History of the Flight
The pilot, on a private flight from Carlisle Airport in fine weather, completed pre-flight checks including latching the left cabin door. After starting the engine and taxiing to holding point A for Runway 19, the pilot made blind radio transmissions as the airport was not yet open. During the takeoff ground roll, the aircraft ran over a joint in the runway surface, and the pilot heard a rattle from the left cabin door. He glanced down and thought he saw a gap at the bottom of the door, but the aircraft reached flying speed and he rotated into a climb. At about 50-100 ft, the left cabin door suddenly opened. The pilot reported tremendous turbulence and visible flexing of the door. He attempted to close the door with both hands while holding the control column between his knees, but failed. After reducing power, he tried again, managed to move the door to a closed position, but could not fully lock the lever. During this attempt, the aircraft stalled with the right wing dropping. The pilot pitched nose-down and applied full power, but there was insufficient height for recovery, and the aircraft struck soft ground outside the airport perimeter. The pilot sustained serious facial injuries and fractures to his right foot. No fire occurred, and airport staff and emergency services quickly attended.
Aircraft Information
The Flight Design CTSW is a high-wing, three-axis microlight with large gull-wing cabin doors hinged at the top and assisted by gas springs. Each door has a latching mechanism with three locking pins actuated by a lever at the lower inside edge. Moving the lever forward locks the door; aft unlocks it. When fully forward, the lever engages a detent with light spring pressure to keep it in place.
Aircraft Examination
AAIB examined the wreckage and found significant wear on the door latching pins where they bear against the door aperture's composite frame when closed. A survey of three other similar CTSW and CTLS aircraft also exhibited similar pin wear. The force required to move the door latch lever from closed (lever out of detent) to open was measured at 20-30 N. Owners of those aircraft reported no uncommanded door openings when the lever was in the detent.
Analysis
The left cabin door likely opened because it was not fully latched closed with the latch handle in the closed detent before takeoff. Airframe vibration during the takeoff roll, including from runway surface joints, probably caused the unlatched lever to migrate enough for the pins to disengage, allowing the door to open. Once unlatched, the gas spring and air loads assisted rapid door opening. The observed pin wear was common to other similar aircraft and not considered contributory to the uncommanded opening.
Conclusion
The aircraft's left cabin door opened in flight shortly after becoming airborne, leading to loss of control and ground impact in which the pilot was seriously injured. The accident highlights the importance of ensuring all aircraft hatches and doors are securely closed prior to flight.