Narrative:

While at cruise altitude, aircraft had been level for approximately 1 1/2 hours. The autoplt pulled up hard. Aircraft reached FL393. I immediately disengaged the autoplt and leveled the aircraft off at FL390. No mention of any conflicts was given by ZAU. The whole event took less than 30 seconds to happen and be level at FL390 again. I do not really know of anything I could have done to prevent this as the autoplt was working normally prior to this occurrence. Afterward I retrimmed the aircraft and engaged the autoplt. It worked fine the rest of the trip. The best advice I can pass along is careful monitoring of all automated system is a must.

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Original NASA ASRS Text

Title: LEAR 35 AUTOPLT MALFUNCTIONS AFTER 1 PT 5 HRS AT CRUISE. ACFT CLBS 300 FT ABOVE ASSIGNED ALT. ALTDEV. ACFT EQUIP PROB AUTOPLT.

Narrative: WHILE AT CRUISE ALT, ACFT HAD BEEN LEVEL FOR APPROX 1 1/2 HRS. THE AUTOPLT PULLED UP HARD. ACFT REACHED FL393. I IMMEDIATELY DISENGAGED THE AUTOPLT AND LEVELED THE ACFT OFF AT FL390. NO MENTION OF ANY CONFLICTS WAS GIVEN BY ZAU. THE WHOLE EVENT TOOK LESS THAN 30 SECONDS TO HAPPEN AND BE LEVEL AT FL390 AGAIN. I DO NOT REALLY KNOW OF ANYTHING I COULD HAVE DONE TO PREVENT THIS AS THE AUTOPLT WAS WORKING NORMALLY PRIOR TO THIS OCCURRENCE. AFTERWARD I RETRIMMED THE ACFT AND ENGAGED THE AUTOPLT. IT WORKED FINE THE REST OF THE TRIP. THE BEST ADVICE I CAN PASS ALONG IS CAREFUL MONITORING OF ALL AUTOMATED SYS IS A MUST.

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.