What happened
The aircraft experienced a sudden loss of engine power immediately following a water takeoff. The pilot was unable to maintain altitude and landed straight ahead into an area of muskeg and tundra. The impact resulted in the collapse of the float structure.
The investigation
A teardown of the engine revealed a metallic debris fragment, described as a 'nugget,' located within the left intake manifold between cylinders 4 and 6. Examination of both intake manifolds showed numerous marks on their interior surfaces. Specific damage included minor denting, scratching, and peening on the inside of the 90-degree elbows at both ends of the crossover tube, as well as on the intake valve ports, seats, and valve faces for cylinders 4, 5, and 6.
Analysis determined that the metallic fragment was composed of the same material as the airbox assembly (Cessna Part Number 1250705). The airbox wall contained two holes, each approximately 2.5 square inches in size. These openings were the result of fatigue cracks. The loading that caused these cracks to propagate originated from membrane and/or bending loads within the airbox wall, which were induced by out-of-plane deformation. This deformation was likely caused by exciting a natural mode of vibration within the wall structure.
Findings
Investigation data indicated that this type of failure was not isolated. Three separate duct failures had been reported to the Federal Aviation Administration in 1983. Two of these reported incidents involved Cessna 210 aircraft, suggesting a systemic issue with the airbox assembly design or material durability under vibrational stress.
Safety message
The incident highlights the critical importance of monitoring for unusual vibrations in aircraft components and the potential for fatigue failures in structural elements subjected to resonant frequencies. Regular inspection of intake systems for debris ingress is essential following such events.