Following a day of routine flying operations on 13 December, the DHC-8-400 aircraft night-stopped at Manchester Airport, U.K. and was parked on a remote stand. The operator’s contracted maintenance organisation completed a routine daily check on the aircraft that evening. This included checking the oil content of the No 1 engine, accessed by opening the outboard main access panel on the engine nacelle. The check was concluded by approximately 21:15 hrs, with the aircraft scheduled to return to service for a 06:10 hrs departure the next morning. The aircraft Technical Log entry for the daily check was signed by the engineer at 00:10 hrs. The operating flight crew arrived at the aircraft at 05:30 hrs and began their normal pre-flight checks. At 05:50 hrs, in accordance with company procedures for the first flight of the day, the commander conducted the pre-departure inspection. As it was still dark, he used a torch to supplement the ambient airport lighting during his inspection. He did not identify any issues with the aircraft and the crew continued with their normal departure routine. The ground crew, who were responsible for pushing the aircraft back off the stand, subsequently arrived and conducted their own walkaround check of the aircraft, also identifying nothing of note. The aircraft was dispatched on time and taxied to runway 23R for takeoff. At approximately 06:24 hrs the aircraft commenced its takeoff roll and then continued on an apparently uneventful flight to Hannover, Germany, landing there at 07:52 hrs. After the aircraft had parked on the stand and the passengers had disembarked, the ground crew informed the cabin crew that a panel was missing from the No 1 engine. The message was relayed to the flight crew, who inspected the aircraft prior to contacting the operator’s maintenance control department. The operator informed Manchester Airport operations staff at 08:36 hrs, who then conducted an inspection of runway 23R. The panel was recovered from a grass area to the side of the runway, approximately 440 m from the runway threshold. Sections of the panel hold-open strut were also recovered from the runway and adjacent paved areas in the same vicinity. Conclusion: Following overnight maintenance work, the outboard engine main access panel on the No 1 engine was not securely closed by the engineer, due to the latch bolts not engaging in the nacelle receiving features when the latches were closed. Contributory factors may have been a slight mismatch in the closure of the panel and the technique used by the engineer of closing the top latches first. The resulting gap around the panel was not identified by the engineer completing the task, possibly as a consequence of the angle at which he was looking down on the closed panel and the lack of contrast of the shadow cast on the dark coloured engine nacelle. The aircraft commander did not identify the incorrect closure of the panel during his subsequent pre-departure inspection, neither did the ground operations crew dispatching the aircraft. During the next takeoff, the panel failed at the hinge attachment points and departed the aircraft striking and damaging the vertical stabiliser, before coming to rest on the runway and its grass verge. The investigation identified a lack of consistency in the way flight crew were instructed on completing pre-departure inspections during their training. A previous accident, where the same engine panel was lost during takeoff, had occurred on the aircraft a month earlier. The circumstances and investigation findings for both accidents were the same. Safety action has been taken by the aircraft manufacturer to add labelling and amend the AMM and the operator has revised its maintenance procedures. In addition, two Safety Recommendations have been made relating to flight crew pre-departure inspection procedures and dissemination of safety information to ground crew, with the intention of preventing recurrence.