Originally Posted by Loose rivets;#1306
The program gave a mini-course on supercooled water and pitot heads, but said little or nothing about airframe icing while encountering the phenomenon.
Severe icing from supercooled droplets can be astonishingly sudden, and flying an aircraft in turbulence, with the added burden of ice, could be one reason the crew had difficulty in staying inside the envelope.
From BEA report no.1:
... the presence of super cooled water at FL350 is not very probable and would necessarily have been limited to small quantities.
I believe that the problem of icing due to supercooled water droplets is well known and understood for about half a century (refer to FAR 25 Appendix C). Airplanes, engines and pitots are designed, protected and extensively tested for safe operation in those conditions.
It seems that the relatively recent icing problem at high altitudes with pitots (and perhaps also engines) is not caused by liquid water but by fine ice particles which present no particular problem for the airframe. From BEA report no.2:
Examination of reported UAS events in cruise has shown that the majority of them occurred outside of the envelope defined in Appendix C. In fact, the certification criteria are not representative of the conditions that are really encountered at high altitude, for example with regard to temperatures. In addition, it appears that some elements, such as the size of the ice crystals within cloud masses, are little known and that it is consequently difficult to evaluate the effect that they may have on some equipment, in particular the Pitot probes. In this context, the tests aimed at the validation of this equipment do not appear to be well-adapted to flights at high altitude.
HN39