Narrative:

Prior to taking off from livermore on an instrument flight; I washed bird droppings off my C-172N. Note that the freezing level was approximately 2;000 ft. There was solid overcast to my destination with a base at 900 ft and tops at 2;500 ft. I climbed to 4;000 ft per clearance. During the 20 minute or so flight; approach asked me about my altitude. They were reading it lower than the altimeter setting indicated; more so than the difference between 29.92 inches and 30-point-whatever would have indicated. I began the GPS lpv approach into stockton and descended to 1;800 ft; leveling off prior to reaching waypoint ipdew; as indicated on the approach plate. Oddly; I broke out below the base of the cloud layer. Approach advised me I was well below the glide path; at 900 ft I believe they said! Accepting I had an altimeter issue; I switched to the alternate static source. The indicated altitude dropped from 1;800 ft to 1;400 ft (I had already begun to climb per ATC's instructions). I continued the approach. Airspeed (also with a static input) and engine RPM correlated throughout the approach. I continued the approach as-published to da; followed by a missed approach; as planned. The flight back to livermore and a subsequent ILS approach were uneventful. I realized on the way back that water; probably from washing the airplane; had frozen and blocked the static port during the initial climb. Thanks to nct for several times contacting me about my altitude; especially just before the final approach fix.

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Original NASA ASRS Text

Title: C172 pilot experiences a large altimeter error; detected by ATC; after washing his aircraft then flying into freezing conditions. Switching to the alternate static source corrects the problem.

Narrative: Prior to taking off from Livermore on an instrument flight; I washed bird droppings off my C-172N. Note that the freezing level was approximately 2;000 FT. There was solid overcast to my destination with a base at 900 FT and tops at 2;500 FT. I climbed to 4;000 FT per clearance. During the 20 minute or so flight; Approach asked me about my altitude. They were reading it lower than the altimeter setting indicated; more so than the difference between 29.92 inches and 30-point-whatever would have indicated. I began the GPS LPV approach into Stockton and descended to 1;800 FT; leveling off prior to reaching Waypoint IPDEW; as indicated on the approach plate. Oddly; I broke out below the base of the cloud layer. Approach advised me I was well below the glide path; at 900 FT I believe they said! Accepting I had an altimeter issue; I switched to the alternate static source. The indicated altitude dropped from 1;800 FT to 1;400 FT (I had already begun to climb per ATC's instructions). I continued the approach. Airspeed (also with a static input) and engine RPM correlated throughout the approach. I continued the approach as-published to DA; followed by a missed approach; as planned. The flight back to Livermore and a subsequent ILS approach were uneventful. I realized on the way back that water; probably from washing the airplane; had frozen and blocked the static port during the initial climb. Thanks to NCT for several times contacting me about my altitude; especially just before the final approach fix.

Data retrieved from NASA's ASRS site as of April 2012 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.