What happened
During the pre-takeoff phase, the pilot deployed the flaps for the run-up procedure. During this process, a mechanical failure occurred within the 90-degree drive assembly adapter, where the flap key on the drive shaft fractured due to an overload. While the pilot retracted the flaps prior to departure, the broken key prevented the right flap from retracting, leaving it in a fully extended position.
Following takeoff, the aircraft performed a left turn. As the aircraft climbed to an altitude between 250 and 300 feet, the airspeed decreased significantly. The aircraft eventually reached a critical speed and entered a stall, which transitioned into a spin that ended with the aircraft impacting the ground.
Findings
Investigation into the mechanical failure revealed that the flap key within the drive shaft assembly failed under stress during the initial extension. Although the pilot could have potentially identified the asymmetric flap configuration through visual inspection or via the flap indicator, the aircraft was operated for takeoff with the right flap stuck in the extended position.
Technical analysis from the manufacturer indicated that the aircraft remained controllable at speeds exceeding 70 knots. However, the flight profile involved a loss of airspeed that dropped below the recommended performance parameters. According to the pilot's operating handbook, the optimal airspeed for a two-engine climb rate is 120 knots, while the best climb angle is 99 knots.