Narrative:

I was in cruise at 11;000. An airmet was in effect for the area for moderate ice between 11;000 and 16;000; along with convective activity (showing on my stormscope) ahead and moving in the same direction I was. I entered IMC at 11;000 ft with no ice; requested and received climb to 13;000 ft in order to stay VMC for better weather avoidance. Temperatures at 11;000 ft were plus 1 degrees C. I reentered IMC at 13;000 ft with no turbulence and; initially; no ice; temperature at minus 1 degree C. After a few minutes; I began picking up a trace of rime ice; but with minimal accumulation. Pre-flight review of the weather icing forecasts; using the websites flight path tool; indicated freezing level along my route of 11;000 and a minimal chance of trace ice at that altitude; with a slight chance of moderate ice at 13;000 ft. Despite the airmet; the combination of the more specific weather forecast and the existence of good escape routes (freezing level 1;000 ft below; bases running around 6;000-8;000 ft; and meas at or below 4;000 ft) led me to be willing to remain at 13;000 ft despite the trace of ice.about 5 minutes later; I started to pick up moderate rime ice. Within about 30 seconds; the airspeed had decayed about 5-8 KTS. With significant congestion on the frequency; it took me another 45 seconds or so to request descent to 11;000 ft. ATC told me to stand by because of crossing jet traffic descending from 13;000 ft behind me. I acknowledged; voicing my hope that the jet 'hustle on down!' after another 30 seconds (and a further 3 or so knot airspeed decay); ATC cleared me to 11;000 ft; and I began an immediate and expeditious descent. I felt that in another 30-60 seconds I would have had to begin descending with or without the ATC clearance; even if that meant some kind of certificate action. (I felt that I would rather face certificate action for an altitude deviation than face the potential consequences of remaining in moderate icing conditions much longer!) in retrospect; I should have requested descent as soon as the trace of ice accumulated at 13;000 ft. Temperatures around minus 1 degree C in visible moisture are pretty ripe for ice and; with ATC as busy as they were; I could have expected a delay in making my request. Moreover; the reason for being at 13;000 ft instead of 11;000 ft was gone (though the 20 KTS ground speed increase at 13;000 ft was very nice to have!). Despite the encounter; I think that the internet websites advanced forecast products were very valuable for planning this flight.

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Original NASA ASRS Text

Title: A Mooney M20 pilot reported encountering ice in cruise flight despite a forecast that indicated ice at his altitude was unlikely. His airspeed began to decay and he got a descent clearance from ATC.

Narrative: I was in cruise at 11;000. An AIRMET was in effect for the area for moderate ice between 11;000 and 16;000; along with convective activity (showing on my Stormscope) ahead and moving in the same direction I was. I entered IMC at 11;000 FT with no ice; requested and received climb to 13;000 FT in order to stay VMC for better weather avoidance. Temperatures at 11;000 FT were plus 1 degrees C. I reentered IMC at 13;000 FT with no turbulence and; initially; no ice; temperature at minus 1 degree C. After a few minutes; I began picking up a trace of rime ice; but with minimal accumulation. Pre-flight review of the weather icing forecasts; using the websites flight path tool; indicated freezing level along my route of 11;000 and a minimal chance of trace ice at that altitude; with a slight chance of moderate ice at 13;000 FT. Despite the AIRMET; the combination of the more specific weather forecast and the existence of good escape routes (freezing level 1;000 FT below; bases running around 6;000-8;000 FT; and MEAs at or below 4;000 FT) led me to be willing to remain at 13;000 FT despite the trace of ice.About 5 minutes later; I started to pick up moderate rime ice. Within about 30 seconds; the airspeed had decayed about 5-8 KTS. With significant congestion on the frequency; it took me another 45 seconds or so to request descent to 11;000 FT. ATC told me to stand by because of crossing jet traffic descending from 13;000 FT behind me. I acknowledged; voicing my hope that the jet 'hustle on down!' After another 30 seconds (and a further 3 or so knot airspeed decay); ATC cleared me to 11;000 FT; and I began an immediate and expeditious descent. I felt that in another 30-60 seconds I would have had to begin descending with or without the ATC clearance; even if that meant some kind of certificate action. (I felt that I would rather face certificate action for an altitude deviation than face the potential consequences of remaining in moderate icing conditions much longer!) In retrospect; I should have requested descent as soon as the trace of ice accumulated at 13;000 FT. Temperatures around minus 1 degree C in visible moisture are pretty ripe for ice and; with ATC as busy as they were; I could have expected a delay in making my request. Moreover; the reason for being at 13;000 FT instead of 11;000 FT was gone (though the 20 KTS ground speed increase at 13;000 FT was very nice to have!). Despite the encounter; I think that the internet websites advanced forecast products were very valuable for planning this flight.

Data retrieved from NASA's ASRS site as of April 2012 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.