Narrative:

I was piloting an small aircraft on an IFR flight from lewiston, identification to phoenix, az. The aircraft had been parked in lewiston for the previous 4 days during an almost continuous rain. Approximately 30 mins into the flight, after climbing thru a mod cloud layer to the assigned altitude first officer 16,000' above the cloud tops, there appeared to be higher tops at 18,000' directly in the flight path. A higher altitude was requested from ATC of 19,000'. Prior WX briefings by phone and in person at the lewiston WX bureau had suggested the nearest cloud tops as reported by PIREPS 30 mins before the departure to be approximately 12,000-14,500'. ATC responded that I would have to hold at 16,000' for about 2 mins due to traffic in the area. Prior to the clearance for 19,000' the aircraft entered the clouds directly in the flight path. Although in passage thru the lower cloud layers produce very light rime icing the passage into this layer produced light to mod clear icing. When clearance was received to climb to 19,000', initiation of the climb displayed a very low airspeed at climb power and standard climb indication on the artificial horizon indicator. The vertical speed indicator showed a 50 FPM climb and the altimeter displayed a slow climb. ATC was informed that I was climbing very slowly. I looked at the outside of the aircraft and could not relate the small amount of clear ice to the slow climbing indications. At 17,000' indication, I requested an ATC mode C readout. They confirmed a similar indication. At 18,000' indication on the altimeter again ATC confirmed the same reading on mode C. The aircraft at this point was above the cloud tops and the vertical speed indicated no climb and the altimeter was failing to climb. This was communicated to ATC who asked if I desired a lower level. I declined since this would place my aircraft into the clouds again and into possible icing. I told ATC I would try to continue to climb to the assigned 19,000' clearance. The ice was quickly sublimating on the aircraft in the sunshine, however the airspeed suddenly dropped to 0. The pitot heat had been on since takeoff, and all circuit breakers appeared intact, therefore I suspected initially that the pitot tube had plugged. ATC was informed and then the vertical speed indicated a 1000 FPM descent despite indication on the artificial horizon that the aircraft was still in a climb. The climb power which was continued appeared to show no fluctuation. The altimeter showed then a slow descent which was not in accord to the visual indications that the aircraft was climbing further above the cloud tops. ATC confirmed that the mode C read 18,000'. I then notified ATC that I suspected my altimeter reading was inaccurate, and that I really did not know what altitude I was currently flying. ATC acknowledged this. At this point almost simultaneous with communication by another pilot overhearing the problem and my own decision, the alternate air source was opened. This produced an immediate spin of the altimeter hands to 28,000'. At first I interpreted this as a fluctuation in the hands with the opening of the alternate air source, but when ATC announced they were reading 28,000' on mode C rather than 18,000 I then realized that the aircraft had been in a continuous climb even though this was not reflected on my altimeter or the mode C reading until alternate air source was used. The vertical speed now indicated a standard rate climb and the airspeed indicator showed an appropriate speed indication. I confirmed the altitude with ATC and request clearance to remain at that altitude. ATC asked if I was on oxygen and I answered in the affirmative. ATC cleared me to proceed at that altitude which I did without further problems. After the cloud cover dissipated over the course of the flight I descended to lower altitude where the static air source appeared to function normally. It now becomes apparent that there was a subtle occlusion of the static air source probably caused by water in the static air lines which became frozen or ice sealed over the static air ports. Once recognized it was corrected by going to alternate air source.obviously, this was not recognizable initially by me or by ATC on mode C altitude readout. In retrospect a similar occurrence occurred on may, 89, which was not appreciated until the flight of may, 89 produced more significant instrument errors. Of concern is the fact that this condition in other aircraft could place those aircraft at higher altitudes than their clearance, and this deviation would not necessarily be recognized by the pilots or ATC mode C reporting. Possibly a regulation requiring the use of alternate air source in IFR flts above the freezing level when cloud or humidity exist. As I understand the use of alternate air source is only recommended when static air source freezing or plugging is suspected.

