Narrative:

The aircraft was cruising at 10000' with the autoplt engaged in the heading and altitude modes. The pilot commenced housekeeping duties, including power plant INS checks, cabin pressurization check, cabin temperature check, notation of takeoff tie on flight log, and also noting on flight log of a 1 hour departure delay due to WX at the departure airport. On frequent occasions the flight encountered moderate to severe turbulence. While checking the en route chart for the next flight leg, I was advised by dulles approach 'you are indicating 10800', descend and maintain 10000', my assigned altitude. At the time of the radio call the flight was encountering moderate with occasional severe turbulence. The reply made to approach was, we show level at 10000'--do you want us to try the other transponder. As the turbulence subsided, I was in fact indicating 10800'. The initial incorrect reading of the altimeter was due to the severe movement of the small transport's shock mounted panel causing the 10800' to appear as 10000'. Upon recognition that I was in fact at the incorrect altitude, a descent was initiated immediately to 10000'. Upon checking of the autoplt annunciator panel it was discovered that at some time during cruise flight the autoplt had switched from the altitude mode to the attitude mode. I do not now if turbulence knocked the altitude hold off, or if the attitude button was bumped in the turbulence, with the possibility of a malfunction in the autoplt system not totally ruled out (the autoplt functioned normally for the remainder of the flight). There is a slight design problem in the autoplt annunciator panel. The close proximity and appearance of the altitude and attitude lights allowed me to miss the discrepancy in the modes during the turbulence. It may also be of interest to you that the flight was being conducted with a single pilot.

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Original NASA ASRS Text

Title: ALT DEVIATION.

Narrative: THE ACFT WAS CRUISING AT 10000' WITH THE AUTOPLT ENGAGED IN THE HDG AND ALT MODES. THE PLT COMMENCED HOUSEKEEPING DUTIES, INCLUDING PWR PLANT INS CHKS, CABIN PRESSURIZATION CHK, CABIN TEMP CHK, NOTATION OF TKOF TIE ON FLT LOG, AND ALSO NOTING ON FLT LOG OF A 1 HR DEP DELAY DUE TO WX AT THE DEP ARPT. ON FREQUENT OCCASIONS THE FLT ENCOUNTERED MODERATE TO SEVERE TURB. WHILE CHKING THE ENRTE CHART FOR THE NEXT FLT LEG, I WAS ADVISED BY DULLES APCH 'YOU ARE INDICATING 10800', DSND AND MAINTAIN 10000', MY ASSIGNED ALT. AT THE TIME OF THE RADIO CALL THE FLT WAS ENCOUNTERING MODERATE WITH OCCASIONAL SEVERE TURB. THE REPLY MADE TO APCH WAS, WE SHOW LEVEL AT 10000'--DO YOU WANT US TO TRY THE OTHER XPONDER. AS THE TURB SUBSIDED, I WAS IN FACT INDICATING 10800'. THE INITIAL INCORRECT READING OF THE ALTIMETER WAS DUE TO THE SEVERE MOVEMENT OF THE SMT'S SHOCK MOUNTED PANEL CAUSING THE 10800' TO APPEAR AS 10000'. UPON RECOGNITION THAT I WAS IN FACT AT THE INCORRECT ALT, A DSNT WAS INITIATED IMMEDIATELY TO 10000'. UPON CHKING OF THE AUTOPLT ANNUNCIATOR PANEL IT WAS DISCOVERED THAT AT SOME TIME DURING CRUISE FLT THE AUTOPLT HAD SWITCHED FROM THE ALT MODE TO THE ATTITUDE MODE. I DO NOT NOW IF TURB KNOCKED THE ALT HOLD OFF, OR IF THE ATTITUDE BUTTON WAS BUMPED IN THE TURB, WITH THE POSSIBILITY OF A MALFUNCTION IN THE AUTOPLT SYS NOT TOTALLY RULED OUT (THE AUTOPLT FUNCTIONED NORMALLY FOR THE REMAINDER OF THE FLT). THERE IS A SLIGHT DESIGN PROB IN THE AUTOPLT ANNUNCIATOR PANEL. THE CLOSE PROX AND APPEARANCE OF THE ALT AND ATTITUDE LIGHTS ALLOWED ME TO MISS THE DISCREPANCY IN THE MODES DURING THE TURB. IT MAY ALSO BE OF INTEREST TO YOU THAT THE FLT WAS BEING CONDUCTED WITH A SINGLE PLT.

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.