Narrative:

The aircraft had just completed maintenance that required interior removal. When the interior trim was replaced; the pitot drain was impinged slightly giving false airspeed and altitude indications. I thought something wasn't quite right in the climb; but was then in IMC. I climbed to indicated FL270; which was then VMC; and sorted out the possible sources. When the problem was corrected; ATC notified me I was at FL310. I then descended to the correct FL270 and the rest of the trip was uneventful. The work was done by a qualified mechanic and I gave it a good look-over myself; but this pressure on the pitot drain was so subtle that neither of us saw it. The changes in the airspeed and altitude were not apparent at lower altitude when I could have returned to the airport and landed. Once in IMC; I flew the plane until I could safely sort out the problem and correct it. ATC made no further comments as I descended to the appropriate altitude and to my knowledge no other flts were affected. Callback conversation with reporter revealed the following information: reporter stated the pressure on the primary pitot static drain fitting from the interior carpet plastic molding was so subtle; neither he or the mechanic noticed. This is a spring loaded closed type drain fitting just outboard of the captain's seat where one's left heel would be placed. Pilot cannot see this drain fitting in flight; only if one gets out of the left seat and looks under; along the carpet molding. Pilot can; however; feel the end fitting with one's hand where the hole in the carpet molding normally would not contact the spring loaded pitot drain fitting. In this case; the hole was just not quite big enough to accommodate the end fitting. As a result; subtle pressure on the end fitting actually opened the pressure side of the pitot line causing false airspeed and altitude indications. Reporter also stated while he was in flight trying to figure out what was wrong; he reached down with his left hand and touched the pitot drain fitting and sure enough; his airspeed and altitude indications jumped-up. He realized then the problem was the carpet molding hole was not large enough. Mechanic told him later they generally enlarge this hole to prevent this from occurring. Reporter added although he does have an alternate pitot system; he just failed to switch over in flight.

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Original NASA ASRS Text

Title: A PIPER P46T PILOT REPORTS OF GETTING FALSE AIRSPEED AND ALTITUDE INDICATIONS AFTER THE ACFT'S INTERIOR TRIM WAS REPLACED.

Narrative: THE ACFT HAD JUST COMPLETED MAINT THAT REQUIRED INTERIOR REMOVAL. WHEN THE INTERIOR TRIM WAS REPLACED; THE PITOT DRAIN WAS IMPINGED SLIGHTLY GIVING FALSE AIRSPD AND ALT INDICATIONS. I THOUGHT SOMETHING WASN'T QUITE RIGHT IN THE CLB; BUT WAS THEN IN IMC. I CLBED TO INDICATED FL270; WHICH WAS THEN VMC; AND SORTED OUT THE POSSIBLE SOURCES. WHEN THE PROB WAS CORRECTED; ATC NOTIFIED ME I WAS AT FL310. I THEN DSNDED TO THE CORRECT FL270 AND THE REST OF THE TRIP WAS UNEVENTFUL. THE WORK WAS DONE BY A QUALIFIED MECH AND I GAVE IT A GOOD LOOK-OVER MYSELF; BUT THIS PRESSURE ON THE PITOT DRAIN WAS SO SUBTLE THAT NEITHER OF US SAW IT. THE CHANGES IN THE AIRSPD AND ALT WERE NOT APPARENT AT LOWER ALT WHEN I COULD HAVE RETURNED TO THE ARPT AND LANDED. ONCE IN IMC; I FLEW THE PLANE UNTIL I COULD SAFELY SORT OUT THE PROB AND CORRECT IT. ATC MADE NO FURTHER COMMENTS AS I DSNDED TO THE APPROPRIATE ALT AND TO MY KNOWLEDGE NO OTHER FLTS WERE AFFECTED. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: REPORTER STATED THE PRESSURE ON THE PRIMARY PITOT STATIC DRAIN FITTING FROM THE INTERIOR CARPET PLASTIC MOLDING WAS SO SUBTLE; NEITHER HE OR THE MECHANIC NOTICED. THIS IS A SPRING LOADED CLOSED TYPE DRAIN FITTING JUST OUTBOARD OF THE CAPTAIN'S SEAT WHERE ONE'S LEFT HEEL WOULD BE PLACED. PILOT CANNOT SEE THIS DRAIN FITTING IN FLIGHT; ONLY IF ONE GETS OUT OF THE LEFT SEAT AND LOOKS UNDER; ALONG THE CARPET MOLDING. PILOT CAN; HOWEVER; FEEL THE END FITTING WITH ONE'S HAND WHERE THE HOLE IN THE CARPET MOLDING NORMALLY WOULD NOT CONTACT THE SPRING LOADED PITOT DRAIN FITTING. IN THIS CASE; THE HOLE WAS JUST NOT QUITE BIG ENOUGH TO ACCOMMODATE THE END FITTING. AS A RESULT; SUBTLE PRESSURE ON THE END FITTING ACTUALLY OPENED THE PRESSURE SIDE OF THE PITOT LINE CAUSING FALSE AIRSPEED AND ALTITUDE INDICATIONS. REPORTER ALSO STATED WHILE HE WAS IN FLIGHT TRYING TO FIGURE OUT WHAT WAS WRONG; HE REACHED DOWN WITH HIS LEFT HAND AND TOUCHED THE PITOT DRAIN FITTING AND SURE ENOUGH; HIS AIRSPEED AND ALTITUDE INDICATIONS JUMPED-UP. HE REALIZED THEN THE PROBLEM WAS THE CARPET MOLDING HOLE WAS NOT LARGE ENOUGH. MECHANIC TOLD HIM LATER THEY GENERALLY ENLARGE THIS HOLE TO PREVENT THIS FROM OCCURRING. REPORTER ADDED ALTHOUGH HE DOES HAVE AN ALTERNATE PITOT SYSTEM; HE JUST FAILED TO SWITCH OVER IN FLIGHT.

Data retrieved from NASA's ASRS site as of January 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.