Narrative:

In cruise from bfl to sjc at 10000 ft on IFR flight plan, in and out of IMC (mostly in), told to descend to 8000 ft. Rime ice was beginning to form, so I asked for a lower altitude. Cleared to 7000 ft but still in IMC and still picking up ice. Although pitot heat worked on preflight, pitot tube seemed to be iced over since airspeed indicator indicated cruise speed, then '0' KIAS within a few mins. Also, the altimeter and vsi were very erratic in indicating climb and descent. Within seconds, vsi would read 2000 FPM climb, then 2000 FPM descent with the altimeter needle spinning between 8000 ft and 6000 ft. I was cleared to 7000 ft and, upon leveling off at what I thought was the correct altitude, (altitude and vertical speed were sporadic) ATC advised I was at 6300 ft and to climb to 7000 ft. I initiated a climb. The altitude and vertical speed seemed to calm down for a min and both ATC and I concurred I was at 7000 ft. I was told I was overtaking traffic that was at 6000 ft which was the reason for my 7000 ft restriction. Possible reasons for my altitude deviation: 1) ice on the pitot tube blocking both ram and static pressure ports. 2) relying on instruments (altitude and vsi) when another instrument on the same system was obviously inoperative at the time (incidentally, at about 4000 ft in VFR, all instruments worked properly). 3) could have asked ATC to give me a level off after indicating to them my instruments were giving me erratic readings. Freezing level for route was 5000 ft sloping to 11000 ft (north to south). PIREPS indicated icing at my altitude but not in my vicinity. They were 30-50 mi away. I could and should have asked for deviations around clouds once I picked up first bit of ice or possibly climbed to VFR and canceled IFR to continue (destination was 3000 scattered). Anyway, I think the ice on pitot tube caused instrument malfunction which caused my altitude deviation. More respect towards the freezing level will be used with/without visible moisture. Even though I was flying a plane that is not certified to fly into known icing conditions, I flew it into visible moisture while being at a forecasted freezing level. Does that constitute a known icing condition or do I have to see an actual report of icing (PIREP, etc) on my route before I violate the 'flying into icing conditions' regulation?

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Original NASA ASRS Text

Title: THE PITOT STATIC INSTS ICED UP IN AN SMA.

Narrative: IN CRUISE FROM BFL TO SJC AT 10000 FT ON IFR FLT PLAN, IN AND OUT OF IMC (MOSTLY IN), TOLD TO DSND TO 8000 FT. RIME ICE WAS BEGINNING TO FORM, SO I ASKED FOR A LOWER ALT. CLRED TO 7000 FT BUT STILL IN IMC AND STILL PICKING UP ICE. ALTHOUGH PITOT HEAT WORKED ON PREFLT, PITOT TUBE SEEMED TO BE ICED OVER SINCE AIRSPD INDICATOR INDICATED CRUISE SPD, THEN '0' KIAS WITHIN A FEW MINS. ALSO, THE ALTIMETER AND VSI WERE VERY ERRATIC IN INDICATING CLB AND DSCNT. WITHIN SECONDS, VSI WOULD READ 2000 FPM CLB, THEN 2000 FPM DSCNT WITH THE ALTIMETER NEEDLE SPINNING BTWN 8000 FT AND 6000 FT. I WAS CLRED TO 7000 FT AND, UPON LEVELING OFF AT WHAT I THOUGHT WAS THE CORRECT ALT, (ALT AND VERT SPD WERE SPORADIC) ATC ADVISED I WAS AT 6300 FT AND TO CLB TO 7000 FT. I INITIATED A CLB. THE ALT AND VERT SPD SEEMED TO CALM DOWN FOR A MIN AND BOTH ATC AND I CONCURRED I WAS AT 7000 FT. I WAS TOLD I WAS OVERTAKING TFC THAT WAS AT 6000 FT WHICH WAS THE REASON FOR MY 7000 FT RESTRICTION. POSSIBLE REASONS FOR MY ALTDEV: 1) ICE ON THE PITOT TUBE BLOCKING BOTH RAM AND STATIC PRESSURE PORTS. 2) RELYING ON INSTS (ALT AND VSI) WHEN ANOTHER INST ON THE SAME SYS WAS OBVIOUSLY INOP AT THE TIME (INCIDENTALLY, AT ABOUT 4000 FT IN VFR, ALL INSTS WORKED PROPERLY). 3) COULD HAVE ASKED ATC TO GIVE ME A LEVEL OFF AFTER INDICATING TO THEM MY INSTS WERE GIVING ME ERRATIC READINGS. FREEZING LEVEL FOR RTE WAS 5000 FT SLOPING TO 11000 FT (N TO S). PIREPS INDICATED ICING AT MY ALT BUT NOT IN MY VICINITY. THEY WERE 30-50 MI AWAY. I COULD AND SHOULD HAVE ASKED FOR DEVS AROUND CLOUDS ONCE I PICKED UP FIRST BIT OF ICE OR POSSIBLY CLBED TO VFR AND CANCELED IFR TO CONTINUE (DEST WAS 3000 SCATTERED). ANYWAY, I THINK THE ICE ON PITOT TUBE CAUSED INST MALFUNCTION WHICH CAUSED MY ALTDEV. MORE RESPECT TOWARDS THE FREEZING LEVEL WILL BE USED WITH/WITHOUT VISIBLE MOISTURE. EVEN THOUGH I WAS FLYING A PLANE THAT IS NOT CERTIFIED TO FLY INTO KNOWN ICING CONDITIONS, I FLEW IT INTO VISIBLE MOISTURE WHILE BEING AT A FORECASTED FREEZING LEVEL. DOES THAT CONSTITUTE A KNOWN ICING CONDITION OR DO I HAVE TO SEE AN ACTUAL RPT OF ICING (PIREP, ETC) ON MY RTE BEFORE I VIOLATE THE 'FLYING INTO ICING CONDITIONS' REG?

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