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Old 5th Aug 2011, 14:01
  #1599 (permalink)  
Join Date: Dec 2002
Location: UK
Posts: 2,044
SaturnV, “… how close would one need to be to a Cb to get that amount of ice crystals.” # 1584.
Some research and flight tests suggest that significant amounts of ice crystals can be found up to 10-15 nm from a Cb (measured from the edge of the red zone).
From experience around and in Cbs with ice crystals, it is most unlikely IMHO that AF 447 entered a Cb core, although the aircraft may have passed through or over the tops of smaller cells.
Ice crystals can be heard – a sort of swishing sound; this is often accompanied with a very wet windscreen (screen heating), which adds to the confusion. Furthermore the ice crystal cloud is very tenuous and might not even be classified as IMC in some circumstances.
Louder, harsher noises come from hard hail often found in the same situations, particularly over the core; this can be heard clearly and might on occasions sound like rain.

The best advice is to stay well clear of Cbs, the avoidance distance (from the edge of the red zone) being proportional to the size of the core. I have encountered signs of outflow cloud up to 60nm in one Caribbean cell and 100nm from a huge cell in Africa; basically any anvil cloud can have a high concentration of crystals.

Just because modern aircraft have high tech radars which automatically detect the severity of ‘this, that and everything else’, and in multicolour, does not imply that technology understands the effect of the conditions on a particular aircraft type, or on a on a type thought not to be susceptible, but now operating with different performance parameters, e.g. engine power, TAT.
Ice crystals caught the industry napping several years ago with engine problems – the engine design had changed, not the weather. At the time there were signs of blocked ‘tubes’ and overpowered heaters (TAT) but no connection was made with pitots.
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