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Original NASA ASRS Text

Title: IFR FLT INTO CLOUDS, STATIC SYSTEM ICED AND BLOCKED GIVING ERRONEOUS READINGS ON AIRSPEED, ALTIMETER AND VERTICAL SPEED. MODE C READOUT WAS INACCURATE UNTIL ALTERNATE STATIC SOURCE WAS USED.

Narrative: I WAS PILOTING AN SMA ON AN IFR FLT FROM LEWISTON, ID TO PHOENIX, AZ. THE ACFT HAD BEEN PARKED IN LEWISTON FOR THE PREVIOUS 4 DAYS DURING AN ALMOST CONTINUOUS RAIN. APPROX 30 MINS INTO THE FLT, AFTER CLBING THRU A MOD CLOUD LAYER TO THE ASSIGNED ALT FO 16,000' ABOVE THE CLOUD TOPS, THERE APPEARED TO BE HIGHER TOPS AT 18,000' DIRECTLY IN THE FLT PATH. A HIGHER ALT WAS REQUESTED FROM ATC OF 19,000'. PRIOR WX BRIEFINGS BY PHONE AND IN PERSON AT THE LEWISTON WX BUREAU HAD SUGGESTED THE NEAREST CLOUD TOPS AS REPORTED BY PIREPS 30 MINS BEFORE THE DEP TO BE APPROX 12,000-14,500'. ATC RESPONDED THAT I WOULD HAVE TO HOLD AT 16,000' FOR ABOUT 2 MINS DUE TO TFC IN THE AREA. PRIOR TO THE CLRNC FOR 19,000' THE ACFT ENTERED THE CLOUDS DIRECTLY IN THE FLT PATH. ALTHOUGH IN PASSAGE THRU THE LOWER CLOUD LAYERS PRODUCE VERY LIGHT RIME ICING THE PASSAGE INTO THIS LAYER PRODUCED LIGHT TO MOD CLEAR ICING. WHEN CLRNC WAS RECEIVED TO CLIMB TO 19,000', INITIATION OF THE CLIMB DISPLAYED A VERY LOW AIRSPEED AT CLIMB POWER AND STANDARD CLIMB INDICATION ON THE ARTIFICIAL HORIZON INDICATOR. THE VERTICAL SPEED INDICATOR SHOWED A 50 FPM CLIMB AND THE ALTIMETER DISPLAYED A SLOW CLIMB. ATC WAS INFORMED THAT I WAS CLBING VERY SLOWLY. I LOOKED AT THE OUTSIDE OF THE ACFT AND COULD NOT RELATE THE SMALL AMOUNT OF CLEAR ICE TO THE SLOW CLIMBING INDICATIONS. AT 17,000' INDICATION, I REQUESTED AN ATC MODE C READOUT. THEY CONFIRMED A SIMILAR INDICATION. AT 18,000' INDICATION ON THE ALTIMETER AGAIN ATC CONFIRMED THE SAME READING ON MODE C. THE ACFT AT THIS POINT WAS ABOVE THE CLOUD TOPS AND THE VERTICAL SPEED INDICATED NO CLIMB AND THE ALTIMETER WAS FAILING TO CLIMB. THIS WAS COMMUNICATED TO ATC WHO ASKED IF I DESIRED A LOWER LEVEL. I DECLINED SINCE THIS WOULD PLACE MY ACFT INTO THE CLOUDS AGAIN AND INTO POSSIBLE ICING. I TOLD ATC I WOULD TRY TO CONTINUE TO CLIMB TO THE ASSIGNED 19,000' CLRNC. THE ICE WAS QUICKLY SUBLIMATING ON THE ACFT IN THE SUNSHINE, HOWEVER THE AIRSPEED SUDDENLY DROPPED TO 0. THE PITOT HEAT HAD BEEN ON SINCE TKOF, AND ALL CIRCUIT BREAKERS APPEARED INTACT, THEREFORE I SUSPECTED INITIALLY THAT THE PITOT TUBE HAD PLUGGED. ATC WAS INFORMED AND THEN THE VERTICAL SPEED INDICATED A 1000 FPM DSCNT DESPITE INDICATION ON THE