however, you will need to explore the issue with your state
  aeronautics office, local government and the owner of the waterway.
  Some specific problems include mooring restrictions imposed by the US
  Army Corps of Engineers, seaplane base licensing requirements imposed
  by state governments and zoning regulations imposed by local
  jurisdictions.

In this kind of situation I would personally seek out a local lawyer that specializes in aviation related matters (a call to your local GA field may get you a name). Unfortunately many local authorities don't always understand the regulations and many don't even know its with in their power to regulate such operations since the cases tend to be few and far between. In other cases older forgotten regulations may have relegated them selves to a dusty file cabinet that no one remembers. Other local sea plane owners may also be of help here.

It is also worth considering if you have the width to actually pull this off. Even the biggest boats I have been on (in and around the 55ft. size) are generally no wider than around 15ft. at the widest point. A Cessna 172 has a 36ft. wingspan which is something worth thinking about.


Can the ball on a turn coordinator be to the full right/left while the wings are level?

Yes, when the rudder is highly deflected. The slip indicator gives us information on the yaw axis of the airplane and "level flight" is generally in relation to a neutral pitch and power situation (i.e. not climbing or descending). An aircraft can be yawed with rudder input alone and subsequently cause the slip ball to deflect as appropriate.


The short answer is you probably wouldn't do it this way: Designing a drive system which allows you to split the work force between two output shafts at an arbitrary ratio (effectively an active differential) is pretty complex, and failure-prone. The failure of the differential would be "problematic" (both rotors stop), and the failure of a single drive link would likely be catastrophic (one rotor stops in the VTOL configuration you have an extreme wing-over from the imbalanced thrust).

The generally preferred design for this type of aircraft has two full engines mounted on the wings (for example the V-22 Osprey).
Similar results can be achieved by vectoring thrust from a jet engine (as in the Harrier "Jump Jet"), which can be done with a single engine.


Are there passenger flights with stops in the middle?

These "direct" flights are actually not unusual, at least in the US. I was surprised by this discovery based on the routes I personally tend to fly, but fortunately we have the Bureau of Transportation Statistics to provide objective information. Using the July 2019 data for all major carriers, I identified direct flights by counting multiples of the same flight number on the same day with the same aircraft, as opposed to single-segment "nonstop" flights. Here's what I found:

Some interesting notes:

Over a third of all flights are direct.
The practice is not limited to low-cost carriers. In fact, the two largest traditional carriers (American and Delta) operate around half of their flights direct and together operate almost two-thirds of all direct flights.
Nevertheless, Southwest uses more direct flights for its size than any other airline, nearly rounding out the other third.
On the other hand, JetBlue, which is commonly considered a low-cost carrier, barely operates any direct flights. Other low-cost carriers use anything from zero (Allegiant) to about a quarter (Spirit, Frontier) direct flights.
The other traditional carriers use direct flights sparingly. Hawaiian flies only nonstop routes, which makes sense.

I may have missed a few direct segments that departed past 12am on the day after the previous segment, but I don't think there's enough of those to substantially alter the results. In any case it would just increase the number of direct legs.
It'd be interesting to run some more analyses to see historical/annual trends in the practice, but that would require someone with more patience and/or computing power than I have. These data sets are huge and can only be downloaded month by month. I chose July 2019 because that was about the peak of commercial passenger aviation.
There's also some fun aviation memorabilia associated with these "direct" flights:
(source)
On airlines with no assigned seats, plastic cards like this used to be provided so that even the passengers continuing on could deplane if time permitted. Somewhere I have a couple old ones from Alaska Airlines. Nowadays I believe the airsickness bag doubles for this purpose in some cases. I suppose if you really didn't want someone to take your seat, you could leave a full one behind.
Note: A previous version of this answer had a much lower count of direct flights. Turns out Excel just wasn't done calculating yet.


What UK aircraft looks like an Islander but with twin fins?

It sounds an awful lot like a C-145, or maybe the 'civilian' version (it's primarily used by militaries), the  PZL Skytruck.

Source

It's used by the U.S. Air Force, so seeing it in Britain would not be surprising - all 3 of the U.S. Air Force's RAF joint use bases are within 100 air miles.

It's twin-engine, with a tailplane and twin fins
Its tail boom does narrow somewhat and distinguish itself, as seen here
It does have somewhat of a chunky body
The wings are fairly thin (front-to-back)


Why do some aircraft stay in airports for a long time?

There are many reasons. At least one is that the airline may not need that airframe type until later that day (the next flight that requires said airframe). Airlines don't just fly any plane they have on any route they have, this would be terribly inefficient and involve using an airplane that is not needed for a given mission. Take any major airline for example that fly large planes to lots of destination in Europe. Lets say that they have a 777 Arrive at EWR from Paris. The next long haul flight that airline has out of EWR is that evening to Dublin. But though out the day the carrier has 2 commuter flights down to Charlottesville VA totaling about 100 passengers between the two flights. They are not going to use a 777 to move 100 people and some light bags, so the plane sits until its needed again.

All planes (even non commercial) are built for a given "mission" much like you would not buy a Ferrari if you wanted a car that was good on gas.


Where is the APU in the case of an S-duct (like the L-1011)?

(Source via aircraft.wikia.com)

APU is number 134 on the cutaway, which you can find near root of left stabilator leading edge. It is ahead of the engine, below the intake duct, on the left side.

By Reedy (Own work) [CC BY-SA 3.0], via Wikimedia Commons

Its exhaust is on the right side close to the stabilator's leading edge.


Why does the radar dome on the E-3 have a white stripe down the middle?

The white strip is the metallic part which houses the electronics and serves as support structure for the antenna. The black part is basically a composite (fiberglass) shell, which covers the antenna. According to RAF:

  The radar is actually the white part of the disc, the black being there to balance and streamline the radar as it rotates

According to this page:

  Each AWACS 707 plane has two fiber glass radomes, joined by a 6 ft. (1.82 m) wide metal spine to form a 30 ft. (9.14 m) diameter radome.

This can be seen in this Northrop Grumman AN/APY-1/2 AWACS multi-mode radar inside the E-3D rotodome.

The Northrop Grumman AN/APY-1/2 AWACS multi-mode radar inside the E-3D rotodome. (Credit: Scott Carson. Twitter.). Image taken from aviation.stackexchange.com

The fiberglass shell is painted over in black as black was one color that did not require metallic particles in its paint composition (which would interfere with radar waves) as discussed in another question in aviation.stackexchange.


Are the aircraft used for the A380 display ready to be delivered to airlines?

In all cases the answer is yes, those aircraft were all outfitted with seats, and at least very close to being ready for delivery to the customer. Installing interiors and seats is a regular part of the production process, and it's unlikely that the plane would be pulled from the line before seats are installed, and worked back in later. From looking at rollout and delivery dates, these aircraft all had about 7 months between rollout and delivery, with the air show appearance happening within a month of delivery. Also, the aircraft are used as static displays while not flying, allowing people to walk through and see the interior.

In 2011, the Korean Air A380 indeed had seats in it. It was delivered in the middle of July.

In 2012, the Malaysian A380 also had seats installed. It was delivered at the end of July.

In 2013, the British Airways A380 probably had seats installed. I can't find any pictures of the inside, but it was delivered shortly after in early July.

Airbus also tends to bring a flight test A380 like MSN001 (first to fly, registered F-WWOW) for display, and has a more unusual interior. The Korean Air A380 was used for the display in 2011 after the flight test plane MSN004 had a bit of a mishap.


Are pilots generally trained for crosswind landings?

Of course yes.

Crosswind landings, like the one in the video you linked, are very common. In fact, landings with no wind or only headwind are rare.

There are several techniques pilots are taught during their extensive training to land aircraft when there is crosswind. The Wikipedia article I referenced above lists them. Since your question isn't about them, we won't discuss them.

Certainly, extreme weather situation are not very common. Some parts of the world experience extreme weather more than other areas. Diverting to other airports is not always an option and in some cases, once an airplane is on the approach, going around becomes impossible.


Are there disadvantages of coaxial rotor on attack helicopters?

The unique configuration of twin coaxial rotors presents both advantages and disadvantages, and the relative weight of each must be considered in light of the aircraft's mission.

