Will the autopilot raise the AoA above the critical AoA to maintain altitude?

Question

NOTE: This question is regarding commercial airliners (eg: Boeing 737, Airbus A320, etc.. This is for any given configuration of the aircraft)

Will the autopilot raise the AoA above the critical AoA to maintain the given altitude or the glideslope(if intercepted)? For example, both engines failed in the case of BA-38 the autopilot tried to stay on glideslope which resulted in a speed loss. (thanks to the pilot's timely action to disconnect the autopilot which avoided it from stalling out and losing too much speed which saved the lives of many people). Also, will the autopilot reduce the AoA on its own if the aircraft stalls out?

Answer 1

Ralph wrote a nice answer for the Boeing 737, which is a conventionally controlled aircraft, meaning the yokes are mechanically linked to the flight control actuators.

Airbus A320

The Airbus A320 however is a fly-by-wire controlled aircraft. This means that both sidestick input and autopilot commands are sent to flight control computers, which then calculate how to move the control surfaces to achieve the desired flight path:

^{(Airbus A320 FCOM - Flight Controls - Description)}

While the autopilot will happily command inputs that would increase the angle of attack above its critical value, the flight control computers have built-in protections against this. These protections are active regardless of whether the pilots control the aircraft via the sidesticks or the autopilot controls the aircraft:

Control with autopilot engaged:

• The ELACs and SECs limit what the autopilot can order. [...]
• All protections of normal laws remain effective.

^{(Airbus A320 FCOM - Flight Controls - Normal Law)}

In the case of increasing angle of attack, the following protection law would apply:

High angle of attack protection

Under normal law, when the angle of attack becomes greater than αprot, the system switches elevator control from normal mode to a protection mode, in which the angle of attack is proportional to sidestick deflection. That is, in the αprot range, from αprot to αmax, the sidestick commands α directly. However, the angle of attack will not exceed αmax, even if the pilot gently pulls the sidestick all the way back. If the pilot releases the sidestick, the angle of attack returns to αprot and stays there.

This protection against stall and windshear has priority over all other protections. The autopilot disconnects at αprot + 1°.

^{(Airbus A320 FCOM - Flight Controls - Normal Law)}

Since the autopilot disconnects itself at αprot + 1°, it would not automatically recover from a stalled condition. However, AoA would reduce now anyway because the flight control computers would not allow any further elevator up actuation until AoA is below αmax again.

In addition to limiting the angle of attack via the elevator, the aircraft will also automatically set maximum thrust at some point. This is called Alpha-Floor protection:

Alpha-Floor protection

Alpha-floor protection automatically sets the thrust at TOGA thrust, when the aircraft reaches a very high angle of attack.

The Flight Augmentation Computer (FAC) generates the signal that triggers the alpha-floor mode. This, in turn, sets TOGA thrust on the engines, regardless of thrust lever position (Refer to 1.22.30 A/THR modes).

The FAC sends this signal when the angle of attack is above a predetermined threshold, that is a function of the configuration.

^{(Airbus A320 FCOM - Auto Flight - Flight Augmentation)}

Boeing 777

The accident you linked was in a Boeing 777, which is also fly-by-wire controlled and therefore also provides flight envelope protections. However, Boeing uses a somewhat different approach, where the protections increase the required force on the yoke to overcome them, rather than limiting what is being transmitted to the flight control actuators. This means, pilots can always overpower the envelope protections in a fly-by-wire Boeing.

The autopilot will however obey the flight envelope protections and therefore not stall the aircraft:

Flight Envelope Protection

There are three forms of flight envelope protection in the autopilot:

• stall protection
• overspeed protection
• roll envelope bank angle protection

An AUTOPILOT caution message and roll or pitch mode failures alert the pilot if the envelope is exceeded, and the autopilot prevents further envelope violations.

^{(Boeing 777 FCOMv2 4.20.19 - Automatic Flight - System Description)}

The stall protection will not allow the autopilot to trim the aircraft into a stalled condition and — similar to the Airbus Alpha-Floor protection — also use the autothrottle to increase airspeed:

Stall Protection

Stall protection reduces the likelihood of inadvertently exceeding the stall angle of attack by providing enhanced crew awareness of the approach to a stall or to a stalled condition.

Stall protection limits the speed to which the airplane can be trimmed. At approximately the minimum maneuvering speed, stall protection limits the trim reference speed so that trim is inhibited in the nose up direction. [...]

The autothrottle can support stall protection if armed and not activated. If speed decreases to near stick shaker activation, the autothrottle automatically activates in the appropriate mode (SPD or THR REF) and advances the thrust to maintain minimum maneuvering speed (approximately the top of the amber band) or the speed set in the mode control panel speed window, whichever is greater. The EICAS message AIRSPEED LOW is displayed.

^{(Boeing 777 FCOMv2 9.20.12 - Flight Controls - System Description)}

In the accident flight, autothrottle inputs where obviously no longer available since both engines had already failed. This also has other implications (e.g. both engine generators fail, which requires emergency electrical power from battery or RAT), which can result in the loss of flight envelope protections:

Flight Control System Secondary Mode

When the PFCs can no longer support the normal mode due to internal faults or lack of required information from other airplane systems, they automatically revert to the secondary mode. [...]

The following functions are not available in the secondary mode:

• autopilot [...]
• envelope protection [...]

^{(Boeing 777 FCOMv2 9.20.7 - Flight Controls - System Description)}

Answer 2

On the 737, some modes will hold altitude (or glideslope) no matter what, while other modes will change over (or "revert") to a descent to maintain a minimum speed.

It's probably a long discussion regarding why "this" mode will revert to a constant IAS descent while "that" mode won't; the easiest case is probably GS Capture: it will track the glideslope NO. MATTER. WHAT. It will not give up the glideslope and allow the aircraft to sink below it unannounced, because that can kill you without warning. If it holds the GS and lets airspeed decay (i.e. with power at idle & not coming up as it should), hopefully the stall warning will get your attention & you add power to avoid the stall and the crash. Obviously, being in that situation, low to the ground & airspeed decreasing, is a place you never want to be, but the engineers apparently say that there, giving up the GS to hold airspeed is even more dangerous than doing the opposite.

Altitude Hold has the same priority, for slightly different reasons. Vertical speed and most VNAV modes, do the opposite, and will revert to a minimum speed rather than flying the aircraft into a stall. (Those modes also have speed reversions at the high end - they will revert to hold VMO or MMO at the expense of the selected V/S or path, rather than holding the latter & overspeeding the aircraft. Alt Hold & GS Capture have no speed reversions high or low & in those modes you could overspeed things with power pushed up long enough.)

Vertical Speed mode is probably easiest to explain here; if I dialed in a 2,000 feet/minute climb and forgot about it, the safest course is probably to continue the climb, at a reduced vertical speed but a safe flying airspeed, rather than stall (probably pretty violently) by pulling harder & harder to hold a climb rate that I no longer have the power (at high altitude) to sustain. So the V/S mode does have a min speed (or, perhaps, an AoA) reversion, where Alt Hold and GS Capture do not.