Do fuel reserves take into account the possibility of a constant headwind? I'm concerned about safety rules for commercial flight in North America. There is supposed to be a fuel reserve of "30 minutes" if I remember correctly. This seems to imply an extra 30 minutes of flying time, but this is not perfectly clear.

Specifically, for example, does this 30 minutes include the worst case scenario of a headwind during the entire flight? If so, what is the max headwind speed used in the calculation?

The fuel "to reach the destination" is computed using winds, head- or tail-wind as appropriate. For "30 minutes of reserve" that is 30 minutes, and it is the same 30 minutes regardless of flying into or with the wind. In 30 minutes of flying with a tailwind you'll fly FARTHER than 30 minutes with a headwind, but 30 minutes is 30 minutes.

-- Edit to answer the question as expanded in a comment:

Wind predictions are actually pretty good now, and "30 minutes" isn't really all that the pilot has to work with.

For FAR Part 91, essentially general aviation (think taking a Cessna on a flight of a few hundred miles as an easy picture), the legal minimum in good weather is 30 minutes.  For FAR Part 121, which governs airline flying, it's 45 minutes (plus other categories as explained in the other answers for things like a destination alternate).  Beyond that, the plan for the flight will almost always include additional fuel above that to account for the routine things that happen to burn more gas: delays waiting to take off, small changes in routing that add a few miles to the planned path, and so on.  That way, even with the routine "stuff" that happens, you don't plan to eat into the 45 minutes reserve.  If the wind forecast is off, then that tends to be another of "those things" that changes the plan.

Realistically, how much are the winds off of the forecasts?  Typically, the forecasts are pretty close.  The computer models that are used now are very detailed, which is possible with all the computing power that's available now as compared to a decade or two ago.  Also, these models are constantly updated by data coming in from aircraft that are airborne.  Various datalinks can transmit the winds and temperature in real time, and ATC will also sometimes ask for current winds at a particular altitude.  All of this combines to give a picture that isn't simply a wind forecast issued in isolation, but a combination of forecast + recent data.

The issue that would arise if an airline were required to plan on the worst-case wind that might be encountered as if it would be present for the entire flight, is that in some cases the flight would significantly under-burn: it would arrive with more fuel than planned.  Why is that an issue?  Maximum Landing Weight.  If your airliner is fully loaded with passengers & cargo, there is a limit to how much fuel it can have on board at landing, driven by the maximum certified landing weight.  On my airplane, that tends to be enough fuel for a bit over two hours of flying.  (Obviously a LOT of factors influence this on each flight -- this is a ballpark number.)  So if I have a full airplane and I want to plan on having as much fuel as possible upon arrival (so that we can hold if necessary, fly the approach, and still have adequate fuel for a missed approach and then a divert, arriving at the alternate with adequate margins), there is a limit to how much I can take off with.  And that limit is driven by how much fuel we're expected to burn enroute.  If my enroute burn is lower than planned and I do NOT have to hold or take a longer route than planned, I can show up too heavy to legally land.

There are two options when that happens: land anyway (safe, but requires an inspection of the aircraft, and it's generally frowned upon to do this without good reason), or fly circles to burn off the gas.  The latter is what is generally done, and is obviously wasteful.  So it's preferred that the enroute burn calculations be as accurate as possible, rather than including unrealistic headwind values.


Does the 747-8 / A380 nose-wheel encounter more weight during braking? Given that the nose wheels of both the Boeing 747-8 and Airbus A380, as is true for almost all aircraft, are not equipped with brakes, do they encounter more weight during braking?

If so, how much more at maximum safe landing weight?

If they were equipped with brakes, would they encounter more weight during braking and what other effects would ensue?

Yes of course. And even with nose wheel brakes the main braking force would still be generated by the main wheels, so a substantial pitching moment is generated.

Now for a crude first-order approximation. We have an answer about the braking forces here, and using those 235 kN per wheel (the 747 has 16 main wheels) means that the center of gravity, which I guess to be at least 6 m above the ground, experiences inertial loads of 3,76 MN. This needs to be reacted in a moment around the main wheels with the lever arm of the wheel base. This is conveniently displayed in the drawing below and is 25.6 m. Thus, the reaction moment around the main wheels requires a nose wheel load of 881 kN in addition to the regular load of the nose wheel (which is maybe 5% of the aircraft's gross weight or 196 kN).

Boeing 747 side view (picture source)

Since the highest friction coefficient is reached at low speed, wing lift or tail downforce will not play a significant role. Braking to a full stop after an aborted take-off at full gross weight will load the nose wheels with a multiple of the static load of more than four.

In a normal landing, the forces involved are much lower and the multiple is close to one. This is really a quick back-of-the-envelope calculation, but the magnitude should still be right.


How is flame stabilisation carried out? How is flame stabilization done in a jet engine combustion chamber?
What are the various ways of doing this?
What are the challenges/ problems faced?

As usual, it is a combination of measures:

Flow speed at the start of the combustion chamber is the lowest in the whole engine, in the order of 30 m/s.
High fuel pressure and many small fuel injector nozzles, such that the mean fuel droplet size is kept as small as possible
Flame holders cause a pocket of stagnated, burning flow which helps to ignite the fuel-air mixture
The combustion chamber is long enough to allow most of the combustion to happen inside, such that radiated heat from the burning gas ignites the fuel-air mixture upstream.
In marginal conditions the ignitors are switched on to help speed up the ignition.
Control of parameters, such that fuel starvation by rapid throttle changes can be avoided. Examples are fuel control units, where a centrifugal feedback valve would control fuel flow depending on the engine's rotational speed, or electronic control (FADEC).

Much can be done by running the engine such that the risk of flameout is minimized. The operational measures to ensure a stable flame would be:

Avoid fuel starvation (empty tank, fuel pump failure, clogged pipes). This one is obvious, but I need to include it for the nitpickers.
Avoid too low pressure for rapid combustion: Pressure speeds up the chemical reaction, and if the aircraft flies high and/or throttles down, the lowered pressure in the combustion chamber might cause a flame-out. High-altitude jet engines have longer combustion chambers as a precaution.
A compressor surge or disturbed inlet air flow can cause a sudden pressure loss in the combustion chamber and a consequent flame-out, the latter especially in supersonic fighters with their long intakes and high angle-of-attack capabilities.
Avoid sudden cooling of the air inside the engine, e.g. by heavy rain. The additional water extracts energy when it evaporates and causes a temperature drop in the combustion chamber which can lead to a flame-out of a throttled-down engine. Another possibility is too much fuel which has been ingested by the intake in a botched aerial refueling attempt.
Avoid clogging of the inside of the engine by ingested dust, like gun smoke or volcanic ash.


What's the difference between multiple disc brake and segmented rotor brakes? Going through FAA's aircraft maintenance manual I came across a topic under braking systems, which had two different types of brakes: multiple discs and segmented rotor disc brakes
Looking at the their construction:
Multiple discs brake

Segmented rotor brakes

These look pretty much of same construction.
Also, reading through the description, I couldn't find much information apart from just that "Segmented rotor disc brakes are used on even higher performance modern aircraft"
Aircraft brakes
The information provided in the above-attached link is the same information as printed in the FAA AMT Handbook VOL 2
How are they any different? Or am I missing something here?

More properly, both types are multiple-disc brakes. Disc brakes, in turn, can be either continuous or segmented.
Regular continuous disc brakes are somewhat similar to car disc brakes, except for the brake pads being replaced by an entire stator disc. The calipers can be placed in one spot, like on a car, or on larger planes spaced symmetrically around the disc, and the areas away from the calipers are only pressed against the rotor by the pressure plate's and the stators' rigidity.
Much of this action is lost as the discs heat up and expand, causing them to warp. When the discs are warped, only the closest part of the disc keeps making contact. The pressure is very inconsistent among the disc, governed by geometry; something will touch, but it's a few radial lines across the disc, not the whole.
Segmented disc brakes return to (semi-)individual brake pads, spaced around the disc evenly, with a caliper per each brake pad. Most importantly, the rotors are split into separate sectors (only loosely connected around the carrier), so they get room to expand in all directions.
When a segmented disc expands from the heat, the sectors almost touch, but without significant warping. And each brake pad is forced against the rotor by its own caliper (even though there's a pressure plate in between); they are less fixed geometry and more fixed pressure. The theoretical contact area is smaller overall, but it's maintained constant even as the brakes heat up. All of the brake pads keep creating friction against the rotors at all times.
I could probably sketch something up, but you can as well do an experiment - push two plates of any material firmly against each other with your hands and rotate them against one another. It won't be difficult to do, even if they're rough, because your hands aren't very rigid, not ensuring constant contact areas. Now place a few clothespins around their perimeter. You'll notice that they'll become much harder to turn against one another, even though the clothespins are weaker than your arms.
Regular disc or multi-disc brakes clamp only a small part of the disc, with rigidity somewhat helping elsewhere, like with two plates held in your hands. Segmented-disc brakes rely entirely on the latter effect, braking at multiple very firmly clamped areas across the disc stack. They provide strong braking torque even when very hot.


Why does turbofan exhaust appear to reverse direction? Yesterday I was at the airport planespotting. I observed the exhaust of a couple of turbofan engines, and found it curious how the fluid seemed to behave as it was leaving the engine and gaining distance to the aircraft.
From observation, this is what it appeared like:

The exhaust left the engine in a high speed, violent manner and quickly slowed down as it got farther from the engine.
The exhaust appeared to move in the other direction, aka going back towards the airplane before it met with new high-velocity exhaust gas and was violently mixed and pushed again further from the plane.

