Maintenance Powerplant; Maintenance Airframe
Time Pressure; Workload
Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Maintenance; Deviation / Discrepancy - Procedural Published Material / Policy
Flight Crew Returned To Gate; Flight Crew Returned To Departure Airport; General Flight Cancelled / Delayed; General Maintenance Action
Procedure; Aircraft; Human Factors
I was performing maintenance to the #1 L/H engine; specifically to [a B737]. One of the maintenance tasks required the #1 Engine L/H outboard cowl to be open for IDG [integrated drive generator] servicing. I noticed no issues opening the cowl. Later that evening I closed and latched the same cowl. Later that night I again checked the engine oil cap and cowl security as I normally do; noticing no issues; to the best of recollection; specifically to the latching security of the #1 L/H engine fan cowl. I had only the forward hold open rod engaged; and rested the cowl against my body for a minute or two while stowing the hold open rod. I lowered the cowl door with no issues; later I moved to the ground and latched the center; forward; and then aft latch. I recall dripping oil splattering lightly on my head from the forward latch closing and using my jacket to wipe my forehead. When opening cowl I clicked the all three hooks into the handle for clearance purposes when closing. Pins; hooks and handles appeared normal. All three latches closed nicely; good torque and a nice secure click when shut. I noticed nothing wrong and was not distracted in any manner. I checked the #1 engine cowl latches more than once later that evening and latches were secure.
Maintenance Technician reported that the fan cowls on #1 engine appeared to be properly latched closed after he had performed maintenance; but the cowling apparently later opened in flight.