Stall speed at high altitude / low speed buffet onset?
 

Hi!

I am kind of confused about the low speed operation at high altitudes. Can someone clarify a few issues?

1. Why does the FCOM of my a/c type (A-320) only publish stall speed (vs1g) in terms of KCAS only for altitudes below FL200? For higher altitudes all there is is the buffet onset chart, with the low/high speed buffet onset in terms of Mach number... Why no stall speed for higher alt? I thought that for a given mass/config A/C stalls at the same EAS regardless of altitude?

2. What is the difference between a "classic" stall buffet and low speed buffet at high alt? Is the latter caused by the shockwave on the wing (akin to the mach buffet)? This would explain why it is given in terms of Mach, not airspeed, right?

3. Is the low speed buffet incorporated into stall warning logic, or does it work only for high AoA and it's up to the pilot to determine the minimum mach number to avoid the low speed Buffet?


Cheers!
Stuck

Perhaps this article in the Boeing Aeromagazine No. 12 answers some of your questions.

For an Airbus FBW airplane like the A320, Vs1g is the lowest speed attainable in normal law at 1g with the sidestick held fully back. At low altitude it is defined by alphamax, an AoA slightly below that for aerodynamic stall. At high altitude (i.e. above a certain Mach number) Vs1g is equal to the 1g buffet onset speed.