I remember the Saturn V's exhaust being pulled back into the flame duct right after engine start as the flame gained speed and the surrounding pressure dropped, and I was curious if something similar was happening with the turbofan?

Source of image: https://space.stackexchange.com/questions/26183/why-does-air-get-sucked-in-for-a-moment-before-engine-start

What you are describing is a characteristic of fluid dynamics known as eddies. As different portions of the fluid flow faster than others, the friction and surface adhesion between the two layers cause swirling of the fluid and reversal of the current, creating a turbulent flow region. It is this same principle that holds a ping pong ball stationary in a moving column of air.
For a simple overview, you can check out this wikipedia page:
https://en.wikipedia.org/wiki/Eddy_(fluid_dynamics)
For a more scholarly exposition, try this:
https://arc.aiaa.org/doi/abs/10.2514/2.1794?journalCode=aiaaj


Why is an ATC operational error called a "deal"? Air Traffic Controllers who lose separation between aircraft call that event a "deal". Why?

When the Conflict Alert function was added to the computer system, it reported conflicts to the area supervisor.  Controllers came to know the feature as "Squeal-a-Deal", but no one could ever tell me why they're called deals.

The first time some of us heard this term was from a controller at a fly-in breakfast, perhaps in the early 80's.  One of the people in the group asked for clarification, and was essentially told that a "deal" was a "big deal" and that a report had to be written and reviewed, and it had the potential of being a factor in a performance rating.

The "deal" in this case was a maintenance truck which was still on a runway, after a 727 was cleared for takeoff.  The 72 driver saw the issue and delayed takeoff, but it was on the tapes, and there were people in the cab who saw it. So paperwork was filed.

Rumor had it that the processes for deviations were relaxed for the new controllers after the strike/firing of controllers under the Regan administration.


Why is wing anti-ice in transport aircraft not required below -40 degrees Celsius? Okay, this confuses me. As it gets colder doesn't it mean more possibility of icing? And is the -40 degrees SAT or TAT?

That -40 is SAT and in degrees Celsius.

The reason that anti-ice is generally no longer needed below that temperature is because at that point, the air is so dry that there isn't enough moisture left to form frost or ice that will stick to the airframe or engine parts.

Normally, it's engine anti-ice, rather than wing anti-ice, that is still on when you get to the point in the climb that the temperature drops below -40. At least in the 737, the engine anti-ice is kept on continuously while the temperature is below +10 TAT and above -40 SAT and you're in IMC or other visible moisture (i.e. rain, snow, etc), while wing anti-ice is only used "as needed" to remove ice buildups from the wings.

Also of note, the "below -40 is too cold" logic only exists in climb and cruise, not in a descent. The rationale there being, you don't want to miss the transition from the air is no longer "too cold for icing" as you descend, and find out that your motors ARE now accumulating ice, when they start to act sick... that makes your day entirely too interesting!


How can I tell the difference between Spitfires and Hurricanes in photos? The Spitfire was one of WW2's iconic aircraft, its fame coming from its spectacular performance in the Battle of Britain. But in that battle, Hawker Hurricanes outnumbered Spitfires 2:1.

Often articles about Spitfires use images of Hurricanes. How can I tell the difference between a Spitfire and Hurricane visually?

There are quite a lot of differences, some more subtle than others, but I tend to find this is the best angle for identification: the side view. Although as we'll see later in this answer, the top/bottom view has a fairly obvious difference, and there's a clue that we can use to identify from the front or rear

So what are you looking at?

On the left is the Hurricane. It's slightly larger than the Spitfire, although that's not always a great help from distance. Perhaps more usefully, the fuselage is fundamentally different shape:  I'll elaborate on this in several areas where we can directly compare.

Firstly, the rear fuselage. At first glance the two are very similar, but the Hurricane has a much more pronounced "drop" to the tail, while the Spitfire's is a "rise".

What do I mean by that? Follow the line of the top of the cockpit canopy: the Hurricane's fuselage behind the canopy drops away, while the bottom of the fuselage is relatively flat. On the Spitfire, it's reversed - the top of the fuselage follows the line of the canopy, while the bottom comes up to meet it. It's a subtle difference at first glance, but once you've seen it once it's a very clear distinction. When looking at the Hurricane's rear fuselage, it almost appears to have a "dog leg", where the rearmost area drops in relation to the middle of the aircraft. The Spitfire's follows a much more continuous line

The Spitfire is also clearly much more sleek - it's a "pencil" shaped fuselage, long and thin and curvy. The Hurricane is more blunt and "solid".

Other differences? Look at the tail. The Spitfire's is relatively small and curves upward from the top of the fuselage in a smooth curve, while the Hurricane's comes off at a sharper angle. Similarly the Hurricane's horizontal stabilizer is higher - almost in line with the top of the fuselage.

The Engine cowling shape, as with the cockpit, is different: where again the Spitfire's continues a nearly straight line from the bottom of the cockpit, the Hurricane's has a distinct curve.

If you can get closer, you will also note that the Hurricane's fuselage is almost "ridged" - it's a metal frame with a fabric covering, and that gives it a bumpy appearance, vaguely visible on the above photo.

What about if you can't get a side view, though? Here's the Spitfire (below) and Hurricane (above) from the front... but you can see the same from the rear

Well from the front, there's a single very nice clue under the fuselage: the radiator housing: that box-like structure hanging from the bottom of the plane. The Hurricane has one, and it's on the centre of the fuselage. The Spitfire's is not, it's under the right wing. On some later models, there are two, one under each wing.

The next photo is included mostly for the wing shape, but the aforementioned radiator boxes are visible too: from this, we can tell the Spitfire is a relatively late war model: a Mk IX (9) onward: although there are other subtle clues that suggest this is a much later model... the 5 bladed propeller indicates a Griffon engine, so a Mk XIV (14) or later. Anyway, we can see the Hurricane's single centre radiator, and the Spitfire with one under each wing

And finally, how about the bottom? Well, the Spitfire's wing is extremely distinctive: it has an "Elliptical" wing, shared with only one or two other aircraft. Once you can recognise the Elliptical wing, you can recognise the Spitfire very easily in flight. The Spitfire's wing, particularly the trailing edge, is very curved compared to most other contemporary aircraft. The Hurricane's wings are much more conventionally shaped, with both leading and trailing edges being straight (a trait shared with aircraft like the Typhoon/Tempest, P-51, Me109, A6M "Zero" and most other aircraft from the WW2 era)

(source: modelingmadness.com)

Late War Spitfires (the above is a Mk 22) are even more distinctive: they have a "bubble" canopy that protrudes entirely from the fuselage.


Is this an accurate depiction of the internal shape of a turbofan engine? This image (so commonly used here that the first image reference for it that pops up in a Google search for "jet engine diagram" points back to ASE!)

From Wikipedia

Shows the housing of the actual "jet" section very tightly packaged around the various sized compressor and turbine stages, leaving a diverging/converging path for the bypass airflow.

It seems to me that it would make more sense for the packaging of the inner jet engine to be a fairly even cylinder so the bypass air flows smoothly around it and out the back of the nacelle as I've attempted to depict with the blue line at the bottom, or a somewhat converging shape as I've depicted with the red line at the top.

Source: personal modification of above mentioned Wiki picture. Don't laugh... I drew that with my thumb-operated trackball.

However, as I've typed this question up and looked at the diagram more, I can see where the converging path around the low-pressure turbine section of the jet would add additional pressure to the bypass air and, therefore, additional energy to the escaping air.

Is this simply an "artist's concept" of a cross-section of a turbofan engine, or is it an accurate depiction of the internals?

Note: I do understand that within the jet section of the engine the shaping needs to be tightly packaged to ensure the jet-internal air flows exactly where it's needed, my question is about the flow of the bypass air only.

Strictly speaking, no. It is an artist's concept to help us understand the engine schematics better. A better figure would look some thing like this :

GE-90 Cutaway; image from web.stanford.edu

The blue colored region more or less shows the 'cold flow' region (where the bypass air flows). This graphic is better for two reasons- not only does it show that much of the flow takes place in the cold flow half-ducts, but it also shows that how the pipes and tubes that are all over the place are covered.

The figure you linked in the question doesn't show the cold air flow ducts, for simplification. Also, the shape and size of the bypass duct varies from design to design (though they are similar overall).

Differences between two and three shaft engines; image from 1.bp.blogspot.com

In the above figure, you can see that the though both the similarly shaped bypass ducts converge, their location and length is different. Of course, in the real aircraft, these ducts host the thrust reversers, as explained by @mins elsewhere.


What should I do if I lose situational awareness while in the pattern? Let's say that I'm doing some pattern work in my favorite little Cessna or Diamond at my favorite little non-towered airport. Everything is going just fine, I'm on the downwind leg...

Or, wait a minute, am I on the crosswind leg? Which runway are we using today? I just heard another plane on the radio that I can't see; what did she say, again? Is she behind me or are we on a collision course?

In short, I suddenly realize that I don't have a full picture of what's going on here.

What are my options here, and how should I choose between them?

Here are my ideas, from best-sounding to worst-sounding:

Just climb out of the pattern, then fly a safe distance away from the airport, descend, and fly back into the pattern. I assume that after starting the climb, a quick radio call wouldn't hurt: "Wobegon traffic, Skyhawk eight three four sierra tango, climbing out of the pattern and going west." (I've always heard that you should never descend or climb directly into the pattern, but I've never heard that you shouldn't climb out of the pattern.)
Keep flying, keep looking out the window, and talk fast on the radio: "Aircraft at Wobegon, where are you again? I'm not sure exactly where I am, do you have me in sight?" (I'd feel stupid saying this, but it's better to speak up and feel stupid than to stay quiet and be stupid.)
Keep flying, keep looking out the window, stay quiet, and just try to figure out what's going on.

