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Service Ceiling

Well perhaps, and I’m only just guessing, you can climb to the published maximum operating altitude, but if you were able to go much further than that, you will stall and look like an idiot in the sky!

On jet planes you can reach the cuffin corner where your stall speed will be equal to your maximum mach number. You will not be able to climb any higher. But as far as I remember this is usually no problem for propeller driven aircraft.

www.ing-golze.de
EDAZ

Right, as long as the plane can climb it will not stall in straight and Level Flight.

Lets say the stall speed of an aircraft is 60 knots. So as long as you can maintain a speed higher than 60 knots you won’t stall.
And as long as you don’t exceed 0.4 mach, you don’t have to worry about compressibility effect which increases the stall speed.
As you go faster than (approximately) mach 0.4 you have the coffin corner effect Sebastian describes (the stall speed becomes higher up to the maximum mach number).

Let say you want to fly at 35.000 feet and the outside temperature is -55 degrees.
Mach = TAS/LSS
LSS=38.95?(Absolute temp in Kelvin) so LSS = 575.
0.4 Mach = TAS/575 —>> TAS = 230 KTAS
230 TAS is about 135 IAS.

So if you are able to fly faster than 60 KIAS and slower than 135 KIAS, you are certain you won’t stall. And this only applies if you are able to get to that altitude. Next thing to worry is the temperature, because the freezing point of 100LL is about -58 degrees C.

Bushpilot C208/C182
FMMI/EHRD, Madagascar

Lets say the stall speed of an aircraft is 60 knots. So as long as you can maintain a speed higher than 60 knots you won’t stall.

In addition, you will not be able to climb unless you are going faster than stall speed, at the absolute ceiling the aircraft will only climb at one indicated airspeed that is somewhere between sea level Vx and Vy.

Interesting stuff about the mach number issue. A mentor of mine tells a story about climbing a T-28 as high as he dared as an young instructor pilot in the 60s, higher than allowed by the service, minus transponder. He then dived down at some high IAS and now says he’s happy nothing bad happened!

Sorry, but the stall is not a function of speed but of Angle of Attack. The coffin corner describes how a jet at a certain very high altitude can not fly “slower” (with a higher AOA!) and also not faster than its critical Mach number, because it will get out of control then. Since these “speeds” (or angle of attack and speeds) are very close together they named that are “coffin corner”.

That’s the short version :-)

The coffin corner is not relevant to any airplane i will every fly (myself)

Last Edited by Flyer59 at 12 Nov 17:36

“at the absolute ceiling the aircraft will only climb at one indicated airspeed that is somewhere between sea level Vx and Vy.”

By definition, the aircraft will no longer climb at the absolute ceiling. It will just maintain level flight, and only at one particular airspeed. This speed will be Vy (max rate of climb = 0 fpm); and also Vx (max climb gradient = 0 ft/NM). At any other speed the rate of climb and the climb gradient will be negative. So, at the absolute ceiling, VY = VX. Actually, the two converge gradually as the aircraft climbs. In most POHs it it stated in the performance section how Vy (IAS) decreases as altitude increases. The Vx speed may also increase slightly, but that is usually not stated in the POH, because the increase is very slight and because Vx is usually only of interest at lower altitudes.

In reality the aircraft may continue to climb as long as it burns fuel, moving the “absolute ceiling” ever higher. With a TB20 or something similar, this “terminal rate of climb” is problably undetectable, but with the Concorde, it was measured in hundreds of feet per minute.

Last Edited by huv at 12 Nov 18:00
huv
EKRK, Denmark

There is no problem with stalling while enroute at the (or any reached) ceiling because, as stated, you need to climb up there in the first place.

And climbing up there is hard because one is close to Vs, and the climb performance is strongly affected by air temperature, the slightest downdraught, etc. Plus obviously weight but you can’t do much about that, other than the fact that you can climb higher later in the flight, due to fuel being burnt off.

I have been to FL200 a number of times. It takes best part of an hour to get there (~30 mins to FL160) from the runway and while the finally reached cruise speed is reasonable (about 140kt TAS) it is very sensitive to downdraughts and a -100fpm downdraught will pull you back down. If you are on autopilot, in the usual altitude hold mode, the AP will try to maintain the altitude and will simply stall the aircraft if you let it get too far. You would have to be asleep (or hypoxic) to not notice the stall warner, however.

I suppose a really inept pilot could stall and spin an aircraft under those conditions, but it may be easily done if one is doing this in icing conditions. I would never try anything like this in IMC, at any temperature between 0C and -20C, because the slightest ice coating will totally kill the climb performance. I recall reading one report of an SR22 which was doing it in icing conditions and went out of control, but they had the chute so had the story to tell… However a normal clean aircraft should not go out of control if coordinated flight is maintained, because when it stalls the nose should just pop back down.

On the plus side, when you are that high, it takes only the slightest descent and your speeds builds up quite nicely. This leads to an technique for more effectively reaching the ceiling – via several steps.

The ceiling is not a practical flight or route planning altitude, and is very sensitive to temperature. For example ISA+15 drops it to about FL180.

By definition, the aircraft will no longer climb at the absolute ceiling

I don’t think that is the official position. The ceiling is defined as the highest one can reach at say +50fpm or +100fpm (depending on the certification authority).

Last Edited by Peter at 12 Nov 18:46
Administrator
Shoreham EGKA, United Kingdom

You can experiment with this minus oxygen (at around 10,000 ft) if you have only 65 HP, but it does take just as long to get there! I did it twice, in the interest of getting over some mountains. It was interesting to play with mixture to eek out a few more feet, particularly with a Stromberg carb that lowers the pressure in the float bowl to (kind of) lean the mixture.

The Vx = Vy IAS at absolute ceiling will be somewhere between the sea level Vx and sea level Vy.

Sorry, but the stall is not a function of speed but of Angle of Attack.

That’s true, I think that to be more precise the lift coefficient is a function of the angle of attack.

Stall speed is more a function to the current configuration like weight of an aircraft and the present loading in flight. But sitting in an aircraft and flying straight and level, I’d rather use the stall speed, because I don’t have an AoA indicator to look at. But that would definitely be nice in turns and in >1 G forces to have an idea about the critical AoA.

An AoA indicator in every aircraft would be a nice tool while flying.

Bushpilot C208/C182
FMMI/EHRD, Madagascar
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