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As per this question:

Altitudes above 60,000 feet are class E airspace.

That means that, if you are lucky enough to have an aircraft that flies that high, once you get there you could theoretically call ATC, cancel your flight plan and continue VFR above flight level 600 without ATC supervision.

At that point, you are on your own, and out of the system. You are literally past the edge of human civilization.

If a commercial airliner, such as an A320 kept ascending, could it reach this altitude? If not, what is physically (ignoring legalities) stopping it from doing so?

Since it is not a legal requirement to use ATC at this altitude, perhaps this could be an airspace future budget airlines occupy to save time and therefore money.

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    $\begingroup$ I changed your title because class E in much of the US starts at 1200ft above ground, which commercial aircraft can certainly reach :-) $\endgroup$
    – Pondlife
    Jun 22, 2018 at 12:31
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    $\begingroup$ The highest flying commercial airliner produced was the Concorde which could reach altitudes of 60.000 feet in supersonic flight. $\endgroup$
    – Adwaenyth
    Jun 22, 2018 at 12:41
  • $\begingroup$ Just for your own fun, "service ceiling" is the specification you're looking for. Now obviously that's not necessarily the physical highest it can go, but the A320 is rated up to around 41000ft so you're unlikely to get another 50% out of it before you run out of lift. $\endgroup$
    – Dan
    Jun 22, 2018 at 13:46
  • $\begingroup$ Coffin corner is up above 41,000 feet for an A320, not something I'd want to play with. $\endgroup$
    – GdD
    Jun 22, 2018 at 13:47
  • $\begingroup$ @Dan Has one ever flown that high? Why would it need to? $\endgroup$
    – Cloud
    Jun 22, 2018 at 14:40

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An airliner's service ceiling is the altitude at which its rate of climb drops to less than 100 fpm. Most commercial airliners can't climb above 40000-45000 on a standard day. If temperatures are above standard, less than that. Corporate aircraft can go higher, just above 50000, so they get to avoid pretty much all the traffic because everybody else is below 40. The Global Express uses low bypass fans and very large wings to do this.

To go toward 60k you need super efficient engines and glider like wings. P&W developed the U2's engine with virtually no blade tip clearance to be able to operate at 70000ft.

Then you have coffin corner, the spread between critical Mach# and stall. It gets pretty small at 60000 ft. In the U2 at 70000 ft the spread between Mcrit and stall is very small, and between Mcrit and the back side of the power curve is just a few knots and they cruise in a tiny speed window, autopilot on in speed mode at all times. Bit too slow and down you go, bit too fast and mach buffet starts.

At an airliner's service ceiling, you have very little reserve thrust margin, and you may be operating just above min drag speed, not so far from the back side of the power curve in cruise. If you are at service ceiling and some speed bleeds off because say you hit some down moving mountain wave and the autopilot pitched up to compensate, and you aren't paying close attention, you may have no choice to descend right away as speed decays with no way to stop the decay without resorting to gravity.

Stalls at this altitude take many 1000s of feet to recover from. In the thin air, a mass has to accelerate over twice the distance through space for the same indicated airspeed increase as down low. If you get stick shaker at that altitude, you have to push over really aggressively for the recovery, not just lower the nose a bit. After a fatal Pinnacle Airlines CRJ200 stall incident that started at 41000, airlines started to focus more on training for high altitude stall recovery.

The high altitude world is a dangerous place, even at altitudes in the 30s.

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  • $\begingroup$ Thanks for the answer John, but "even at altitudes in the 30s" seems a bit off, since that is where most commercial aviation happens, and we can establish that is not dangerous. $\endgroup$
    – Cloud
    Jun 22, 2018 at 14:44
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    $\begingroup$ @Cloud as someone who primarily flies in them, I can assure you that the flight levels are dangerous. Factors include low temperature, low partial pressure of oxygen, and low air density. Concerns include losing pressurization, aerodynamic stall, and exceeding critical Mach, and exceeding engine limitations. We use technology and techniques to mitigate those dangers. $\endgroup$
    – J W
    Jun 22, 2018 at 15:44
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    $\begingroup$ It's not "dangerous" when the limitations are observed by competent crews, but the limitations get tighter and tighter the higher you go. And you can get into a lot of trouble even at 35000 ft. A pilot without high altitude training can be dangerous. Shortly after the Pinnacle incident we started getting sim training for shaker events and it was done at 35000 ft. It was a minimum 3000 foot altitude loss just to recover from shaker (an RJ), let alone pusher or worse a real stall, because you need to pretty much dive to build enough speed to be able to pull up without hitting shaker again. $\endgroup$
    – John K
    Jun 22, 2018 at 16:29

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