It is the advice of the FAA that you should be looking out the window quite a bit when VFR in the pattern. According to this AC related to traffic patterns:

  Collision Avoidance. The pilot in command’s (PIC) primary
  responsibility is to see and avoid other aircraft and to help them see
  and avoid his or her aircraft. Keep lights and strobes on. The use of
  any traffic pattern procedure does not alter the responsibility of
  each pilot to see and avoid other aircraft. Pilots are encouraged to
  participate in “Operation Lights On,” a voluntary pilot safety program
  described in the AIM, paragraph 4-3-23, that is designed to improve
  the “see-and-avoid” capabilities.

If you are worried you should (over) self announce, if you lose track of an aircraft you should do every thing in your power to make sure that the other aircraft know where you are in the pattern.

  Self-Announce Position and/or Intentions. “Self-announce” is a
  procedure whereby pilots broadcast their aircraft call sign, position,
  altitude, and intended flight activity or ground operation on the
  designated CTAF. This procedure is used almost exclusively at airports
  that do not have an operative control tower or an FSS on the airport.

The FAA also offers lots of advice on how to fly a pattern here.


Are there any airports with multi-level taxiways? There are many airports where roads (and pedestrian walkways) are routed underneath taxiways, runways, etc. But are there any airports where airplane taxiways go under/over each other?
I can't imagine there would be (for many logistical and sanity reasons), but in this world of unusual design and construction paradigms, I've learned to not say that something has never been done.  This could either be airports that are currently in use, or ones where they were used at one time, but are no longer active.

If you allow taxiway to also mean a surface where an airplane rolls but not under its own power, then there's the lower decks of an aircraft carrier, and Schätz's carousel hangars.  Note that even Schätz prefers scissors lifts to multilevel tarmac.
Multilevel tarmac would means ramps between levels, which is commonplace for surface vehicles but rare for airplanes, both in airframe design and in pilot training.


Is a Mode C or S Transponder a type of Mode 3 Transponder? Mode A transponders are sometimes referred to as Mode 3/A transponders. According to NATO STANAG 4193 (not publicly available, summary from Wikipedia), the "3" in that description refers to the mode in the Identification of Friend or Foe ("IFF") system:

Mode 1 – 2-digit octal "mission code" that identifies the aircraft type or mission.
Mode 2 – 4-digit octal unit code or tail number
Mode 3/A – 4-digit octal identification code for the aircraft, assigned by the air traffic controller (the only mode shared with civilian aircraft)
Mode 4 – 3-pulse reply, delay is based on the encrypted challenge
Mode 5 – Cryptographically secured version of Mode S and ADS-B GPS position.

In flight school I was taught that Mode C was an improvement on Mode A through the addition of data fields that provide aircraft altitude. I extrapolated that to Mode S being an improvement on Mode C through the addition of yet other data fields and making the interrogation targeted at a specific aircraft, and Mode ES/ADS-B being a further improvement through automatic broadcasting.

I have, however, never seen Mode S referred to as Mode 3/S. So, is Mode S a variation within the Mode 3 "protocol" or is it a stand-alone mode outside of the five IFF modes?

Modes 3/A and C are very similar, but the interrogation pulses are timed differently and the responses mean different things.

If you send a mode 3/A pulse, you get a mode 3/A reply (or nothing). The response is a 12-bit number called the squawk code, which is conventionally written as four octal digits, plus an optional ident bit.

If you send a mode C pulse, you get a mode C reply (or nothing). The response is a 12-bit redundant encoding of the pressure altitude between -1200 and +126700 feet, in 100 foot increments.

Note that mode C does not include a squawk code; it is only the altitude. In practice, though, every mode C radar and transponder is also capable of mode 3/A. Likewise, every "mode C" radar always sends both types of pulses and internally combines the two responses, so it looks like a single system. (TCAS only sends mode C pulses since it doesn't care about squawk codes.)

Mode S is almost a completely different system. It includes selective addressing, data frames, message types, and much longer messages (56 bits). It also uses a different, more efficient encoding to allow moving all that extra data without losing efficiency. Of course, for backward compatibility, all mode S transponders can also do mode 3/A and C using the old 12-bit system if needed. Likewise, Mode S radars also send mode 3/A and C pulses, but they're modified very slightly so that mode S transponders will know to ignore them.

1090 Extended Squitter is a relatively minor extension of mode S. A "squitter" means a transponder that sends replies without being interrogated, and "extended" means it added some new message types with 112 bits per message (to hold ADSB data) instead of the previous 56. That's it.


What is the fuel temperature in the tanks during a flight? I'm trying to estimate the temperature of fuel stored in wing tanks during the flight. In my situation, I'm considering two wing tanks of 2000 liters each, one on each wing, at 20°C before the flight

I've run some calculations (I can provide additional numbers if necessary), and I end up with a temperature of 17°C after more than two hours of cruise (Mach 0.5, FL250)

Does this number sound correct to you? I was expecting a much lower temperature. I took a constant mass of fuel, which is not the reality, but even 15°C for instance seems rather hot. What are the usual temperatures of fuel in airliners tanks?

Quite cold, in fact.  External air temps at altitude reach -30 to -40 deg C.  The fuel in the tanks wil cool down and reach those temps within a couple of hours exposure.  Jet A has an icing inhibitor added to it to prevent freezing of the fuel in the tanks during extended periods in the stratosphere.

From a heat transfer standpoint, I would probably analyze this as a rectangular bar made from a composite sandwich of aluminum with a Jet-A center subject to convective cooling by air moving at a typical true airspeed, say 450KTAS at STP for FL250.  For simplicity we would ignore compressibility effects.  That may be where you erred.


How quickly can someone learn to fly? I'm a US-citizen living in Japan, and the costs here are insane ($500 per hour). So I'm thinking to go to the US or Australia for my pilot's license, but how long am I'm going to need to live there? Could it be done in 2-3 months? Less?

This question discusses the cost of learning to fly, and it's mentioned that people often take a year (flying once a week) to see the hours. My question is: reasonably speaking, how quickly can someone get their license?  I'm not actually looking to go from "zero to hero" in record time, I do want to learn the material and enjoy the process. But wanted to have a rough idea of the lower limit so I can plan for the total costs (housing, time away from work).

If you need 40-50 hours of flight time, and you fly 2-3 times a week, what is the typical duration of these instructional flights?  How many hours can someone realistically book in a week (I hear flights get cancelled)?

In reply to the comment about individual ability… I have a high-IQ and have taught myself many things throughout my life. I'm a software engineer with 20+ years experience (created my first commercial software application around age 12), I've authored several top-selling Japanese language books, I taught myself photography and have two Page-One credits in the NYTimes, I learned freediving and 9 months later I won an event in a competition in Japan and ranked 11th nationally, etc. When I put my mind to something I'm focused and disciplined. How much does it really depend on my ability and how much depends on class hours and flight time?

I did it in 17 days - start to finish.

Did flight school in Arizona.  Before I went, I got the PPL study guide, MS Flight Simulator complete with a yoke, pedals, and throttle quadrant... and put in dozens of hours just learning the instrument scan and practicing virtual stuff.

When I got to the flight school, I already knew the parts of the plane, and what each control did.  I knew about maps and plotting courses.  I understood the physics of flight.  I knew about VORs, VASIs, NDBs... I knew about stalls and corrective procedures - from an academic standpoint.

As such, my under-the-hood training was by-the-numbers.  My seat of the pants stuff was only hampered the first few days by the awful flying conditions of AZ in July heat.

That said, I aced the written, and did my 3 required solos in two days.  I went to a flight school run by a Gold Seal licensed FAA examiner so I didn't have to depend on the FAA to squeeze me in.  And I passed.  17 days.

ONLY flew tricycle gear stuff.  No taildraggers.

Other than the serious prep beforehand, there were three main drivers for my success:

I flew at LEAST once a day - many times twice.
I wanted this more than anything I had ever wanted before.
I only had enough money and vacation accrued to be there for 3 weeks total - no more.

Best of luck!


Why are aircraft callsigns pronounced over the radio in a non-phonetic alphabet way? Why is it numbers are not always pronounced in a phonetic way? For example a flight number of 2390 is pronounced "twenty-three ninety" instead of "two three niner zero"

The FAA's ATC orders (section 2-4-20) say that the 'group form' should be used by controllers when talking to any aircraft with an FAA authorized call sign. So you would expect to hear it with airline traffic but not private flights:

  Air carrier and other civil aircraft having FAA authorized call signs. State the call sign followed by the flight number in group
  form.

  NOTE- “Group form” is the pronunciation of a series of numbers
  as the whole number, or pairs of numbers they represent rather than
  pronouncing each separate digit. The use of group form may, however,
  be negated by four-digit identifiers or the placement of zeros in the
  identifier.

  EXAMPLE− “American Fifty−Two.” “Delta One Hundred.” “Eastern
  Metro One Ten.” “General Motors Thirty Fifteen.” “United One Zero
  One.” “Delta Zero One Zero.” “TWA Ten Zero Four.”


What purpose does a control area really serve related to managing air traffic flow? I can understand that we have control zones at busy airports to streamline the flow of landing and departing traffic. But in Control areas of many airports i have seen that routes go straight through the control area upto control zone. I mean no noticeable change occurs in route geometry. I can't understand what purpose are these control areas serving the air traffic services personnel? In other words, are'nt control zones alone enough to streamline landing and departing traffic at any airport? If not, why not? I am really confused about this.

