B757-200
Flight Crew Last 90 Days 150; Flight Crew Total 11000; Flight Crew Type 2500
Troubleshooting; Distraction; Situational Awareness; Training / Qualification; Communication Breakdown
Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Published Material / Policy
General Maintenance Action
Aircraft; Company Policy; Procedure
We received the aircraft with a defect of high oil consumption on left engine; which was previously monitored. We discussed with the Mechanic; who indicated previous consumption trends at approximately .35 quarts per hour and the inbound flight to this station spiked to approximately .7 quarts per hour and required the addition of four quarts of oil. Maintenance consulted with Maintenance Control and conducted an idle engine run for approximately five minutes with no leaks noted. The high power run; which is the preferred test; was not available due to staffing requirements. After consulting with Maintenance Control; the Mechanic seemed reluctant; but indicated the aircraft was airworthy. We were advised a rate of .6 quart per hour was a 'red line' in domestic operation and required troubleshooting before continued operation. Maintenance Control suggested the trend was below the .6 rate; so it was airworthy; even under ETOPS operations. They advised the further diagnosis would occur at the next station. My concerns were the following: 1. Departing with a known trend beyond a limit. 2. Interpretation of the trend to be applicable; even though most recent flight it spiked upward. 3. Unknown additional limitations that could affect ETOPS operation. 4. The perception of possibly operating under a very marginal condition due to economic reasons. The next station is much better equipped to manage an engine change than our current station. Because there was a lack of clear guidance regarding the oil consumption and Maintenance Control interpreted the usage 'trend;' we thought it was reasonable to proceed on the flight.  I did; however; have strong concerns about ETOPS operation with a known oil consumption issue; possible ETOPS implications; and a steep trend increase on the most recent flight. During the return flight; we observed the oil quantity on the left engine decrease a bit more rapidly than the left; although other parameters seemed normal. Upon arrival at the next station I met with the Mechanic who seemed very surprised the aircraft was operated inbound and especially ETOPS. The left engine required six quarts of oil and seemed to confirm the increasing usage trend to an unsafe level. The aircraft was taken out of service. My concern relates to the lack of clear guidance and the inability to make the safest decision possible. I felt the area subject to interpretation (usage trend) might have compelled a decision to continue the flight to the Maintenance Station; which had logistical and economic benefits (maintenance facilities). However; I don't believe this was necessarily the safest and most prudent options. From a safety stand point; I felt uncomfortable departing overwater with a known issue that appeared to be deteriorating.
A B757 arrived after an ETOPS flight having used seven quarts of oil in the left engine. Maintenance talked the crew into a return flight where the aircraft was taken out of service because of the continued high oil usage.