Fast:

Not necessarily upset, but shaker and/or stall..but most likely, in the shaker and unable to accelerate without descending. In some fleets they call this "terminal Mach decay," terminal meaning "The End," not the place you park at the end of your trip.

If you've ever seen this in a sim, you set a speed that is below L/D max, then let "turbulence" slow you 5 or 10 knots.

You add power and find you are now 15 knots slow. So, you give it all she's got. Problem is, there is so much induced drag (from you increasing pitch to maintain altitude) that you no longer have more thrust available than drag.

And it gets even slower.

So, you either:

(1) hang on to altitude thinking "I can't get violated" and end up in the shaker (or worse), or

(2) declare a mayday for turbulence (unable to maintain assigned altitude), kick-off altitude hold, and let the airplane accelerate in a descent. Once back to a normal speed, you ask ATC if you can climb back to your assigned altitude

The first one ends up being the second one, so it's best to just go with 2.

Thanks for the clear explaination. I have never flown the Boeing only the 320. But it seems to me that the Bus anyway would react long before this condition would occur, perhaps I am wrong in this thinking. Something to try in a sim somtime. However in anycase I would always be willing to sacrafice altitude for stability. Not that busting the FL is good but if you are going to stall and perhaps lose stability then it seems like the best trade you have.

Thinking back of course we would never try to keep alt in the small birds if it got rough enough, but this is because we like to keep the wings on

I should point out, this happens at very high altitudes and/or gross weights, or when the atmosphere is warmer than standard, ie, all things that limit climb (or acceleration) performance.

I flew the Bus too (and am supposed to fly it again after mil-leave). If you get the Bus slow enough, without the automatic protections of the Bus, the same thing would happen. Actually, it might even happen with the protections...can't remember that particular feature (ie, if it limits the minimum-speed at level-off based on it's knowledge of power-available vs. thrust required).

Any time I'm trying to reach a level-off altitude and my airplane (whether a fighter or an airliner) can do 400 fpm or less, I'm concerned about getting slower than approximately the clean-maneuvering speed you would use on downwind in the traffic pattern, and not being able to correct with power alone.

Where does low speed Mach buffet fit into this ?

Doug:

I've never heard of low-speed Mach buffet, but there is such a thing as high-speed Mach buffet.

High-speed Mach buffet happens on aircraft that typically have relatively thick airfoils (like airliners). I've felt it in both the 727 and 747-400. If you get close to redline speed, the air that goes over the top of the wing, following Bernoulli's equations, speeds up....to the point that it (locally) goes supersonic. This happens over the thickest part of the wing, or canopy, or cockpit windows (which is supposedly why the 727 cockpit is so loud). It then slows down as it matches-up with the flow at the trailing edge.

You may have seen this at airshows when a fighter comes ripping by at about 550 knots in humid conditions: you see momentary "cloud" or "wave" forms over the thickest parts of the wing, intakes, fuselage, or canopy. If he pulls some g as he does it, it becomes more pronounced, as the air going over the top of the curved surface must now speed up even more (for more lift coefficient).

During the time the air was supersonic, though, it usually detaches the boundary layer, so that when it slows down again, it is turbulent. A traditional stall produces buffet because of detached laminar flow and the resulting turbulence. So, even though the mechanism was different (One is extreme angle of attack, the other is almost no angle of attack), the symptom is the same: you feel buffet.

In airliners, this buffeting air usually is felt as aileron "buzz," as swept-wings have a natural tendency to stall wingtips-first. The turbulence is right in front of the ailerons so it gets transmitted through them.

We were talking about this over in the power question thread where a DC-8 was dived through Mach 1 and it exhibited a lot of this same buzz according to the pilots. It can also happen on the rudders. This explanation is supplemental to understanding why airliners have such difficulty going supersonic- as shocks form they add a lot of drag to the airplane. When they start to occur what seemed like easy going a few knots before suddenly becomes very difficult.

In addition, the propulsive efficiency of jet engines (particularly turbofans) goes way down as you go supersonic (because the Fan and compressor blades are supersonic from root to tip, and max coeffient of lift drops about 40%).

Ties-in with an earlier post you made that airliners can only go transonic and possible supersonic in a dive. Concur.

BTW: all claims of supersonic flight in airliners are, to my knowledge, recorded as "probable." The explanation I was given is that the pitot-static systems are not designed for Mach-wave phenomenon; have never been flight-tested at such speeds; and are not regularly calibrated to such speeds. But under unusual circumstances (extreme dives and late attempts to arrest it), it is quite likely. In WWII, P-47, P-51, and Spitfire pilots reported hitting 700 mph in dives. That's what the gauge read...but it was wrong. It wasn't designed to work at that speed (well, no one knew how to design them then...everything was sub-sonic!!)

The airliners I have read stories about going probable supersonic are:

DC-8
707
720
727
747-200
747SP

What you discribed in bold is what has been has been explained to me in the past as "low speed mach buffet", some supersonic airflow over parts of the airplane induced by angle of attack. Except that, at very high altitudes, the AOA increase required to initiate low speed mach buffet is very little.

I don't have a reference text available to back me up, but I'm pretty sure it's called high-speed buffet.

In the 747, getting aileron buzz at 350 IAS and M0.86 around FL320 (and a TAS of 530+ knots) isn't what I would call "Low Speed."

Perhaps some think of it as "Low Speed" because at a relatively "low" Indicated Speed, you are getting the effects of something at Mach 1.

Again, semantics. I can see a case for calling it either. But in the case of where I accidentally did it (in Mach-mode in a descent instead of transitioning to IAS), the Capt's response was "You're too fast...slow down!!"

Hey all thanks for replying to my stuff. This has turned out to be a really informative thread! I don't know a lot about high speed aerodynamics, and I'd like to learn more - especially before I upgrade to captain. I want to know everything before I get in the left seat. Where can I find more information on this kind of stuff?