Narrative:

While replacing an autofeather arming potentiometer on #1 power lever on aircraft, it was found that the drive gear part number AP240-36 would need replacement since its retaining set screw was stripped out and could not be removed. The potentiometer was found to be faulty after troubleshooting and trying to adjust it reference DHC8-200 amm 1-82-2ER (engine rigging manual). No maintenance procedure was found for potentiometer replacement -- only adjustment of and operations check of the autofeather system in air traffic area's 76 and 61 of the amm. The question that arose while replacing the gear was if it would need a preload when installed. Gear part number AP24Q-36 is called an anti backlash gear and is 2 gears mounted on 1 hub with springs in the center, allowing the gears to rotate in opposite direction and be spring loaded back towards each other. The gear was installed on the potentiometer shaft and installed with no preload. The potentiometer adjusted, engines ran, and autofeather operated normally, arming at the prescribed parameters. Later that morning, after the aircraft had departed, I discussed the possibility of a preload being needed with our base manager and the lack of no manual procedures for replacing the potentiometer. He faxed an inquiry on this matter. Upon arriving to work on jul/wed/02, I found a copy of the inquiry reply in my mail box. The information showed that preload on the anti backlash gear was required at installation. This information was derived from the manufacturer's production installation drawings (drawing #87610388). Also, it was noted on the facsimile cover sheet that the information would be forwarded to the appropriate department so that it could be added to the amm.

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Original NASA ASRS Text

Title: A DEHAVILLAND 8-100 AUTOFEATHER POTENTIOMETER AND DRIVE GEARS WERE INSTALLED WITH NO MANUAL REF TO DRIVE GEAR PRELOAD.

Narrative: WHILE REPLACING AN AUTOFEATHER ARMING POTENTIOMETER ON #1 PWR LEVER ON ACFT, IT WAS FOUND THAT THE DRIVE GEAR PART NUMBER AP240-36 WOULD NEED REPLACEMENT SINCE ITS RETAINING SET SCREW WAS STRIPPED OUT AND COULD NOT BE REMOVED. THE POTENTIOMETER WAS FOUND TO BE FAULTY AFTER TROUBLESHOOTING AND TRYING TO ADJUST IT REF DHC8-200 AMM 1-82-2ER (ENG RIGGING MANUAL). NO MAINT PROC WAS FOUND FOR POTENTIOMETER REPLACEMENT -- ONLY ADJUSTMENT OF AND OPS CHK OF THE AUTOFEATHER SYS IN ATA'S 76 AND 61 OF THE AMM. THE QUESTION THAT AROSE WHILE REPLACING THE GEAR WAS IF IT WOULD NEED A PRELOAD WHEN INSTALLED. GEAR PART NUMBER AP24Q-36 IS CALLED AN ANTI BACKLASH GEAR AND IS 2 GEARS MOUNTED ON 1 HUB WITH SPRINGS IN THE CTR, ALLOWING THE GEARS TO ROTATE IN OPPOSITE DIRECTION AND BE SPRING LOADED BACK TOWARDS EACH OTHER. THE GEAR WAS INSTALLED ON THE POTENTIOMETER SHAFT AND INSTALLED WITH NO PRELOAD. THE POTENTIOMETER ADJUSTED, ENGS RAN, AND AUTOFEATHER OPERATED NORMALLY, ARMING AT THE PRESCRIBED PARAMETERS. LATER THAT MORNING, AFTER THE ACFT HAD DEPARTED, I DISCUSSED THE POSSIBILITY OF A PRELOAD BEING NEEDED WITH OUR BASE MGR AND THE LACK OF NO MANUAL PROCS FOR REPLACING THE POTENTIOMETER. HE FAXED AN INQUIRY ON THIS MATTER. UPON ARRIVING TO WORK ON JUL/WED/02, I FOUND A COPY OF THE INQUIRY REPLY IN MY MAIL BOX. THE INFO SHOWED THAT PRELOAD ON THE ANTI BACKLASH GEAR WAS REQUIRED AT INSTALLATION. THIS INFO WAS DERIVED FROM THE MANUFACTURER'S PRODUCTION INSTALLATION DRAWINGS (DRAWING #87610388). ALSO, IT WAS NOTED ON THE FAX COVER SHEET THAT THE INFO WOULD BE FORWARDED TO THE APPROPRIATE DEPT SO THAT IT COULD BE ADDED TO THE AMM.

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.