Date: 201212
Local time of day: 0001-0600
Location: ZZZ.Airport, US
Weather elements / visibility: Rain
Light: Night
Aircraft: B737-700
Operator: Air Carrier
Operating under: Part 121
Mission: Passenger
Flight phase: Parked
Aircraft component: Oil Indicating System
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Technician
Reporter qualification: Maintenance Powerplant; Maintenance Airframe
Human factors: Workload; Troubleshooting; Fatigue
Anomaly: Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Returned To Departure Airport; General Maintenance Action
Contributing factors: Aircraft; Human Factors
Primary problem: Aircraft

Narrative: Really cold rainy and busy; heavy night and a bit tired. My B737-700 aircraft had a scheduled Maintenance Overnight Check [with] an in the [Log] book write up: 'Left Engine Oil Bypass Light illuminated prior to landing.' I checked circuit breakers (c/b) and opened the engine. Checked oil clogging Indicators; non-popped; checked oil filter bypass switch and drains clogging; transmitter electrical connectors; oil pressure sensor and Cannon plugs. I proceeded to call fellow Mechanic Y to assist me in removing the scavenge oil filter bowl; but we were unable to remove the filter bowl; so I then told Mechanic Y to proceed to run the engine and monitor the Indications in the flight deck while I visually Inspected the engine during the run for several minutes. Having found no abnormalities and no Indications of a problem and no oil bypass light came on. I then called my Supervisor and told him everything we had done and that we could not duplicate the problem and that everything checked out good; but that we were unable to remove the scavenge filter bowl and that it might break during the removal so if he could call Maintenance Control and let them know the situation and what our next step would be. Minutes later in the presence of Mechanic Y; our Supervisor told us that Maintenance Controller X; had checked history and had found none and that after all we had done and if we could not duplicate the problem; to go ahead and return Aircraft to schedule service and that I could reference his name in the Sign-off. So my Sign-off was as follows 'Ran Engine per MM for several minutes; could not duplicate; Aircraft Released to Service per Maintenance Controller X.' The next day I arrived to work; unbeknown to me; I looked at the work board for the night and saw that Aircraft X had had an air return and that it now required and engine change so seeing all this I decided to write a report. I believe that I tried to do everything I could and even got assistance from a fellow Mechanic; informed my Supervisor and Maintenance Control. I should have removed the filter bowl at all cost; even if it meant delaying the aircraft; safety always comes first. I honestly regret not doing this; a lot of safety issues; time; money; manpower and heartache could have been avoided.




Synopsis: A Line Mechanic describes his efforts to determine the cause of a Left Engine Oil By-pass Light that illuminated on a B737-700 aircraft prior to landing. Aircraft was released the following day; but required an air turnback; resulting in an engine change.