This article states:

An occurrence was reported where an Airbus A321 aeroplane encountered a blockage of two Angle Of Attack (AOA) probes during climb, leading to activation of the Alpha Protection (Alpha Prot) while the Mach number increased. The flight crew managed to regain full control and the flight landed uneventfully.

  • What is Alpha protection?
  • Is it part of another major system (like auto-pilot)?
  • What can it do?
  • How is it activated?

Alpha is also called "Angle of Attack". It is the angle between the relative wind and the chord line of the wing. When the angle of attack is increased to a certain point, the airflow over the wing is no longer smooth. Turbulence is created after the wing, and it loses lift. This is known as a stall.

"Alpha protection" is a system which prevents the angle of attack from getting too high. This is done by software monitoring the plane's angle of attack sensor. When a high alpha situation is detected, the software lowers the nose of the plane to maintain high but still safe angle of attack. It also commands full thrust to all engines. This system is designed to prevent pilots from stalling the aircraft and to allow them to get the best possible performance in emergency e.g. in a wind-shear.

This is not part of the autopilot. It is integrated into the flight control computer and activated at all times. Autopilot are functions which fly the plane in a certain way to reduce the pilot's workload in normal situations (e.g. climb to 13,000 feet, turn left to heading 340).

Pilots can opt to turn off such protection systems should it malfunction, or a very special occasion exists where these protections should be switched off, such as performing at an airshow.

Notable Accidents

In the Airbus, the A320 was the first "fly-by-wire" aircraft. During a demonstration flight AF296 when showing off the alpha-protection system the pilots took the airplane too low and too slow and disabled the part of the system that should have commanded full thrust. The plane didn't stall but was having too little energy to climb anyway. It crashed gently into the trees.

In an acceptance flight, the pilots were testing the alpha-protection at low altitude. The system should have prevented the plane from pitching too high. However, to the surprise of the crew, it failed. The plane stalled with insufficient altitude to recover. The system did not work because water got into the angle-of-attack probes during previous improper washing procedure, froze during the flight and blocked the probes.

In AF447, the system switched to "alternate law" and no longer provided stall protection, because the pitot tubes (airspeed sensors) froze over. The plane stalled from 38,000 feet to the water after the pilots failed to apply correct procedure for unreliable airspeed.

  • 8
    $\begingroup$ Re final para: For AF296 the alpha protection prevented a stall, but by then the aircraft was too low and too slow to clear the trees ahead. The pilot claimed the engines did not respond and that the altimeter was incorrect, so far as I know he didn't blame the crash on alpha protection. $\endgroup$ Feb 3 '15 at 13:12
  • 1
    $\begingroup$ For XL888T the problem was a frozen AoA sensor that had earlier been improperly washed with a pressure washer. This became an issue when the pilots carried out unauthorised and improvised testing at low level. $\endgroup$ Feb 3 '15 at 13:17
  • 1
    $\begingroup$ @RedGrittyBrick re AF396: from what I recall from the documentary, the pilot claimed that when he pulled back on the stick, it pitched down. XL888T: Yup the sensor was frozen, so the system (as a whole) failed. $\endgroup$
    – kevin
    Feb 3 '15 at 13:21
  • 1
    $\begingroup$ I've not seen where Michel Asseline claimed that and the video footage doesn't show any pitching-down youtube.com/watch?v=-cv2ud1339E. Of course, as the aft of the fuselage is dragged through the trees it must eventually affect aircraft attitude. That's not alpha protection. $\endgroup$ Feb 3 '15 at 13:43
  • 2
    $\begingroup$ AF296 is notable by two things: That the Alpha protection saved all but three people on board and that overconfidence in technology causes accidents. They were trying to demonstrate alpha protection, but they've exceeded it's capabilities. If it didn't activate, the crash would have been worse. Also note, that they manually deactivated the other bit that commands full throttle. If they didn't, they would have made it, because the engines would have spooled up earlier. $\endgroup$
    – Jan Hudec
    Feb 3 '15 at 14:34

Alpha protection is a safety measure that prevents an airplane going over certain values of angle of attack. Usually "alpha" is used to designate the angle of attack.

When an airplane is flying, if it exceeds a certain value of angle of attack it will enter a stall (less lift, sometimes suddenly, and the plane falls like a rock). Airbus introduced this safety check into the flight control system that blocks the airplane from getting closer to dangerous values.

Anyhow pilots are able to recover from a stall.

So alpha protection is just software on board.

  • 5
    $\begingroup$ Not all aircraft fall "like a rock" when stalling. Some just pitch gently down and resume flight, others mush along with a higher but stable sink rate. Only bad designs will show nasty stall characteristics, and, unfortunately, those have been built, too. $\endgroup$ Feb 3 '15 at 19:09
  • $\begingroup$ "Falling like a rock" is not the same as having "the aerodynamics of a rock". I tried to express with falling that the gravity is only partially compensated by lift, not saying that the post-stall behaviour of the airplane is the same behaviour as a rock. This behaviour of falling can be felt actually by people when airplane is affected by gust, where the AoA seen by airplane really varies suddently. Screaming is a good way to measure it :) $\endgroup$ Feb 3 '15 at 20:05
  • 1
    $\begingroup$ @TrebiaProject. That's usually a downdraft, not a sudden loss of lift. The wings are still producing lift, but the whole air mass is moving downward, pushing the plane down along with it. $\endgroup$
    – reirab
    Feb 3 '15 at 21:13
  • $\begingroup$ And then there's the Ercoupe... $\endgroup$
    – copper.hat
    Feb 4 '15 at 7:52
  • $\begingroup$ @Trebia Peoject let's compare stall recovery of a paper airplane to a 737. The issue is weight to surface area. This affects altitude loss in the event of a stall. A paper airplane recovers in inches, the larger plane needs several thousand feet. Really comparing a goldfish to a whale. So massive planes must AVOID stall. Higher efficiency wings (higher AR) wings will stall at a lower AOA compared with older designs. Pilots must be properly trained. Alpha protection is needed. But I would definitely look into beefing up those slats. $\endgroup$ Aug 23 '19 at 10:51

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.