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[Chris Scott]

Iced Pitots

HN39,
Thanks for your characteristically perceptive response to my post #1224.

Quote:
"Sensor Validation points to the possibility of pitot overpressure due to drain blockage, but adds that he doesn’t know by which percentage. Until someone comes up with that information, I’m inclined to think that the effect is quite small."
That's my assumption too (low percentage). My own pilot's-eye view during walk-rounds (until they got too high for close inspection!) was that the holes' exit ends were often contaminated slightly by oxidation and/or burnt de-icing fluid. I doubt their area of cross-section is controllable in everyday service. It makes absolute sense that an unblocked drain-hole bleeds some dynamic pressure, requiring a correction. But if a partial blockage is acceptable, my guess is that a complete one would not in itself make a serious difference.

Quote:
"To my knowledge, stall warnings are not phase advanced (stickpushers sometimes are)".
Maybe not, and my assumption was influenced more by experience on VC10s and OneElevens than what little I've found in Airbus Tech Manuals. We used to get stick-shaker occasionally on the OneEleven in approach turbulence; and pre-stall ignition in cruise turbulence on the VC10, particularly if we were operating with a buffet margin below 1.35g for the weight. We were told these were phase-advanced warnings/protections, but the manuals are not to hand...

You quote the French text from the part of the BEA Report which states that the "two distinct signatures" of speed anomalies:
"Les anomalies de vitesses peuvent se caractériser par deux signatures distinctes :
- chutes intermittentes (pics),
- chute suivie d’un palier (période continue)."
Readers of my post will have noted that it is the word "pics" (translated to "peaks" in the English version) that creates the ambiguity in the English text:
" - intermittent falls (peaks);"
In the English, I'm led to think that, because "peaks" is in brackets, it refers to sharp peaks and sharp troughs of a negative (under-reading) curve. If it had been written "intermittent falls/peaks", that would have suggested alternate under-reading/over-reading.
In the French, the latter interpretation seems even less likely: they would have needed to write something like "chutes ou pics intermittentes". One of the definitions of "pic" in my Petit Larousse is "FIG: Maximum d'intensité atteint par un phénomène...". In this case, I think the maximum intensity is in the negative sense.

Apologies to (any) readers for the descent into semantics, but − unless the BEA author is prepared to clarify it him/herself − it's a necessary one. (The designers of FBW logic necessarily have to avoid possible ambiguities of this kind!)

Chris