The main purpose of controlled airspace is NOT to "streamline" traffic, but to protect traffic. Control areas are not there to "service air traffic service personnel", but to enforce certain rules upon the pilots.
In and around busy airports, the density of aircraft in the sky is relatively high, meaning there would be a relative high risk of collision if everyone were just flying around randomly. To mitigate this risk, various types of controlled airspace is established (control zones, terminal areas, control areas etc.)
Within controlled airspace, aircraft are subject to an ATC clearance, which more or less eliminates any risk of collision. Crucially, outside of controlled airspace, pilots can, essentially, fly pretty much however they want, and the only way to not hit another aircraft is if you happen to see it out the window.
Control areas are not just established around airports, but in any area where the density of air traffic is high. Typically, this means pretty much everywhere at higher levels, and around major airports at lower levels.


Are we at peak speed efficiency for jet airliners at Mach 0.85? The cruising speed of large jet airliners has not increased in the past four decades. The 747 cruised at Mach 0.85 and the new Dreamliner 787 also cruises at Mach 0.85 even though it was designed 40 years later (1969->2009) and uses advanced, composite materials and more powerful and efficient engines.  Meanwhile, faster airliner designs have been shelved, most notably the Sonic Cruiser (Mach 0.98).

This seems to indicate that as long as we value fuel efficiency over travel time, that we have reached peak speed efficiency at Mach 0.85 and that the only difference will be the comfort (or discomfort) of travel at that speed.

Are there fixed physical limitations at Mach 0.85 that make this the most efficient speed for jet airliners? Does this mean that even with advances in materials science and jet engine technology, we will not see significant transoceanic speed increases in the foreseeable future - so long as fuel efficiency is primary? Will airline travel forty years from now -- 2054 -- still be at Mach 0.85?

Short answer: Increasing fuel prices would have driven down the most economical speed, but advances in aerodynamics have compensated for that and the maximum cruise Mach number is still at Mach 0.85. But there is a much simpler reason why this Mach 0.85 number seems so immutable.

Please note that we are talking about the maximum cruise Mach number; all airliners will fly more economical at slightly lower Mach numbers between 0.78 and 0.82.

Now we need to define efficiency. For airliners this is haul capacity per time, and can be expressed by the product of payload, range and speed. When none of the three constituents can be changed without making the product more costly to achieve, you have reached the point of best efficiency.

Engine Technology

The biggest change in the last 50 years was in engine technology. Bypass ratios have increased to values close to 10, the turbine inlet temperature is now 300°C higher than in the early jet age, and electronic control has improved especially the off-design performance of engines. This has made it less attractive to increase flight speed, so the optimum cruise Mach would be lower if no other effects had influenced it.

Another effect is the lower fuel consumption. Early designs had to use a high wing chord to pack enough fuel volume for transoceanic flights. The much lower consumption of modern engines has given them ranges which are more than what is needed to reach every point on earth directly. This made it possible to reduce wing area, thickness and mass, and to save some friction drag, helping fuel economy again. The lower thickness helps to make higher cruise Mach numbers possible.

Aerodynamics

Aerodynamics is the second factor. With the A310 supercritical airfoils were introduced, and they shifted the operating Mach number up because now a limited region of supersonic flow on the wing could be tolerated without a significant rise in drag. However, the benefit from that was used to reduce wing sweep, increase airfoil thickness and to attach bigger fuselages to those wings. In the end, the newer airplanes had the same cruise Mach numbers as their predecessors. Something made Mach 0.85 too attractive to abandon.

A second effect of supercritical airfoils is their blunter nose, which helps to reach a higher maximum lift coefficient. This also contributed to reducing wing chord, because now the wing area could be made smaller for the same landing speed.

The Real Reason

Every new airliner needs to compete with the older models. On fuel economy this is no problem, but speed is also important. A higher cruise Mach number allows  shorter times for connections. Now you have to know that the electronic reservation systems would list connections sorted by flight time, shortest first. When a travel agent wanted to book a flight, he or she would rarely look beyond the first screen and pick one of the first flights listed. If a new type made the flight slip down in the listings, it was a non-seller. This is even true in the times of online bookings: The airlines don't make much profit from the bargain hunters in Economy, their target are the business people flying in Business or First Class. And those still order mostly through travel agencies, so the rules have not changed. This is why that Mach 0.85 speed seems to be so firmly set; flying any faster would drive up fuel consumption disproportionally, and even the newest jets will mostly be flown around Mach 0.82 to Mach 0.84.

Making this speed possible is still a struggle for the design engineers, especially when you consider that larger aircraft would need thicker wings for structural efficiency, but need to be fitted with wings thinner than optimal to let marketing claim a cruise speed of Mach 0.85.


How are light weight fillers made for fixing dents on airplane wings? Say an airplane wing was damaged by hail, it's really costly to replace the panel due to inaccessible rivets and what not, and the back of the panel is inaccessible to mechanically remove the dent.  How are light weight fillers made to fill the gap? Not like the heavy stuff like Bondo that is (was?) used to fill dents on cars, piled on and then sanded smooth.  Airplanes need something light weight.

Lightweight body fillers such as Poly-Fiber's SuperFil are two-part epoxy fillers. The reason they are lightweight is that a significant portion of the filler in the epoxy is composed of microballoons, also known as glass microspheres.


When are winds given with respect to true vs. magnetic north? Wind information can be reported by various sources (ATIS, METAR, TAF, spoken on the radio, etc). I was taught that officially some of these sources are relative to magnetic north and others to true north (although practically this isn't always followed). Which sources are supposed to use which reference system?

The general rule is:

If you read it, it's true. If you hear it, it's magnetic.

All charts and textual sources (METAR, TAF, winds aloft, surface analysis charts, etc) use true north as the reference.

ATIS/AWOS/ASOS broadcasts, or any information a controller gives you over the radio, is magnetic.

  Wind direction broadcast over FAA radios is in reference to magnetic north.

AIM Section 7-1-11 (page 7-1-26 in the 5/26/16 edition)

One exception to the "if you hear it" rule is that a FSS briefer will read you the winds referenced to true north, since they're just reading you the charts/textual information.

(This is at least true in the United States... other countries may vary in some instances)


Is procedure turn considered published approach course? Procedure turn is not completely lined on the chart and only marked with the barbed arrow. Is it still considered a segment of a published approach course? Can you report to ATC that you're established on the course when you're flying PT?

I’ve been doing a lot of practice approaches lately and if a procedure turn (or hold) is required, they tell me to 'report procedure turn (hold) inbound'. Then they clear me for the approach. I think this is why:

AIM 5−4−7. Instrument Approach Procedures

  b. When operating on an unpublished route or while being radar vectored, the pilot, when an approach clearance is received, must, in addition to complying with the minimum altitudes for IFR operations (14 CFR Section 91.177), maintain the last assigned altitude unless a different altitude is assigned by ATC, or until the aircraft is established on a segment of a published route or IAP. After the aircraft is so established, published altitudes apply to descent within each succeeding route or approach segment unless a different altitude is assigned by ATC. Notwithstanding this pilot responsibility, for aircraft operating on unpublished routes or while being radar vectored, ATC will, except when conducting a radar approach, issue an IFR approach clearance only after the aircraft is established on a segment of a published route or IAP, or assign an altitude to maintain until the aircraft is established on a segment of a published route or instrument approach procedure. For this purpose, the procedure turn of a published IAP must not be considered a segment of that IAP until the aircraft reaches the initial fix or navigation facility upon which the procedure turn is predicated.


Why didn't the Boeing 757 share the 767 fuselage? (Flight) Early 767 design was a trijet.

The numbers-built for the 757 and 767 is +1,000 each. They are both equally successful.

But why didn't the 757 feature a slightly shorter 767 fuselage to cut down on production cost, especially after ETOPS and the selection of twin-jet design for both?

After all, the 767 isn't terribly wide, it seats 7 in a row compared to 6 for the Boeing 757. Counting the aisles that's a 7:9 ratio. And their 2-class seating capacity overlaps across the variants.

A 757 with a slightly shorter 767 fuselage could have just had a lower certified MTOW and reduced fuel carrying capacity and it would have met its mission. The long 757-300 always seemed out of place for me.

(Own work based on Wikipedia) 2-class seating across variants.

Boeing had already a fuselage for the 757 that fit the bill. It came originally from the 707 via the 727. To accommodate more baggage, the rear fuselage was deeper, as on the 737. During development Boeing engineers were afraid that directional stability might be insufficient with a deeper forward fuselage, so the lower half of the fuselage was kept from the 727 instead of switching to the deeper fuselage of the 737. A twin-aisle fuselage would had restricted the possible variants with fewer seats too much.

From a contribution by OldAeroGuy to this discussion on Airliners.net:

  The 757 entered design using the 727 fuselage as baseline as it was
  originally a 727 derivative, so it had the 727 dimensions for fore and
  aft lower lobes. It was later decided to evaluate a deeper forebody
  lower lobe, ala the 737. The forward fuselage cross section decision
  was made to meet a manufacturing commitment before all wind tunnel
  testing was complete. Early indications were that if the deeper lower
  lobe was used for the forebody, directional stability would have been
  reduced to an undesireable level. When all testing was finished, it
  was shown that a deeper forward lobe would have been OK.
  Unfortunately, it was too late to revise the manufacturing plans.

  I was a member of the wind tunnel testing team at the time the 757 was
  being developed and was aware of the situation. After the 757 was
  certified, I asked the 757 Chief Engineer what design decisions he
  would like to change with the benefit of hindsight. The forward lower
  lobe decision was the first on his list.

