737 Pre-stall buffet at high altitude. Not at landing config low alt. Why?
 

During stalling recovery practice in the 737-300 simulator at high altitude (37,000 ft for example) there is very strong buffet preceding stick shaker operation. So strong that it is obviously cannot be mistaken for normal turbulence.

On the other hand when the simulator is set up for a practice approach to the stall recovery on final approach in landing configuration where action is taken at stick shaker, there is no discernable pre-stall buffet.

No doubt there is an obvious answer to why a heavy pre-stall buffet at high altitude; but no buffet in landing configuration at low altitude.
Presumably, something to do with flap and leading edge extension at low altitude that changes stall characteristics - but why is this so?

Does this occur in real life or is it a simulator fidelity problem?

Sounds like 370 is exceeding buffet margins...Approach to landing is normal stall characteristics...as in a C-172...

Ask your Instructor about high alt buffet margins AKA: Coffin Corner...

B-727 is the same...

Remember those clowns in the CRJ who took it up to FL410 on a ferry flt?

It just rolled over after the stick-shaker....

It's a weight/temp thing at high alt...on approach, the margin is much wider...

To clarify the initial post. My question was why is there marked pre-stall buffet in a level flight clean configuration stall at (say) 37,000 ft. or make it 31,000 ft if you like. Regardless of weight. Yet no buffet in a dirty stall at say 1000 ft. I didn't mean to confuse the original purpose of the question with G buffets at high altitude.

I remember an article in the Boeing magazine about angle of attack (*). It explained that on older models the stickshaker was set a a fixed AoA. On more recent airplanes the stickshaker threshold value reduces with increasing Mach to stay ahead of Mach buffet.

Since the Mach buffet is caused by transonic effects at high Mach number, it only occurs at high altitude (see buffet onset envelope in your manuals). You don't encounter it at low altitude.

(*) Boeing AERO Magazine: Operational use of angle of attack on modern commercial jet transport airplanes

I've experienced Mach buffet as a passenger in a delayed southbound 737-800. Crossing the Pyrenees, in a northerley jetstream (which I presume suddenly reduced). Moderate buffet with pitching, shockwaves clearly visible from the wing leading edges :eek:. Reminded me of one of my early aeronautics lessons :uhoh:

I would not place too much trust in a simulator`s interpretation of flight characteristics on the edges of the flight envelope.

Hi Tee Emm,

Have a look at "Stick Shaker" explanation Aero 12 - Angle of Attack
"The early stall warning system thresholds were not set to be effective at cruise altitudes and speeds because they did not correct for Mach number (fig. 10). This kept the system simple. The stick shaker was set at an AOA effective for low altitudes but at too high a value for cruise. Natural stall buffet was found to give satisfactory warning at higher Mach numbers."

Would not suspect a simulator fidelity problem.

My understanding is that the stick shaker speed is calculated that way (ie below low speed buffet at altitude).

This is why the min manoeuvre speed is calculated differently - based upon stick shaker below approx 20,000ft and based upon low speed buffet above approx 20,000 ft.

Thanks everyone who replied so far. The Boeing article on angle of attack explains everything. TM