Narrative:

Filed IFR flight plan from buf to arb after briefing from buffalo FSS forecasting freezing level of 7000', occasional icing in clouds or precipitation, occasional turbulence. Preflight demonstrated no deficiencies. Turned on pitot heat on climb out with estimated ceiling 2500' MSL. Climbed through mild turbulence 6000' MSL and continued to assigned altitude of 10000'. Began to pick up mild to moderate ice after a few mins at 10000'. Noted that airspeed indicator registered 130 KTS, lower than expected for manifold, RPM settings. Requested lower altitude form buffalo departure because of icing, cleared to 8000' (or perhaps 6000') by departure. Noted airspeed indicator showing falling airspeed despite nose-down attitude and appropriate mp and RPM settings. Notified departure of impending airspeed indicator failure. Departure asked if I wished to continue on or come back to buffalo. I told them that I would return to buf. I did not declare an emergency, was vectored to an ILS 5 approach, recovered normal airspeed indicator function at 4000' MSL, and landed uneventfully. Taxied to FBO and checked pitot heat tube heat tube which was warm, not hot to touch. Discussed situation with FBO mechanic who felt that, if there was a problem, it was most likely in the tube and not in wiring, etc. He stated that he probably could look at the pitot heat system and would have to order tube if defective. Discussed WX situation and buf FSS in person, reviewed latest available WX information, and discussed flight options with them in light of probable pitot tube dysfunction. After this review, decision was made that flight below freezing level at 6000' MSL could be accomplished safely. I waited an additional hour for the WX to move further e-ward, and then departed on IFR flight plan to arb west/O further incident. Upon landing at arb, pitot tube was hot to the touch. Inspection of the pitot tube function by maintenance FBO demonstrated normal function. The cause of the pitot tube dysfunction which led to airspeed indicator while IMC remains unknown. I double-checked pitot tube toggle switch position when I first noted inappropriate airspeed indicator function and it was properly positioned. In retrospect, I should have performed the following steps: 1) switched on pitot tube heat while still on ground prior to departure to allow sufficient time for it to heat up; 2) retoggled pitot heatswitch in addition to verifying it was in on position when airspeed indicator malfunction first noted. Callback conversation with reporter revealed the following: reporter stated that several other pilots also experienced pitot heat problems on this same rental aircraft. While the specific cause of the problem was not found, general servicing and inspection of the electrical system has apparently eliminated the problem. The reporter was advised in the callback that declaration of an emergency would likely have been wise.

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Original NASA ASRS Text

Title: PLT EXPERIENCES PITOT HEAT FAILURE WHEN IN ICING CONDITIONS.

Narrative: FILED IFR FLT PLAN FROM BUF TO ARB AFTER BRIEFING FROM BUFFALO FSS FORECASTING FREEZING LEVEL OF 7000', OCCASIONAL ICING IN CLOUDS OR PRECIPITATION, OCCASIONAL TURB. PREFLT DEMONSTRATED NO DEFICIENCIES. TURNED ON PITOT HEAT ON CLBOUT WITH ESTIMATED CEILING 2500' MSL. CLBED THROUGH MILD TURB 6000' MSL AND CONTINUED TO ASSIGNED ALT OF 10000'. BEGAN TO PICK UP MILD TO MODERATE ICE AFTER A FEW MINS AT 10000'. NOTED THAT AIRSPD INDICATOR REGISTERED 130 KTS, LOWER THAN EXPECTED FOR MANIFOLD, RPM SETTINGS. REQUESTED LOWER ALT FORM BUFFALO DEP BECAUSE OF ICING, CLRED TO 8000' (OR PERHAPS 6000') BY DEP. NOTED AIRSPD INDICATOR SHOWING FALLING AIRSPD DESPITE NOSE-DOWN ATTITUDE AND APPROPRIATE MP AND RPM SETTINGS. NOTIFIED DEP OF IMPENDING AIRSPD INDICATOR FAILURE. DEP ASKED IF I WISHED TO CONTINUE ON OR COME BACK TO BUFFALO. I TOLD THEM THAT I WOULD RETURN TO BUF. I DID NOT DECLARE AN EMER, WAS VECTORED TO AN ILS 5 APCH, RECOVERED NORMAL AIRSPD INDICATOR FUNCTION AT 4000' MSL, AND LANDED UNEVENTFULLY. TAXIED TO FBO AND CHKED PITOT HEAT TUBE HEAT TUBE WHICH WAS WARM, NOT HOT TO TOUCH. DISCUSSED SITUATION WITH FBO MECH WHO FELT THAT, IF THERE WAS A PROB, IT WAS MOST LIKELY IN THE TUBE AND NOT IN WIRING, ETC. HE STATED THAT HE PROBABLY COULD LOOK AT THE PITOT HEAT SYS AND WOULD HAVE TO ORDER TUBE IF DEFECTIVE. DISCUSSED WX SITUATION AND BUF FSS IN PERSON, REVIEWED LATEST AVAILABLE WX INFO, AND DISCUSSED FLT OPTIONS WITH THEM IN LIGHT OF PROBABLE PITOT TUBE DYSFUNCTION. AFTER THIS REVIEW, DECISION WAS MADE THAT FLT BELOW FREEZING LEVEL AT 6000' MSL COULD BE ACCOMPLISHED SAFELY. I WAITED AN ADDITIONAL HR FOR THE WX TO MOVE FURTHER E-WARD, AND THEN DEPARTED ON IFR FLT PLAN TO ARB W/O FURTHER INCIDENT. UPON LNDG AT ARB, PITOT TUBE WAS HOT TO THE TOUCH. INSPECTION OF THE PITOT TUBE FUNCTION BY MAINT FBO DEMONSTRATED NORMAL FUNCTION. THE CAUSE OF THE PITOT TUBE DYSFUNCTION WHICH LED TO AIRSPD INDICATOR WHILE IMC REMAINS UNKNOWN. I DOUBLE-CHKED PITOT TUBE TOGGLE SWITCH POS WHEN I FIRST NOTED INAPPROPRIATE AIRSPD INDICATOR FUNCTION AND IT WAS PROPERLY POSITIONED. IN RETROSPECT, I SHOULD HAVE PERFORMED THE FOLLOWING STEPS: 1) SWITCHED ON PITOT TUBE HEAT WHILE STILL ON GND PRIOR TO DEP TO ALLOW SUFFICIENT TIME FOR IT TO HEAT UP; 2) RETOGGLED PITOT HEATSWITCH IN ADDITION TO VERIFYING IT WAS IN ON POS WHEN AIRSPD INDICATOR MALFUNCTION FIRST NOTED. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING: RPTR STATED THAT SEVERAL OTHER PLTS ALSO EXPERIENCED PITOT HEAT PROBS ON THIS SAME RENTAL ACFT. WHILE THE SPECIFIC CAUSE OF THE PROB WAS NOT FOUND, GENERAL SERVICING AND INSPECTION OF THE ELECTRICAL SYS HAS APPARENTLY ELIMINATED THE PROB. THE RPTR WAS ADVISED IN THE CALLBACK THAT DECLARATION OF AN EMER WOULD LIKELY HAVE BEEN WISE.

Data retrieved from NASA's ASRS site as of August 2007 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.