I am going to assume the reader has a basic understanding of helicopter controls (cyclic, collective, antitorque).

Disadvantages

Aeromechanical complexity

A coaxial rotor has a much more complex hub than a regular one. It does not just need to invert the rotation direction, it has two sets of cyclic and collective controls that also mimic the effect of a tail rotor.

Since the lower rotor operates in the down-wash of the upper one, its collective pitch needs to be higher to produce the same lift (and thus torque). This is important because a torque imbalance will make the aircraft yaw uncommanded. In fact, this is how yaw is achieved when the pilot steps on the pedals, by letting the rotors generate differential lift, but ensuring their combined total stays the same so the aircraft retains level flight.

The cyclic controls suffer the same effects, that is the least of their problems, see below.

Rotor self-intersection

A single rotor in forward flight will assume a slightly conical shape due to the lifting force on the blades. This cone will also be tilted sideways with respect to the direction of flight because the blades see different airspeeds at different points in their cycle. That is to say, the blades will generate more lift while they advance into the direction of flight, and less while they retreat on the opposite side of the helicopter. The higher the forward speed, the larger will this effect be (In fact, there will usually be an area on the rotor where the blades do not produce lift because the airflow over them is reversed: the forward speed of the helicopter is greater than the local speed of the blade).

In a coaxial design, both rotors turn in opposite directions, so their tilts will be in opposite directions too: the clockwise-turning one will tilt right, and the anticlockwise-turning one will tilt left (the actual locations of maximum bending are not quite as clear cut because of blade inertia, but this serves as a basic explanation).

The resultant problem is pretty obvious: the two rotors can intersect each other. This is a Bad Thing(TM) and will lead to all sorts of issues like rapid unplanned dissasembly of the rest of the rotor blades. A pilot in this situation will most likely make use of the conveniently installed ejection mechanism.

The actual forward speed at which self-intersection will occur is not published (to the best of my knowledge), but is probably somewhere above 250km/h. This is only taking forward flight into account... sudden collective or cyclic inputs at high but not critical airpeeds may have the same effect.

The overall effect is that coaxial rotor helicopters are more limited in their flight envelopes than conventional ones. Although this is not a fatal drawback,  agility is highly valued for attack helicopters.

Yaw Response

This is somewhat speculative, but the lack of a dedicated tail rotor may limit the yaw rate at low forward airspeeds, where most of the yawing toque would come from the tail rotor.
At high speeds the weathervaning effect of the tail becomes dominant anyway, making this a moot point.

Advantages

Simpler tail design

The lack of a tail rotor frees the helicopter from having to transmit torque to it, thus heavily simplifying the boom design. In fact, some coaxial designs make a particular point of exploiting this advantage, having very short boom, like the Ka-25 and Ka-27, which are navy helicopters meant to fit into the tight confines of a shipboard hangar.

Less weakpoints

The loss of a tail rotor is not, like many movies would suggest, an unrecoverable situation. A helicopter can be stabilised with sufficient forward speed if it has enough tail volume.

That being said, losing a tail rotor in a combat situation will most likely result in a mission kill. And it's not just the rotor that's vulnerable, the boom contains the transmission rods for it, so any damage along that structure can be potentially fatal.

For a coaxial helicopter,the loss of the tail will lead to some stability issues, but nothing catastrophic.

Whether the advantages of the design outweigh its drawbacks is ultimately a matter of preference and of the fit between the aircraft and its intended mission. The adoption rate of a design is not necessarily an indicative of its merits, especially when military procurement is involved, as there are countless other factors at play.


What is the process for requesting flight following in Canada?

The general procedure in Canada is the same as in the USA: to contact the nearest ATC unit and ask for a radar service.

For any VFR flight beyond 25nm from the point of departure it is mandatory in Canada to file either a flight plan or a flight itinerary before departure - (CAR602.73(2)).

The provision of radar service to VFR traffic is typically at the controller's discretion and dependent on workload.

VFR Traffic in the Montreal area is generally expected to follow the suggestion published in the Canada Flight Supplement to contact the Montreal ACC 30 minutes prior to departure to obtain a discreet squawk code. This procedure becomes mandatory if you hope to penetrate the Class C airspace.

Generally it's worth reading the appropriate national publications and/or getting advice from an FBO at your point of departure when flying in a foreign country (Canada).


How do planes know what altitude they're cruising at?

Above Transition Altitude (e.g. this is 5000ft in Germany) the altitude is measured in flight levels (FL) - each FL equals 100ft and is measured above an artificial QNH of 1013,25 hPa.
If you cruise for example at FL380 that will be 38000ft above the pressure level of 1013,25 hPa. This system assures that 2 aircraft which are 10 FL apart (e.g. FL370 and FL380) always have the required separation of 1000ft.
During descent when passing Transition Level you will change your altimeter setting to local QNH which ATC will tell you.


How many versions of the P-51 dorsal fin fillet are documented?

A quick search of Google Images seems to show only three major variations on the tail for production P-51 variants:

Razorback (all P-51A, B, and C variants; no dorsal strake and a higher rear fuselage line):

Bubble canopy without the strake (seen on early D-variants)

With the strake (seen on later production D variants, integrated fully into K and H models and retrofitted to earlier Ds, sometimes to the Razorback B/Cs):

There is some difference in the prominence and exact shape of the fin fillet as the design continued to be developed; for all B/C/D variants it was a thin piece essentially bolted onto the strakeless frame. For the H variant in the bottom picture, it was integrated more fully into the fairing of the vertical stabilizer.

There's one more I've seen and I'm not sure if there's just the one example, but there is a P-51 "XR" variant, "Precious Metal", with dual counter-rotating props and further aerodynamic refinement, including a fairing behind the modified bubble canopy that narrows into a continuous strake along the aircraft's rear spine:

Other custom racing variants have existed over the years, all heavily modified from USAF production variants.


why are turboprop/turboshaft engines rated only in horsepower and never in ft-lbs of torque like diesel engines or electric motors?

The reason a Diesel engine has a torque figure and a horsepower power figure is because max torque and max horsepower occur at different engine RPMs. Have a look at any performance curves of a piston engine and this will be evident. Hence, to define in a few numbers the maximum performance of the engine, you need to give both figures.

However, turboprop engines for propeller aircraft, and turboshaft for helicopters, operate differently. Maximum efficiency for a rotor or propeller occurs at a certain RPM. Operating at different RPMs is inefficient. Hence, propellers and helicopter rotors are designed to operate at a fixed RPM. As load changes, the blade pitch angle is changed so it “bites” into more or less air. Hence, RPM can be kept constant, despite the load (and hence torque demand) changing. Now, since the power turbine is running at a constant fixed RPM, only 1 performance figure is needed (SHP or TQ) because the other can simply be derived from it.

Torque is just as important in a turboshaft or turboprop as a piston engine. These engines all have a torque sensor that usually works by measuring the twist in the power turbine shaft. Most helicopters do an engine performance check daily that involves reading TQ of the gauge prior to flight to ensure adequate performance.


Why is mixture lean recommended on the ground?

The new insurer for the Cessna 210 I fly is only charging \$300 to add me to the policy—instead of $1,300 the old one wanted—so I’ve been reading the 210 manual for the first time in several years. I noticed that the Owner’s Manual doesn’t mention anything about leaning on the ground and gives advice about leaning in the air that has been superseded by newer best practices. I looked in the manuals for the Cherokee and Cessna 182 and they don’t mention anything about leaning on the ground either.

One of the first things I learned when flying the Cessna 182 is to lean aggressively on the ground. There were several flights when I first started flying it that were not able to be started because, on runup, we couldn’t get the mags to clear. After a couple of expensive trips to the A&P, I started leaning aggressively on the ground. At the time, I was too busy with the firehose of information to even think about why I needed to lean, just that it saved a lot of money if I did. This post is an explanation of why you need to lean on the ground—especially older small-bore engines.

Fuel is different from the 1960s and 1970s

The POH for the Cherokee and 182 specify 80/87 fuel. The 210 specifies 100/130 and that 100/130 low lead is also approved. So what do these numbers mean?

Shell explains it really well, (bolding is mine)…

  Avgas grades are defined primarily by their octane rating. Two ratings are applied to aviation gasolines (the lean mixture rating and the rich mixture rating) which results in a multiple numbering system e.g. Avgas 100/130 (in this case the lean mixture performance rating is 100 and the rich mixture rating is 130).

