Even though it's mid May, there might still be some "cloud residue" waiting for the unsuspecting. Pireps reported negative ice. Even though it wasn't forcast, the conditions were certainly present and when given the descent, I considered my options before starting down. In this case, it was of only minor concern since the cloud layer was verified to be less than two thousand feet thick and I'd be in a descent the entire way down. Once out, there was over 7000 feet of warmer air between me and the ground. Still, I picked up this much cloud residue in about two minutes coming down at about 1000fpm and 150kts:
Remember, flying in air temperatures of even a few degrees *above* freezing can result in the accumulation of airframe icing. Before penetrating any visible moisture when the OAT is less than 10Cº, I check the following:
1) What are my options? I.e. can I stay out of the clouds longer, can I find some clear airspace, how thick is the layer, what is the temperature below/where is the freezing layer, and so on.
2) Pitot heat is anti-ice, meaning it's designed to prevent the formation of frozen contaminants. Mine comes on prior to entering visible moisture any time the OAT is less than 10ºC.
3) Stay fast. I have test data for my aircraft that shows that 3/8" of rime ice increased the stall speed by 3 knots. But that was only really valid for that particular day, since ice can form in numerous different shapes and have associated effects. What I really learned from re-running the test card after picking up the ice during an IFR descent was that a tailplane stall was a definite possibility. If you have contamination on the wing, choose a long runway, land fast, and avoid the use of flaps if possible.
Above all, avoid it if you can. The work airplane has bleed air anti-ice for the wings and tail, and I've been in conditions there where the unprotected surfaces had a measured six inches of ice accumulation in the time it takes to do an approach (i.e. about 15 minutes). Days like that are much better spent hangar flying :-)