Narrative:

We were proceeding southwest on A700, mach .76, approximately 200 mi northeast of trout, at FL410, deviating (with clearance) east of A700 to remain clear of WX topping FL410, experiencing moderate turbulence, autoplt engaged, learjet 60. Observed the aircraft slowly climb to 150 ft above selected altitude, pause for a min or 2, and slowly initiate a descent back to FL410. Aircraft continued to descend through FL410, began smoothly decreasing pitch altitude, increasing airspeed and rate of descent, at 200 ft below FL410 I manually engaged 'trim by wire' to increase pitch altitude (without disengaging autoplt). However, this was not producing a positive enough correction. I then disengaged autoplt and hand flew the aircraft level at FL410. Re-engaged autoplt at FL410. It functioned properly the rest of the flight (and subsequent flts). Altitude loss was 800 ft. I am confident there was no traffic in the area. We were 30 mi east of A700, received no reports from ATC although probably out of radar coverage. It is fortunate that I observed the initial deviation from altitude, there was no autoplt aural warning since the autoplt was still engaged although no longer holding altitude. The pitch down was smooth but firm, producing a rapidly increasing airspeed that reached but did not exceed mmo due to immediate corrective action. Callback conversation with reporter revealed the following information: reporter stated that this problem did not happen again to him and that he had flown in similar turbulence subsequently. However, he stated that another flight crew operating the same type of aircraft and autoplt had reported similar incidents. In fact, this subject came up in a meeting with the operators of the lear 60 in which the manufacturer advised that they were aware of the problem and looking into it. The autoplt maker believes that the problem is not the autoplt, but that it is the air data computer for the flight management system. So far there has been no known resolution to the problem. The air data information seems to 'kick' off line for 7 seconds!

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Original NASA ASRS Text

Title: ALT EXCURSION FROM ASSIGNED.

Narrative: WE WERE PROCEEDING SW ON A700, MACH .76, APPROX 200 MI NE OF TROUT, AT FL410, DEVIATING (WITH CLRNC) E OF A700 TO REMAIN CLR OF WX TOPPING FL410, EXPERIENCING MODERATE TURB, AUTOPLT ENGAGED, LEARJET 60. OBSERVED THE ACFT SLOWLY CLB TO 150 FT ABOVE SELECTED ALT, PAUSE FOR A MIN OR 2, AND SLOWLY INITIATE A DSCNT BACK TO FL410. ACFT CONTINUED TO DSND THROUGH FL410, BEGAN SMOOTHLY DECREASING PITCH ALT, INCREASING AIRSPD AND RATE OF DSCNT, AT 200 FT BELOW FL410 I MANUALLY ENGAGED 'TRIM BY WIRE' TO INCREASE PITCH ALT (WITHOUT DISENGAGING AUTOPLT). HOWEVER, THIS WAS NOT PRODUCING A POSITIVE ENOUGH CORRECTION. I THEN DISENGAGED AUTOPLT AND HAND FLEW THE ACFT LEVEL AT FL410. RE-ENGAGED AUTOPLT AT FL410. IT FUNCTIONED PROPERLY THE REST OF THE FLT (AND SUBSEQUENT FLTS). ALT LOSS WAS 800 FT. I AM CONFIDENT THERE WAS NO TFC IN THE AREA. WE WERE 30 MI E OF A700, RECEIVED NO RPTS FROM ATC ALTHOUGH PROBABLY OUT OF RADAR COVERAGE. IT IS FORTUNATE THAT I OBSERVED THE INITIAL DEV FROM ALT, THERE WAS NO AUTOPLT AURAL WARNING SINCE THE AUTOPLT WAS STILL ENGAGED ALTHOUGH NO LONGER HOLDING ALT. THE PITCH DOWN WAS SMOOTH BUT FIRM, PRODUCING A RAPIDLY INCREASING AIRSPD THAT REACHED BUT DID NOT EXCEED MMO DUE TO IMMEDIATE CORRECTIVE ACTION. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATED THAT THIS PROB DID NOT HAPPEN AGAIN TO HIM AND THAT HE HAD FLOWN IN SIMILAR TURB SUBSEQUENTLY. HOWEVER, HE STATED THAT ANOTHER FLC OPERATING THE SAME TYPE OF ACFT AND AUTOPLT HAD RPTED SIMILAR INCIDENTS. IN FACT, THIS SUBJECT CAME UP IN A MEETING WITH THE OPERATORS OF THE LEAR 60 IN WHICH THE MANUFACTURER ADVISED THAT THEY WERE AWARE OF THE PROB AND LOOKING INTO IT. THE AUTOPLT MAKER BELIEVES THAT THE PROB IS NOT THE AUTOPLT, BUT THAT IT IS THE AIR DATA COMPUTER FOR THE FLT MGMNT SYS. SO FAR THERE HAS BEEN NO KNOWN RESOLUTION TO THE PROB. THE AIR DATA INFO SEEMS TO 'KICK' OFF LINE FOR 7 SECONDS!

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.