  In the past, there were many different grades of aviation gasoline in general use e.g. 80/87, 91/96, 100/130,108/135 and 115/145. However, with decreasing demand these have been rationalised down to one principle grade, Avgas 100/130. (To avoid confusion and to minimise errors in handling aviation gasoline, it is common practice to designate the grade by just the lean mixture performance, i.e. Avgas 100/130 becomes Avgas 100).

  Some years ago, an additional grade was introduced to allow one fuel to be used in engines originally designed for grades with lower lead contents: this grade is called Avgas 100LL, the LL standing for ‘low lead’.

  All equipment and facilities handling avgas are colour coded and display prominently the API markings denoting the actual grade carried. Currently the two major grades in use internationally are Avgas 100LL and Avgas 100. To ease identification the fuels are dyed i.e. Avgas 100LL is coloured blue, while Avgas 100 is coloured green.

EAA explains the change as well…

  Avgas 80/87 is used in low compression ratio aircraft engines, and contains little or no lead (up to .5 grams of lead per gallon is allowed, but none is required). It is red in color, and should not be used in any automotive engine due to a low motor octane number of about 80.

  Avgas 100/130 can be used in some automotive engines. It has both research and motor octane numbers slightly over 100. Avgas 100/130 is green in color, contains four grams of lead per gallon, and is becoming hard to find.

  Avgas 100 LL (the LL stands for “low-lead”) contains two grams per gallon, half the lead contained in the avgas 100/130 it was designed to replace. It has research and motor octane numbers very similar to 100/130 avgas. The color is blue. This product sometimes has a high level of aromatics.

  Avgas 115/145 was developed for high performance piston aircraft engines used in World War II and the Korean conflict. It is very hard to find today due to lack of demand, and is usually only produced on special order. The color is purple.

Purvis Brothers has similar information in metric units:

  Tetraethyl lead content:

    80/87 Max of .14g/l
    100/130 has 1.12 g/l
    100LL has .56 g/l

So why do plugs foul?

The engines on the Cherokee and the 182 were designed for fuel that has little or no lead. 100LL has at least four times as much lead as 80/87. At annual there is a substantial amount of lead that needs to be removed from them. On the other hand, the 210 was designed for fuel that had twice as much lead. I don’t recall ever fouling the plugs and they don’t usually have much in the way of lead deposits at annual. I spoke to two other 210 drivers and they don’t recall fouling the plugs either—though they do lean aggressively on the ground.

As Steve Ells explains “The additive in Avgas that helps scavenge the lead in the fuel doesn’t do much until the combustion temperatures get up to around 1,400 degrees F.”

At idle, your engine isn’t generating much heat. If you remember from studying for your PP test, fuel is a coolant. So the more fuel you give an idling engine the cooler it runs. Leaning the engine makes it run hotter—which is what you want on the ground.

Leaning

None of the owner’s manuals I’ve looked at mention leaning on the ground. Probably because, when the planes were manufactured, fouling plugs wasn’t an issue. Fuel prices were low, so saving a half-gallon or so wasn’t that big of a deal. The truth is that a full-rich mixture combined with the low power setting is the perfect scenario for lead fouling your spark plugs and valve stems.

So why don’t instructors teach new students to lean aggressively? Maybe so low time pilots won’t forget to go full-rich before take-off. But if you lean appropriately, this will never be a problem. Lean the plane at taxi RPM (usually 1000 RPM) so that it starts to stumble. Then enrich just a bit. You’ll be able to taxi and do your runup at this mixture setting without worrying about taking off with less than full mixture. As soon as you advance the throttle a couple hundred RPM the engine will stumble. If you tried to advance to takeoff power, the engine would die from lack of fuel, before you even started your take-off run. To avoid the embarrassment of stalling when I leave the runup area, I put my hand on the mixture knob when I make my call to the tower or CTAF. And I don’t take it off until I’ve pushed the mixture to full rich for takeoff. (Or less that that for high-density altitude.)

John D. Collins explains his procedure for high density altitude:

  At sea level up to a density altitude of approximately 3000 feet, I would push the mixture full rich for takeoff. At higher density altitudes, I would do a full power run up and set the mixture to achieve the highest RPM as this will give you the most power available. As you climb above 3000 MSL, I would lean the engine for the highest RPM.

How octane numbers are calculated.

From Shell:

  Engine knock, which describes explosive detonation of the fuel/air mixture or preignition, can cause severe engine damage and subsequent failure in a short period of time. Anti-knock ratings are expressed as Octane Numbers for those of 100 or less and as Performance Numbers for those ratings above 100. These numbers relate the fuels performance compared to a reference fuel of pure isooctane. Because the anti-knock characteristics are influenced by the air/fuel mixture ratio, ratings are developed for both rich mixture performance and lean mixture performance. Rich mixture settings yield higher octane or performance numbers since the added fuel acts as an internal coolant and suppresses knock. Prior to 1975, both numbers were reported as the grade designation but current specification utilize only the lean mixture rating. Currently, ASTM (American Society for Testing and Materials) specifies five grades – 80, 82UL, 91, 100, and 100LL (low lead). In practice only 100LL is widely available. Production of Grade 80 has generally ceased due to small demand.

I don’t know when 80/87 started to become less available, but Service Letter L185B from Lycoming dated January 19, 1988 states that “Whenever 80/87 is not available you should use the lowest lead 100 grade fuel available.” It goes on to say that, “higher lead fuels can result in increased engine deposits both in the combustion chamber and in the engine oil. It may require increased spark pull maintenance and more frequent oil changes.…Operation at full rich mixture requires more frequent maintenance periods; therefore it is important to use proper approved mixture leaning procedures.”

The Service Letter doesn’t mention leaning on the ground, but it does mention that:

  At engine speeds of 1000-1200 RPM on the ground, the spark plug temperatures are hot enough to activate the lead scavenging agents in the fuel which retards the formation of salt deposits on the spark plugs and exhaust valve stems.

Since the leaner the mixture, the hotter the cylinder temperatures, leaning aggressively would help with reducing lead deposits.

The letter provides a method for leaning in the air to reduce deposits.


Why bother tracking TSN after an overhaul?

No, if the part is not subject to a life limit of its root component.

In the airline business, there are actually two terms used in the industry - "overhaul" and "restoration" which can generally be defined as follows:

If a part doesn't have a structural life limit, then the part can be rebuilt to new specifications an indefinite number of times, and each time, it is equivalent to a new part and will generally be called "overhauled" or "zero timed".  Mechanics can track "time since new" but they don't really have to and if parts move around it probably won't be anyway.  Piston aircraft cylinders are like this.  If you buy a rebuilt cylinder, nobody will know how many hours are on it since it was first made.

However a lot of components, especially components that operate under stress on transport aircraft, can have structural life limits applied as a result of endurance tests (where, say, a series of housings under endurance test will start to develop cracks at X thousand cycles or hours, and a life limit will be applied at 1/3rd of the endurance result, which is the safety factor applied to structural cracks that develop in test).

When these parts get rebuilt, they aren't called "overhauled", they are called "restored".  A structural actuator may get a restoration interval applied where it has to be rebuilt (because parts wore out during endurance testing) but once the root component like the basic housing gets to the life limit, the entire part is scrap.  In these cases the rebuild is not an overhaul but a restoration.  The hours or cycles have to be tracked since new since once it hits the life limit it is scrap even if it was rebuilt (restored) the previous month (which an airline won't let happen).


Is there any load and balance and attitude that can put positive G on one wing and negative on the other?

Just imagine an aerobatic aircraft going straight up and rolling at the same trime - sure it will have positive lift on one side and negative on the other while accelerating into the roll or when stopping the rolling motion. Note that during a constant rolling motion while in a vertical climb the overall rolling moment and the overall lift both are zero.

Also, wing twist will cause very little lift or even a small downforce at the wingtip at low angle of attack. I remember that the wingtips of the SB-8 and SB-10 gliders would point downwards from the negative lift at high speed. The high aspect ratio glass fiber wing would bend a lot and indicate the local lift with a local change in curvature.

