Lear 35 autoplt malfunctions after 1 pt 5 hours at cruise. Aircraft climbs 300 ft above assigned altitude. Altitude deviation. Aircraft equipment problem autoplt.

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Attributes
ACN	338810
Time
Date	199606
Day	Thu
Local Time Of Day	0601 To 1200
Place
Locale Reference	atc facility : obk
State Reference	IL
Altitude	msl bound lower : 39000 msl bound upper : 39300
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zau
Operator	general aviation : corporate
Make Model Name	Learjet 35
Operating Under FAR Part	Part 91
Flight Phase	cruise other
Flight Plan	IFR
Person 1
Affiliation	Other
Function	flight crew : captain oversight : pic
Qualification	pilot : atp pilot : cfi
Experience	flight time last 90 days : 90 flight time total : 6400 flight time type : 2800
ASRS Report	338810
Person 2
Affiliation	Other
Function	flight crew : first officer
Qualification	pilot : commercial pilot : instrument
Events
Anomaly	aircraft equipment problem : less severe altitude deviation : excursion from assigned altitude non adherence : clearance
Independent Detector	other flight crewa
Resolutory Action	flight crew : returned to intended course or assigned course
Consequence	Other
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	other

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.

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.