AF447

[Hyperveloce]

Flight AF 422 (FGLZT) Paris/Roissy -Bogota

Quick translation of an ASR summary of the AF census of Pitot problems (08/2008)

Cruising at FL 380, weather radar ON (CAL mode ?), PAX ON, entering a cirrus veil. Suddendly, severe turbulence, A/P manually disengaged, deployment of the airbrakes to avoid MMO, Mach 0.78 selected, airbrakes retracted and A/P reengaged. Then again, new brutal acceleration, A/P automatically OFF, many red echoes on the weather radar, ice is beginning to accrete on the windshield. Altitude is maintained +/- 200 ft.
Many thunderbolts around, key phrase to the PNC in the cabin.
In a blink of an eye, the airspeed rolls back in the red area, recovers its value, then again the two airspeed indications are lost on the PFD and stall alarms are sounded. I was about to implement the stall procedure, before I even had the opportunity to send a MAYDAY, when the airspeeds recovers and are back on displays, then VFR conditions. All flight systems are back, verbal exchange with the cabin crew, they had the time to fasten their seat belt. Message ATC, Dispatch, ATL writing.
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fortunate to get the airspeeds back soon enough.
different pitot freezing event durations (between 2 and 4-5 mn, less than 1 mn for the last BA type probe recently), but no time indication here (sounds sudden and of short duration), and the stall alarms occur immediately
I don't get why people focus on airspeeds problems in the media, ok it is a problem but a problem with a clear solution (pich & thrust), but the very difficult troubleshooting (fault isolation) and the false stall alarms in this context seem to be the real evil, a problem pilots are seemingly not very well trained to.
Jeff
PS) These Pitot problems seem to have increased very rapidly in 2008 on the Thales AA, despite new maintenance rules adopted in 2003, why ? (this may be an explanation for the apparent lack of reactivity of the authorities, they may have been taken by surprise). Problems with the drain manufacturing qualifity were observed on certain AA probes, but if the pollution removal was correctly operated, does this suggest that the problem could be a gradual corrosion (aging accentuated by the initial manufacturing process) ?
PPS) I don't know whether AF had correctly evaluated the problems posed to air safety as seen through the cases it was gathering or through the Air Caraïbe cases, but I would like to know whether these ASR related to Pitot problems had been communicated to the DGAC, BEA, Airbus and to the EASA.
PPPS) CS-25 -General Certification Considerations -
.../...
(ii) Airspeed.
Display of airspeed in the cockpit is a critical function.
Loss of all airspeed display, including standby, must be assessed in accordance with CS 25.1333(b). *
Loss of primary airspeed display for both pilots must be improbable
Displaying hazardously misleading airspeed simultaneously on both pilots’ displays, coupled with the loss of stall warning or overspeed warning functions, must be Extremely Improbable.
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could this kind of requirement be the reason why the stall warning or overspeed functions are still available in alternate law 2, even if the airspeeds are corrupted (ADR Disagree), even if this stall/overspeed warnings are largely plagued by false alarms ?
In the same vein, I really wonder why the (false) speed trends are still displayed on the PFD when the airspeeds are corrupted ? The AF 908's PF reacted to stall alarms because he was reading -50 kts of speed trend while the stall warning sounded at FL350 (these -50kts are most likely a measure of the roll back due to the obstruction, but indeed this must be a worrying signal at high altitude where the aerodynamic margin is reduced). This makes two misleading signals (plus the misleading procedure about stall warnings) and an immediate decision to take. Plus a possible airspeed chase by the A/THR to set the stage (and degrade the aerodynamic margin). Plus night hand flight & possible turbulences. Plus workload and attention split.