However, if you were flying an approach, the aircraft should have flown too slowly to allow for that condition, unless it also started to roll violently from the lift asymmetry. If you had to apply full aileron to counter a gust-induced rolling moment, that very aileron deflection would avoid any negative lift on one of the wingtips. This only leaves the possibility that you had negative lift for a very short moment and over a small part of the wing when the gust hit you and before you countered the rolling motion with the ailerons.


What would be the best fan duct design to achieve maximum thrust?

It's not easy to find research on small ducted fans. I did come across this interesting masters thesis from 2009, which does show some relevant results - at low propeller speeds.

Duct design

The document describes three duct designs, one with a variable bypass inlet and two simpler ones. The simplest one is made from PVC tuning and a foam inlet lip:

Geometric information is given as:

Inner diameter = 14.5"
Duct Chord = 4"
Duct Length (C/D) = 0.28

Maximum Thrust

One of the results of the project is that a ducted propeller delivers more thrust than an un-ducted one. Here is one of the results, listing $C_T$ as a function of tip Mach numbers - which are really low and therefore hard to generalise into practical rotor and fan configurations.

Tip Clearance

Best results were obtained with a propeller with square tips with the smallest gap between propeller and duct that could be achieved. This was a 3-blade propeller, but the research project was severely limited by the small motor used, and therefore low maximum torque that could be exerted on the propellers.


What fomula use for calculating pressure altitude?

The formula from wikipedia is a rough estimation. Think of it as a rule of thumb. It's useful if you need numbers for rough calculations. However, the exact formula is as shown below:

According to that formula, I calculated pressure altitude difference for 28.92 and it's 938 feet, as opposed to 1000 feet. Additionally, as you can tell from the formula, the relationship is not a constant linear value. So difference of 1 inch of HG is not always 938 feet, it's changing.

So, the FAA guide is accurate. And I would say your app is accurate enough also, however probably omitting some numbers after decimal, hence the small difference. Of course, you need to add field elevation on top of those numbers as well.


In a helicopter does forward flight produce a rolling moment?

Yes cyclic is used to compensate.

The advancing blade is allowed to "flap" up to compensate for its extra lift, and the retreating blade flaps down to compensate for its decreasing lift.  On an articulated rotor, each blade has a flapping hinge for this purpose.  On a two blade teetering rotor, the blades being on the same axis are flapping up and down in harmony and the teetering pivot at the center takes care of it.

If the cyclic was perfectly centered (swash plate horizontal) the rotor disc would be flying a long with a slight tilt away from the advancing side as the machine flies straight.  But this would impart a lateral moment that would make the machine turn to the left as it flies forward.  So to compensate the pilot is initially applying a small amount of compensating right cyclic without thinking about it (on a machine where the right side advances), so effectively the advancing blade gets a slight reduction in pitch and the retreating blade gets a slight increase in pitch, becoming more and more the faster you go.

As you get faster and faster, either the retreating blade runs out of pitch authority or wants to stall as its relative speed gets too low, or the advancing blade's tip becomes trans-sonic. One or the other.  This is what limits helicopter speeds.


Why is air mixed with bypass air on the A/C of an aircraft?

In short

Bypass bleed air is a fraction of the bleed air coming from the core compressor. It's named like this because it doesn't cross the air cycle machine of the packs as does the regular bleed air.

On the following picture this bypass bleed air is identified as trim air. Its purpose is to be mixed with cooler bleed air from the packs to modulate the temperature in the different zones.

Mixing is done by the mixing and distribution unit (see more below).

Use of trim air on the Airbus A320 family. (Source: A319/A320/A321 ECS Familiarization and Training Manual)

Boeing has open a new approach for air conditioning and pressurization using ram air rather than bleed air. The first aircraft with this approach is the B787, this requires dedicated compressors.

Details

The air conditioning and pressurization systems on large airliners are quite similar. Here is the overall picture:

Air conditioning and a part of the pressurization are obtained using the environmental control system (ECS)
Bleed air, the high pressure air taken from the engine cores and APU, is very hot.
Ram air, the cool low pressure air taken from the aircraft belly, is used to cool the bleed air in heat exchangers. This ram air, which gets warmer in the process, is rejected in the atmosphere without entering the cabin. A fan is used to create the ram air flow when the aircraft is on the ground.
Air conditioning packs are mainly air cycle machines (refreshers) that receive pre-cooled bleed air and deliver air at ambient temperature. There are usually 2 or 3 packs for redundancy.Some bleed air bypasses the ACM and can be used later by the mixing unit.
Air from the packs goes to the mixing and distribution unit which role is to mix air from packs with air already in the cabin, and deliver the result to different zones of the aircraft. The unit can vary temperature according to zones by mixing packs output air and bypass bleed air. The unit also mixes air from the ground supply when at the gate and engines and APU are not running.
Air in the cabin is recirculated by two or more fans which return it to the mixing unit.
As air is pumped into the aircraft by the mixing unit, air also exits the aircraft through one or several outflow valve(s). The exact quantity of air leaving the aircraft is controlled so that the pressure inside the aircraft is at the required value.This value is usually expressed as cabin altitude, that is the altitude we would find this pressure in the standard atmosphere. Pressure is about 1013 hPa at sea level and then decreases continuously when altitude increases.However the cabin altitude doesn't climb as high as the aircraft because there would be an insufficient oxygen pressure in the cabin. The cabin altitude is kept proportional to the aircraft altitude, but usually the minimum of pressure is a value equivalent to 6,000 to 6,500 ft.
All those elements are driven by controllers that can be monitored and programmed by the crew.

Let's look at the ECS for the Airbus A320.

1. Location of elements

(When not specified, the source of the images is the A319/A320/A321 ECS Familiarization and Training Manual)

2. Packs

A view of a pack with the triangular ram air inlet (the ram air outlet has been removed):

The pack includes:

Two air-to-air heat exchangers: main (MHX) and primary (PHX). Ram air is used as the heat receiver fluid for both.
A centrifugal compressor (COMP) and a turbine (TURB). They compose the air cycle machine (ACM) which cools and reduces pressure of bleed air.
A reheater (RH), a condenser (COND) and a water extractor (WE) to dry bleed air.

3. Air cycle machine

4. Pack operation

A pack works this way:

Hot high pressure bleed air is directed to the PHX which cools it a bit so that it can be processed more efficiently by the ACM.
Cooled air is sent to the compressor. Increasing the pressure allows for more heat to be extracted in the next step. Air gets very hot again.
Compressed hot air is sent to the MHX which remove as much heat as possible, possibly below the ram air temperature, which is great while on the ground.
Cool air, which contains water vapor, goes to a reheater and a condenser to create liquid water, water is collected by the water separator. This water is reinjected into the ram air to cool it before it is used in the exchangers.
Cool air finally rotates the turbine which extracts energy from it to drive the compressor and a ram air fan. The energy loss produces a cool low pressure air which can be used in the cabin and cockpit.

The exact temperature and moisture of air delivered by the pack are obtained by mixing air from the ACM turbine, cooler air from the PHX and warmest bypass bleed air taken upstream the MHX.

Water extraction is controlled by adjusting the temperature, in the same way, of the airflow going through the condenser.

Bypass bleed air is also delivered by the pack to the mixing unit for zone temperature trimming.

5. Water extractor

Collected water is reinjected into ram air flow.

6. Air temperature and pressure at each step

FCV: Flow control valve (bleed air input)


Why the dynamic pressure is not mentioned in the explanation of lift by Bernoulli's principle?

In short, "dynamic pressure is not pressure at all" (A Princeton paper on dynamic pressure and Bernoulli's theorem).  "Dynamic pressure" is a term that designates the decrease in pressure, as a consequence of the fluid's / medium's flow velocity.

I recommend that you also examine what NASA has to say:

  Integrating this differential equation:

  ps + .5 * r * u^2 = constant = pt

  This equation looks exactly like the incompressible form of Bernoulli's equation. Each term in this equation has the dimensions of a pressure (force/area); ps is the static pressure, the constant pt is called the total pressure, and

  .5 * r * u^2

  is called the dynamic pressure because it is a pressure term
  associated with the velocity u of the flow. Dynamic pressure is often
  assigned the letter q in aerodynamics:

  q = .5 * r * u^2


What are these four tubes for in the intake for an Extra 300?

