Narrative:

Last night I worked an MEL on the autopilot on aircraft X. When I initially tested the autoplt; I found the aileron servo failed the aileron servo test 05. It came up with a cw and ccw command fail; the aileron servo also it would not drive the control wheel. I changed out the aileron servo. Though replacing the aileron servo does not require an autoplt yaw damper engagement transfer test per the manual; after replacing the servo and powering up the aircraft; I pulled the wow C/B's; selected a heading and engaged the autopilot. (These are procedures in the autoplt yaw damper engagement transfer test.) when I was moving the heading bug ccw the control wheel moved cw; and vice versa. (I did not do this full test at that time as it is not the return to service test for an aileron servo replacement.) at first I was puzzled by this; and thought the controls were moving backwards; then I grabbed the manual and did the required aileron servo test 05; and it passed the test. (This is the test the manual calls out for an aileron servo replacement.) after it passed the test; I thought to myself; I guess I was wrong about the direction; and thought; just to make sure; I wanted to go back and verify with aircraft Y. We were very busy and I got side tracked. At xa:50 am I remembered that I wanted to double check this with aircraft Y; I ran out and found to my horror that the aileron servo is moving the control wheel on aircraft X backwards to the heading bug commands. The aircraft was 10 minutes from departure. I explained my concern to the flight crew and confirmed the control wheel was indeed turning backwards to heading bug commands. I called maintenance control and we agreed that we could put the autopilot on MEL. I don't believe that I made any mistakes in the installation; as the cables were transferred one at a time; and they are threaded differently between the top and bottom cables. I suspect that the aileron servo is wired backwards. I made a recommendation to maintenance control that this aircraft be taken off line as soon as possible. If a flight crew attempts to engage the autoplt as it is; it will put the aircraft into a roll.callback conversation with reporter revealed the following information: reporter stated that on the dhc-8-200 the aileron servo is located under and forward of the co-pilot's seat. The autoflt controller interface unit is part of this system. Although the replaced aileron servo passed the aileron servo test 05; he just had a gut 'feeling' something was not right; especially when earlier he had performed the not required autoplt-yaw damper engagement transfer test and noticed that; when he was moving the heading bug counter-clockwise (ccw); the control wheel moved clockwise (cw). Reporter also stated he was one; of only two; mechanics working that night. But he was able to get back to the aircraft just prior to departure. That's when he and the flight crew realized the seriousness of the situation. Reporter stated the mfg tore down the aileron servo unit and found the servo wired backwards. As a result; the mfg recalled the aileron servo units and found 13 more wired backwards. Reporter does not know if they were all from his carrier. He does not know if the dhc-8-400 models have the same aileron servo unit. He believes his company will be making changes to require the autopilot-yaw damper engagement transfer test be part of the aileron servo test. Reporter is hoping dehavilland will also make changes to their manuals.

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Original NASA ASRS Text

Title: A DEHAVILLAND DHC-8 MAINT TECH; WHILE WORKING AN MEL ITEM; NOTICES DURING THE AUTOPILOT-YAW DAMPER ENGAGEMENT TRANSFER TEST THAT THE CONTROL WHEEL MOVED IN THE OPPOSITE DIRECTION OF THE HEADING INPUT.