A wider and higher fuselage causes more drag directly but will also require a larger tail surface because of its shorter length, causing more mass and drag indirectly. Note that the Airbus A318, the shortest member of the A320 family has a taller vertical tail than all other members. Using the slender fuselage and the RB-211 engines made the 757 very fuel efficient, which was very important for Boeing just after the second oil shock of 1979-1980. A larger fuselage would had reduced efficiency.

From this Aviation Week article:

  A single aisle worked better for the lower end, but did not stretch
  very well. A twin aisle worked better for the upper end, but equally
  did not shrink well. As a result, for almost six years in the 1970s,
  the company exhaustively studied two concepts: a single-aisle twin
  dubbed the 7N7 and a widebody twin called the 7X7.

  Most observers at the time believed Boeing would develop one or the
  other but not both, at least not immediately. It was therefore with
  some surprise that between 1978-79, over a period of less than eight
  months, the company ambitiously began the simultaneous development of
  both aircraft. The 7X7 became the 767 in July 1978, while the 757,
  formerly the 7N7, received the production go-ahead the following
  March.


What prevents a commercial jet from electronically "resetting" the way a computer sometimes does? I've had a fear of flying for the past several years and one of the ways this irrational phobia manifests is concern over the airplane power-cycling in mid-air: engines cutting out, control equipment powering off, etc.

Occasionally a computer or smartphone will crash and reboot itself, or in a worse case, the power supply or battery will fail and the device will not turn back on.

Does this ever happen with commercial jets? I expect that they are electronically wired up in a fundamentally different way from computers, so that one system failure does not affect the rest, but I've never asked a pilot.

I'm a programmer and private pilot, so maybe I can help dispel some of those fears.

The computers that run a commercial airplane are conceptually much simpler than the one that runs your phone. This means far less chance of a bug in the software, just because there's less for the programmer to keep track of.
If your phone restarts, it doesn't imperil anybody's life. So, the testing and Q.A. for such a device is basically whatever the company wants to do. On the other hand, computers in aviation are much more thoroughly tested before they can be certified to fly.
Similarly to point 1, the computers on an airplane each have only one job. A lot of the crashes on a typical PC or smartphone come from different apps stepping on each other's toes. (The operating system is supposed to keep each app apart so they can't do that, but point 2 applies to operating systems as well.)
The airplane isn't one single computer, like your phone. Yes, your phone probably has multiple processors, each with multiple cores, but they're tightly coupled together in order to form one computer. The computers on an airplane are networked together, but they are separate computers. If your phone crashes, even if the guy sitting next to you is on the same network, it doesn't affect him, does it? Similarly, if the FADEC for one engine fails (a vanishingly rare occurrence, because FADECs are conceptually among the simplest of computers), all that will happen is that one engine will shut down, with no effect on the rest of the plane.
On the other hand, the really important computers (such as fly-by-wire controllers) have multiple redundancies. So, if one fails, the rest of them can pick up the slack. Even the pilot wouldn't notice except for the warning light that would show in the cockpit.

If you want to see what actually happens when things fail on an airplane, take a look at the following videos:

Complete avionics failure over the ocean.

   The pilot flew the airplane by hand while he shut down and restarted the avionics. The flight then proceeded normally.

Autopilot failure. (Ironically, the same guy as the previous link!)

   The pilot actually let the autopilot have it for a few seconds, just out of curiosity as to where it would take them. But once it started banking more than it was supposed to, he disengaged the autopilot and took over manual control with no issue.

Brand new pilot (less than 60 hours total flying time) suffers complete electrical failure at night

   Engine stayed running despite losing all electrical power to the entire plane. He landed safely.

Edit: Well, just two hours ago as I type this, another video of an in-flight failure just happened to pop up in my YouTube feed:

Generator failure in flight

   He switched to his backup generator, and other than an annoying whine in the headphones, had no other issue with the flight.


Why do the Comet's engines point outwards? As it's visible from the images below,
when looking at the plane from the top, the engines point outwards and they are banana-shaped, why is that?

(unfortunately I couldn't find a real photo straight from the back of the airplane, from that angle it is way more noticeable),

https://upload.wikimedia.org/wikipedia/commons/9/93/De_Havilland_Comet.svg

The engines were mounted well inboard to reduce the hazard of asymmetric thrust.

Four turbines are placed so close to centerline of plane that even if two on one side cut out, pilot has little trouble maintaining straight, level flight.  [photo caption, p. 101]
...
The Comet's engines are snugged in so closely to the fuselage that there's no need for the customary trim tab on the rudder. [p. 102]

-- Francis, Decon. "I Saw This Jet Liner Fly 500 m.p.h." Popular Science, 156(5), May 1950, pp. 98–104.
Protecting against that hazard also explains the deflection of the engines' exhausts, which places them closer to a line intersecting the airplane's center of mass.
Another reason for the deflection could be to prevent jet blast from overheating or otherwise damaging the fuselage's skin just aft of the exhaust.  The dH Ghost turbojets were built a decade before the first high-bypass engines, so their hot exhaust had no surrounding cooler air.
A third, faint, reason might be to facilitate egress from the over-wing emergency exit windows, sliding aft and down instead of climbing forwards.  But avoiding asymmetric thrust is the reason that is documented in the literature of the time.  (By the way, that entire Pop Sci article is a hoot.)


What to do to maintain my Spanish ULM (GA) gyroplane licence valid in the USA when moving there? I have an ultralight gyroplane (GA) Spanish license. What sort of conversion or validation process do I need to follow to be able to continue flying after moving to the USA?

As a caveat here, I know nothing about gyroplanes. But based on the US regulations it looks like there are a few different possibilities.

If your ULM license is equivalent to an ICAO private license then you could get an FAA foreign-based private license as described in this question. I believe you would get category rotorcraft and class gyroplane (see 14 CFR 61.5) but I'm not completely sure.

If your ULM license is not equivalent to an ICAO private license then it depends on the type of gyroplane you want to fly. If it's considered an ultralight then you don't need any license at all. If it's an LSA then you would need a sport or recreational pilot license. If it's a 'regular' gyroplane, you would need a private license.

If you do need to get an FAA license (sport, recreational or private) you should be able to use time you logged in Spain to count towards it.


Who decides how fast commercial aircraft fly: the pilots or ATC? Once a commercial aircraft is in the cruise phase, who determines the speed of the aircraft?
Do pilots have the authority to fly at up to the maximum permissible speed of that plane, or does ATC decide on that depending on the approximate time it will take to reach the destination? Is it stated in the flight plan that the pilot cannot go beyond some specific speed even if the aircraft is capable of it?

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

R: "DLH123, maintain speed Mach .78"

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

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

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

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

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

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

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

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

The FAA CFR 91.117 says:

  §91.117   Aircraft speed.

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

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

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


How does maintenance differ for military fighters/bombers vs. commercial airliners? And why do military fighters/bomber require so many maintenance hours compared to commercial airlines, which seems like just kicking the tires? What do they actually do?

How does maintenance differ for fighters vs. bombers?

As many of the comments and answers have illuminated, "different missions drive different design priorities." Examples of differences in specific maintenance activities would be greatly appreciated.

I think a few things need to be examined here:

1) The type of flying.  A commercial aircraft is closer to a military C-17 than a B-52 or F-16 in how it is used.  It has several stops throughout the day for loading and unloading.  The maintenance in between stops does not need to be very extensive - because it has just landed, presumably working fine, the things that need to be checked primarily involve what has the most chance of decaying during a flight (tires, oil, general wear and tear, etc).  Fighters/bombers don't generally fly several legs in a day.  Fighters might double/triple turn, and in between each of these flights the maintenance inspection is a much lower requirement than between each day of flying.

2) Component Requirements.  Supersonic jet engines need maintenance more often than high bypass commercial engines.  They are stressed much more, and work on a much tighter margin of error, thus fighter/bomber aircraft need maintenance more often.

3) Age.  Military fighter/bomber aircraft are on average much older than the average commercial aircraft.  B-52's are from the 50's, and B-1's are from the 80's (along with most of the fighter fleet).  Older aircraft require more maintenance, and there is no avoiding it.

Between fighters and bombers, they each have different flying styles.  Fighter engines work a lot harder than bomber engines (except the B-1, which essentially has the same engine as an F-16), and thus require maintenance more often.  Bombers are much larger, and have more systems that can break.

Hope this helps.

Update: Incorporating a suggestion from the comments below, it is probably useful to specifically mention a few more specific maintenance items unique to combat aircraft

1) Ejection Seats:  Most military combat aircraft carry ejection seats, which have a very specific maintenance routine.  This may include repacking the associated parachute, and checking to ensure that all components are functional/in good repair.

2) Weapons:  The obvious difference between fighters/bombers and commercial airliners is that they carry weapons.  This means that there is a software package that needs to be maintained (that controls the weapons), the aircraft hardware needs to be maintained (ie bomb bays, missile racks, hard points, etc), and the weapon itself requires building/deconstruction.

3) Flight Computer:  Combat aircraft often have tactically specific functions in their flight computer that a commercial airliner would not.  There are routines to help calculate bomb trajectory, to link with other combat aircraft and many other military specific tasks that a commercial airliner would not need.

I would agree with an earlier statement stating that performance has a slightly higher emphasis than reliability in military aircraft than in commercial aircraft.  It's easy to see how the requirement to carry bombs/missiles increases aircraft gross weight, which lowers fuel efficiency.  Every internal bomb bay could potentially be a fuel tank extending range.  Hard points, even when bombs aren't installed, cause drag lowering performance.  In the end, designers have to determine which capability is more important - endurance, payload, power, etc.  You may want to install a triply redundant INS in a fighter aircraft, but you're adding weight that decreases performance, and taking space which could be devoted to other avionics (like radar).


