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Old 2nd May 2011, 16:10
  #536 (permalink)  
Chris Scott
Join Date: Jan 2008
Location: Blighty (Nth. Downs)
Age: 74
Posts: 2,084
Frozen pitots

Quote from deSitter:
What RR_NDB is referring to is N274US, a Northwest 727-25 on a ferry to Buffalo, that crashed after stalling following near simultaneous freezing of all three independent pitot systems. The pitot heating was not selected because the 3 crew members got tangled up in the pre-flight checklist.

Hope you will not mind me adding some words of explanation for those not familiar with pitot-probes or the above accident (even I was young then), since freezing of pitot tubes is presumed to be material to AF447.

The tubes froze completely at some stage in the climb, trapping the current dynamic pressure on the dynamic side of each (mechanical) pressure capsule. Because the aircraft was climbing, the static pressure continued to fall steadily, transmitted to the static side of each aneroid (pressure) capsule from the (unfrozen) static ports. The increasing differential pressure across each capsule led to a steady increase in the IAS reading on the cockpit ASIs, even to the point of an overspeed warning. The PF allegedly flew indicated speed in preference to attitude, with a stall being the inevitable result. (Unlike its British T-tail counterparts, the B727 did not have a stall-ident/stick-pusher system, although it did have a stick shaker.)

It appears that the scenario was dependent on the whole of each pitot head being frozen, including the small water drain-hole which is normally placed near the aft end of a pitot tube. If that hole remains open, the dynamic pressure is able to dissipate; leading to an under-reading, or zero-reading, of airspeed.

In the case of AF447, history of previous anomalies on A330s and A340s suggests that the drain holes may have remained open. Also, when maintaining a pressure altitude, the static pressure remains constant by definition (although an automatic altimeter correction for speed-related errors at the static ports may complicate the issue slightly). The result may have lead to an under-reading of airspeed. However, I'm not aware that the BEA has discussed publicly the issue and/or effects of drain holes being frozen or remaining unfrozen. It would certainly be crucial during climb or descent.

In those days (the early 1970s), US jet transports like the B727 and B707/720 were still being certificated with scratch-foil flight-recorders capable of handling fewer than 10 parameters. Their British counterparts (BAC 1-11, VC10 and Trident) had analogue, wire-recording FDRs with many times the number of parameters. They paved the way for what became the SESMA programme, which monitors crew/aircraft performance on all flights. I think the Americans can lay claim to having caught up with the Brits by now...

Last edited by Jetdriver; 2nd May 2011 at 16:39.
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