Narrative:

On 9/tue/89 at mid pm, I was installing an APU on a line aircraft that had been removed for maintenance. The installation was done in accordance with the aircraft maintenance manual. The maintenance manual called for a torque of 800-1140 inches/pound for the rear attach bolts and 260-320 inches/pound for the forward attach bolts. The torque wrenches supplied for our use are the beam type and the scales read in ft/pound. Therefore, the inches/pound reading had to be converted to ft/pound readings. In calculating 1000 inches/pound to ft/pound, I miscalculated--and even though I rechked my mathematics, I failed to catch my mistake. I torqued the rear mount bolts to 42 ft/pound or 504 inches/pound. This is about 300 inches/pound lower than the lowest called out torque value. The forward mount bolts were torqued to the proper value of 300 inches/pound, or 25 ft/pound. The next morning, 9/wed/89, at am hours, I realized that I had made the mistake. I immediately called the maintenance supervisor on duty. I idented myself and explained the situation to him. Unfortunately the airplane had already been dispatched on a flight. He felt that it was not an unsafe condition, but suggested that the bolts be torqued to the proper valve when the aircraft returns. It's my suggestion that the company policy be changed--that any time a critical torque value is called out, such as APU mount bolts, engine mount bolts or flight control mount bolts, that the quality control inspector be present to witness the torque applied. I feel that another set of eyes might have prevented this situation, because working on line aircraft you are under pressure, trying to meet the flight schedule. Also I have noticed that metric figures are appearing in the maintenance manuals. It's very confusing to be working with 3 sets of figures. It would be very helpful if everything was standardized.

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Original NASA ASRS Text

Title: INCORRECT MATHEMATICAL TRANSPOSITION REGARDING TORQUE REQUIRED RESULTS IN A MAINTENANCE DISCREPANCY DURING APU INSTALLATION IN ACR ACFT.

Narrative: ON 9/TUE/89 AT MID PM, I WAS INSTALLING AN APU ON A LINE ACFT THAT HAD BEEN REMOVED FOR MAINT. THE INSTALLATION WAS DONE IN ACCORDANCE WITH THE ACFT MAINT MANUAL. THE MAINT MANUAL CALLED FOR A TORQUE OF 800-1140 INCHES/LB FOR THE REAR ATTACH BOLTS AND 260-320 INCHES/LB FOR THE FORWARD ATTACH BOLTS. THE TORQUE WRENCHES SUPPLIED FOR OUR USE ARE THE BEAM TYPE AND THE SCALES READ IN FT/LB. THEREFORE, THE INCHES/LB READING HAD TO BE CONVERTED TO FT/LB READINGS. IN CALCULATING 1000 INCHES/LB TO FT/LB, I MISCALCULATED--AND EVEN THOUGH I RECHKED MY MATHEMATICS, I FAILED TO CATCH MY MISTAKE. I TORQUED THE REAR MOUNT BOLTS TO 42 FT/LB OR 504 INCHES/LB. THIS IS ABOUT 300 INCHES/LB LOWER THAN THE LOWEST CALLED OUT TORQUE VALUE. THE FORWARD MOUNT BOLTS WERE TORQUED TO THE PROPER VALUE OF 300 INCHES/LB, OR 25 FT/LB. THE NEXT MORNING, 9/WED/89, AT AM HRS, I REALIZED THAT I HAD MADE THE MISTAKE. I IMMEDIATELY CALLED THE MAINT SUPVR ON DUTY. I IDENTED MYSELF AND EXPLAINED THE SITUATION TO HIM. UNFORTUNATELY THE AIRPLANE HAD ALREADY BEEN DISPATCHED ON A FLT. HE FELT THAT IT WAS NOT AN UNSAFE CONDITION, BUT SUGGESTED THAT THE BOLTS BE TORQUED TO THE PROPER VALVE WHEN THE ACFT RETURNS. IT'S MY SUGGESTION THAT THE COMPANY POLICY BE CHANGED--THAT ANY TIME A CRITICAL TORQUE VALUE IS CALLED OUT, SUCH AS APU MOUNT BOLTS, ENG MOUNT BOLTS OR FLT CONTROL MOUNT BOLTS, THAT THE QUALITY CTL INSPECTOR BE PRESENT TO WITNESS THE TORQUE APPLIED. I FEEL THAT ANOTHER SET OF EYES MIGHT HAVE PREVENTED THIS SITUATION, BECAUSE WORKING ON LINE ACFT YOU ARE UNDER PRESSURE, TRYING TO MEET THE FLT SCHEDULE. ALSO I HAVE NOTICED THAT METRIC FIGURES ARE APPEARING IN THE MAINT MANUALS. IT'S VERY CONFUSING TO BE WORKING WITH 3 SETS OF FIGURES. IT WOULD BE VERY HELPFUL IF EVERYTHING WAS STANDARDIZED.

Data retrieved from NASA's ASRS site as of August 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.