Narrative:

On aug/xa/99, engineering department notified us that the cable tensiometers may have been incorrectly calibrated by the air carrier instrument shop since 1993. These tensiometers are used to calibrate air carrier aircraft. I was the lead mechanic for some of the mechanics doing this calibration. Engineering and quality assurance are presently investigating this issue. Management has been aware from the beginning of the problem. Callback conversation with reporter revealed the following information: the reporter stated that at the present time, all of the tensiometers are now being sent to an outside vendor for calibration. The reporter said the shop was using a hydraulic ram type of unit to put tension on a cable and the tensiometer was calibrated using the hydraulic ram pressure indicator. The reporter said the hydraulic ram leaked and the pressure indicator was inaccurate. The reporter stated that unknown to the engineering department boeing changed the military specification using the hydraulic ram method of calibrating the tensiometers. The reporter said the vendors are still using a hanging weight type of instrument which still can induce errors. The reporter said the new equipment has been ordered and the maintenance manual updated. Callback conversation with reporter acn 445055 revealed the following information: the reporter stated the present tensiometers are being calibrated by an outside vendor who is using the hanging weight method that is maybe within 10 pound accuracy. The reporter said the hydraulic ram method is the most accurate way to calibrate these instruments but the unit used in the shop is worn out and leaking. The reporter stated the new equipment now on order has a 1 pound margin of error where the old unit was +/-10 pounds. The reporter stated the biggest problem in getting action over all these yrs was the shop supervisor who wanted to make no corrections. The reporter said some mechanics who report from the overhaul docks that after using a tensiometer the cables are drooping and are then rigged to what feels right. The reporter said the manual for calibration has not been revised since 1980.

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Original NASA ASRS Text

Title: A NUMBER OF B737 AND DC9 ACFT OVERHAULED BY ACR MAINT HAD FLT CTL CABLES TENSIONED WITH IMPROPERLY CALIBRATED INSTS.

Narrative: ON AUG/XA/99, ENGINEERING DEPT NOTIFIED US THAT THE CABLE TENSIOMETERS MAY HAVE BEEN INCORRECTLY CALIBRATED BY THE ACR INST SHOP SINCE 1993. THESE TENSIOMETERS ARE USED TO CALIBRATE ACR ACFT. I WAS THE LEAD MECH FOR SOME OF THE MECHS DOING THIS CALIBRATION. ENGINEERING AND QUALITY ASSURANCE ARE PRESENTLY INVESTIGATING THIS ISSUE. MGMNT HAS BEEN AWARE FROM THE BEGINNING OF THE PROB. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THAT AT THE PRESENT TIME, ALL OF THE TENSIOMETERS ARE NOW BEING SENT TO AN OUTSIDE VENDOR FOR CALIBRATION. THE RPTR SAID THE SHOP WAS USING A HYD RAM TYPE OF UNIT TO PUT TENSION ON A CABLE AND THE TENSIOMETER WAS CALIBRATED USING THE HYD RAM PRESSURE INDICATOR. THE RPTR SAID THE HYD RAM LEAKED AND THE PRESSURE INDICATOR WAS INACCURATE. THE RPTR STATED THAT UNKNOWN TO THE ENGINEERING DEPT BOEING CHANGED THE MIL SPEC USING THE HYD RAM METHOD OF CALIBRATING THE TENSIOMETERS. THE RPTR SAID THE VENDORS ARE STILL USING A HANGING WT TYPE OF INST WHICH STILL CAN INDUCE ERRORS. THE RPTR SAID THE NEW EQUIP HAS BEEN ORDERED AND THE MAINT MANUAL UPDATED. CALLBACK CONVERSATION WITH RPTR ACN 445055 REVEALED THE FOLLOWING INFO: THE RPTR STATED THE PRESENT TENSIOMETERS ARE BEING CALIBRATED BY AN OUTSIDE VENDOR WHO IS USING THE HANGING WT METHOD THAT IS MAYBE WITHIN 10 LB ACCURACY. THE RPTR SAID THE HYD RAM METHOD IS THE MOST ACCURATE WAY TO CALIBRATE THESE INSTS BUT THE UNIT USED IN THE SHOP IS WORN OUT AND LEAKING. THE RPTR STATED THE NEW EQUIP NOW ON ORDER HAS A 1 LB MARGIN OF ERROR WHERE THE OLD UNIT WAS +/-10 LBS. THE RPTR STATED THE BIGGEST PROB IN GETTING ACTION OVER ALL THESE YRS WAS THE SHOP SUPVR WHO WANTED TO MAKE NO CORRECTIONS. THE RPTR SAID SOME MECHS WHO RPT FROM THE OVERHAUL DOCKS THAT AFTER USING A TENSIOMETER THE CABLES ARE DROOPING AND ARE THEN RIGGED TO WHAT FEELS RIGHT. THE RPTR SAID THE MANUAL FOR CALIBRATION HAS NOT BEEN REVISED SINCE 1980.

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.