Those are the impact tubes for the fuel metering for the RSA-type fuel injection system:

Above is the face of the fuel servo. If you had more light you would have also seen the throttle valve as shown above. When the throttle opens, air rushes in, and the higher pressure in the impact tubes would deliver more fuel. See schematic and description below:

... As a result, the diaphragm moves to the left, opening the ball valve. Contributing to this force is the impact pressure that is picked up by the impact tubes. [Figure 2-32] This pressure differential is referred to as the "air metering force." This force is accomplished by channeling the impact and venturi suction pressures to opposite sides of a diaphragm.

Source: FAA-H-8083-32A. Aviation Maintenance Technician Handbook-Powerplant Volume 1.


Can the real time operating system used in aviation equipment be certified by itself?

A DO-178C compliant RTOS (such as LynxOS-178 mentioned by @mins) is going to have a certification package you provide along with your DO-178C artifacts for your software.

Note that there is no such thing as a DO-178C "certificate". DO-178C is a process that generates certification artifacts that are packaged and provided to the Certification Authority as part of some approval application (TSO, TC, STC, PMA)

It is also usefull to recognize that most software on equipment installed on an aircraft is not going to run on an RTOS, but rather run directly on the hardware (bare metal)


What is the relation between airspeed and rate of turn?

Independent of airspeed, angle of bank gives you the "G" force in a level coordinated turn. A 60 degree bank = 2 G's, at any speed.

At low speed, a given G force will give you a greater heading change than it would at a high speed. Driving your car at 10 mph you can complete a U-turn, 180 degrees of heading change, in a couple seconds without pinning passengers against the door, but at highway speeds, even a hard turn that does impose big side-loads will take much longer to give you that much heading change.

Related to all this is turn radius: at low speed, your turn radius for any given loading (bank angle) is smaller than it would be at a higher speed with that same loading/bank angle. Thus, even though you're going slower, you have less distance (circumference of the circle) to cover, and so you are able to complete the circle (or one 360th of it) more quickly.

((For the pedantic ones, everything here assumes level, coordinated turns. Change those assumptions enough, things can be different, but those aren't particularly instructive cases until the base case is understood.))


Excerpt from "First-Hand: Evolution of the 2-Person Crew Jet Transport Flight Deck"

  Early in the preliminary design effort, Boeing had taken a careful look at the in service performance and accident history of previous airplane designs. The conclusions were interesting:

  In flight troubleshooting sometimes led to more serious problems, occasionally jeopardizing the safety of the flight.
  The flight engineer’s intense focus on systems problems often distracted one or both of the pilots, sometimes resulting in no one “flying the airplane.”
  Jet engine reliability was much higher than piston engines and they required little or no in flight troubleshooting or adjustment.
  Certain high workload periods during departure and arrival left little time for outside watch nor much time for handling abnormal events.

  In support of industry work to establish rational external vision requirements for transport airplanes, Boeing developed a computer model that determined the relative collision threat value of the visual space around the aircraft. Because airplanes are in motion and at a positive angle of attack, the computer model showed that the collision threat is concentrated generally forward and below the longitudinal axis of the airplane.

  In response to these findings the design team took a different approach to the 737 flight deck:

  A concerted effort would be made to simplify systems designs to increase reliability and to eliminate or greatly reduce the need for in flight trouble shooting.
  A Flight Deck design group would participate in all flight deck design decisions and would have considerable authority in the design of controls, indicators, and operating concepts across all airplane systems.
  Workload assessment would be used as an integral element of design to adjust functionality and location of equipment and controls.
  Operating procedures would be simplified and shortened to reduce distractions and provide ample time for outside watch.
  The autopilot would be improved to better support operations in the terminal area not just in cruise.
  The pilot’s stations would be designed to maximize external vision in the primary collision threat zone.

So it seems that a lot of the changes that facilitated the movement to a 2-crew flight deck were procedural ones rather than engineering challenges. Jet engines by that time didn't require constant attention and adjustments. It seems that this movement towards a reduced cockpit count was the result of "too many cooks in the kitchen", and while not explicitly mentioned it seems like Eastern Air Flight 401 was a driving factor (Edit: or at least they wanted to prevent something like that, turns out EA401 didn't happen until 1972 when the 737 was introduced in 1968).

I'm guessing though that this won't completely answer what you are looking for, I'm sure you'd want to see the specific input that the "Flight Deck Design Group" offered for the placement of controls and indicators. I guess one way of figuring this out would be to compare the cockpit layout of the 727 and 737, as far as I can find, those design decisions aren't published.


Many small 4 to 6 seats propeller airplanes can overcome their brake/wheel holding capability, on concrete and ashpalt.  My Cessna Cardinal is 180HP, I do engine pre-takeoff runup checks at 1700-1800 RPM. Going to 2600, the plane will start moving.  Many other planes of similar size are 300 HP, and they definitely won't hold. We only have two 5" or 6" disc brakes, no power assist, pushing the pedal tops with our toes. Or a hand-pull parking brake.


Why is there conflicting nomenclature for the 1,000 ft runway markings?

First, let’s clear up some terminology. Touchdown zones are only on runways with precision approaches. Aiming Point Markings are at 1,000' in USA (ICAO standard is different and is runway length dependent) and are also on non-precsion and visual runways. If you fly a small GA airplane, you will often land in the first 1,000' using the Aiming Point as a guide. The markings are at a fixed distance (500') from each other so that’s probably why you have heard that term. I have most of the FAA publications and ACs on my computer and the only place that "fixed distance markers" is referenced is in the Chart Supplement for some runways that have non-standard distances for the markers, so I am curious as to where you have read that term. EDIT: Per the edited question, I changed my search to “fixed distance marker“ and found a half dozen references. All of them are in Knowledge Test questions and none in other FAA publications.

AIM 2−3−3. Runway Markings

  e. Runway Touchdown Zone Markers. The touchdown zone markings identify
  the touchdown zone for landing operations and are coded to provide
  distance information in 500 feet (150m) increments. These markings
  consist of groups of one, two, and three rectangular bars
  symmetrically arranged in pairs about the runway centerline, as shown
  in FIG 2−3−1, Precision Instrument Runway Markings.

  TOUCHDOWN ZONE− The first 3,000 feet of the runway beginning at the
  threshold. The area is used for determination of Touchdown Zone
  Elevation in the development of straight-in landing minimums for
  instrument approaches.


Why do flying wires hum louder in a sideslip?

Here is one reason.
It once was common for suspension wires to be flattened rather than circular in cross-section, to reduce drag. those wires were then aligned with the airflow past the wing and fuselage in cruise. However, any time that airflow was not aligned with the wires, they would present an angle of attack to the slipstream and generate lift, which would pull them out of position, stretch the wires, and twist them to increase the local AoA. Then the wires would shed a vortex, stall (killing the lift), untwist, snap back, and then get pulled out of position again, etc., etc. and the result was the wires would "sing".
You can think of this as a special case of aerodynamic flutter.


4 Boeing 707 for $50,000. Is that possible?

Its surely possibly and there are a few things I can think of off hand,

Missing Log Books: generally speaking if an aircraft is missing its log books you need to do quite a bit of tear down and rework on it to make it airworthy once again. In many cases the cost of this far out weighs the cost of a logbook/airworth plane so not always done. As such this can severely drive down the price of the aircraft making it close to worthless. In line with this, without the log books you can not sell any of the parts for replacement either. With out the ability to properly tag the part they are only worth their weight in scrap. Considering they seem unflyable and are fairly large to chop up and ship scrapping it can also become a costly maneuver.
The tax man...: Airplanes (and all manner of vehicles) often change hands for strange dollar amounts due to various tax regulations. That does not look like the case here but it does happen.
Simply Outdated: keep in mind the 707 is a fairly old plane by anyones standards these days. While it may have avionics chances are they are no newer than when it was parked there to collect dust, like it or not the best LORAN C unit in the world may have cost a fortune to install back then but is worth next to nothing now.

These things dont exactly go up for sale all that often but here is a pretty nice one listed for 6.2 Million and here is reference to one listed for only 700K. Please keep in mind that there are countless variables for this kind of thing that could drive the price both up and down drastically.


Are there schools that would hire a non-CFI ground instructor?

