GreatBear Said: "UNCTUOUS. I'm still having trouble picturing the auto throttle creep necessary to your "favoured" Mach tuck scenario when, by measuring ground speed until the upset (A/P disconnect, ten seconds after the last position report), the A/C seemed to be traveling at a constant cruise speed... see diagram. Perhaps creep towards the upper right of the coffin corner (+20kts? +40knts? +more? to a flick-roll) occurred in a very short time-span? Short enough not to influence the 0200 to 0210 distance traveled segment?
BBC 178
MARTIN ALDER (A330 pilot). Our speed range is quite limited. Typically it could be ten knots either side of the cruise speed.
If, over a 6 minute period (say), the average of the undetected CAS speed increase was 5 knots, it would only make a half-mile extra distance gained along track. As the pitot became increasingly blocked, the rate of the displayed speed decrease
trend (requiring auto-thrust increments to retain a system-stable displayed speed) would accelerate - so logically the last half of that 10+kt undetected speed-gain would occur over the last few minutes. i.e. not a uniform acceleration.
ACARS messages are queued for transmission and can be sent out of sequence. Positions relayed with messages are probably related to msg formatting times.... rather than Tx times. Pls correct me if I'm wrong there. However precision in the reported positions might be an unsupportable assumption anyway.
BOAC said: "Pilots neglected to add power (i.e. left throttles in CLB) after dial-selecting a slowdown to turbulence penetration speed"
BOAC's query: "excuse a dumb q from a non-AB chap - isn't the A/T supposed to do that?
The BBC assertion seems to be (sequentially) that:
a. a lower speed (turb penetration of M0.82) was dialled in due to radar-perceived down-track cell activity
b. at some time before they reached that speed, the
"pure precipitation instant freeze-over" of some/all pitots occurred, causing the autopilot and autothrust disconnect.
c. in order to preclude a stall, the pilots would have then had to set 85% power and a 5 deg nose-up attitude (says Alder). Because of all the distracting alarms and reversion to ALT1, they possibly neglected to set the power, lost speed (unbeknownst to them) and stalled. i.e. they'd have been very acclimatized to autothrottles taking care of power to sustain selected speeds (another automation trap). A selection of max power at FL350 DURING a high AoA stall would stand a good chance of compressor stalling one engine (at least). That would introduce a severe asymmetric component and increase the chance of an autorotative "departure".
BBC208 - In 10 previous incidents of airspeed failure, the crews failed to increase thrust immediately.
TONY CABLE. In quite a number of them, it’s clear that the crews were very slow to get onto manual throttle operation.
BOAC said: Unctuous- I am now totally confused as to what you are presenting regarding BBC/ACARS. First you 'present' "An ACARS message from the airplane recorded the sudden onset of critical Mach" with no comment and then you tell GB "Well think it through. Why would there be in any airspeed pitot compromised scenario? It's not a missing message, it's a disabled capability."
Sorry, didn't mean to mince words/semantics, but I did say
"recorded" rather than
"reported" (in both the original and the response to the query above). I was trying to get across the timing of the triggering event (causing immediate autopilot and autothrust drop-outs), rather than infer that there was a specific
Mach crit report.... which of course there couldn't have been - as (at that time) there was no airspeed data from which to derive it.
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Ref henra's comment about the time taken to accelerate a 210tonne A330 to Mcrit, I've really no idea. The whole leg could've involved a quite slow acceleration (over 30 minutes say) or it may have been <10 mins. All such timing depends upon the rate of ice crystal accretion within the pitots, requiring autothrottle offset for a sustained display speed.
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