AF447 Thread No. 3

[Chris Scott]

Back in the Loop?

spagiola post #117 (27/1733z):
"Did the resumption of the stall horn once the speeds once again become valid after commanding pitch down (at 2:12:02+15) confuse the PF into thinking he was doing the wrong thing?" [my emphasis]
Quote from MurphyWasRight:
"Until today were you aware that the stall warning could go away if a stall developed to the point that indicated airspeed dropped below 60KT?
And that it would return as you recovered?"

I think it may well have done, particularly considering the situation he had found himself in. The irony is that the AoA-based stall warning had been inhibited, indirectly, by the system perceiving an IAS below 60kts. On the occasions when a reduction of AoA was achieved, this may have cruelly coincided with a recovery in the (false) IAS to have enabled the (warbling, audio) stall warning to resume.

(In any case, because of the under-reading of IAS, the threshold of stall warning would have probably been too high for the altitude, as has been discussed by HazelNuts39 and others on previous threads [see Page 46 Of BEA Interim Report No 2]. So it is arguable that the stall warning, even when not inhibited, may have been inactive at times when it should have been active.)

This accident seems to demonstrate an Achilles-heel (weakness) in the stall warning system for Pitch-Alternate Law and Direct Law that needs to be addressed. As I wrote in my previous post, why is it considered necessary to inhibit AoA-probe data IN FLIGHT when the IAS is measured below 60 kts? (On the GROUND, it must be.) It seems to me that there are sufficient indications available to establish GROUND/FLIGHT status automatically, as used by numerous other systems on the aircraft.

Quote from MurphyWasRight:
"Until today were you aware that the stall warning could go away if a stall developed to the point that indicated airspeed dropped below 60KT?
And that it would return as you recovered?"
Although what you say in your last sentence may have happened, it seems to have been by (tragic) coincidence of the vagaries of the UAS. If the erroneous IAS reading had remained near zero, the stall warning would not have returned.

Checkbard, post at 27/2129z, quote:
"[....] aircraft which crashed (or very nearly so) because the pilots failed to realise that the trim had been set full nose up, and couldn't understand why they lost pitch control."
This seems possible, if unlikely. The limited pitch-down commands from the PF may have not been sufficient to cause the THS to run to a less nose-up trim-state than the 13 degrees. In my post at 27/2011z, however, I asked:
"is there any possibility that the THS motor stalled during down-elevator inputs?"
This has been known to happen in older types, but seems improbable in this case.

Quote from TyroPicard, re my question as to why the AoA data is inhibited IN FLIGHT below 60kts IAS:
"...without much airflow at really low IAS gravity affects the position of the AoA vane just as it does on the ground? The designers had to choose a speed - they chose 60kts."
You may be right, but, if so, surely they could be balanced and damped. G-forces could be the problem? (The VC10 has non-mechanical AoA probes, as you may remember.)

Yellow_Pen (27/2222z),
Agree that pitch and thrust can be relied on in fairly level flight. Once you are climbing or descending steeply, particularly in a stall, I guess it gets tough as a recovery tool? Maybe AoA is the only answer, but it would be a very radical step in airline ops.

Checkboard (27/2226z and 2302z).
Think it was me who started a discussion on use of FPV as an indication of AoA last year. It's not that easy to use in changing bank, and inaccurate when wind-to-TAS ratio is significant. But it's much better than nothing. Unfortunately, AF447 seems to have lost FPV-capability during 0211z.
Re the FPV VS data, I also used to think it was inertial, but remember being corrected by one of my copilots! Too near bedtime to check: sorry.

Svarin ("The Last Effort"?),
Your posts have always given much food for thought. Don't go off mushroom-picking at this stage, please, just because you want a life...

Chris