ARTIFICIAL HORIZON THAT THE ACFT WAS STILL IN A CLIMB. THE CLIMB POWER WHICH WAS CONTINUED APPEARED TO SHOW NO FLUCTUATION. THE ALTIMETER SHOWED THEN A SLOW DSCNT WHICH WAS NOT IN ACCORD TO THE VISUAL INDICATIONS THAT THE ACFT WAS CLBING FURTHER ABOVE THE CLOUD TOPS. ATC CONFIRMED THAT THE MODE C READ 18,000'. I THEN NOTIFIED ATC THAT I SUSPECTED MY ALTIMETER READING WAS INACCURATE, AND THAT I REALLY DID NOT KNOW WHAT ALT I WAS CURRENTLY FLYING. ATC ACKNOWLEDGED THIS. AT THIS POINT ALMOST SIMULTANEOUS WITH COM BY ANOTHER PLT OVERHEARING THE PROBLEM AND MY OWN DECISION, THE ALTERNATE AIR SOURCE WAS OPENED. THIS PRODUCED AN IMMEDIATE SPIN OF THE ALTIMETER HANDS TO 28,000'. AT FIRST I INTERPRETED THIS AS A FLUCTUATION IN THE HANDS WITH THE OPENING OF THE ALTERNATE AIR SOURCE, BUT WHEN ATC ANNOUNCED THEY WERE READING 28,000' ON MODE C RATHER THAN 18,000 I THEN REALIZED THAT THE ACFT HAD BEEN IN A CONTINUOUS CLIMB EVEN THOUGH THIS WAS NOT REFLECTED ON MY ALTIMETER OR THE MODE C READING UNTIL ALTERNATE AIR SOURCE WAS USED. THE VERTICAL SPEED NOW INDICATED A STANDARD RATE CLIMB AND THE AIRSPEED INDICATOR SHOWED AN APPROPRIATE SPEED INDICATION. I CONFIRMED THE ALT WITH ATC AND REQUEST CLRNC TO REMAIN AT THAT ALT. ATC ASKED IF I WAS ON OXYGEN AND I ANSWERED IN THE AFFIRMATIVE. ATC CLRED ME TO PROCEED AT THAT ALT WHICH I DID WITHOUT FURTHER PROBLEMS. AFTER THE CLOUD COVER DISSIPATED OVER THE COURSE OF THE FLT I DESCENDED TO LOWER ALT WHERE THE STATIC AIR SOURCE APPEARED TO FUNCTION NORMALLY. IT NOW BECOMES APPARENT THAT THERE WAS A SUBTLE OCCLUSION OF THE STATIC AIR SOURCE PROBABLY CAUSED BY WATER IN THE STATIC AIR LINES WHICH BECAME FROZEN OR ICE SEALED OVER THE STATIC AIR PORTS. ONCE RECOGNIZED IT WAS CORRECTED BY GOING TO ALTERNATE AIR SOURCE.OBVIOUSLY, THIS WAS NOT RECOGNIZABLE INITIALLY BY ME OR BY ATC ON MODE C ALT READOUT. IN RETROSPECT A SIMILAR OCCURRENCE OCCURRED ON MAY, 89, WHICH WAS NOT APPRECIATED UNTIL THE FLT OF MAY, 89 PRODUCED MORE SIGNIFICANT INSTRUMENT ERRORS. OF CONCERN IS THE FACT THAT THIS CONDITION IN OTHER ACFT COULD PLACE THOSE ACFT AT HIGHER ALTS THAN THEIR CLRNC, AND THIS DEVIATION WOULD NOT NECESSARILY BE RECOGNIZED BY THE PLTS OR ATC MODE C REPORTING. POSSIBLY A REGULATION REQUIRING THE USE OF ALTERNATE AIR SOURCE IN IFR FLTS ABOVE THE FREEZING LEVEL WHEN CLOUD OR HUMIDITY EXIST. AS I UNDERSTAND THE USE OF ALTERNATE AIR SOURCE IS ONLY RECOMMENDED WHEN STATIC AIR SOURCE FREEZING OR PLUGGING IS SUSPECTED.

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.