The airline transport pilot and the flight instructor were conducting an instructional flight in a Cessna 206 with an observer on board. They proceeded to an airport to conduct instrument approaches.
After crossing the final approach fix and before arriving at the missed approach point, the engine lost power. The instructor took control and maneuvered the airplane over a set of power lines.
He attempted to maneuver under a second set of power lines when the right wing hit one of the lines. The airplane touched down on a road in Sugar Land, Texas, and its left wing hit a passing vehicle.
The landing gear struck the concrete median, yawing the airplane clockwise, and the airplane’s tail struck a second vehicle.
Post-accident engine examination revealed the No. 1 cylinder intake valve had failed. Fractured pieces of the intake valve had been drawn into the intake plenum.
The No. 1 cylinder was extensively damaged, and all of the piston heads had impact marks or pieces of valve embedded in the heads.
The No. 1 cylinder, No. 1 piston, its intake and exhaust valve train components (springs, pushrods, and rocker arms), and 12 hydraulic lifters were sent to the engine manufacturer for metallurgical examination, which revealed that the No. 1 cylinder intake valve had failed in fatigue. However, the root cause of the failure could not be determined due to the extensive damage after the valve failure.
Both the intake and exhaust rocker arms had nonconforming shoe heights.
In addition, nine of the hydraulic lifters failed the leak-down test, including the No. 1 intake.
Maintenance information revealed the engine had been overhauled four years earlier, during which all 12 hydraulic lifters were replaced. The intake and exhaust rocker arms had been reworked and reused, and about 0.01 inch of material had been removed from the shoe pads during the overhaul.
While the nonconforming rocker arm shoe height and lifter may have contributed to a malfunction of the valve, the investigation could not determine with certainty the reason for the fatigue failure. The fatigue failure of the No. 1 cylinder intake valve resulted in the loss of engine power.
Probable cause: Fatigue failure of the No. 1 cylinder intake valve, which resulted in the loss of engine power.
This September 2018 accident report is provided by the National Transportation Safety Board. Published as an educational tool, it is intended to help pilots learn from the misfortunes of others.