Flight Crew Commercial; Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Aircraft Equipment Problem Critical
Aircraft Aircraft Damaged; Flight Crew Diverted; Flight Crew Landed in Emergency Condition; Flight Crew Returned To Departure Airport
Aircraft
Preflight; taxi; and takeoff were uneventful; and followed SOP (Standard Operating Procedures). Approximately 1 hour into cruise; and under radar control with Center; we passed over a waypoint and noted our time and fuel remaining for the FPR [Flight Plan Route]. In addition; we selected the full engine display and immediately noted the left engine oil quantity indicated 10 quarts. The right engine indicated 18 (normal). The Captain and I concurred that 10 was not normal. Within 5 minutes; it indicated 9. The Captain sent an ACARS message to our Dispatcher. I suggested a SAT phone call to [Operations Control] which included Tech/Maintenance; which was done. We all concurred that if the left engine oil quantity dropped to 8; we would return to ZZZ. Within minutes; the oil quantity indicated 8. The Captain passed control of the aircraft to me and began coordinating our return; calling our [Relief Pilot] off break; advising our flight attendants; and informing the passengers. I contacted Center; requested and received immediate clearance to ZZZ. With the Captain concurrence; I reduced the left engine to idle; and descended to FL210; selecting 270 knots (avoided the need for max N1 on the right engine). The Captain continued to coordinate with Dispatch (desk 81) requesting ARFF as a precaution; as well as the required information for HAZMAT loaded in the forward cargo. The Captain resumed his role as Pilot Flying for the remainder of the flight. We [requested priority handling] with Approach Control; and made an uneventful (overweight) approach and landing on Runway XXR. Our debrief was simple: It was a textbook R9; with a few lessons learned along the way.Apparent oil leak on a high-pressure oil line (per Maintenance).
Flight Crew encountered engine oil leak and landed uneventfully.