Stall speed and Buffet boundary at high altitude

[TheDrop]

Stall speed and Buffet boundary at high altitude ... "Coffins corner" stuff.

I fly a jet capable of FL 510 (FL490 JAR OPS with no radiation detectors). Have never gone up there, but occasionally we go to FL 450 or 470.

How is the real low speed buffet determined ? At a certain mass, the speeds are as follows according to the Flight Manual (all speeds are KIAS or indicated mach):

Vref full flaps flaps is 121
Stall speed full flaps is 93

Stall speed clean 105

Buffet boundary for low speed buffet at FL450 at the same mass is Mach .58 / 158 knots.

At FL450, ISA +2, climbing at 500 fpm, as IAS went down to 180, the stall warning came on, kicking off the auto pilot. Recovered with slight altitude loss to get 200 knots.

From 105 to 158 is compressibility IAS>EAS, I reckon

But what explains the gap from 158 to 180 knots ?

[411A]

Hmmm, seems like you got a bit slow. Not good at higher altitudes, expecially at higher weights (mass for our European friends). As you found out, height loss may be required to regain IAS/Mach as little if any excess thrust is available high up.
The Boeing 707 guys found out about this many years ago when they tried to overfly thunderstorms in the USA.
'Tis called "jet upset", if you're not careful.
Is your buffet boundry info presented in tabulated, or graphic form?
Graphs are MUCH better, IMHO.

[TheDrop]

It is in a graphical form, both low and high speed buffet. An article i business News and Aviation, january this year, was about Coffins Corner, exactly on this aeroplane. The weird thing is that the ADC shows IAS and not EAS - that gives you a false indication of being safe - stalling at 180 knots ? I knew about the IAS-EAS compressibility factor, but why is it not placarded or warned more ? Anyway, now I know, you don't want to get below 190 at FL450.

By the waym this would not as much be coffins corner, as there were still M .69 to M .79 to increse spead, that is about a 60 knot margin.

[quid]

TD-

Most stall warning systems (stick shaker) are set to go off at a certain value ABOVE stall. The spread you mentioned 158-180 is not an unusual margin between stick shaker and actual stall.

[411A]

All buffet boundry charts that I have seen have the speeds in IAS/IMN, but on the other hand, some may be different on other aircraft.
Have generally figured that 1.3 G maneuvering capability was about right normally, 1.25 IF the air was very smooth, to be used with caution.

[Earthmover]

411A, please don't think that we Europeans actually like having 'mass' and 'hectopascals' forced upon us - Jeez!

Sorry, off thread

[411A]

I know Earthmover...have flown in Europe a LOT, and still have not met anyone who liked the idea.

[saudipc-9]

Just a thought that stalls are the result of exceeding the critical angle of attack. Now you were at FL450 and climbing at 500fpm what was your attitude? At that height I cannot imagine that you had alot of excess thrust to spare which would mean you might have had the nose up too much (or the auto pilot did to maintain what you had asked of it)
Thoughts?

[Dick Whittingham]

If you assume CLmax remains constant then stall EAS remains constant. In IAS terms this means stall speed increases slightly as you climb, from the compressibility corection. However, CLmax does not remain constant. At quite low mach it begins to degrade, so stall speed rises, even in EAS. The low speed buffet boundary curves toward the high speed boundary, usually Mmo. Where they meet is coffin corner. Buffet boundaries are given at a percentage above the stall speed and turbulence boundaries equate to load factor increases, so are tighter.

Dick w

[TheDrop]

Thank you all, very useful.

I Think all this low speed buffet is underestimated to some extent, mainly there is so much focus on Vref awareness throughout the flight, but it seems there is no use for Vref on cruise as low speed buffet apparently is way above Vref. One should really have an enroute calculation/lookup for the low speed buffet at the flight level chosen for cruise.

The AoA indicator is for some reason not of much use here as the stall warning came on when the indicator had not even reached the middle of the green area of AoA range.

Another thing, quite relevant, will not the same happen to Vmca - if so, engine failure at altitude is not just a question of maintaining altitude, but also to know what is Vmca at present level. Believe I heard of a 747 with one engine out, and they didn't descend in time to below one engine out maximum (due to lack of knowledge) so they went below Vmca for 3 engines and flipped (but recovered in the last moment)