Factual Information
On 8 June 2024, a Tropicair Ltd DHC-6-300 Twin Otter, registration P2-BBM, operated a flight that ended in a runway excursion during landing at Kikori Airstrip in Gulf Province, Papua New Guinea. The aircraft was on a scheduled service. The flight departed Jacksons International Airport (Port Moresby) and landed at Kerema Airport before proceeding to Kikori.
Upon arrival at Kikori, the flight crew conducted an approach to Runway 30. During the landing flare, the aircraft deviated from the runway and came to rest in a drainage ditch adjacent to the airstrip. The aircraft sustained damage to the nose cone and other structures. No fire occurred. All occupants evacuated through the main cabin door and the cockpit sliding window.
Analysis
The investigation focused on the sequence of events during the landing. Recorded data showed that the aircraft was established on a normal final approach. However, during the flare, the propeller condition levers were not set to the standard flight idle (ground fine) position. Instead, the levers were inadvertently moved to the 'Disc' position, which prevented the propellers from achieving the necessary low pitch for effective reverse thrust and deceleration.
The design of the propeller condition lever gate did not provide clear differentiation between the 'Disc' and 'Lo Pitch' positions, contributing to the crew's error. The aircraft's weight and balance were within limits, and no pre-existing mechanical issues were identified.
Conclusions
The runway excursion resulted from the flight crew's failure to select the correct propeller condition lever position during landing. The confusion was exacerbated by the ambiguous gate design.
Probable Cause
The investigation identified that the cause of the runway excursion was the flight crew’s failure to perform the standard flight idle (ground fine) propeller selection during the landing flare. The flight crew inadvertently selected the propeller condition levers to the ‘Disc’ position instead of the ‘Lo Pitch’ (Flight Idle) position during the landing sequence. Contributing factors included the flight crew’s confusion with the propeller condition lever gate configuration, as the current design does not provide clear differentiation between ‘Disc’ and ‘Lo Pitch’ positions.