Narrative:

How the problem arose: an engine over-torque resulting in daily inspections of the rgb chip detector. Inspections were to be taken place out on the ramp when the aircraft returned at the end of the day; for up to 25 flight hours. To make the inspection more efficient; we were removing the rgb scavenge oil strainer plug to drain the oil in the rgb; or else; when we would pull the chip detector oil would be all over the aircraft engine cowling and ramp. Evidently it was the oil strainer plug that had shifted during flight to cause a loss in oil pressure. It is physically impossible to install the oil strainer incorrectly because it will not join the housing properly and the bolt will not go flush with the rgb to secure it. I had been asked by my superiors if we did a leak check and I responded no. The maintenance manual does not instruct to do so. I believe that because the strainer plug was getting removed every day; that the pre-formed o-ring may have worn just enough to cause the plug to shift during flight. Once again the manual does not say to replace the o-ring. Considering the aircraft was in-flight for over 1/2 hour before the drop in oil pressure brings me to this conclusion. To prevent a recurrence; the o-ring should be replaced every time it is removed; and a leak check is mandatory.

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Original NASA ASRS Text

Title: A MECHANIC WORKING A CESSNA 208 CARAVAN DESCRIBES THE CHAIN OF EVENTS THAT MAY HAVE CONTRIBUTED TO THE LOSS OF OIL PRESSURE IN THE PT-6 ENGINE REDUCTION GEARBOX.

Narrative: HOW THE PROB AROSE: AN ENG OVER-TORQUE RESULTING IN DAILY INSPECTIONS OF THE RGB CHIP DETECTOR. INSPECTIONS WERE TO BE TAKEN PLACE OUT ON THE RAMP WHEN THE ACFT RETURNED AT THE END OF THE DAY; FOR UP TO 25 FLT HRS. TO MAKE THE INSPECTION MORE EFFICIENT; WE WERE REMOVING THE RGB SCAVENGE OIL STRAINER PLUG TO DRAIN THE OIL IN THE RGB; OR ELSE; WHEN WE WOULD PULL THE CHIP DETECTOR OIL WOULD BE ALL OVER THE ACFT ENG COWLING AND RAMP. EVIDENTLY IT WAS THE OIL STRAINER PLUG THAT HAD SHIFTED DURING FLT TO CAUSE A LOSS IN OIL PRESSURE. IT IS PHYSICALLY IMPOSSIBLE TO INSTALL THE OIL STRAINER INCORRECTLY BECAUSE IT WILL NOT JOIN THE HOUSING PROPERLY AND THE BOLT WILL NOT GO FLUSH WITH THE RGB TO SECURE IT. I HAD BEEN ASKED BY MY SUPERIORS IF WE DID A LEAK CHK AND I RESPONDED NO. THE MAINT MANUAL DOES NOT INSTRUCT TO DO SO. I BELIEVE THAT BECAUSE THE STRAINER PLUG WAS GETTING REMOVED EVERY DAY; THAT THE PRE-FORMED O-RING MAY HAVE WORN JUST ENOUGH TO CAUSE THE PLUG TO SHIFT DURING FLT. ONCE AGAIN THE MANUAL DOES NOT SAY TO REPLACE THE O-RING. CONSIDERING THE ACFT WAS INFLT FOR OVER 1/2 HR BEFORE THE DROP IN OIL PRESSURE BRINGS ME TO THIS CONCLUSION. TO PREVENT A RECURRENCE; THE O-RING SHOULD BE REPLACED EVERY TIME IT IS REMOVED; AND A LEAK CHK IS MANDATORY.

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