Archives For AF447

AirAsia QZ8501 CVR (Image source - AP Photo-Achmad Ibrahim)

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.

Notes

*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.

Triggered transmission of flight data

Why something as simple as control stick design can break an aircrew’s situational awareness

One of the less often considered aspects of situational awareness in the cockpit is the element of knowing what the ‘guy in the other seat is doing’. This is a particularly important part of cockpit error management because without a shared understanding of what someone is doing it’s kind of difficult to detect errors.

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Air France Tail plane (Image Source: Agencia Brasil CCLA 2.5)

Requirements completeness and the AF447 stall warning

Reading through the BEA’s precis of the data contained on Air France’s AF447 Flight Data Recorder you find that during the final minutes of AF447 the aircrafts stall warning ceased, even though the aircraft was still stalled, thereby removed a significant cue to the aircrew that they had flown the aircraft into a deep stall.

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Because they have typically pitch unity ratios (1:1) scales, aircraft primary flight displays provide a pitch display that is limited by the vertical field of view. This display can move very rapidly and be difficult to use in unusual attitude recoveries becoming another adverse performance shaping factor for aircrew in such a scenario. Trials by the USAF have conclusively demonstrated that an articulated style of pitch ladder can reduce disorientation of aircrew in such situations.

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AF A330-200 F-GZCP (Image Source: P. Kierzkowski)

Knowing the outcome of an accident flight does not ‘explain’ the accident

Hindsight bias and it’s mutually reinforcing cognitive cousin the just world hypothesis are traditional parts of public comment on a major air accident investigation when pilot error is revealed as a causal factor. The public comment in various forum after the release of the BEA’s precis on AF447 is no exception.

This post is part of the Airbus aircraft family and system safety thread.

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