Narrative:

I was captain on an air carrier medium large transport en route from lga to ind. We made an intentional unscheduled landing at day due to a decrease in oil quantity in the right engine. The oil quantity showed 12-13 quarts after engine start at lga. Upon reaching cruise altitude engine readings were taken and all fluid quantities were also noted as being normal. After the readings were taken a normal scan of all INS continued. During one of the scans the first officer noticed the right oil quantity gauge reading just above 4 quarts. I called for the abnormal and checked the right oil quantity circuit breaker. It was not tripped and we performed the abnormal which said to monitor the quantity and continue if oil temperature and pressure were normal. The oil temperature and pressure were normal, so we continued. At this time were over cmh at FL310. The oil quantity seemed to be stable at 4-5 quarts for awhile. After receiving a clearance to FL280 and starting down, the oil quantity decreased to below 4, then 6 mins later to 3 quarts. At this time we were still 165 DME from ind and approximately 30 DME from day. I thought the right engine oil would be exhausted before reaching ind, so I elected to make a precautionary landing in day. The cabin crew, ATC and company were notified. The cabin crew did an excellent job securing the cabin, and the first officer informed the passenger. ATC handled us right away (no emergency was declared) and an uneventful landing was completed at day. After clearing the runway I shut the right engine down and taxied to the gate. The oil quantity gauge was still reading approximately 3 quarts. Maintenance found stud thread failure on the only stud in the gearbos that oil could drain from. My decision to land at day was based upon the following: my preference to land with 2 engines rather than 1, and the fact that day was so close that if no more oil was lost the engine might not be damaged by oil exhaustion. Supplemental information from acn 155886: don't rely on automation, use your eyes. If we had not caught the loss, the first indication would have been low oil pressure and a resultant engine shutdown.

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Original NASA ASRS Text

Title: FLT CREW OF ACR MLG DETECTS LOW ENGINE OIL INDICATION DURING NORMAL INS SCAN WHILE AT CRUISE. NOTING A CONTINUING REDUCTION IN OIL QUANTITY INDICATION, FLT CREW ELECTS TO DIVERT TO A PRECAUTIONARY LNDG.

Narrative: I WAS CAPT ON AN ACR MLG ENRTE FROM LGA TO IND. WE MADE AN INTENTIONAL UNSCHEDULED LNDG AT DAY DUE TO A DECREASE IN OIL QUANTITY IN THE RIGHT ENG. THE OIL QUANTITY SHOWED 12-13 QUARTS AFTER ENG START AT LGA. UPON REACHING CRUISE ALT ENG READINGS WERE TAKEN AND ALL FLUID QUANTITIES WERE ALSO NOTED AS BEING NORMAL. AFTER THE READINGS WERE TAKEN A NORMAL SCAN OF ALL INS CONTINUED. DURING ONE OF THE SCANS THE F/O NOTICED THE RIGHT OIL QUANTITY GAUGE READING JUST ABOVE 4 QUARTS. I CALLED FOR THE ABNORMAL AND CHKED THE RIGHT OIL QUANTITY CB. IT WAS NOT TRIPPED AND WE PERFORMED THE ABNORMAL WHICH SAID TO MONITOR THE QUANTITY AND CONTINUE IF OIL TEMPERATURE AND PRESSURE WERE NORMAL. THE OIL TEMPERATURE AND PRESSURE WERE NORMAL, SO WE CONTINUED. AT THIS TIME WERE OVER CMH AT FL310. THE OIL QUANTITY SEEMED TO BE STABLE AT 4-5 QUARTS FOR AWHILE. AFTER RECEIVING A CLRNC TO FL280 AND STARTING DOWN, THE OIL QUANTITY DECREASED TO BELOW 4, THEN 6 MINS LATER TO 3 QUARTS. AT THIS TIME WE WERE STILL 165 DME FROM IND AND APPROX 30 DME FROM DAY. I THOUGHT THE RIGHT ENG OIL WOULD BE EXHAUSTED BEFORE REACHING IND, SO I ELECTED TO MAKE A PRECAUTIONARY LNDG IN DAY. THE CABIN CREW, ATC AND COMPANY WERE NOTIFIED. THE CABIN CREW DID AN EXCELLENT JOB SECURING THE CABIN, AND THE F/O INFORMED THE PAX. ATC HANDLED US RIGHT AWAY (NO EMER WAS DECLARED) AND AN UNEVENTFUL LNDG WAS COMPLETED AT DAY. AFTER CLRING THE RWY I SHUT THE RIGHT ENG DOWN AND TAXIED TO THE GATE. THE OIL QUANTITY GAUGE WAS STILL READING APPROX 3 QUARTS. MAINT FOUND STUD THREAD FAILURE ON THE ONLY STUD IN THE GEARBOS THAT OIL COULD DRAIN FROM. MY DECISION TO LAND AT DAY WAS BASED UPON THE FOLLOWING: MY PREFERENCE TO LAND WITH 2 ENGS RATHER THAN 1, AND THE FACT THAT DAY WAS SO CLOSE THAT IF NO MORE OIL WAS LOST THE ENG MIGHT NOT BE DAMAGED BY OIL EXHAUSTION. SUPPLEMENTAL INFO FROM ACN 155886: DON'T RELY ON AUTOMATION, USE YOUR EYES. IF WE HAD NOT CAUGHT THE LOSS, THE FIRST INDICATION WOULD HAVE BEEN LOW OIL PRESSURE AND A RESULTANT ENG SHUTDOWN.

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.