"Air-frost"

[covec]

In a similar vein to the De-icing question on this Forum, does anyone have any thoughts/advice on handling air-frost?

I've cancelled a couple of night trips before after finding the aircraft covered in heavy frost, despite the fact that 10 minutes previously it was frost-free! (N Scotland in December, by the coast).

I got to wondering what would have been the outcome if I had been airborne, whether the frost would still have formed, whether Bernoulli would have won through....?

[sycamore]

It would probably form on the lower wing surfaces under the tanks if you stay airborne long enough/ cold enough to cold-soak.Look at the underside of any airliner after landing (if the ground temp. is below freezing) and you will see large areas of frost. Not usually a problem on the lower surface in flt., but if a/c is fuelled and left out it will form on the top, so it must be removed..

[mad_jock]

I have had that as well and cleared the leading edges etc. And gone flying, after you got up a bit there was usually a temp inversion and the airframe became clear very quickly.

I presume it was cold air flowing down the mountains in the evening coupled to the warm air over the sea rising.

Whats more scary is on a cavok night flying along doing the xc bit of the night rating and it starts to form on the wings with no viz moisture around. I have had this a couple of times and decided I would prefer to be in the bar than in the air. So ended the flight as quickly as possible.

MJ

[hoey5o]

Air frost also described as hoar frost can only form (according to the books) on the ground ie in static conditions. The static air on and over a very cold aircraft surface is cooled below its dewpoint and condenses out to form dew and if its below 0c the dew becomes frost (ie ice).
There is generally no hoar frost on a wing in the air at night or day because the air is not static, its rushing past at 100kts etc so has no opportunity to adhere to the aircraft.

I am not aware of any aircraft ever icing up in flight in clear air with hoar frost.

Remember at night many types of precipitaion/clouds cannot be seen even when you're in them, virga (rain not striking the ground) is the best example. If you ice up in the air at night it will be because you are in some form of moisture however it would be visible if it were day time.

Finally, I did say generally as Sycamore is correct that a type of hoar frost can form under some airline jet wings due to fuel soak but the temperatures involved are in the minus 60s at very high altitudes. Just to correct Sycamore it does not have to be below 0 on the ground. MY aicraft will show ice under the wing at 20c as its the fuel temp thats critical here not the outside air temp.