Narrative:

I was piloting an small aircraft on an IFR flight to lewiston, identification. At approximately xa hours local time, and 50-60 DME mi from lws VOR at an assigned altitude of 20,000', I saw a cloud bank with tops extending to possibly 30,000'. Since previously I had encountered light rime icing in passing through the clouds with occasional rain, I did not want to enter known icing conditions since I was currently en route above the cloud tops. Therefore, I requested and received from ATC, seattle center, a clearance for deviation to the south of cloud tops. I proceeded south and was able to pass along the fringe of the cloud bank at an altitude displayed on my altimeter of 20,050. After my instrument scan, I entered the attenuated edge of the cloud and was picking up light to mod rime ice. ATC then called asking my altitude. On my immediate instrument scan I noted that the artificial horizon indicator showed level flight, the vertical speed indicator showed 0 climb route, but my altimeter hands were showing a rapid climb in a matter of seconds of 2500-3500'. I immediately responded to ATC, stating that I had to have climbed to get over or around the cloud tops. I felt at the time that I had climbed the aircraft above the assigned altitude and had not recognized the change in altitude. I could not account for the rapid rate of altitude change in less than 1 minute, since my aircraft can only climb about 1000 FPM. At the time I was in the edge of the cloud layer, ATC responded but I could not understand their reply since the transmission was breaking up. I told ATC this and requested a clearance at the higher altitude. This I received and proceeded to return to the airway. In addition I reported to ATC that the cloud bank which I encircled if flown through would obviously produce heavy rime icing. The rest of the flight from phoenix to lewiston was without incident. In considering the events of may 89, I later became aware of a problem with my static air source. This was only called to my attention when another problem with altitude display was encountered during a return flight from lewiston, identification, on may 89, reported in a separate NASA report enclosed. My present interpretation of the event was that moisture was in the static air source lines or the port became frozen during my level flight at 20,000'. This was above the freezing level. A gradual climb was unseen because of fixation of the altimeter, vertical speed indicator due static air source occlusion. Also ATC would not have recognized the climb on mode C, since the encoding altimeter would report only a pressure change in the system which did not occur until later in the flight. At the point where the aircraft enter the cloud edge possibly an air inversion occurred or a spontaneous unplugging of the static air lines occurred with resultant rapid change in altimeter reading to a higher altitude in only a minute or less. Further, this also explains the level flight displayed on the artificial horizon indicator and the 0 climb rate on the vertical speed indicator.

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Original NASA ASRS Text

Title: FLT DEVIATION FOR WX, UNKNOWING CLIMB AS PITOT STATIC SYSTEM BECAME BLOCKED. SUDDEN UNPLUGGING OF STATIC PORT GAVE CORRECT ALTIMETER INDICATION, 3500' HIGHER THAN PREVIOUSLY INDICATED. NOTICED BY BOTH PLT AND CTLR.

