Narrative:

IFR at 9000 ft into heavy snow; -10 degrees C light snow on leading edge of wings and noticed snow breaking off cowling/propeller and hitting windscreen intermittently. Engine lost power suddenly; checklist procedure did nothing to change engine loss of power; engine was running but only at idle and unable to hold attitude. Had 30 gals of fuel. Declared emergency with ZZZ and vectored to airport within glide range. At 5500 ft noticed snow melting from wings; was checking magnetos when engine regained power at 5000 ft. Have lo presti cowling; did not close ram air; suspect ram air became clogged from instant infusion of snow sliding off cowling into ram air duct past filter T so almost all air was blocked. As air became warmer at lower altitude I surmise snow melted and engine regained power and was running normally. When engine regained power egt on 2 cylinders remained much colder than other 2 for about 1 min as I suspect snow/water ended up forced into 2 manifolds until completely melted. Called ZZZ and requested landing there as that was original destination and they have VNAV and ILS; rest of flight (20 mins) and approach was nominal. Engine egt returned to normal and engine ran perfect; mechanic has run engine since and reports it appears to be working normally. Should notify all aircraft owners to close ram air in heavy snow conditions and check if lo presti cowling mods are more susceptible.

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Original NASA ASRS Text

Title: M20F PLT LOSES PARTIAL POWER WHILE IMC IN SNOW. REGAINS POWER AFTER DESCENDING TO LOWER ALT.

Narrative: IFR AT 9000 FT INTO HVY SNOW; -10 DEGS C LIGHT SNOW ON LEADING EDGE OF WINGS AND NOTICED SNOW BREAKING OFF COWLING/PROP AND HITTING WINDSCREEN INTERMITTENTLY. ENG LOST PWR SUDDENLY; CHKLIST PROC DID NOTHING TO CHANGE ENG LOSS OF PWR; ENG WAS RUNNING BUT ONLY AT IDLE AND UNABLE TO HOLD ATTITUDE. HAD 30 GALS OF FUEL. DECLARED EMER WITH ZZZ AND VECTORED TO ARPT WITHIN GLIDE RANGE. AT 5500 FT NOTICED SNOW MELTING FROM WINGS; WAS CHKING MAGNETOS WHEN ENG REGAINED PWR AT 5000 FT. HAVE LO PRESTI COWLING; DID NOT CLOSE RAM AIR; SUSPECT RAM AIR BECAME CLOGGED FROM INSTANT INFUSION OF SNOW SLIDING OFF COWLING INTO RAM AIR DUCT PAST FILTER T SO ALMOST ALL AIR WAS BLOCKED. AS AIR BECAME WARMER AT LOWER ALT I SURMISE SNOW MELTED AND ENG REGAINED PWR AND WAS RUNNING NORMALLY. WHEN ENG REGAINED PWR EGT ON 2 CYLINDERS REMAINED MUCH COLDER THAN OTHER 2 FOR ABOUT 1 MIN AS I SUSPECT SNOW/WATER ENDED UP FORCED INTO 2 MANIFOLDS UNTIL COMPLETELY MELTED. CALLED ZZZ AND REQUESTED LNDG THERE AS THAT WAS ORIGINAL DEST AND THEY HAVE VNAV AND ILS; REST OF FLT (20 MINS) AND APCH WAS NOMINAL. ENG EGT RETURNED TO NORMAL AND ENG RAN PERFECT; MECH HAS RUN ENG SINCE AND RPTS IT APPEARS TO BE WORKING NORMALLY. SHOULD NOTIFY ALL ACFT OWNERS TO CLOSE RAM AIR IN HVY SNOW CONDITIONS AND CHK IF LO PRESTI COWLING MODS ARE MORE SUSCEPTIBLE.

Data retrieved from NASA's ASRS site as of May 2009 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.