Are we expected to fly distanced leg (instead of timed leg) in holding pattern in GPS approaches? The standard inbound/outbound holding leg are 1 min leg each (no wind condition). I noticed that a distance (4 nm in the following example) is noted in many GPS approaches. Do we need to fly the leg as 4 nm, or the number conveys other meanings?

E.g. http://flightaware.com/resources/airport/TCY/IAP/RNAV+(GPS)+RWY+12/pdf

Short Answer:
Yes, the pilot is expected to fly the hold as depicted, with an outbound leg based on distance (see AIM 5-3-7, paragraph b). Most holds depicted on GPS instrument approaches in the US now have a specified distance for the outbound hold leg, instead of a timed leg.
Longer Answer:
The traditional hold leg distance is determined by timing. See AIM 5-3-7 § j. 4.:

4. Timing.
(a) Inbound Leg.
(1) At or below 14,000 feet MSL: 1 minute.
(2) Above 14,000 feet MSL: 11/2 minutes.
NOTE- The initial outbound leg should be flown for 1 minute or 1 1/2 minutes (appropriate to altitude). Timing for subsequent outbound legs
should be adjusted, as necessary, to achieve proper inbound leg time.
Pilots may use any navigational means available; i.e., DME, RNAV,
etc., to ensure the appropriate inbound leg times.
(b) Outbound leg timing begins over/abeam the fix, whichever occurs
later. If the abeam position cannot be determined, start timing when
turn to outbound is completed.

In years past this was common for all holds, unless otherwise specified by the published hold instructions, or by ATC. All otherwise unspecified holds were assumed to be timed holds, including for GPS approaches. I only remember a few distance-defined holds in my local area from 10 years ago; now all those holds associated with GPS instrument approaches are distance defined.
Which brings us to the AIM 5-3-7 § j. 5. (emphasis on the last two sentences is mine):

5. Distance Measuring Equipment (DME)/ GPS Along-Track Distance (ATD). DME/GPS holding is subject to the same entry and holding procedures except that distances (nautical miles) are used in lieu of time values. The outbound course of the DME/GPS holding pattern is called the outbound leg of the pattern. The controller or the instrument approach procedure chart will specify the length of the outbound leg. The end of the outbound leg is determined by the DME or ATD readout. The holding fix on conventional procedures, or controller defined holding based on a conventional navigation aid with DME, is a specified course or radial and distances are from the DME station for both the inbound and outbound ends of the holding pattern. When flying published GPS overlay or stand alone procedures with distance specified, the holding fix will be a waypoint in the database and the end of the outbound leg will be determined by the ATD. Some GPS overlay and early stand alone procedures may have timing specified.

See also AIM Fig 5-3-7 and the associated note, which address the type of distance holds found on GPS approaches:

NOTE-
The inbound course is always toward the waypoint and the ATD is zero at the waypoint. The end of the outbound leg of the holding pattern is reached when the ATD reads the specified distance.

The holds found on GPS approaches will generally be distance based. The AIM states that some might still be time based, but I am no longer aware of any such holds in actuality. Every approach that I know of that used to have a time based hold on a GPS approach now has a distance based hold.
All holds should be flown as published or as instructed by ATC. If ATC does not instruct you otherwise, you are expected to fly the hold as published. In this case, that means you are expected to fly the hold as a distance-based hold. If you are not sure how you should fly the hold, ask ATC. They will provide you with the complete holding instructions if requested. See AIM 5-3-7 § b.:

b. If the holding pattern is charted and the controller doesn't issue
complete holding instructions, the pilot is expected to hold as
depicted on the appropriate chart. When the pattern is charted, the
controller may omit all holding instructions except the charted
holding direction and the statement AS PUBLISHED; e.g., HOLD EAST AS
PUBLISHED. Controllers shall always issue complete holding
instructions when pilots request them.


Can I do an instrument rating checkride in an experimental aircraft? The EAA has a document that strongly suggests that IFR operation in Experimental Aircraft is legal.  The necessary equipment is described in 91.205.  Provided all this is met, as well as all other IFR requirements (e.g., pitot check, appropriate engine, etc.), I think instrument rated pilots are ok to fly IFR in an experimental.

However, in a question related to a checkride in an LSA, Can I do an IFR checkride in an airplane that isn't certified for IMC? , I learned about another concerning aspect.  It says:

  the Instrument Practical Test Standards (FAA-S-8081-4E PTS Instrument):  Aircraft and Equipment Required for the Practical Test The instrument rating applicant is required by 14 CFR part 61 to provide an airworthy, certificated aircraft for use during the practical test.

[my boldface].  Putting all this together suggests that I can train and fly in true IMC in IFR in an experimental aircraft, but I cannot take my checkride in one.  Is this interpretation incorrect?  Are there (m)any cases in which checkrides were done in experimentals?

PS: I sometimes wonder whether we are training to become pilots or lawyers...

Yes, but only if the examiner agrees. 61.45 says:

  (a) General. Except as provided in paragraph (a)(2) of this section or
  when permitted to accomplish the entire flight increment of the
  practical test in a flight simulator or a flight training device, an
  applicant for a certificate or rating issued under this part must
  furnish:

  (1) An aircraft of U.S. registry for each required test that—

  [...]

  (ii) Has a standard airworthiness certificate or special airworthiness
  certificate in the limited, primary, or light-sport category.

  (2) At the discretion of the examiner who administers the practical
  test, the applicant may furnish—

  (i) An aircraft that has an airworthiness certificate other than a
  standard airworthiness certificate or special airworthiness
  certificate in the limited, primary, or light-sport category, but that
  otherwise meets the requirements of paragraph (a)(1) of this section;

This article describes an IR checkride in a Van's RV6.


What free online educational resources should pilots in training be aware of? Obviously this website is great as is boldmethod and several of the aviation youtube channels. Also the FAA wings website is great as is all the FAA handbooks and advisory circulars. I also like to use the ASA study guides for the oral exams. Are there any other high quality sources of information that pilots should be taking advantage of, preferably free or low cost?

The Airplane Owners And Pilots Association has lots of great articles on both training and new legislation that comes down the pike. The new information is really nice as it lets you know what may be or has changed since the last time you studied the books. Its worth it to join as well (its less than $50 a year and if you have just started to train they are about to bombard you with mailers anyway).

Along with publishing the FAR's the FAA publishes lots of Briefings and really great supplemental material and handbooks like this one on airspaces.

There are lots of Youtube channels out there with great information like FlightChops and the like, but note they are not in any way approved training and may not cover every aspect of what they are doing.

I like to read GeneralAviationNews.com. Its a nice source of informative articles on what is happening around general aviation, airplane ownership advice, mechanical engine advice, and all all manners of legislation that are being discussed as well as fun fly ins and things like that.


What type is this Syrian light attack aircraft? I saw the following 2012 photo of a light ground attack jet used by the Syrian regime in an attack on Tel Rafat:

I do not recognize the type, however, and it does not appear to be any of the regular ex-Soviet types normally flown by the Syrian air force (MiG-29, MiG-25, MiG-23, MiG-21, Su-24, Su-22). Does anybody know what it is?

Its a Syrian Aero L-39 Albatros.

You can see the same image (just a bit sooner on the second missle lauch) when searching Google for Syrian L-39 (second result).


Is this Part 23 Final Rule document the final regulation and what are the main improvements? Is the FAA's new Part 23 Final Rule the final new Part 23 document? Or will another document be released? The formatting is totally different and kind of a discussion can be found in all the paragraphs.

Also, what are the main improvements in this new version of Part 23?

The document itself is final, but it isn't the same thing as the actual regulations.

The same thing happened with the recent part 107 UAV regulations: the final rule document came first, then the regulations were published later. That means, at some point soon (?) the part 23 regulations will be updated to include the final rules you found, but I have no idea exactly when that will happen. The Part 23 final rule itself says:

  DATES: Effective [INSERT DATE 8 MONTHS AFTER DATE OF PUBLICATION IN
  THE FEDERAL REGISTER].

AOPA has a good explanation of what the rule changes will mean.


How exactly did engineers come to the final design of jets like the F-16 or SR-71? I ask this question because I read somewhere that SR-71 was designed in 60's with hand calculations and drafters and when engineers later tried to improve the design with modern computers they found that they got original design exactly correct!
I take this with a pinch of salt but I find it intriguing that engineers were able to do this 60 years ago without CFD tools and CAD programs. Can anyone give me an insight on how they got each curve and line correct? Was it purely a lot of hit and trial in the wind tunnels? What technology or math tools did they use?
Can a mechanical engineering graduate with GA design experience succeed in designing jets like F-16 considering available literature, reports and findings,modern engineering tools and computers available in 2021?

The short answer is that if we can program it into a computer then we can do it manually, it just takes a lot longer.
Back in the day, a "computer" was a clerical professional or mathematician and with paper, pencil, log tables, slide rule and the like on their desk. The data centre of the day was a large hall filled with rows of desks with a computer working industriously at each one, each carrying out a precisely specified routine. Supervisors collected their calculations and passed them on to the next calculator in the chain, who carried out the next routine. It could take a couple of years to fully calculate out a large airframe for production.
The mechanical desktop calculator looked rather like a small old-style cash register and made their lives a lot easier. Electronic desktop calculators raised their game another step.
The computer was one of the first professions to be wholly usurped by the electronics revolution. Programmers long afterwards talked of writing their code in a modular fashion, as routines and sub-routines.
Nowadays we can figure out the basic airframe in a few weeks at worst, while the wind tunnel models are being 3D printed, and iterate until we achieve joy unbounded. However design for manufacturing has become immensely more sophisticated, not to mention standards-bound, and the timescale has if anything increased.
But all computers suffer from one particular phenomenon, known in the trade as GIGO - Garbage In, Garbage Out. You really do need a competent and experienced designer at the top of the food chain. A GA designer turning their hand to a warplane has a steep learning curve; the likes of SR-71 design lead Kelly Johnson do not turn up still wet behind the ears. (It was not always so; UK company de Havilland badgered the Air Ministry for years, were fobbed off and insulted at every turn for being mere civilian designers, so they built their brainchild as a private venture and called it the Mosquito....)


