Suppose it was somehow possible build a runway at FL300. Yes, doing so would be incredibly expensive, and it would be a mammoth civil engineering task, and the downsides would probably by far outweigh its advantages in practice; but for the time being, let's say that the desire to do it exists, and the issue of money has been resolved (as if by magic).

Let's also ignore the issue of getting the passengers, cargo, etc., to and from the actual aircraft, and focus on the aircraft operation itself.

Having constructed this runway-in-the-sky, what difficulties, compared to a similar runway built at ground level, would pilots and airplanes face when using it to "land" (if we can even use that term here) and take off, respectively, with a typical passenger jet (turbofan) aircraft?

It has been suggested that this is a duplicate of Would building an airport at 30,000 ft save money? (which also, in the question body, allows for one at 10,000 ft). I believe that this question is not a duplicate of that one because that question focuses on the cost of building such an airport and the savings in fuel expenditures because airplanes would already be near cruising altitude, while this question focuses on the challenges of landing at and taking off from an extremely-high-altitude airport while specifically ignoring the economic aspects.

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    $\begingroup$ Possible duplicate of Would building an airport at 30,000 ft save money? $\endgroup$ – abelenky Aug 23 '19 at 13:11
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    $\begingroup$ This was well covered by comments and answers on another recent question. $\endgroup$ – abelenky Aug 23 '19 at 13:11
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    $\begingroup$ @abelenky I don't think I agree. The difficulties that the pilots and airplanes would face were mentioned in answers to that question, but really only in passing, I think. $\endgroup$ – Terran Swett Aug 23 '19 at 13:25
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    $\begingroup$ Maybe some S.H.I.E.L.D. agent is around and can tell how their Helicarrier (a flying aircraft carrier in the Marvel Cinematic Universum) works. (Sorry, couldn't resist.) $\endgroup$ – PerlDuck Aug 23 '19 at 13:26
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    $\begingroup$ @abelenky Adding to TannerSwett's comment above, the linked question seems to me to be about monetary savings from such an extremely high-altitude airport, while this question is about actually operating an airplane into and out of such an airport if one could be built (but excludes "ground" operations such as baggage handling, passengers, etc., as well as the monetary aspect). Related, yes; duplicate, I don't really see how. Looking at Bianfable's answer to this and their answer to the other one (where it's highest voted and accepted at this time), the two answers are completely different. $\endgroup$ – user Aug 23 '19 at 14:19

These are the main challenges I can think of:

  • High takeoff and landing speed: To generate enough lift for takeoff in the thinner air at FL300 (gravity has almost not decreased, so required lift stays almost the same), very high speeds are required. Combine this with less thrust from the engines available and you need very long runways. It should still be possible to accelerate and takeoff eventually since FL300 is still below the maximum altitude most jet airliners can climb to (coffin corner).
  • Tires: The tires used on aircraft have a maximum speed which would be exceeded when taking off or landing. Other tires could be developed, but it is a major challenge to make these work in the cold temperatures (rubber does not like freezing temperatures, that is why Mars rovers do not use rubber tires).
  • Cold Temperature: It is cold at FL300, around 220K on average or around -50°C. This is not only problematic for the tires, but also for fuel, which would be constantly close to its freezing point. A heat exchanger between e.g. engine oil and fuel would be mandatory to operate an aircraft from here.
  • Wind: Wind speeds can easily reach 150kt at FL300. The runway would have to be perfectly aligned with the wind to have a pure headwind component. Otherwise, the crosswind would quickly exceed the limits for the aircraft.
  • Engine Failure: FL300 is above the maximum altitude for OEI (One Engine Inoperative) flight, which implies one cannot return to this airport after an engine failure. An aircraft would be forced to drift down to a normal airport for landing.
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    $\begingroup$ Not to mention that at -50C an engine that's shut down for an hour or so during unloading/loading/refueling would see some issues with lubrication. Engine heating blankets, fuel tank heaters, hot tankers for fueling, etc. $\endgroup$ – Zeiss Ikon Aug 23 '19 at 13:24
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    $\begingroup$ Cruise configuration is probably the only configuration here that allows for positive rate. So we are talking about Fast & Furious 6 length of runway here. Should probably also mention that Dom's crew would all require oxygen bottles at this altitude. $\endgroup$ – JZYL Aug 23 '19 at 13:36
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    $\begingroup$ "so I ignored everything related boarding and baggage handling" Indeed, the whole point of that restriction in the question was to attempt to narrow the question down sufficiently to be (a) answerable and (b) actually about aviation. $\endgroup$ – user Aug 23 '19 at 14:14
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    $\begingroup$ Assuming that this airport really is floating in the sky or built on the top of a very large tower, wouldn't it be enough for takeoff to just push the plane over the edge with just enough horizontal momentum for it to have just enough lift to not exceed its Vne while diving down, gaining speed, thus gaining lift, etc.? Still doesn't solve the landing problem though. $\endgroup$ – Jörg W Mittag Aug 23 '19 at 20:53
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    $\begingroup$ If you can point the runway straight into a 150kt headwind it alleviates the first two problems noticeably :) $\endgroup$ – hobbs Aug 24 '19 at 15:41

Flight range would be limited

Aircraft would not be able to take off with full fuel, because of the difficulty of taking off in the high altitude.

In fact some planes could not take off at all at max weight, not even with unlimited runway, because with a full fuel load they are at "coffin corner" at that altitude, and incapable of (safely) attaining the altitude. Planes typically take off and fly 27,000, 29,000, 33,000, 35,000 etc. as they burn off fuel and become capable of the higher altitude.

Taking off, a too-heavy airplane would have to run off the end of the runway. Then nose down to reduce the severity of the immediate stall, and sacrifice altitude to gain speed and get to a more practical altitude. The runway would need to end at a ramp arching down a cliff, which will make RTOs a lot more dicey.


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