Date: 201305
Local time of day: 0001-0600
Location: ZZZZ.ARTCC, US
Flight conditions: VMC
Light: Night
Aircraft: B767-200
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Cruise
ATC / advisory: Center ZZZZ
Aircraft component: Oil Indicating System
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: First Officer; Pilot Not Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Situational Awareness; Troubleshooting
Anomaly: Aircraft Equipment Problem Critical
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Landed As Precaution; Flight Crew Returned To Departure Airport; General Flight Cancelled / Delayed; General Maintenance Action
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: We took off. Left Engine oil quantity was 16 quarts. After leveling off at FL370 the oil quantity in the Left Engine began to decrease until it got down to 12 quarts. At that time I (pilot not flying) talked with the Relief Pilot who was the pilot flying at that point. We decided to send a message to the Dispatcher and have him confer with Maintenance Control. They responded with a message saying if it got down to 11 quarts we should be concerned with the situation. At that time we (the two First Officers) decided we had best wake up the Captain and get him in the loop. The Captain came back up and took over the pilot flying duties. We advised him of what was going on and since we were just about to go feet wet onto NATX he felt it better to not continue over the North Atlantic. We both totally agreed with him. About that time our Dispatcher attempted to call on the Satellite phone. I took the call and couldn't hear him. I called back and the reception was so bad I hung up and tried for a third time. This was better; but still horrible quality. The Dispatcher got Maintenance Control on the line with us and inquired about the situation. At that time the oil went down to 11 quarts and I advised Maintenance Control/Dispatch (we were at the 02+35 point in the flight). They agreed with our turning back off the track and wanted us to return to [our departure airport]. I told that to the Captain and he told Center that we needed to turn around and head back to [our departure airport]. We were given a turn-back to [the previous] intersection. Shortly after we made our turn to [the previous intersection] the Captain started the APU to have a backup power source in the event of a possible engine shutdown. The Captain also began looking for alternate airports to divert to; as back ups. Approximately one hour later we had a Status message come up on the EICAS. It seems now the APU had lost all of its oil as well. We then shut that down. By the time the APU was shut down we were back in CAVU weather all along the east coast of Canada and the U.S. so there were plenty of options. We had no further problems on the way back to [our departure airport]. In fact the oil never went below 11 quarts after we turned around. As we began our decent into [our departure airport] the oil quantity began to rise but only to 15 quarts. We made an uneventful landing and taxied to the gate and shutdown. After shut down the Left Engine oil went back up to 19 quarts (which is where it was prior to engine start).Maintenance came out and checked the actual oil quantity and said that it would only take about a half quart of oil till it was topped off. Therefore; it was an indication problem that possibly had to do with a stopped up vent. That would account for the change in reading with a change in temperature and altitude. I feel that we made the correct decision due to the fact that we had no other way to check the oil quantity while in flight and; in the middle of the night over the ocean is no place to find out that your 'hunch' that it was just an indication problem was wrong. [We had] false reading from the Left Engine oil quantity sending unit. ETOPS requirements. Air turn back.




Synopsis: A First Officer describes an in-flight event involving a decreasing Oil Quantity Indication on the Left Engine of a B767-200 aircraft while over water; the loss of their APU and the decision process to return to their departure airport. Oil quantity returned to normal after engine shutdown. False readings from Left Engine oil quantity sending unit.