Originally Posted by
Svarin
to Takata :
How do you know the aircraft was actually aware of ADR DISAGREE at 0210Z ? The sequence of ACARS messages says NAV ADR DISAGREE (which sounds somehow something else than F/CTL ADR DISAGREE ?) at 0212Z, giving us a full minute at least between A/P OFF A/THR OFF and your proposed going into ALTN LAW 2.
I would rather suggest that the aircraft was, at 0210Z, just after AP disconnect, in ALTN LAW 1, not yet aware of ADR DISAGREE condition.
The ADR DISAGREEment is time stamped 02:12Z (with the uncertainty associated to ACARS time stamping). If two ADR are reported in disagreement, this imply that one ADR had already been voted out (but it was not reported as faulty like in an NAV ADR FAULT) ? The switching to ALTN LAW is time stamped 02:10Z, so you suggest that they were in ALTN LAW 1 and not ALTN LAW 2. This seems logical to me too.
Which would mean that Low Speed Protection was active. Precisely, while the pilots were thinking "we're hand flying now", the aircraft applied "gentle progressive nose down input" without any kind of warning other than... STALL STALL STALL... Interesting ?
Anyone could disprove this, and do not hesitate to go very technical.
In ALTN LAW 1, the LOW SPEED STABILITY protection is still active (pitch in direct law) so if the AP monitor a CAS that is close to/under the stall speed (in the Air Caraïbe case, the CAS plunged from 273 to 75 kts), it can order a "gentle progressive nose down input" to regain a margin versus the stall speed, an order which can be overriden.
Is it possible to simulate or compute the evolution of the (real) airspeed from that event ? (for example using the Air Caraïbe observed biases or the airspeed).
It also raises the reaction of the crew. Did they start to apply the procedure about "unreliable airspeeds" from 02:10Z ? if so did they believed to the instruction that says to "rely on the stall warning that may be triggered in ALTN or direct law because it is not affected by unreliable airspeeds but based on the AoA " ? (and let AP (re)gain speed leading to overspeed). Which part of their attention did they devote to control the aircraft, to try to understand what was the problem with their NAV and instruments & to apply the procedure ? Did they try to restart PRIM1 to regain the normal law during the first 3mn from 02:10Z ? Did they let a critical overspeed situation to unfold and exited the flight enveloppe this way ? (in the very center of a cluster of Cb). Under this scenario, the upper range of allowed airspeeds should have been exceeded rapidly after the initiation of the Low Speed Stability protection ? Is it a matter of 20 s, 30 s, one minute if the pilots let the AP do ? Would it mean that at 02:11Z or 02:12Z max, the aircraft was already in a critical overspeed ? If we have at least 2 mn of ACARS/flight left, would it mean that the overspeed was somewhat contained ?
After the ADR disagreement, they were in ALTN 2 with a very limited protection for their control surfaces (if the AP reads 75 kts instead of 275 kts like for Air Caraïbe, the rudder can be allowed 31.6° instead of ~7° max on the graph given by PJ12). If the VS was lost in mid air, the things must have unfolded very quickly from then.
Jeff