AF 447 Search to resume

[Chris Scott]

Iced Pitots

There seems to have been wide agreement here for a day or two that − in level flight, right at the beginning of this event − whichever ASIs were showing erroneous readings are more likely to have been over-reading the actual IAS/CAS, than under-reading it. Thus, if the crew were misled by the false readings, they might have slowed the aeroplane into a stall near the cruise altitude. This would be likely to produce a far steeper descent profile, fitting Machinbird's and mm43's suggestion that the search should not ignore areas close to the LKP. One or two of you have even described the well-known effect of a blocked pitot in the climb (increasing over-reading). Remember: this event started in level flight.

My argument, originally indicated in a PS to my "Au Sud?" post #1175, 48 hours ago (and again in post #1180) , is that iced pitot probes are more likely to result in UNDER-readings. If these (falsely) showed the IAS as being dangerously low, this might have led the crew instinctively to opt for lowering the nose rapidly, leaving the cruise altitude (FL350) into an accelerating descent.

So what does the BEA think? They studied the 13 best-documented A330/340 events involving ASI (ADR) anomalies. Their results, detailed in Interim Report #2 (December), include the following. (My annotations are in square brackets, and under-scorings are all mine.)

"The speed anomalies can be characterised by two distinct signatures:
- intermittent falls (peaks);[sic]
- Fall followed by levelling off (continuous period).
They were accompanied by an instant increase in displayed static temperature (and total when recorded), and with a 'drop' in altitude on airplanes equipped with altimetric correction (A330-200)."
[These phenomena imply under-reading of CAS – ref. 1.6.11.6 of the same report.]
"In both cases, the lower speed limits recorded were below 100 knots.
The maximum continuous duration of invalid reported speeds was three minutes and twenty seconds."

"...Nine cases of triggering of the stall warning were observed...
...(this) triggers when the angle of attack passes a variable threshold value. All of these warnings are explicable by the fact that the aircraft is in Alternate law at cruise Mach, and in turbulent zones. Only one... was caused by clear inputs on the controls."
[This seems to imply that the warning is genuine, but apparently triggered by a high angle-of-attack (or rate of increase??) in turbulence.]

So, with a possible exception where they have ambiguously used the term "peaks" (apparently when describing intermittent falls, so something has been lost in the English translation), they are describing false, often alarming, falls in displayed IAS/CAS. These are often combined with (phase-advanced??) stall warnings caused by rapid AoA fluctuations, presumably in what many pilots would call "severe" turbulence.

One last quote:
"The pilot flying gives priority to piloting the airplane and to the flight path, by maintaining a cruise altitude or by performing a descent to increase the margins for evolution within the flight envelope..." [To get out of 'coffin-corner'.]
"The descent can also be decided following the triggering of a stall warning."

In the descent, with rising static pressure, the (possibly) trapped pitot pressure would lead the ASI to under-read even more.

Chris