What are the regulations regarding the safety briefings and harness designs for open-door helicopters? I'm reading about a helicopter crash that happened yesterday in New York City. The helicopter landed in the East River and apparently inverted. The pilot was able to escape, but all five passengers died.

I'm having a really hard time processing the following quote from the CNN article:

  "One of the most difficult parts of the rescue were that five people were tightly harnessed," Fire Department of New York Commissioner Daniel Nigro said. "People had to be cut out."

How can it be that the passengers didn't undo their harnesses? I can't understand why their harnesses were still on. What are the regulations regarding the safety briefings and harness designs for open-door helicopters?

The flight was a "doors-off" photographic "experience" conducted by a company called FlyNYON, as seen in this video. The doors of the helicopter are either removed completely or latched in the open position, and the passengers are secured with tethered harnesses which allow them to sit or stand in the open doorway whilst in flight. Unfortunately, the harnesses used by FlyNYON do not have any quick-release mechanism (supposedly because of the risk of suicides) and so the only means for a passenger to free themselves from the harness is to either undo the carabiner-type shackle in the middle of their back or cut through the harness with a knife!

Note: FlyNYON operate these flights as chartered "photographic missions", not as tourist/sightseeing flights which would require quick-release belts and harnesses.

Helicopter harnesses ‘did too good a job,’ trapped crash victims underwater

  All of the victims were upside down and wearing tight-fitting safety harnesses when rescue divers found them trapped dozens of feet underwater inside the capsized chopper, FDNY Commissioner Daniel Nigro said.

When a Device Meant for Helicopter Safety Becomes a Death Grip

  The helicopter that crashed was owned by Liberty Helicopters and flown by FlyNYON, a company that specializes in doors-off helicopter photo tours. The company promoted its safety standards on its website and singled out its “proprietary eight-point Safety Harness System” worn by passengers. It is a complicated system of straps, carabiners and an emergency blade for cutting it off in case of trouble.

Doors-Off Helicopter Flights Under Scrutiny After East River Crash

(Update)
FAA statement on 16 March 2018

  Helicopter operators, pilots & consumers should be aware of the hazard from supplemental restraint devices during an emergency evacuation during “doors off” flights. The FAA will order operators & pilots to take immediate action to control/mitigate this risk. Until then, the FAA will order no more “doors off” operations that involve restraints that cannot be released quickly in an emergency. Additionally, the FAA will conduct a top to bottom review of its rules governing these flights to examine any potential misapplication that could create safety gaps for passengers.


What type of aircraft is depicted in the Taylor Swift music video "Look What You Made Me Do"? It looks like some sort of business jet or small regional airliner, but I can't tell anything more specific than that.  About the only thing I'm certain about is that it's a T-tailed twinjet with rear-mounted engines.

View of the starboard side of the aircraft in question:

Frontal view of the aircraft's empennage:

Apologies for the poor lighting and resolution (especially in the first image).

According to Reddit, it is a Gulfstream G650 with the wings cut off, from a Taylor Swift music video “Look what you made me do”.

It has 8 windows like a G650, but is not a real aircraft.

the engine fans don’t look right
the wing leading edge has non-flush rivets and there are too many of them
the window size, shape, and spacing are wrong
there is no front entry door

And no one would be crazy enough to actually cut the wings off a 65 million dollar aircraft.

I don't think it is CGI because of how poorly some of the details are rendered. CGI should be more accurate.

It’s probably a Hollywood prop left over from some movie.


Should you immediately attain max glide speed after engine failure? For the Cessna 172 Skyhawk maximum glide is attained at a speed of 65 KIAS. If you're flying well above this speed, let's say 90 KIAS, and suffer an engine failure, what would be the best practice: immediately slow down in order to achieve 65 KIAS as soon as possible, or slow down slowly?

According to the amplified emergency procedures of the Cessna 172 Model N POH (emphasis added):

  After an engine failure in flight, the best glide speed shown in Figure 3-1 should be established as quickly as possible.

The same POH lists 65 KIAS as the best-glide speed.

Time spent away from best glide means less glide range. If increasing time, not glide range, is your goal then the minimum sink rate speed would be what you would want to fly. In an emergency, time and glide range give you more options.

The POH and physics agree: yes, establish best-glide speed immediately after an engine failure.


Which 2-seat 4th generation fighter/engine/configuration (de-militarized) would the best supersonic long-range civilian private transport? Seeing recently -with some surprise- that former military jets of fairly recent vintage can be bought e.g. here and operated by (skilled+licensed²+rich³) private individuals after all classified/restricted tech has been removed, I was wondering what plane would be best suited for a Max-Fuel configuration.

I think I use the right terminology and assumptions, but for clarification:

2-pilot-seat configuration: "Learner configuration", like in driving school, where both seats can "pilot".
I'm assuming, for every modern 2-seat airplane a "learner" version/configuration_kit exists.
Super-Cruise: I believe is a supersonic mode of travel without using the Afterburner
Gen-4: I believe this includes F16 and newer (and foreign equivalents)
Gen-3 and older: There aren't any Super-Cruise-capable specimens?
I believe, all civilian runways are long enough for Gen-4 planes
I'm assuming, using afterburner absolutely kills max range.

For example, Wikipedia says (ballpark numbers) an F-18 weighs 15 tons empty, and has a max take-off weight of 30 tons. Internal fuel capacity + maximum external fuel capacity is 12 tons. I'm assuming the remaining 3 tons are the ammo for the Vulcan Gun, flares, the pilot and other stuff you can drop or shoot in order to have a minimum onboard defense/offense power, and not just fuel. Wikipedia seems to say in the specs, that it can travel ~2300km with ~6tons of fuel.

I'm assuming, that a de-militarized version of this 15ton bird will be quite a lot lighter, i mean, no [gun, ammo, flares, armor, ECM, cameras, etc.] needed. I'm guesstimating that could slim the plane down to say 12 tons. I'm aware, that not all the remaining ~6 tons could be used as additional fuel weight, but by converting the gun/weapons bay to big tanks, and running fuel lines to the missile-mounts, surely a large chunk of that could be converted to fuel weight.

So, the above looks like an F18 learner could be "converted" to a max-range-model 2-seater "sports-car-for-the-weekend-trip" with a fuel capacity of ~16 tons and therefore a range of ~5700km. That gets you over every ocean.

Now all of the above does not take into account the fuel usage at Super-Cruise (iiuc), and I have no idea which plane/engine is the most "efficient" in this speed envelope.

SO, basically, my question is:
Which plane would a "bezos-rich" person buy, if he wanted something for super-sonic weekend trips with his (also former air-force-pilot) friend, and what modifications could/would be possible to extend (super-sonic) range to achieve the best average speed. Is it fast enough, or even possible (w.r.t. time-zones)  to have brunch on both sides of an ocean?

@ManuH pointed to a great example from pre-jet-times, the business exec version of the Cavalier_Mustang

@RonBeyer: Don Kirlin bought 40+ air-worthy F18s from the Australian Airforce

Edited to address comments.

"Best" is subjective. At the very least, you need to prioritize your criteria for the question to be fully answerable.

I'll give a short answer based on your current criteria:

Civilians can potentially buy any fighter jet, provided it's demilitarized, depending on their relationship with the government. The US certainly won't sell an F-22 or F-35 to just any individual or country, but George W. Bush might have a shot. Absolute monarchs, of which there are quite a few left, might buy a jet for their military and appropriate it to themselves.
No recent fighter jet is even remotely close to being flyable by someone with only a PPL, even with every endorsement out there. The ground is severely obstructed in these planes while landing, their stall speed is uncomfortably high, their control sensitivity out of the ballpark. The only civilian-piloted case for them would be a retired fighter pilot.
No fighter jet is capable of long-range supersonic flight. The supersonic range of all fighters is extremely short - a few hundred miles for the best-case. The only military aircraft capable of sustained supersonic flight are the B-1 and the Tu-160.
Not much weight can be removed from a fighter jet before it's easier to buy two fighter engines and build an experimental aircraft around them instead. The gun and the main radar is about it.
If one had to choose specifically a fighter jet, the best choice is perhaps the Su-34, thanks to its amenities, as well as its long range and large useful load.
Of course, with unlimited choice, the Concorde would still be the best. Out of military aircraft, the Tu-160 - it's got enough in common with a supersonic airliner to convert, and it's the largest.
If SST and bombers are out of the picture, you'll get where you're going fastest in a fast bizjet like the Global 8000. The F-18 is not suited for the job at all, as it's not even able to supercruise. A fighter jet can be fun to fly (if you can do that) for the performance, not for its usefulness as a transport.


Are retired airline pilots allowed and able to help in an emergency situation? My dad is a pilot who flew over 15000 hours in 777.
Will he be able/allowed to help the crews if the same plane is in emergency?

