What happened
On 6 October 2023, a Manta Air Viking Air DHC-6-200 floatplane, registration 8Q-RAL, was conducting a charter flight from Soneva Jani to Velana International Airport. While descending through approximately 350 feet on final approach to the water aerodrome, the aircraft experienced a sudden loss of power from the right engine.
The loss of power occurred immediately after the crew extended full flaps and moved the propeller levers to the maximum RPM position. This caused the aircraft to react violently, yawing to the right and banking significantly. The crew briefly lost control of the aircraft, which the pilot described as feeling like a potential spin. As the aircraft veered toward land-based structures, the pilot attempted to pitch down to gain airspeed and avoid a stall, eventually declaring a MAYDAY.
The aircraft impacted the water with a hard bounce. To prevent structural damage to the floats, the pilot attempted to land on both floats simultaneously. Despite the intensity of the impact, there were no injuries to the eight people on board, and no damage was sustained by the aircraft.
The investigation
The AICC investigation focused on the mechanical failure of the right engine's Constant Speed Unit (CSU). Investigators found that the right engine's power lever became stuck, and the propeller remained locked in the feathered position.
Technical analysis of the CSU, conducted by Woodward Inc. under NTSB oversight, revealed that the beta valve plunger assembly had failed. Specifically, the clevis piece had completely separated from the plunger at the brazende joint due to a shear overload of the copper braze material. This separation occurred while the propeller was operating in the Beta Range.
Findings
- The primary cause of the incident was the failure of the CSU due to the separation of the beta valve clevis from the plunger assembly.
- The left engine remained fully operational throughout the event.
- A previous maintenance entry from two days prior noted a defect in the engine's reversing system, which may have been an early indicator of the impending CSU failure.