HN39 question:
Would the crew wait for Alt2 law?
I don't think they would have any choice. While in Normal Law, their control inputs would be largely overridden in a Vmo/Mmo protection pitch up. This protection goes away in Alt2 Law.
TheShadow says:
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).
Hi Shadow. No guarantees that the Vmo/Mmo pitchup scenario was the cause, but if possible, it sure influences where to look for the aircraft and thus should be considered in the search plans.
Now, I sense a common mis-perception about pitot blockages in your writing.
Yes, if you block the pitot inlet
only, the indications will drop. But, if you block both the inlet and the tiny bleed holes drilled into the pitot tube to clear moisture from the tube, you end up with trapped air in the pitot system and that is a different ball of wax. Once you have trapped air behind the blockage, you can build pressure. Both kinds of blockages can and have happened.
Now lets look at the energy balance involved in getting AF447 from cruise speed down to the water in approximately 5 minutes in a configuration resembling a deep stall impact (not stated specifically by the BEA but can be inferred from the BEA's Line of Flight statement and the condition of recovered wreckage). How do you burn up that much energy that fast? One of my initial realizations about this accident was that I would have been hard pressed to bleed off that much energy in an F-4, and an A330 is a much cleaner aircraft. The only reasonable scenario it seems to me, is that the wing was operating at such an AOA that it was acting as a giant speed brake. Any time spent operating in a Mach tuck condition and then somehow recovering to end up in a deep stall, would eat up a lot of the total time available to burn up energy.
Vmo=330 knots, Mmo= .86, but the aircraft was demonstrated to higher speeds during its certification. (I believe it was demonstrated to M.93 but I don't have the reference at hand). The aircraft would not exhibit tuck below its demonstrated Mach number. It seems unlikely that the aircraft would accelerate above its demonstrated Mach number in level flight without application of a whole lot of power for a number of minutes that likely exceeds the ability of the 3 pitot systems to stay in sync while providing bogus information.
Add to this the relatively low G capability of the airframe and low IAS limits, and I doubt the ability of an A330 to survive a severe case of Mach tuck. It was intended that the aircraft should never encounter this situation and its certification was directed at ensuring that it would not encounter Mach tuck.