Our local flight school had a dedicated AGI/IGI for a while. Lots of CFIs don”t like doing ground instruction and, if they are building hours, every hour on the ground is an hour they are not building time. I like teaching and so I just took and passed all three tests and need to fly over to a FSDO to get my certificate.

For the Fundamentals of Instruction Knowledge Test, study the Gleim book. If you have time, and if you want to be a CFI, at some point you will need to read FAA-H-8083-9A Aviation Instructor's Handbook but you don’t need to read it to pass the exam. A lot of the information in the book is wrong, so I had a hard time memorizing the wrong answer to questions. You probably won’t have to worry about that.

For the IGI and AGI it probably helps to read FAA books or whatever you prefer for getting your aviation knowledge. And you definitely need to study FAR 61 and 91 and know the VFR and IFR charts. I used the Gleim books Flight/Ground Instructor and Instrument Pilot and there were only a few questions that were completely new to me.

I’d say that about half of the questions are really stupid. e.g.

The force acting opposite of lift is:
A. Apples
B. Gravity
C. Oranges

Another quarter were interpolation/math questions that you will never ever need while flying. The study guides do a good job preparing you for them. The trick is to read the question really carefully. e.g. Are they asking you to look on the chart for landing distance or do you need to do another calculation to get ground roll.

There are a bunch of E6B questions that you just need to be really careful in your arithmetic and reading the question.

There are a few that I really had no idea what the question was, let alone what the right answer was. I got about half of those right. There were also a couple on each test where none of the answers was the correct answer.

I took the Commercial and CFI since they are good for two years and I won’t have to study them again if I decide to get a CFI.

My results: FOI 84%, AGI 88%, IGI 90%, CFI 87%, COM 92%.

I studied for a month, which was a bit of overkill.

Sporty’s and King Schools let you take practice tests for free. I put a lot of my notes up on my website so you can save time when studying by printing and memorizing them.


200 km/h = 55,6 m/s
The density of air is taken as 1,23 kg/m3
You have a mass flow rate of 55,6 x 3 x 3 x 1,23 = 615 kg/s
In other words, in every second, you're moving a mass of air of 615 kg. The kinetic energy of that moving mass is 0,5 x 615 x 55,6^2 = 951 kilojoule. In a second, that's 951 kilowatt.
A lot of power, and that, assuming your fan has an efficiency of 100% and friction losses are zero...


How does carb heat increase the risk of dirt ingestion?

Carb heat is not only about heating the air going into the carb, it's also a meant to be an alternate air path in case the air intake becomes fouled or the air filter gets clogged by debris, snow or ice. Fuel injected airplane engines typically have an alternate air intake that bypasses the air filter for the same reason, it's part of the checklist to test the alternate air.

The reason there isn't a separate filter for the carburetor heat air is that the same thing that clogged up the main filter could clog up the secondary, it's better to have unfiltered air for a short time than take the chance of losing your engine.


What is a raw data takeoff and is there a benefit to performing them?

Raw data means data read on the basic instruments. Not-raw data means enhanced information built from raw data and/or flight management system.

Flight director bars are not raw data, but a computation from basic air data, desired airspeed, roll angle limit, etc. At the next higher level, the autopilot can use the FD data to fly the aircraft. Similarly desired speed can be determined automatically and thrust can be managed by the auto-throttle.

Taking off and climbing with FD, auto-throttle and autopilot

Takeoff with FD and AP, just after liftoff (A320 simulator). Source: Youtube

The green bars of the FD shows what should be the attitude of the aircraft, SRS (speed reference system) guidance law has been used to determine the desired 165 kt speed. The pilot may fly the aircraft manually using these non-raw data.

In the present case the pilot has also activated another layer of assistance: Autopilot and auto-throttle. The AP follows the FD indications, and the thrust is managed by the auto-throttle so that the desired speed is achieved safely as soon as possible.

The indication in the top-left corner means autopilot, flight director and auto-throttle are currently selected / active.

T/O with raw data

Flying with raw data implies not using the FD or the SRS mode, nor the AP other modes or the auto-throttle.

The crew must determine the attitude to be used after the T/O rotation, and during the climb. They also need to manage the thrust required. For example, the roll angle has to be determined manually, depending on all factors discussed above and the desired horizontal track.

Takeoff without FD (A320). Source: Youtube

Benefits

Not using computer assistance helps maintaining and developing pilot skills and prevents dependency on automation that can always fails. The correct attitude, speed and thrust depend on many limits  — some are displayed in the speed indicator band and the attitude indicator when they have video displays — that the pilots must have in mind.

So it's usual that pilots perform manual take-offs and landings and raw-data operations. The frequency of such permanent training may be influenced by the company rules (standard operating procedures, SOP) which can either encourage or discourage crews to use the airplane automatic features at certain phases of the flight.

On modern aircraft there are some mechanisms can't be inactivated as the direct transmission between controls and corresponding actuators have been replaced, partially or totally, by electrical links. The conversion between actual pilot movements and electrical command signals is subject to some level of filtering provided by the fly-by-wire system.

Other raw data operations

Raw data (ILS) approach.
Raw data navigation.

This post consistently uses Airbus-based explanations, as terminology varies between aircraft manufacturers (but principles are mostly the same for this topic).


Where can I get information on ATC facility activity, like the number of flights handled in a month?

The FAA's Air Traffic Activity Data System (ATADS) contains broad statistical information on activity in the national airspace system which can be used to get an idea of how "busy" a particular Center, TRACON, or Tower facility is.

The caveat is this data only encompasses aircraft talking to Air Traffic Control, so it won't be able to tell you anything about uncontrolled fields except possibly what you can glean from Center/TRACON activity information for the greater area.

I'm not sure where they get their data from, but Fltplan.com has annual reported data for each airport in their Airports & FBO's database:

For example, this is for KBRY (which is uncontrolled):


Does Smeaton's coefficient have a modern accepted value or it is dependent of the air density?

There is no modern equivalent to the Smeaton coefficient, therefore it has no "modern" value.

The Smeaton coefficient is supposed to represent a "coefficient of air pressure" (and thus indirectly density), but it is a false constant: There is no single value because air pressure is not a constant value, but rather a variable (depending on altitude, temperature, etc.).

As you noted lift is calculated by a different formula today, using actual density of the fluid (air) you're moving through in place of the Smeaton coefficient.

$ L = Cl \cdot \frac{(A \cdot \rho)}{2} \cdot V^2$

You can calculate the Smeaton coefficient at any given location/altitude with simple algebra if you know the air density: Solve the modern lift equation, plug the values into the "old" Wright Brothers era equation, and solve for $k$.

The density-based calculation also has the benefit of working in other fluids besides air. (The same equation can give you hydrodynamic lift for boats and submarines by plugging in the density of water - itself a dynamic value depending on temperature, salinity, etc.)


Could you help me identify a sound that began before pushback?

Well, expanding off my comment above, I was boarding a flight with Delta and the pilot said over the PA that the people sitting over the wing should not be concerned about a whining sound that they would hear in a few minutes as it was the hydraulic pumps working.


Why would an airport be depicted with symbology for runways longer than 8,069 feet even though it is reported on the sectional as 7,200 feet?

The FAA chart user's guide says:

   Hard-surfaced runways greater than 8069' or some multiple runways less than 8069'

It's the latter case of some multiple runways less than 8069'.

This is also confirmed in the post, "What is significant about the number 8069 ft?"

  Specialists also place these polygons around the runway pattern of aerodromes with multiple runways that are less than 8069 feet, in cases where the multiple runway pattern does not fit into the largest, circular aerodrome symbol.

(FAA FAQ; emphasis mine.)

Most likely MKO doesn't fit in the standard circle  because of 36-18 and/or 4-22 intersecting the circumference.

(Chart Supplement via skyvector.com)


How heavy are modern cantilever monoplane wings?

Wing structure is roughly 20% of empty weight, which is roughly half of gross weight. I clipped the two sources some time ago and no longer have references.


You're confusing the airspace height with airspace altitude.

The definition of Class C (in the US) is

  3-2-4 (a) Generally, that airspace from the surface to 4,000 feet
  above the airport elevation (charted in MSL)

If you look at chart for the Class C for Colorado Springs (elevation 6,187), the surface area of the airspace goes up to 10,200 ft MSL, approximately 4,000ft above the airport elevation of 6187.