Narrative: LAST NIGHT I WORKED AN MEL ON THE AUTOPILOT ON ACFT X. WHEN I INITIALLY TESTED THE AUTOPLT; I FOUND THE AILERON SERVO FAILED THE AILERON SERVO TEST 05. IT CAME UP WITH A CW AND CCW COMMAND FAIL; THE AILERON SERVO ALSO IT WOULD NOT DRIVE THE CONTROL WHEEL. I CHANGED OUT THE AILERON SERVO. THOUGH REPLACING THE AILERON SERVO DOES NOT REQUIRE AN AUTOPLT YAW DAMPER ENGAGEMENT TRANSFER TEST PER THE MANUAL; AFTER REPLACING THE SERVO AND POWERING UP THE ACFT; I PULLED THE WOW C/B'S; SELECTED A HEADING AND ENGAGED THE AUTOPILOT. (THESE ARE PROCEDURES IN THE AUTOPLT YAW DAMPER ENGAGEMENT TRANSFER TEST.) WHEN I WAS MOVING THE HEADING BUG CCW THE CONTROL WHEEL MOVED CW; AND VICE VERSA. (I DID NOT DO THIS FULL TEST AT THAT TIME AS IT IS NOT THE RETURN TO SERVICE TEST FOR AN AILERON SERVO REPLACEMENT.) AT FIRST I WAS PUZZLED BY THIS; AND THOUGHT THE CONTROLS WERE MOVING BACKWARDS; THEN I GRABBED THE MANUAL AND DID THE REQUIRED AILERON SERVO TEST 05; AND IT PASSED THE TEST. (THIS IS THE TEST THE MANUAL CALLS OUT FOR AN AILERON SERVO REPLACEMENT.) AFTER IT PASSED THE TEST; I THOUGHT TO MYSELF; I GUESS I WAS WRONG ABOUT THE DIRECTION; AND THOUGHT; JUST TO MAKE SURE; I WANTED TO GO BACK AND VERIFY WITH ACFT Y. WE WERE VERY BUSY AND I GOT SIDE TRACKED. AT XA:50 AM I REMEMBERED THAT I WANTED TO DOUBLE CHECK THIS WITH ACFT Y; I RAN OUT AND FOUND TO MY HORROR THAT THE AILERON SERVO IS MOVING THE CONTROL WHEEL ON ACFT X BACKWARDS TO THE HEADING BUG COMMANDS. THE ACFT WAS 10 MINUTES FROM DEPARTURE. I EXPLAINED MY CONCERN TO THE FLIGHT CREW AND CONFIRMED THE CONTROL WHEEL WAS INDEED TURNING BACKWARDS TO HEADING BUG COMMANDS. I CALLED MAINT CONTROL AND WE AGREED THAT WE COULD PUT THE AUTOPILOT ON MEL. I DON'T BELIEVE THAT I MADE ANY MISTAKES IN THE INSTALLATION; AS THE CABLES WERE TRANSFERRED ONE AT A TIME; AND THEY ARE THREADED DIFFERENTLY BETWEEN THE TOP AND BOTTOM CABLES. I SUSPECT THAT THE AILERON SERVO IS WIRED BACKWARDS. I MADE A RECOMMENDATION TO MAINT CTL THAT THIS ACFT BE TAKEN OFF LINE ASAP. IF A FLIGHT CREW ATTEMPTS TO ENGAGE THE AUTOPLT AS IT IS; IT WILL PUT THE ACFT INTO A ROLL.CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: REPORTER STATED THAT ON THE DHC-8-200 THE AILERON SERVO IS LOCATED UNDER AND FORWARD OF THE CO-PILOT'S SEAT. THE AUTOFLT CONTROLLER INTERFACE UNIT IS PART OF THIS SYSTEM. ALTHOUGH THE REPLACED AILERON SERVO PASSED THE AILERON SERVO TEST 05; HE JUST HAD A GUT 'FEELING' SOMETHING WAS NOT RIGHT; ESPECIALLY WHEN EARLIER HE HAD PERFORMED THE NOT REQUIRED AUTOPLT-YAW DAMPER ENGAGEMENT TRANSFER TEST AND NOTICED THAT; WHEN HE WAS MOVING THE HEADING BUG COUNTER-CLOCKWISE (CCW); THE CONTROL WHEEL MOVED CLOCKWISE (CW). REPORTER ALSO STATED HE WAS ONE; OF ONLY TWO; MECHANICS WORKING THAT NIGHT. BUT HE WAS ABLE TO GET BACK TO THE ACFT JUST PRIOR TO DEPARTURE. THAT'S WHEN HE AND THE FLIGHT CREW REALIZED THE SERIOUSNESS OF THE SITUATION. REPORTER STATED THE MFG TORE DOWN THE AILERON SERVO UNIT AND FOUND THE SERVO WIRED BACKWARDS. AS A RESULT; THE MFG RECALLED THE AILERON SERVO UNITS AND FOUND 13 MORE WIRED BACKWARDS. REPORTER DOES NOT KNOW IF THEY WERE ALL FROM HIS CARRIER. HE DOES NOT KNOW IF THE DHC-8-400 MODELS HAVE THE SAME AILERON SERVO UNIT. HE BELIEVES HIS COMPANY WILL BE MAKING CHANGES TO REQUIRE THE AUTOPILOT-YAW DAMPER ENGAGEMENT TRANSFER TEST BE PART OF THE AILERON SERVO TEST. REPORTER IS HOPING DEHAVILLAND WILL ALSO MAKE CHANGES TO THEIR MANUALS.

Data retrieved from NASA's ASRS site as of May 2009 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.