High speed buffet at high altitude level flight lead to a stall?

[Centaurus]

Slight thread drift here but I would like to know the answer to the following question. In the Boeing 737 simulator, we practice high altitude (37,000 ft) approach to stalls (clean) and recovery, and low altitude stall in the landing configuration at 1000 ft on a typical final approach.

The former is not a terrain clearance situation obviously, while the landing configuration stall requires recovery with minimum loss of height.

The buffet preceding the stick shaker during the clean high altitude is quite severe and unmistakeable from weather related turbulence. Taking the worse case, recovery action is taken at stick shaker.

To gain an acceptably safe speed before recovery to level flight the pilot needs to know what speed to look for. The clue may be found in the FCTM under the sub heading Holding airspeeds Not Available from the FMC and states "above FL 250 use VREF 40 + 100 knots to provide at least a 0.3 g margin to initial buffet (full maneuver capability).

Typically in the B737 Classic with a VEF 40 of 130 knots, it means losing height until the IAS reaches 230 knots. This includes high thrust and a nose attitude in the dive of around zero degrees. This results in an average loss of 3000 feet providing the pilot does not allow the nose to rise during the descent and thus delay reaching 230 knots or cause a secondary stall buffet because of over-controlling by a ham-fisted pilot.

However in the case of the landing configuration wings level stall (in the simulator that is) there is no pre-stall buffet - only a stick shaker.
Question: Why is there no pre-stall buffet in the landing configuration stall? Because I don't know.

[Brian Abraham]

I wonder if this is a separate issue with the so-called Shock Stall not being a stall at all but just a buffeting

JammedStab, with a spare $1,350 you can purchase "Introduction to the Abrupt Wing Stall Program" which may make a believer of you.

Pprune doesn't seem to like the link, so search the title. It gives a fair introduction to the publication.

[HazelNuts39]

Question: Why is there no pre-stall buffet in the landing configuration stall?

Probably because the stickshaker has been set to operate before natural stall warning occurs.

But it is also possible that the stall in landing configuration is not preceded by buffet. The low altitude pre-stall buffet occurs when the horizontal tail enters the turbulent wake from regions of separated flow on the wing. On some airplanes in some configurations that does not occur.

High altitude buffet is an entirely different mechanism. It results from a cyclic variation of lift when a shockwave causes flow separation which then changes the position of the shockwave. It can occur at any angle of attack. The angle of attack at which it starts decreases with increasing Mach number.

With increasing altitude (i.e. increasing Mach number for the low-altitude stall-IAS) the low altitude pre-stall buffet gradually changes into Mach-related high altitude buffet. That transition starts when the critical Mach number is exceeded, i.e. when the local airflow velocity exceeds the speed of sound. For the model shown in the graph above at an AoA of 21° that is at a free-stream Mach number of around 0.4.

[rudderrudderrat]

Hi HN39,

Probably because the stickshaker has been set to operate before natural stall warning occurs.

Correct.
At low Mach Nos, the stick shaker will operate before the natural buffet. (Clean or Landing Config and low ALT).
At high Alt (hence higher Mach No) the natural buffet normally occurs before the stall warning.

Hi Centaurus,

or cause a secondary stall buffet because of over-controlling by a ham-fisted pilot.

It is very difficult to judge how much "g" we are pulling in the simulator because we only feel 1g in the sim at all times. In real life I think you'd find your "ham fisted" pilots would be more gentle because of the feed back loop through their "g" sensitive bodies.

[Centaurus]

A bit superfluous I know, but thank you for all your replies. I now understand more.

[roulishollandais]

Originally Posted by Brian Abraham

with a spare $1,350 you can purchase "Introduction to the Abrupt Wing Stall Program" which may make a believer of you

Thanks for these references. Are they other documents since 2003 ?

Thank you to Hazelnuts39 picture.

We know that low and high "stalls" are experienced by pilots as sudden modification of lift and pitch.

To simplify, could we remember that only pressure and pressure positions build the forces and the moments modifying lift and pitch.
These forces and moments are resulting from these pressure variations.
The low stall from vortices, the high stall from the pressure cliff Inside the wave shock. The wave shock is not a straight line as shematized in airline books, but a larger cliff.

