Narrative:

All checks completed with normal indications. We boarded and pushed on time. We started both engines prior to taxi. All engine indications were normal. The engine oil quantities were normal (left engine - 20 units; right engine - 21 units). [We had a] normal taxi and takeoff; normal departure and climb. After 14 to 17 minutes and passing 18;000 ft; I scanned the engine instruments again and noticed the left engine oil quantity was indicating 13 units while the right engine oil quantity was indicating 20 units. I pointed out the low oil quantity indication to my first officer and that we needed to watch that engine for a possible engine oil leak. As we continued to climb; the left engine quantity dropped to 12 units. We leveled at 35;000 ft as assigned and the left engine oil quantity dropped to 11 units. At that point we told center that we had a low engine oil indication and that we would be returning. The controller gave us an initial turn to the left and the left engine oil quantity changed to 10 units. We declared an emergency and reduced the left engine RPM to idle. We coordinated with all agencies and were given a vector back and a decent. I briefed and flew a single engine approach with the left engine in idle. I started the APU on final approach and made a good landing below max landing weight. At touch down; the left engine oil quantity indicated 8 units. I cleared the runway; the oil indication was 7 units and I shut down the left engine and taxied to the gate. At shutdown; the engine oil pressure and temperature on both engines was normal. The emergency was terminated.I felt that we must have had a large leak in the left engine oil system. The lead mechanic that met the aircraft came to the cockpit. He immediately stated that they had found the problem. He stated that the left engine oil filler cap had not been placed on the engine oil filler pipe.

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Original NASA ASRS Text

Title: B757 Captain experiences oil loss from the left engine during climb and elects to return before all the oil is depleted. Maintenance discovers that the oil tank filler cap has been left off.

Narrative: All checks completed with normal indications. We boarded and pushed on time. We started both engines prior to taxi. All engine indications were normal. The engine oil quantities were normal (left engine - 20 units; right engine - 21 units). [We had a] normal taxi and takeoff; normal departure and climb. After 14 to 17 minutes and passing 18;000 FT; I scanned the engine instruments again and noticed the left engine oil quantity was indicating 13 units while the right engine oil quantity was indicating 20 units. I pointed out the low oil quantity indication to my First Officer and that we needed to watch that engine for a possible engine oil leak. As we continued to climb; the left engine quantity dropped to 12 units. We leveled at 35;000 FT as assigned and the left engine oil quantity dropped to 11 units. At that point we told Center that we had a low engine oil indication and that we would be returning. The Controller gave us an initial turn to the left and the left engine oil quantity changed to 10 units. We declared an emergency and reduced the left engine RPM to idle. We coordinated with all agencies and were given a vector back and a decent. I briefed and flew a single engine approach with the left engine in idle. I started the APU on final approach and made a good landing below max landing weight. At touch down; the left engine oil quantity indicated 8 units. I cleared the runway; the oil indication was 7 units and I shut down the left engine and taxied to the gate. At shutdown; the engine oil pressure and temperature on both engines was normal. The emergency was terminated.I felt that we must have had a large leak in the left engine oil system. The Lead Mechanic that met the aircraft came to the cockpit. He immediately stated that they had found the problem. He stated that the left engine oil filler cap had not been placed on the engine oil filler pipe.

Data retrieved from NASA's ASRS site as of April 2012 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.