Stall warning and Alternate law
According to an investigator from Indonesia’s National Transportation Safety Committee (NTSC) several alarms, including the stall warning, could be 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*. The most likely explanation for hearing the stall warning is that the aircraft had entered what’s called Alternate law and (obviously) was entering or in 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’ laws are removed (along with others) and it becomes physically possible for a pilot (or the environment**) to stall the aircraft, hence the presence of a stall warning function in Alternate law.
While we need to wait for the investigators to complete their full investigation, it now looks increasingly likely that the pilots had lost a significant amount of the protection offered by Airbus automation, right at the moment when they needed it the most. The parallel of this disaster with the loss of Air France AF447 are disturbing to say the least.
This post is part of the Airbus aircraft family and system safety thread.
*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, however as far as I am aware ‘stall warning’ is a mode dependent function.
**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 the normal protections provided, it may prove to be a hazardous regime to operate in.