Originally Posted by
PPL-Pilot
Thanks, PDR, for the helpful links.
If I am understanding correctly, airframe icing can't occur in clear air as water vapour needs to be supercooled - that is, in liquid form below zero degrees celsius. Therefore, for the sake of general aviation, and when considering flight under IFR (with an IR(R) or similar rating), icing could be considered as flight in cloud (or precipitation) below zero.
Is that a fair (and safe) summary?
Basically, yes. If the temperature drops below about -20C, you’re also unlikely to get icing.
Also, especially in faster aircraft, you can get a small temperature increase from the friction of the airframe going through the air.
If using an OAT sensor in flight, you need to bear in mind the position of the sensor. There are two on my aircraft and they often read more than 5 C different from each other. The higher reading one is on the side of the cowling next to the engine and above the exhausts. Quite why they put it there, or why there are two different ones is a mystery......
On my aircraft, they display in different parts of the avionics and it’s important to know which one to ignore!