Originally Posted by
funfly
As a GA pilot I have few concerns about the effect of height on stalling speed.
However following the AF447 thread I understand that on commercial aircraft at altitudes abouve FL 350 the margin between stalling speed and maximum speed can be extremely small - perhaps down to less than 10 kts in some cases. The implication being that these margins leave very little for variations in air properties and the phrase 'coffin corner' applies to a situation where the altitude and air temperature allows the maximum speed to fall below the stalling speed.
I understand the theory but I find that my inclinatioin to fly commercially has virtually been removed by this 'little' information. Can anyone who knows enlighten me as to how close commercial flight comes to flying at the top limit and what the margins would be on a conventional flight at, say, FL350 otherwise I don't think that I will fly commercially again.
The kinds of narrow margins people have mentioned in the context of AF447 only occur AFTEr a variety of things go wrong.
Under normal circumstances a cruise speed is selected which provides a margin at the lower speed end equivalent to the ability to pull 1.3'g' before encountering buffet. If you consider that stall warning (or buffet) is usually required to give 5% warning of stall, then you could consider that the typical minimum margin to stall for a selected cruise speed is 1.05 * sqrt (1.3) = 1.197 - basically, 20% speed margin to stall at the lower end. That's comparable to the minimums typically applied for takeoff (1.13 to 1.20) and landing (1.23 to 1.30) so high altitude flight, properly planned and without extreme events, is no more at risk of stalling than any other phase of flight.
At the high speed end, you cannot cruise at higher than Mmo (max operating Mach number) - its an absolute flight manual limitation. But for certification the aircraft must be flown safely to a speed called Mdf (demonstrated flutter Mach number), must be shown to have acceptable handling to Mfc (which lies between Mmo and Mdf), must be shown to be structurally sound at the design Mach number, MD, and must be shown to be able to start at mmo and be subject to various "upsets" - including severe gusts - without infringing the various high speed limits or causing a hazardous condition. So there's a considerable margin at the "top end" also - typically of the order of 0.05 Mach. So an aircraft with a cruise "speed limit" of Mmo=0.85 will have been shown to be safe out to 0.90. Oh, and the flutter calculations at MD have to have a 15% speed margin on top, and the structural calculations have a 40-50% safety margin too.
It takes HELL of a lot to go wrong before any normal civil airliner starts to worry about "coffin corner" - the problem is fairly well understood and catered for these days. The aircraft it really was an issue for were military types pushing the envelope (such as the early U-2, which did cruise precariously close to "coffin corner"). No-one does that in a civil transport.