Originally Posted by
jcjeant
Now that is a very good find indeed. From the Full Narrative:
The crew received a master warning and master caution alert, and the autopilot (AP), autothrust (ATH) and flight directors disengaged. The crew reported airspeed fluctuations on the Captain’s, First Officer’s (FO), and the standby airspeed indicators. They reported receiving a stall warning, noted the flight law switched to Alternate Law, and saw messages indicating NAV ADR DISAGREE and NAV IAS DISCREPANCY. They reported the airspeed fluctuations and warnings lasted about one minute, and they controlled the airplane by pitch and power reference, per applicable checklist procedures until normal airspeed indications returned.
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Flight law switched to alternate law twice in the initial 35 seconds of the event........ About 35 seconds after the initial fluctuation the flight control laws returned to normal,
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The crew could re-engage the autopilot, because at least two ADR values were consistent and valid. The EFCS was able to temporarily return to Normal law because the discrepancy between the ADRs was shorter than the confirmation time to latch the Alternate law. When the airspeed discrepancies returned, including the longer duration No.3 airspeed drop, the autopilot disconnected, and Alternate law then latched for the remainder of the flight.
So it does appear the aircraft will return to Normal law (which would almost certainly have saved the day in AF 447)from ALT 2 but only if the time which the speeds disagree is short enough. Suspect that needs a rethink, apart from that it's hard to find fault with the aircraft.
Also, there is a note at the bottom which shows the NTSB had made a link between this incident and the AF accident some time ago.