Why are there different starter limitations for dry cranking and manual start on the A320?

Per the Airbus maintenance training manual, the crank limit whether it's dry (CRANK) or wet (IGN/START + MAN START) is 2 minutes.

An ECAM warning will notify of exceeding the limit: ENG 1 (2) START FAULT (condition: starter time exceeded).

This video shows the cranking of an A320 jet engine as a test after changing the IDG filter.


How should Isopropyl Alcohol be used as an AvGas Fuel System Icing Inhibitor?

FAA Advisory Circular AC No: 23.1521-1B deals with use of IPA for Part 23 aircraft. It states:

  ASTM D 910, Standard Specification for Aviation Gasolines, allows the use of isopropyl alcohol conforming to the requirements of ASTM D 4171, specifications for Fuel System Icing Inhibitor, as a fuel system icing inhibitor. Accordingly, isopropyl alcohol conforming to ASTM D 4171 may be used in concentrations up to 1 percent by volume, to benefit safety, as an icing inhibitor in automobile gasoline.

However, it is left to the aircraft manufacturers (according to ASTM 910 too) to determine whether or not to use IPA as an additive. For example, in a letter regarding Lycoming engines in certain of their aircraft, Cessna says,

  Isopropyl alcohol in amounts not to exceed 1% by volume can be added only to aviation fuel  (not  automotive  fuel)  to  prevent  ice  formation  in  fuel  lines  and  tanks.  Although approved  for  use  in  Lycoming  engines,  do  not  use  isopropyl  alcohol  in  the  aircraft  fuel systems unless approved by the aircraft manufacturer.

Note that Cessna allows the addition of IPA only to aviation fuel for the applicable aircraft. In UK, the MoD Defense Standard 91-90 again leaves the matter to the individual manufacturers, saying,

  The concentration of Isopropyl Alcohol shall be recommended by the aircraft
  manufacturer at the time of delivery to the purchaser and typically not exceeding 1% v/v.

Though almost all these cap the IPA at 1%, the best way is to refer to the individual aircraft's appropriate manual as it would have the necessary details. But generally, I think that manufacturers (of large aircraft, atleast) prefer not using IPA these days due to environmental concerns.


The following information is based on US FAA regulations and training programs.
In the US, there is no age limit (minimum or maximum) you must be in order to fly at the controls of an aircraft. There are age limits specified in Title 14 of the Code of Federal Regulations Part 61. Here are some of the particulars:

14 or older to solo a ballon or glider.
16 or older to get a balloon or glider private pilot certificate.
16 or older to solo a powered aircraft.
17 or older to get a powered aircraft private pilot certificate.
18 or older to get a commercial pilot certificate.

In the meantime, you can get your child into one of several youth aviation programs like Civil Air Patrol, or the Young Eagles.


Does radar altimeter work over the ocean?

Yes.  In the Navy, our primary source of altitude indication under 5,000ft is the radar altimeter.  The AGL over the ocean conveniently also happens to be the MSL as well.


Why do jet engines use kerosene rather than gasoline?

You can persuade a turbine engine to run on just about anything that can burn. So the decision of which fuel to actually use depends on the side factors including, but not limited to:

availability
cost
emissions
hot section temperature
chemical reactions with engine parts

Specific examples:

Coal dust is rather difficult to pump around, and the rampies don't like shovelling
liquid hydrogen (used in the Space Shuttle) requires a lot of storage and has the nasty habit of freezing anything it touches, like rampies.
ethylacetylenedecaborane is unpleasantly toxic (rampies union again) and the combustion byproducts were rather abrasive to the engine's innards
trimethylaluminum would reduce the engine complexity (no igniters needed) because it has the nasty habit of igniting instantly upon contact with air, so leaks are rather dangerous.
natural gas is commonly used as a turbine fuel in pumping stations: it's already there and thus is "free". The required pressure vessels make it impractical to use as an aircraft fuel.

So kerosene basically became the standard turbine fuel because it's:

cheap: kerosene makes up a rather large fraction of crude oil. When you measure your fuel load in tons a few cents per litre makes a difference.
safe to handle: relatively non-toxic, doesn't ignite all that easily
storable and transportable in common structural metals
doesn't clog up the engine


Can ATC terminate flight following AND approve (arbitrary) frequency change while still inside Class C airspace?

Jim S asks an interesting question.  Can ATC ask you to do something which is contrary to the regulations?  The answer is they can, and just because they do, does not change the regulations.

The practice of advising aircraft to switch to advisory frequency and squawk VFR is common here, in Class C airspace, and I have experienced it in Class B airspace as well.

Discussing this with the ATC supervisor once, it was explained to me this way: The controller knows where you are going, knows your intent, and believes that he can manage the traffic in the area effectively if you leave the  frequency. So for the controller it is an issue of risk and workload management.

With students, this creates a good learning opportunity to have them "monitor" one radio, while using another.  No official guidance on this, but it is common practice. So if you are not familiar with this, have an experienced pilot or CFI demo it to you.

Keep in mind that this is the same process for IFR aircraft making approaches to non-towered airports.

Arguably, ATC can authorize deviations from 91.130, as @DaveCFII points out. Certainly that covers temporary frequency changes to check WX and things like that, but it could be interpreted to have broader scope is that it could be used to terminate communications near to the point where an aircraft leaves Class C.

Having said this, I would prefer that ATC state something like, "communications deviation approved."  Of course that will not happen on a busy frequency.

Having said all this, it is important to remember that the pilot has final authority over a flight, and the FAA in the Granby LOI makes it clear that ATC does not have the requirement to point out to pilots what regulations apply to their operation.  So a pilot who is uncomfortable with this procedure, might monitor ATC on one radio, and broadcast their intents on the advisory frequency. This way they can fully comply with the regulations. Most aircraft have dual COM radios.  If not, delay your frequency change until you are clear of Class C, to remain in strict compliance.

Ref: Granby LOI  https://www.faa.gov/about/office_org/headquarters_offices/agc/practice_areas/regulations/interpretations/data/interps/2006/granby%20-%20(2006)%20legal%20interpretation.pdf


Are there advantages when operating with CG near forward or aft limits?

A CG near the aft reduces drag by reducing the necessary downforce that has to be supplied (usually) by the tailplane.
This is also a point for competition gliding (more often than not you have water ballast in the vertical tailplane to compensate for those love handles). Aerobatic gliders sometimes feature aft lead ballast to enable maneuvers like tailspins which are not possible with a forward CG.

Aft CGs come with reduced stability, which can be a severe flight-safety concern (e.g. in case of unrecoverable spins).
Some fighter aircraft feature inherently unstable aerodynamics (see super-maneuverability) which offer great agility (e.g. turning rates), but need constant input by a flight constrol system (and woe if that fails, e.g. via Pilot Induced Oscillation).

A forward CG results in greater flight stability and reduced danger of sudden stalling, but also increased drag (because of the increased downforce required from the tailplane).  In extreme cases this can limit the maneuverability as well and cause a pancake landing (the tailplane can only produce so much lift/downforce before itself stalls, at which point it is no longer holding your nose up).


What airfoil simulation software is suitable for a cut trailing edge?

One tool you might want to consider is MIT's XFoil. I'm not sure I fully understand what you'd like to do, but this tool is used in many of MIT undergraduate aerodynamics classes.

If you think it might work for your purposes, here is a course by the program's creator from MIT's OpenCourseWare site. Lecture 3 is a tutorial on how to use XFoil. You might want to browse the OpenCourseWare site for more advanced uses of XFoil, too.

Good luck!


How can we calculate the fuel that we need?

You need to know the fuel burn per distance covered which depends, among other things, on aircraft weight, speed, altitude and wind (all of which probably change over time), and then you add it up over the trip distance for the planned route (which is likely to differ from shortest possible great circle route for various reasons like airspace organisation and weather). As mentioned by OverD, regulations require some extra bits on top to cover for unforeseen circumstances. As also observed by OverD,  this is also dependent on the individual aircraft - e.g. old engines might need more fuel than new ones, a dent in the fuselage might create additional drag, an inoperative component might require extra fuel or off-optimum altitude etc. etc.

