2D
You can expect ice to form on your aircraft with an OAT in the range +4 to -20 degrees C
Could one really get ice accumulation on the airframe in the range +4C to 0C, at typical GA airspeeds (below say 150kt)?
Assuming the OAT gauge is actually accurate (easy enough to test), it should not happen because the temp drop due to the pressure drop above the wing is less than 1C. This was discussed in the American newsgroups extensively some time ago and I recall seeing some data showing that the temp drop due to expansion is below 1C at GA speeds and with aerofoils in common use.
Having said that, the average UK training aircraft is not legally checked / maintained for other than VFR flight so the OAT gauge may never have been checked.
Perhaps the other thing worth mentioning is that even in clear air, the pitot tube can freeze up below 0C, so pitot heat should be used anywhere near freezing. I've seen this myself, and put the pitot heat on below +5C as a habit.
I find that the met forecast icing level being below the MSA is what stops most of my winter flying... TKS is probably the best money spent
