The autoplt came on without being turned on by the flight crew. The autoplt could not be turned off ...

Attributes
ACN	311536
Time
Date	199507
Day	Thu
Local Time Of Day	1201 To 1800
Place
Locale Reference	airport : mzt
State Reference	FO
Altitude	agl bound lower : 0 agl bound upper : 0
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	tower : ryy
Operator	common carrier : air carrier
Make Model Name	Commercial Fixed Wing
Operating Under FAR Part	Part 121
Flight Phase	ground : preflight
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 200 flight time total : 20000 flight time type : 5000
ASRS Report	311536
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : commercial pilot : instrument
Events
Anomaly	aircraft equipment problem : less severe other anomaly other
Independent Detector	other other : unspecified cockpit
Resolutory Action	flight crew : overcame equipment problem
Consequence	Other
Supplementary
Air Traffic Incident	other

Narrative:

On ramp in mzt, mexico, first officer and I noticed autoplt switch on forward panel came 'on' uncommanded. We turned switch off only to have it come on again. Control wheel did rapid movement and was indeed locked onto autoplt as we were unable to move control wheel. We attempted to disconnect autoplt with control wheel button. Autoplt would stay off as long as button was depressed. Once button was released, autoplt came back on. Unable to disconnect autoplt except through circuit breakers to autoplt and flight guidance system. Callback conversation with reporter revealed the following information: reporter stated that their company technical representative from mcdonnell douglas said what he described could not have happened. After the technical representative was briefed by technicians, he said maybe it could happen. However, after they placed the defective autoplt control box into another aircraft, he finally became a believer. Analysis revealed that the coil in the autoplt's control box had shorted out locking the system on, even though the circuit breaker was popped open. The reporter was advised by the technical representative that if they would have been airborne, the control wheel would not have locked up, but could have been overridden.

Original NASA ASRS Text

Title: THE AUTOPLT CAME ON WITHOUT BEING TURNED ON BY THE FLC. THE AUTOPLT COULD NOT BE TURNED OFF IN THE NORMAL MANNER BUT ONLY WITH THE CIRCUIT BREAKER.

Narrative: ON RAMP IN MZT, MEXICO, FO AND I NOTICED AUTOPLT SWITCH ON FORWARD PANEL CAME 'ON' UNCOMMANDED. WE TURNED SWITCH OFF ONLY TO HAVE IT COME ON AGAIN. CTL WHEEL DID RAPID MOVEMENT AND WAS INDEED LOCKED ONTO AUTOPLT AS WE WERE UNABLE TO MOVE CTL WHEEL. WE ATTEMPTED TO DISCONNECT AUTOPLT WITH CTL WHEEL BUTTON. AUTOPLT WOULD STAY OFF AS LONG AS BUTTON WAS DEPRESSED. ONCE BUTTON WAS RELEASED, AUTOPLT CAME BACK ON. UNABLE TO DISCONNECT AUTOPLT EXCEPT THROUGH CIRCUIT BREAKERS TO AUTOPLT AND FLT GUIDANCE SYS. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATED THAT THEIR COMPANY TECHNICAL REPRESENTATIVE FROM MCDONNELL DOUGLAS SAID WHAT HE DESCRIBED COULD NOT HAVE HAPPENED. AFTER THE TECHNICAL REPRESENTATIVE WAS BRIEFED BY TECHNICIANS, HE SAID MAYBE IT COULD HAPPEN. HOWEVER, AFTER THEY PLACED THE DEFECTIVE AUTOPLT CTL BOX INTO ANOTHER ACFT, HE FINALLY BECAME A BELIEVER. ANALYSIS REVEALED THAT THE COIL IN THE AUTOPLT'S CTL BOX HAD SHORTED OUT LOCKING THE SYS ON, EVEN THOUGH THE CIRCUIT BREAKER WAS POPPED OPEN. THE RPTR WAS ADVISED BY THE TECHNICAL REPRESENTATIVE THAT IF THEY WOULD HAVE BEEN AIRBORNE, THE CTL WHEEL WOULD NOT HAVE LOCKED UP, BUT COULD HAVE BEEN OVERRIDDEN.

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.

The autoplt came on without being turned on by the flight crew. The autoplt could not be turned off in the normal manner but only with the circuit breaker.