The manufacturer, in your example Boeing, provides the aircraft‘s performance data (e.g. fuel mileage etc.) to the operators.


What is the history of Concorde motto "May Concorde bring people closer" and medal?

That medal is a copy of one that was presented to Queen Elizabeth II by M. Ziegler (S.N.I.A.S) during a state visit by the queen to France in 1972, sometime between 15–19 May. The most likely date would have been 18 May, when she visited a SNIAS helicoper factory and was accompanied by Mr. Ziegler.

It was presented to commemorate the intention to build the 'Concorde' for commercial supersonic flying and currently sits in the Royal Collection Trust. "M. Ziegler" is Monsieur Henri Ziegler, who passed away in 1998 at age 91.

"In 1968, he was appointed to run Sud-Aviation, a leading aircraft manufacturer in the south of France, and charged with reorganizing the badly splintered aircraft industry. Sud-Aviation was later absorbed by the larger Societe Nationale Industrielle Aerospatiale, or Snia, which in 1984, as the core of the French aerospace industry, was renamed Aerospatiale S.A."

That is where the "S.N.I.A.S." comes from, "Société nationale industrielle aérospatiale". The Concorde was built by British Aircraft Corporation (BAC) and Sud Aviation. Mr. Ziegler was instrumental to the partnership, and was seen by some, at the time, as a traitor to France for suggesting it.

I think it is fair to say that this motto was coined by Ziegler for the creation of the medal, likely to express his gratitude for England's role in the partnership, but it is not the official motto of the Concorde.

British Airways' slogan for the Concorde was "Arrive before you leave." I honestly don't remember, nor can I find, a French slogan for the Concorde, but the slogan for Air France is "France is in the air."

It was a lot of fun researching the answer for this.


When do we use IATA codes and when do we use ICAO codes?

It helps to know the objective of both ICAO and IATA to understand when which code is used.

ICAO (International Civil Aviation Organization) is a UN-body which focusses on international harmonization of civil aviation regulations.

ICAO codes are used for "official" purposes such as Air Traffic Control; E.g. flight plans use ICAO codes for airports and airline flight identification.

IATA (International Air Transport Association) is a trade association that focusses on making air traffic businesses safe, secure, reliable and efficient.

IATA codes are mainly used for ticketing. E.g. travel itineraries use IATA codes for airports and IATA flight numbers.


Tyre wear and land gear shock reduced by wheel spin up

I don't believe there is problem with gyro precession in spinning wheels; it's more of a vibration issue.  On most airliners the brakes are applied automatically after liftoff to stop the mains and the main reason is unpleasant vibrations as the wheels slow down without the damping effect of being pushed into the pavement.  On less complex aircraft pilots are taught to tap the brakes after liftoff to stop the main wheels from shaking (to the extent that they do) as they wind down.

Airliner wheels get statically balanced after tire installation but they don't usually get balanced dynamically like car tires, and there is often minor vibration that is not noticeable while rolling but can be quite noticeable while spinning on a gear leg dangling in the open when the wheel's natural frequency comes into phase with the gear, scaring the crap out of the pax.

Nose wheels are typically stopped by a snubber in the nose gear well upon retraction so they spin down until then.  Being so much smaller, they aren't as big a problem but if the static balance is not good you can get a pretty noticeable vibration under your feet that grows and then declines as the wheels slow down, then stops as soon as the gear is up. If it's bad enough crews will snag it.

Anyway, to have all that mass spinning all the time the gear is out?  Even if there was a simple way to do it, without the usual weight/cost/complexity issues, airlines would probably be forced to dynamically balance their wheel assemblies and purchase hugely expensive equipement.

Cheaper to just let them wear down and recap them.  Aircraft tires can be retreaded indefinitely as long as the carcass is sound.


Why do we not have autonomous helicopters?

Don't we? Most consumer and commercial UAV are helicopters.

If you mean military-only models, consider what jobs military-only helos do. It's troop transport or close combat - both necessarily manned functions. Commercial drones suffice for non-military-specific jobs.

If you wonder why they have 4 rotors rather than 1-2, it's lower complexity - quads can use simple fixed rotors and blades, and only vary power on each rotor. Larger drones with turbine power may well go to 1-2 rotors.


White has some significant thermal advantages over color. In most cases, this is probably just a benefit in terms of keeping the cabin slightly cooler. In the case of 'plastic' airplanes (those built with composite construction), some airframes require the use of white paint on upper surfaces to keep some elements within limits. Early Diamond Aircraft designs had a 38° C outside air temperature limit, past which the main spar is not considered structurally sound.

As a result, Diamond would only manufacture aircraft with predominantly white upper surfaces, as darker surfaces could result in significant heat gain.

Avweb has an article on the DA-20 that shows the bright red over-temp indicator that raises the maximum temperature to 55° C, added as a Service Bulletin.


Why are end-fire glideslopes less useable for low-minimum approaches than image-type glideslopes?

End-fire CAT III is available.
A 1990 paper[1] mentions that the slotted cable end-fire system is able to provide the highest levels of performance for any runway. One of the references is the WAC Model 105 from 1986. And indeed the product page confirms the CAT III option. I was also able to find the Leonardo Model 2100.
But, there's truth to what @UnrecognizedFallingObject wrote, at least for the US.
A 2008 assessment[2] of how the US evaluates the G/S performance has found that the current methodology does not account for the end-fire signal structure, the requests for waivers is in most cases declined, the rewriting of the standards would be too costly, and for the FAA to "have the latitude to apply alternative analysis methods". The assessment also reached out to Rockwell Collins, an auto land manufacturer, to understand how auto land systems utilize the G/S, since information about this is very limited (commercially sensitive information). And it turns out past the threshold the auto land system does not use the G/S, which is in favor of the end-fire evaluation issue – "In other words, the specific computational method used to estimate RDH is not important as long as it is representative." [bold emphasis mine]
Of note in that report from 2008:

As of this writing, all endfire glide slope systems operating in the U.S. are certified only for Category I use.

1: Watts, C. B. "Applications of slotted cable antennas in the instrument landing system." IEEE Aerospace and Electronic Systems Magazine 5.5 (1990): 16-20.
2: Edwards, Jamie S., Michael F. DiBenedetto, and FAA Aeronautical Center. "ASSESSMENT OF THE EFFECTIVENESS OF THE RDH/ARDH EVALUATION METHODOLOGY FOR THE ILS GLIDE SLOPE." (2008). (PDF)


There are two problems linked to the wind after accidents:

Inflating the slides.
Running away from the aircraft when on the ground

@DavidRicherby listed the reasons related to running away upwind to try to avoid the effects of flames and fumes (visibility, heat, toxicity). This is part of the IATA guidelines for post-evacuation:

  Post-evacuation. Once outside the aircraft, the cabin crew is
  responsible for the passengers until relieved by the authorities or
  emergency services. Until this help arrives, the cabin crew should:
  - Direct passengers upwind and away from the aircraft;
  - Assemble passengers;
  - Direct passengers away from fuel, fire and vehicles;

Source

While it appears running upwind is a good choice, deploying slides upwind has not always been a good idea.

A study was conducted by the National Aerospace Laboratory (NLR) in Netherlands, about slides issues during evacuation. It was published by the Flight Safety Foundation in 2005:

Analysis of Evacuation-slide Problems
Calls Attention to Recurrent Issues

The scope of the search was the period 1970–2003 and covered aircraft operations worldwide. NLR researchers identified 150 accidents in which slides were used. A total of 89 slide problems were identified:

One problem was slides not inflating due to the wind:

  Wind had an adverse effect on the use of slides in 11 slide problems
  (12.4 percent) analyzed by NLR researchers. Typically, wind blew the
  slide against the side of the aircraft, preventing its use.

To allow using the upwind side of the cabin for slide deployment, today, aircraft certification include this requirement for slides:

  4.20 Wind. The device must be shown, in 25-knot winds directed from the most critical angle, to deploy and, with the assistance of only
  one person who has evacuated down the device, to remain usable after
  full deployment to evacuate occupants safely to the ground.

Source

See also:

IATA - Cabin Operations Safety - Best Practices Guide
EASA ETSO-C69, Emergency evacuation slides, ramps, ramp/slides, and slide/rafts