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Old 14th May 2011, 19:00
  #1340 (permalink)  
Join Date: Jun 2009
Location: Paris
Posts: 691
Originally Posted by Turbine D
Pitot tubes: I read somewhere about other A330/A340 problems relating to speed errors, i.e., pitot tube abnormalities. What I remember (not from the BEA reports) was that one crew thought the pitots might be icing, and at some point through the episode, switched the pitot tube heating setting from "automatic" to "on". Then everything settled back down and the flight proceeded normally. What exactly is the difference between the automatic setting and the setting regarding pitot tube heating?
In auto mode, the amount of heat provided by the heaters is modulated following what the "probes" themselves are sensing. When anti-ice is selected ON, full heating is provided, whatever the "probes" are sensing. It makes sense if you are considering that the "probes" are freezing because the heaters were not delivering the correct amount of heat at the first place.

In fact, it seems that those probes freezing at cruise are not related to temperature (really) changing but to those specific (very small) ice particles melting when such conditions are encountered (mostly in tropical weather). Hence, TAT will increase very fast and reach a level close to 0C (iced particles temp melted on the TAT probes). The drawback is that you can not fly with anti-ice ON all the time as it can also damage all the systems by overheating the probes. Hence, it should be selected on purpose.
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