There appears to be some misunderstanding of the character of high altitude/high Mach No stalls on current transport aircraft. On designs using supercritical wing sections on which upper surface flow breakdown starts at or near the TE and for which the design span loading leads to the critical maximum local lift coefficients being located about two thirds semispan the loss of lift is only slightly aft of the CG, so any pitch up is indeed mild. This is true no matter which side of the Atlantic the design comes from. The limiting factor in cruise conditions is buffet and in fact “stall” is usually defined by buffet reaching +/- 1g vertical acceleration at the pilot’s seat. The A330 high altitude/cruise Mach No stall data published in the second “Expert’s Report” (in French only unfortunately) shows this occurred at about 9 deg AOA on a simulated AF447 test, accompanied by +/- 0.5g lateral acceleration. I’m no pilot, but surely this sort of environment is not “benign”?. The authorities obviously think that this level of buffet plus a verbal stall warning announcement constitutes compliance with FAR 25.201 d (2).
The recent publication of a joint Boeing/Airbus paper “Stalling Transport Aircraft” (Society of Flight Test Engineers) makes interesting reading in this respect. For those who don’t want to bother, here are some relevant extracts (my emphasis)
<At high altitude/high Mach number, the Vs1g cannot be determined due to early triggering of buffeting. This buffeting is caused by local shock waves which excite the structural modes of the aircraft. Theoretically a ClMAX may exist at high Mach number but the associated level of buffeting prevents efforts to identify it during flight tests for aircraft structural integrity and safety reasons.
Nevertheless, according to the regulation a minimum of 1.3g maneuverability up to “buffet onset” must be demonstrated for each flyable Mach number. Therefore the ClMAX for a given Mach number will generally be defined by the level of buffeting corresponding to ±0.1g vertical acceleration level measured at the pilot’s seat (so called “buffet onset”) and is named ClBUFFET
Pitch up
“Pitch-up” can be observed during stall mainly on aircraft fitted with swept wings. It is due to the sudden loss of lift on the outer part of the wings, which creates a nose-up moment (see Figure 7). This phenomenon can also occur at high altitude/high Mach well before ClBUFFET is reached and is again due to shock waves destroying the lift on the outer part of the wings.
Expected 1g stall characteristics of large aircraft
Typical stall characteristics of transport aircraft in 1g non-accelerated flight (ref FAR Part 23.201) begin with the onset of initial buffet. This is best described as light airframe buffet which begins a few knots prior to stick shaker. As the aircraft approaches Clmax the level of buffet generally increases and can become severe to deterrent in nature. It is not uncommon to see buffet with a repetitive load factor of ± 1 G in the vertical direction and ± .5 G in the lateral direction (see Figure 20). It feels similar to driving an automobile across railroad ties. Buffet on large airplanes tends to be much greater than experienced in smaller aircraft. This is due to wing airflow separation and turbulent airflow vortices which produce a strong excitation forcing function on the wing. This excites the fundamental frequency of the fuselage leading to large vertical and horizontal deflections. It can be very evident on the flight deck, where anything not securely tied down, such as an errant water bottle, can get hurtled into the air.
Stall identification is deterrent buffet for most recent models in the clean wing configuration
Certification Stall Requirements for FBW Aircraft
Stalls for certification are performed initially at forward CG to determine the stall speeds that will be used for speed computations. For FBW aircraft, stalls must be performed in Normal Law to demonstrate the ability to control the aircraft throughout the stall and recovery, should an inadvertent stall occur despite the protections (for example, in the case of severe windshear). Because the angle of attack limits in Normal Law prevent the aircraft from stalling, a specific version of the Normal Law is created which shifts the limiting AOA to a higher value (usually by 10 degrees). Additional stalls are then performed in Degraded Control Laws, in all configurations, and at aft CG. For stalls in Augmented Control Laws, it is essential to place the stick forward of neutral during recovery, since the C* law is g-demand law, and neutral stick is a 1.0g command. If the stick is not placed forward of neutral, more nose up elevator than desired will be applied at the stall break.
Stalls at high altitude are not required in the classic sense. Instead, a series of wind-up turns are performed at constant Mach number to determine buffet boundaries. This determines how much g the airplane can sustain before the airplane begins to buffet, which is useful to the operational pilot when flying at maximum altitude for a given weight. If the airplane is flown too slowly, there will be an increase in buffet to the point that altitude will have to be sacrificed to regain speed. The same holds true if the airplane enters turbulence which applies g loads higher than the buffet boundary for a given Mach/altitude/weight combination
Stalls at High Mach Number
Stalls at Mach numbers normally associated with cruise flight (0.78-0.89 Mach) are not possible in level flight because Mach number decreases as the airplane decelerates. This increases Clmax and effectively increases the margin to the stall. It is difficult to tell when the test point will end, because the end point is shifting. Angle of attack limits for safety are equally difficult to predict.
As the airplane decelerates, the level of buffet increases significantly and rapidly becomes deterrent buffet. The g-break may be difficult to recognize, either from the g trace, or the Cl trace, therefore a rapid change in vertical velocity may be the first good cue of the stall. Pitch-up may be present during deceleration, complicating the pilot’s ability to smoothly control pitch with increasing levels of buffet.>