Notwithstanding what Machinbird says at post
1641 above and the other quoted remarks below, I have a problem with identifying the circumstances leading to (and the possibility) of an initial Vmo/Mmo protective pitch-up (see argument below the quotes).
Confiture said:
A bit of study on the Vmo/Mmo protection mode indicates that activation of this protection will disengage autopilot as it pitches the aircraft up. This means, that the soonest such an event could have occurred would be at the very beginning of the ACARS sequence, thus perhaps explaining this message at the very beginning:
2:10:10WRN/WN0906010210 221002006AUTO FLT AP OFF
Hazelnuts said:
1st interim report, p.47: "some messages concerning the aircraft’s configuration such as stall or overspeed warnings are not recorded."
Machinbirs said:
Thank you for pointing out that BEA quote HN39. If such warnings existed, that would have invalidated a Vmo/Mmo protection pitch-up scenario. I was having trouble finding an ACARS reference on the subject. The only other thing that might contra-indicate such a pitch up is the rudder limiter position which probably would have locked at the polled airspeed indication existing when Normal Law was abandoned.
I wonder what BEA used to exclude such a pitch up scenario in the process of setting the search area? To me, being left 20 degrees nose up at FL384, M 0.66, at night, with no visible horizon and on the outskirts of a big cell, with no valid airspeed, in a fly by wire aircraft that has just shifted to Alt2 Law seems a bit hairy. Any delay in pushing the nose down would almost certainly result in a stall, particularly without AOA indications to see how the wing is flying. (Just had to put in a plug for AOA indicators).
If AF447 promptly entered a stall, it might flop around a bit initially but as long as the stall remained, it would likely cause further deceleration until the AOA was quite high and would then result in a stable attitude pretty much along the lines of the impact attitude described by BEA. The flight path over the ground would likely describe an inward spiral starting from an initial circle of no more than 10NM diameter and would be fairly close to the LKP.
It seems that there may well be valid potential to searching the "donut hole" in the last search pattern.
. I'm not really following why the Airbus Flt Ctrl
pitch-up protection against an imminent Vmo/Mm0 encounter should kick in if the airspeeds being fed to the ADIRS were:
.
...a. Initially in agreement, albeit wrong due to pitot icing - either internally (supercooled ice-crystal build-up) or externally (BBC's pure water icing theory)
.
...b. Latterly being maintained at the selected cruise speed by autothrust (even though the actual airspeed/mach was much higher).
...
..c. Ultimately mismatching the three pitot-derived speeds sufficiently to cause a disagree and the Autopilot to disconnect.
. I imagine that the flt ctrl protections are keyed by CAS and Mach, and if these weren't uniformly trending dangerously high (due to pitot blockage), what is there to cause the protections to cook off and intervene - by allowing the AP to pitch the nose up then disconnect? Isn't it more likely (per posts
1208 and
1471 and
1476 and
1489 ) that the aircraft accelerated into a nose-down pitch (i.e. mach tuck) because there was nothing (
no high CAS or mach) detected to trigger any such protection. If the flight crew responded to a pitch-down by misinterpreting it as a stall and went TOGA/stick fwd, then that would have embedded the A330 in compressibility (with all its nasty L.o.C. follow-ons).
. The above debate disregards where the THS may have been trimmed to and what instantaneous pitch effect it may have had upon the aircraft at autopilot disconnect. That's another ball-of-wax that's unclear to me. i.e. where's the THS
being trimmed to when the datum indicated airspeed is artificially low - and how much deflected elevator can the autopilot's baro-hold handle in order to maintain the dialled in flight level (before disconnecting)?
.