Date: 201606
Local time of day: 1201-1800
Location: ZZZ.Airport, US
Light: Daylight
Aircraft: SF 340B
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Initial Climb
Airspace: Class C ZZZ
ATC / advisory: Tower ZZZ
Aircraft component: Nacelle/Pylon Skin
Component problem: Improperly Operated
Reporter organization: Air Carrier
Reporter function: Technician
Reporter qualification: Maintenance Airframe; Maintenance Powerplant
Anomaly: Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Maintenance
Detector: Person Flight Crew
When detected: In-flight
Result: Aircraft Aircraft Damaged; Flight Crew Returned To Departure Airport
Contributing factors: Aircraft; Procedure
Primary problem: Procedure

Narrative: In between flights I performed the daily oil check on the RH engine. This is done by accessing the outboard door on the #2 engine. I opened the door and installed the stay arm in the front only to keep it open while I serviced the engine with oil. On the completion of servicing I reinstalled the oil cap and closed the door and latched it closed. After returning the spent oil can to the truck I came back out to the airplane to grab the ladder and I reopened the door by accessing the latches and double checked that the oil cap was installed properly. I then closed the door a second time and pushed against it to ensure the latches were seated properly in the adjacent alignment holes. I then returned the ladder and came in to the office to finish the paperwork and record the amount of oil. This was at approximately XA:55. The plane departed at approximately XB:20 and shortly after takeoff they contacted the ops phone and said they would be turning back and would need maintenance when they landed. I immediately went outside and waited. I witnessed them landing and saw the nacelle door open and immediately called my first line supervisor. This call was made at approximately XB:42 even before the plane finished taxiing back to the gate. When it got to the gate I immediately recognized that there was damage to the nacelle door and that both locking (latch) hinges were in the locked position. It is my understanding that the FA saw the nacelle open after takeoff but before 200ft and immediately called the pilots to notify them. I am baffled as to why this event occurred. I am positive that after having the second thought to double check the oil cap that I properly seated and latched the nacelle door. Before [my Supervisor] arrived the FO for that flight relayed to me that that was his first flight of that day and that he did a thorough walk around which included verifying that both engine nacelles were latched seated and closed. What he described as the 'first flight of the day' walk around. I am not sure what the crew did but I think they executed a pattern and returned to the field without declaring an emergency. It may be a poor description of how I feel; but again I am baffled because I am positive the nacelle door was latched and closed properly. I don't even know how to speculate as to how it came open in flight other than the center of the nacelle possibly buckled failed and pulled it out of the alignment holes. I want to add that if I wasn't positive about latching the nacelle door that I would own it immediately and say so.




Synopsis: A maintenance Technician reported that after accomplishing an oil service on a Saab SF340B; the aircraft departed then returned to the airport due to the oil service door coming open in flight.