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Old 11th Jun 2009, 13:05
  #81 (permalink)  
Join Date: Dec 2002
Location: UK
Posts: 1,844
Re; “ Why must we assume that it was ice that did provoque an unreliable Airpseed situation ”

Supporting Blacksheep’s answer (#73, #79), see the presentation Instrument External Probes.
For ref, the certification Appx C for icing is on page 126 of CS 25 Large Aircraft, also see Fig 5 (page 182) and compare the events (outside the cert envelope) with the information on The Ice Particle Threat to Engines in Flight and other similar references to engine problems which involved TAT anomalies.

The relevance of TAT freezing is as an indicator of the ambient conditions (temp/ice/water near large Cbs), also that the displayed temperature will always change (increasing temp).
Whereas with pitot icing the indications may not change unless altitude is changed (trapped pressure) or there could be fluctuating indications as shown in the first link. Thus there could have been many more pitot icing events which went unnoticed – and involve a range of aircraft types depending on the type of pitot probe and how the data is ‘managed’ before being displayed.

Also see The Ice Crystal Weather Threat to Engines slides 7 & 19 - TAT anomalies & Appx C.
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