<< >> NASA ASRS Report 960841, July 2011

B737-300 flight crew experiences an autopilot malfunction at FL370 that commences with a pitch up then pitch down. The autopilot is disconnected using the yoke switch but continues to pitch down. Brute force is used to overpower and disengage the autopilot and return the aircraft to FL370. A previous identical malfunction had been signed off by maintenance as 'bite check ok'.

Attributes
ACN	960841
Time
Date	201107
Local Time Of Day	0601-1200
Place
Locale Reference	ZZZ.ARTCC
State Reference	US
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Make Model Name	B737-300
Operating Under FAR Part	Part 121
Flight Phase	Cruise
Flight Plan	IFR
Component
Aircraft Component	Autopilot
Person 1
Function	Captain
Qualification	Flight Crew Air Transport Pilot (ATP)
Experience	Flight Crew Last 90 Days 239 Flight Crew Type 11200
Person 2
Function	First Officer
Experience	Flight Crew Last 90 Days 155 Flight Crew Type 3500
Events
Anomaly	Aircraft Equipment Problem Critical Deviation - Altitude Excursion From Assigned Altitude

Narrative:

Cruising at FL370; the autopilot initially climbed and then descended at approximately 1;500 FPM. The pilot flying hit the autopilot disconnect switch on the yoke; the horn sounded; but autopilot did not disengage. The pilot not flying had to use extreme force to override and disengage the autopilot. This was an exact copy of the previous discrepancy on the aircraft's last flight. Write-up was; 'ran bite check no faults found.

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Original NASA ASRS Text

Title: B737-300 flight crew experiences an autopilot malfunction at FL370 that commences with a pitch up then pitch down. The autopilot is disconnected using the yoke switch but continues to pitch down. Brute force is used to overpower and disengage the autopilot and return the aircraft to FL370. A previous identical malfunction had been signed off by maintenance as 'bite check OK'.

Narrative: Cruising at FL370; the autopilot initially climbed and then descended at approximately 1;500 FPM. The pilot flying hit the autopilot disconnect switch on the yoke; the horn sounded; but autopilot did not disengage. The pilot not flying had to use extreme force to override and disengage the autopilot. This was an exact copy of the previous discrepancy on the aircraft's last flight. Write-up was; 'Ran bite check no faults found.

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.