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One of the A330/A340's several means of flight-envelope protection is something called "alpha floor" protection. In essence, if the aircraft's angle of attack is excessively high, TOGA thrust is automatically commanded on all engines to help ward off a stall.

There are a number of situations where alpha-floor protection is inactive, most of which are cases where automatically applying TOGA thrust would be unnecessary or would result in control difficulties. One of these, however, is... whenever the aircraft is above mach 0.53?!

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(Excerpt from A340 FCOM, highlighted by user14897 here at AvSE.)

Given that it is, in fact, possible for an aircraft to be simultaneously above mach 0.53 and in danger of stalling (especially at high altitudes, where a given mach number corresponds to a much-lower IAS - and, thus, a much-higher 1-G AoA - than it does at lower altitudes), why is alpha-floor protection disabled beyond mach 0.53?

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    $\begingroup$ I would be suspicious that, at higher Mach numbers, there is a risk of high power settings aggravating the problem at high AoA, which is why the design limits its implementation to regimes of the envelope with limited compressibility effects. There is also the possibility that you may not be able to achieve angles of attack near the critical AoA w/o exceeding the flight load limits and causing structural damage. $\endgroup$ Commented Mar 6, 2023 at 4:33
  • $\begingroup$ @CarloFelicione: Structural overload would be a reason to disable alpha-floor protection above a certain IAS, not a certain mach number, and compressibility effects shouldn't be a major problem for modern jetliners with benign transonic-flight characteristics. $\endgroup$
    – Vikki
    Commented Mar 6, 2023 at 4:49
  • $\begingroup$ But high AoA at high Mach numbers can result in undesirable aerodynamic effects as well. $\endgroup$ Commented Mar 6, 2023 at 4:51

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Try to see it this way: ALPHA FLOOR is a protection function of the AUTO THRUST SYSTEM, and it protects the aircraft from excessive speed decay. It does not primarily protect the aircraft from stalling. That can of course a possible outcome, if you don't increase thrust to accelerate.

Stall protection is primarily done by the low speed protection (high AOA protection of normal law), where the aircraft commands nose down so it does not decelerate below V_ALPHA_PROT (thus maintaining actual AOA <= ALPHA_PROT). And that protection is working above M.53.

One example for ALPHA FLOOR: you are in approach with Flaps Full, Idle Thrust (Manual Thrust) and nearing V_APP. Environmental influences (loss of headwind, increase of tailwind, gusts) or a shallow descent or even level-off could even further let your speed drop. That is where ALPHA FLOOR kicks in, because the PRIMs calculate, that you are in a low-energy state. To prevent the aircraft from automatically pitching down, the auto thrust commands TOGA. If everything goes well (as the engineers intended the protection to work), the speed decay is prevented just in time, so that the low speed protection does not need to be activated. In this case, alpha floor makes sense.

Other example: you are in cruise flight, near MAX REC ALT and near green dot speed. It does not take a lot (sudden change in wind, turbulence, excessive pitch input in manual flight (which you should not do at cruising altitude)) to let the speed drop to V_ALPHA_PROT (which is very close to green dot in this thin air near max altitude). If you are flying in alternate law (which we do in the simulator for stall training), the stall warning will activate instantly. Maybe the Auto Flight System disengages, because the speed drops below V_LS minus 5. In both cases, it would do more harm than good to have TOGA thrust, which can cause a pitch up moment, that is hard to control manually at these high altitudes. The only thing to do to save the airplane is to pitch down (nose down to unload, g-load wise), recover speed, increase thrust smoothly and recover the flight path.

These are two edge cases. And somewhere in between, there is a point where alpha floor does not make sense anymore as the best way of protecting the aircraft in all possible scenarios. And apparently, the engineers determined M.53 to be this limit.

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