Narrative: I WAS PILOTING AN SMA ON AN IFR FLT TO LEWISTON, ID. AT APPROX XA HOURS LOCAL TIME, AND 50-60 DME MI FROM LWS VOR AT AN ASSIGNED ALT OF 20,000', I SAW A CLOUD BANK WITH TOPS EXTENDING TO POSSIBLY 30,000'. SINCE PREVIOUSLY I HAD ENCOUNTERED LIGHT RIME ICING IN PASSING THROUGH THE CLOUDS WITH OCCASIONAL RAIN, I DID NOT WANT TO ENTER KNOWN ICING CONDITIONS SINCE I WAS CURRENTLY ENRTE ABOVE THE CLOUD TOPS. THEREFORE, I REQUESTED AND RECEIVED FROM ATC, SEATTLE CENTER, A CLRNC FOR DEVIATION TO THE S OF CLOUD TOPS. I PROCEEDED S AND WAS ABLE TO PASS ALONG THE FRINGE OF THE CLOUD BANK AT AN ALT DISPLAYED ON MY ALTIMETER OF 20,050. AFTER MY INSTRUMENT SCAN, I ENTERED THE ATTENUATED EDGE OF THE CLOUD AND WAS PICKING UP LIGHT TO MOD RIME ICE. ATC THEN CALLED ASKING MY ALT. ON MY IMMEDIATE INSTRUMENT SCAN I NOTED THAT THE ARTIFICIAL HORIZON INDICATOR SHOWED LEVEL FLT, THE VERTICAL SPEED INDICATOR SHOWED 0 CLIMB RTE, BUT MY ALTIMETER HANDS WERE SHOWING A RAPID CLIMB IN A MATTER OF SECONDS OF 2500-3500'. I IMMEDIATELY RESPONDED TO ATC, STATING THAT I HAD TO HAVE CLIMBED TO GET OVER OR AROUND THE CLOUD TOPS. I FELT AT THE TIME THAT I HAD CLIMBED THE ACFT ABOVE THE ASSIGNED ALT AND HAD NOT RECOGNIZED THE CHANGE IN ALT. I COULD NOT ACCOUNT FOR THE RAPID RATE OF ALT CHANGE IN LESS THAN 1 MINUTE, SINCE MY ACFT CAN ONLY CLIMB ABOUT 1000 FPM. AT THE TIME I WAS IN THE EDGE OF THE CLOUD LAYER, ATC RESPONDED BUT I COULD NOT UNDERSTAND THEIR REPLY SINCE THE XMISSION WAS BREAKING UP. I TOLD ATC THIS AND REQUESTED A CLRNC AT THE HIGHER ALT. THIS I RECEIVED AND PROCEEDED TO RETURN TO THE AIRWAY. IN ADDITION I REPORTED TO ATC THAT THE CLOUD BANK WHICH I ENCIRCLED IF FLOWN THROUGH WOULD OBVIOUSLY PRODUCE HEAVY RIME ICING. THE REST OF THE FLT FROM PHOENIX TO LEWISTON WAS WITHOUT INCIDENT. IN CONSIDERING THE EVENTS OF MAY 89, I LATER BECAME AWARE OF A PROBLEM WITH MY STATIC AIR SOURCE. THIS WAS ONLY CALLED TO MY ATTN WHEN ANOTHER PROBLEM WITH ALT DISPLAY WAS ENCOUNTERED DURING A RETURN FLT FROM LEWISTON, ID, ON MAY 89, REPORTED IN A SEPARATE NASA REPORT ENCLOSED. MY PRESENT INTERPRETATION OF THE EVENT WAS THAT MOISTURE WAS IN THE STATIC AIR SOURCE LINES OR THE PORT BECAME FROZEN DURING MY LEVEL FLT AT 20,000'. THIS WAS ABOVE THE FREEZING LEVEL. A GRADUAL CLIMB WAS UNSEEN BECAUSE OF FIXATION OF THE ALTIMETER, VERTICAL SPEED INDICATOR DUE STATIC AIR SOURCE OCCLUSION. ALSO ATC WOULD NOT HAVE RECOGNIZED THE CLIMB ON MODE C, SINCE THE ENCODING ALTIMETER WOULD REPORT ONLY A PRESSURE CHANGE IN THE SYSTEM WHICH DID NOT OCCUR UNTIL LATER IN THE FLT. AT THE POINT WHERE THE ACFT ENTER THE CLOUD EDGE POSSIBLY AN AIR INVERSION OCCURRED OR A SPONTANEOUS UNPLUGGING OF THE STATIC AIR LINES OCCURRED WITH RESULTANT RAPID CHANGE IN ALTIMETER READING TO A HIGHER ALT IN ONLY A MINUTE OR LESS. FURTHER, THIS ALSO EXPLAINS THE LEVEL FLT DISPLAYED ON THE ARTIFICIAL HORIZON INDICATOR AND THE 0 CLIMB RATE ON THE VERTICAL SPEED INDICATOR.

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.