Incident Overview
The aircraft experienced a cowl door separation during takeoff. As the airplane accelerated through rotation speed (Vr), the outboard, forward cowl door on the left (number 1) engine detached from the engine nacelle and struck the horizontal stabilizer. The takeoff was the first flight following the aircraft's return to service.
Pilot and Passenger Reports
The pilot reported an abnormal vibration as the aircraft accelerated through Vr. Several passengers pressed their flight attendant call buttons and alerted the flight attendants, who subsequently informed the cockpit crew about the cowling separation. The flight crew executed a return landing at the departure airport, which was unremarkable.
Post-Event Inspection
Following the incident, maintenance personnel examined the cowl door latching system. The over-center type latches on the inboard door were found in the latched position with intact and undamaged hooks. The corresponding latch receptacles on the outboard door were visibly undamaged. The latches were painted red.
Maintenance Context
A Remain Over Night (RON) check had been performed during hours of darkness the previous night. This check required that the cowling doors be opened. The following morning, the aircraft was handed over from the maintenance graveyard shift to the day shift. At that time, maintenance items remained to be completed in areas of the aircraft other than the number 1 engine.