Both vortices and wave shock pressures modifications are resulting from fractal structure of lift (Liebchaber) in cavitations and some resonances around speed of sound. Both allow 'continuity' in the équations of air density around the whole aircraft until steady air distance.

Critical reading is welcome

[John Farley]

Agreed, but it is too simplistic to separate mach buffet and angle of attack, because the shock waves vary with angle of attack.

Exactly.

Whatever stall you are talking about, reducing the angle of attack will reduce the severity of whatever symptoms one is talking about.

[Aluminium shuffler]

Mach stall is, as stated earlier, caused by the shock wave on the top of the wing causing the airflow to become non-laminar, much like a conventional low speed, high AOA stall. Where the confusion about Mach stall and AOA is creeping in I think is in that AOA will have a very swift and significant effect on the stall - the camber of the wing is accelerating the airlfow over the top surface - we all know that from the first principles lessons. That acceleration brings near-transonic airflow up to transonic, creating the shock wave. At that point, even a small increase in AOA will have a huge effect in further increasing that top flow acceleration and increase the localised Mach number, moving and exacerbating the shock wave and exponentially deepening the mach stall. Reduce the angle of attack ad the effect will be mitigated somewhat, though not eliminated until the Mach number is reduced to get rid of the shock wave entirely.

[roulishollandais]

Originally Posted by Aluminium_shuffler

the camber of the wing is accelerating the airlfow over the top surface - we all know that from the first principles lessons

But we know now it is wrong (discussed in "theory of lift).

Shock wave is a local overpressure of air, and AoA is necessary to build it.

[JammedStab]

It is well written out in the 727 FCTM. I can't copy and paste it but it says that high mach affects stalls favourably. Separation starts at a more inboard location and at lower AOA leading to earlier warning and more pronounced pitchdown. It then mentions about the buffet occurring before artificial stall warning and says "Thus, the improved buffet warning and increased stall margins due to mach number effects give greater stall protection during high altitude flight"


So there you go. At least on this type, you have a greater stall margin. Then again, perhaps a lower margin for having to explain to somebody why you got buffeting in the first place.

One question. Once while cruising in a 737-200 we got above out target mach to maybe .76 but for sure it was below the barber pole. But all of a sudden we started getting some airframe vibration until the speed was reduced. Is this normal for this to happen when clearly below the barber pole?

[porch monkey]

I suspect you were way beyond the barber pole if you were at .86 in a 737-200. You meant 727, yes?

[JammedStab]

Sorry, I meant around mach .76 which was below the barber pole.

[Machinbird]

Once while cruising in a 737-200 we got above out target mach to maybe .76 but for sure it was below the barber pole. But all of a sudden we started getting some airframe vibration until the speed was reduced. Is this normal for this to happen when clearly below the barber pole?

Couple of possibilities come to mind.
1. Limit cycle oscillation of a control surface being excited by the higher than customary speed.
2. Unstable shock waves as you apparently are suspecting.

For those who have not had the pleasure of going supersonic, the transition to supersonic in the bird I used to fly was easily perceived by a slight change in pitch attitude nose down and a sudden smoothing out of the aircraft (before it had been like running down a bit of a rough road at high speed). That aircraft did not exhibit noticeable wing drop in the transonic range.

The transition to subsonic from supersonic had an important characteristic. Whatever g we were pulling suddenly increased by ~50% with virtually no aerodynamic warning. Although this created overstress incidents, we did not experience stalls due to the relatively high Q. (This was in a non-FBW aircraft normally operated below FL450.)

[IcePack]

The A330 "hums/buzzes" if you get her much above Mach .83
So somewhere around the airframe the airflow is going sonic,but buffet due to airflow separation will occur at higher speed.
As we know some older jet aircraft suffered from jet upsets whilst knowledge of high altitude flight was being gained. So yes aircraft can high speed stall at high altitude & speed. Don't they teach that now in the ATPL?

[roulishollandais]

To start with chaos vortices and shockwaves

To math addicts
http://greenfluids.syr.edu/docs/GreRowSmi_2007.pdf
http://arxiv.org/pdf/1202.2989v1.pd
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