Stall warning and Alternate law
This post is part of the Airbus aircraft family and system safety thread.
According to an investigator from Indonesia’s National Transportation Safety Committee (NTSC) several alarms, including the stall warning, could be heard going off on the Cockpit Voice Recorder’s tape.
Now why is that so significant?
Because one should not hear a stall warning when the aircraft’s so called Normal protection laws are running and preventing the aircraft from being flown outside the flight envelope (1). One explanation for hearing the stall warning is that the aircraft had entered what’s called Alternate law and (obviously) was entering into a stall. For those that don’t know much about the Airbus, Alternate law is a fallback mode that kicks in after the loss of onboard equipment. In alternate the aircraft’s normal ‘alpha protection’ (2) laws are removed (along with others) and it becomes physically possible for a pilot (or the environment (3)) to stall the aircraft, hence the importance of a stall warning.
An alternative explanation to the stall is that the external environment was so extreme that the stall warning was triggered even while the aircraft was in Normal law. In Normal law stall warning is designed only to activate at alpha angles higher than those at which alpha max protection would be triggered. Such a scenario would require the environment to cause the aircraft to pitch up through alpha max. A ready example of such an event is what happens to an aircraft in the presence of a micro or macro burst wind-shear event (4).
While we need to wait for the investigators to complete their full investigation, it now seems possible that the pilots had lost a significant amount of the protection offered by Airbus automation, just when they needed it the most. The parallels of this disaster with the loss of Air France AF447 are disturbing to say the least.
1. The stall warning is set to trigger at 23 degrees angle of attack or ‘alpha’, regardless of airspeed in Normal law. The stall warning will sound at airspeeds less than Vs1g, the lowest speed at which the aircraft can generate more lift than the weight of the aircraft (i.e Vsw<Vs1g). In Alternate and Direct law this threshold is modified so that the stall warning will occur at speeds above Vs1g, e.g Vsw>Vs1g. Note that the actual stall speed of the aircraft is load factor affected, e.g 0.8g load factor will reduce the stall speed.
2. ‘Alpha’ protection means protecting the aircraft from excessive angles of attack, expressed as the alpha angle, that could cause the aircraft to enter a stall. The system also protects against wind shear at low altitude.
2. Plenty of aircraft have entered and been flown safely in Alternate law (for various reason) so just entering Alternate Law is not necessarily a problem. However when the circumstance require the crew to respond to abnormal situations without relying on normal protections, it may prove to be a hazardous regime to operate in.
3. See for example the CSC A319/B-6054 incident, in which the aircraft entered a microburst on final approach that induced sufficient pitch up for the stall warning to be triggered during normal law.