United 232 might be close to what you're thinking about: a United flight training instructor on board helped the pilots to land the aircraft, although he wasn't retired. But for this scenario to happen you need a fairly specific combination of circumstances:

The passengers have to know there's an emergency
Your father has to believe he could help significantly
The cabin crew have to decide it's worth passing your father's offer to the pilots
The pilots have to decide it's worth bringing another, unknown pilot into the situation

The basic question would be: is your father's help going to be important enough that it's worth increasing the communication and CRM workload for the crew? That's not necessarily an easy decision to make, especially if time is very limited or the nature of the emergency isn't clear.

Being "allowed" to help isn't really a consideration here, by the way: pilots have very broad emergency authority to do whatever they think is best.

Overall, it's certainly possible and it would be the pilot in command's decision to accept your father's offer or not. It's just very, very unlikely.


Why are First/Business Class seats at the front of aircraft where it is the least safe? According to this image, taken from The Science of Survivability (Page 79) from NOAA, the statistically safest seats to sit in are at the rear of the plane (69% survival rate vs 49% at the front).

From a business perspective (and ignoring any ethical considerations of whether passengers should pay more for a better chance of survival), it would seem to me to make more sense to put your highest paying and likely most regular passengers in the safest part of the plane, the rear.
Yet almost exclusively they are placed in the front, the un-safest part.

So why are First/Business class seats in the un-safer front of the craft and not the safer rear?

There are several reasons:

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


May a Cessna 182 land on a major, high-traffic airport, say EDDF (Frankfurt)? Some 30 years ago a family's friend owned a TR-182 Skylane (Turbo,
retractable gear) and we sometimes flew around here in the Rhine-Main
area around Frankfurt. Of course he used EDFE (Egelsbach) for take-off
and landing. I remember asking him if it would be possible/legal to land
in Frankfurt with his Cessna 182.

As I recall, he said they would likely reject such a landing as they
wouldn't really appreciate us occupying a runway and make the big planes
wait for our tiny one. Even if we were allowed, the landing and parking
fees would be in such high regions that no one in their right mind would
be willing to pay them.

The only exception, he said, would be an emergency. In that case we
could very well land in Frankfurt but must be prepared for massive
interrogation afterwards if the emergency wasn't obvious.

I'm not asking about a C182 specifically but about "small" GA planes
(say MTOW below 2-3 t) on "big" German airports such as Frankfurt.

There is the General Aviation Terminal (GAT) in Frankfurt that is handling "executive and private aviation flights" as they say. General aviation airplanes and helicopters are handled at this GAT that is open 24 hours per day. I think the best is to contact them.

Some additional rules related to GA in Frankfurt can be found here. Bus service is mandatory for passengers between aircraft and GAT. The list of services includes a sentence Lashing of small aircraft. So probably some not very big are expected to arrive.

Landing fees in this airport are used solely to refinance airport infrastructures and the resulting operating expenses. They are regulated, and, while not exactly very cheap, they are not astronomic either.


Why is New York-London used in time measurement for aircraft? Looking around aviation news and history, I see that many commercial aircraft are touted to have this or that much time fly between New York and London. What caused the route between these two cities to be used in measuring the "speed" of the aircraft?

They are (and historically, were even more so) two of the biggest cities in the business world (ranked Alpha++). That specific route has some of the highest rates of business travel and is, therefore, a fairly prestigious, famous route which is popular with fairly wealthy, prestigious people.

They are also two of the most famous cities in the world, and most people know roughly where they are on a map. The fact they're separated by an ocean which is large but was just small enough to be possible to cross safely in the early days of aviation also helped, because it gives the impression of being further than it is.

For example it's a comparable distance to London-Dubai, but that doesn't "feel" as big due to being overland through Europe. Equally while many people know roughly where Dubai is nowadays, it wasn't anywhere near as famous 30-80 years ago.


Which should I get a FAA glider licence or a Canadian glider licence? I plan to operate in the Caribbean (Antigua, under ECCAA jurisdiction, which does not have a glider rating of its own) and not in the USA or Canada. I am looking at which one would be require less work and time on my side. I plan to give rides in my country and practice the sport.

This is heavily dependent on the local jurisdictions you plan to operate in and what their laws are.

First off you must find out if your desired countries of operation accept foreign certificates. This varies heavily all over the world and can range from full no questions asked acceptance, to you needing to simply take the test again (in that country) through any various case you can think of.

Second you need to find out what certificate you need. Here in the US to give rides (and receive compensation for such) you need a commercial certificate. This looks like it extends to gliders as well.

Hours.

The FAA requires a few more hours than the Canadian certificate will,

FAA

  Are at least 16 years of age; and
  Have logged at least 10 hours of
  flight time in a glider and that flight time must include at least 20
  total glider flights, and
  Have 2 hours of solo flight time in a
  glider, and
  Have passed the FAA written examination; and
  Have passed the flight exam with a FAA Examiner.

Canada

  A minimum of 6 hours of flight training under the direction and supervision of an instructor
  At least 2 hours of solo flight time, including a minimum of 20 takeoffs and 20 landings

Right off the bad Canada requires 4 less hours which, no matter how you spin it will be cheaper (since you have to pay for less time).

Here is the US if you are not a citizen you will need to pass an NTSB background check to train. I do not know if Canada has a similar requirement.

Travel, assuming you are in the Caribbean already it may be cheaper to travel to Florida (or the like) to train then go all the way up to Canada for your training.

Edit:

According to this document the ECCAA does issue glider ratings

  2.1.2.3 RATINGS ISSUED

  (a) The Authority may issue the following ratings for pilots
  (i) Category ratings in the following aircraft:

  (a) Aeroplane;
  (b) Rotorcraft;
  (c) Glider;
  (d) Lighter-than-air;
  (e) Power lift.


Will the landing gear work mechanically in case of total power loss? In case of a total power failure in all the aircraft systems like engine failure and APU failure, would it be possible to use mechanical means (manually) to open the landing gear bay door and deploy the landing gear through mechanical means?

I know it’s possible to glide the flight if the engines failed. But, wondering how they land.

Not a real example, but this comes from the FAA, including some nice drawings around page 13-20:

The emergency extension system lowers the landing gear if  the main
power system fails. There are numerous ways in  which this is done
depending on the size and complexity of  the aircraft. Some aircraft
have an emergency release handle  in the flight deck that is connected
through a mechanical  linkage to the gear uplocks. When the handle is
operated, it  releases the uplocks and allows the gear to free-fall to
the  extended position under the force created by gravity acting  upon
the gear. Other aircraft use a non-mechanical back-up,  such as
pneumatic power, to unlatch the gear.
The popular small aircraft retraction system ..... uses a free-fall valve for emergency  gear extension. Activated from the flight deck,
when the  free-fall valve is opened, hydraulic fluid is allowed to
flow  from the gear-up side of the actuators to the gear-down side  of
the actuators, independent of the power pack. Pressure  holding the
gear up is relieved, and the gear extends due to  its weight. Air
moving past the gear aids in the extension  and helps push the gear
into the down-and-locked position.
Large and high performance aircraft
are equipped with  redundant hydraulic systems. This makes emergency
extension less common since a different source of hydraulic  power can
be selected if the gear does not function normally.
In some small aircraft, the design configuration makes  emergency
extension of the gear by gravity and air loads  alone impossible or
impractical. Force of some kind must  therefore be applied. Manual
extension systems, wherein  the pilot mechanically cranks the gear
into position, are  common. Consult the aircraft maintenance manual
for all  emergency landing gear extension system descriptions of
operation, performance standards, and emergency extension  tests as
required.


Is this the relationship between rpm and thrust of a Turbofan? In this graph from the FAA Airplane Handbook Chapter 15, page 5:

It looks like it is talking about "jet engines" but is it engine type specific or it is for both turbojet/fan/prop/shaft? It looks right about the more rpm the engine is running, the more thrust it would produce, but is the relationship shown here for jet engine or piston engine?

The basic way jet engine and piston engine produce thrust on an airplane is the same. They accelerate a given amount of air which in reaction provide a forward force called thrust. The faster the fan or the propeller spins, the more air it can accelerate thus increasing thrust. There is a good answer there. The reason why they don't operate at the same percentage of available RPM is efficiency. It might be true that the thrust increase with RPM for any aircraft engine, but propeller have huge losses at high speed the propulsive efficiency going down dramatically. And this is mostly due to the tip vortex appearing at the tip of each blade as you can see in the picture below. As turbofan are enclosed in a casing, those tip vertices don't develop and you have less losses.

Now to your question, as you can see the curve start a 50% of the maximal RPM with a thrust rating very low. This is typically the case for turbofan where the fan design has been optimised for higher RPM. At low RPM the AOA ouf the blade is not optimal and the efficiency is quite bad.


Why does it feel like a plane accelerates just before touching down? On commercial flights it often feels like the plane gets faster just before touching down. What creates that sensation?

The aircraft flares just before touching down. It descends with a constant velocity, and just before touching down pulls the nose up to reduce the descent. This results in a higher angle of attack, more lift, and a vertical deceleration of the airplane. A passenger perceives this vertical deceleration as a force. Direction of the force is straight down and the aircraft is nose up, you're leaning back, so there is a component of (gravity + vertical deceleration) that pushes you into the back of your seat.

This same effect is used in flight simulators with a motion base. Upon accelerating, the simulator pushes forward like the aircraft does, but also slowly rotates backwards so that the pilot feels sustained seat back pressure. He cannot see the leaning back angle inside the simulator because the horizon of the projected outside visual image does not move. As happens in the aircraft: look forward in the aircraft and your view of the inside of the aircraft is always the same, irrespective of aircraft angle.

So for your eyes there is no apparent tilt, the inner ear senses rearwards force, and there is increased pressure from the seat backrest. The brain translates this into perceived forward acceleration.