ERA25LA064 · AMS Flight D O O CARAT A

The pilot of the motor glider stated that about 1 hour after departure, the motor glider’s engine lost partial power and began to run rough. Shortly thereafter, the engine lost power completely and the pilot’s attempt to restore power was unsuccessful. He located a nearby private airport on his GPS and attempted to perform a forced landing to its turf runway. As he approached the runway, he observed cattle and a fence on the runway and realized it was not an actual airport but was unable to make any corrections due to altitude and airspeed. During the landing, the motor glider landed hard and came to rest inverted resulting in substantial damage to the fuselage. Postaccident examination of the motor glider’s engine revealed that the No. 4 cylinder intake valve had separated from its two keepers/safety wedges and had fractured into three sections. The valve face was located inside the cylinder, and the valve stem was deformed and found in the intake manifold. One valve keeper was found inside the valve cover and exhibited abrasion at its rotocap contact surface; the second keeper was not located. The intake rocker arm adjustment screw was backed off, and its corresponding jamb nut was found in the valve cover. The exhaust rocker arm adjustment screw for the No. 4 cylinder was also backed off, though its jamb nut remained in place. These conditions differed from those found on the remaining three cylinders. Abrasion marks on the intake and exhaust rocker arm adjustment areas were consistent with contact between the rocker arms and rotocaps during engine operation. The engine manufacturer’s maintenance publications required a minimum 1 mm clearance between the rocker arm and rotocap when the valve was fully depressed and specified that the clearance be visually inspected at intervals not exceeding 100 hours. The engine had accumulated 107.1 tachometer hours since the valves were last replaced and 54.9 hours since the most motor glider’s most recent condition inspection. Based on this information, the wear and damage observed during the postaccident examination of the engine likely the result of improper clearance between the rocker arm and rotocap, which resulted in unseating of the keepers and valve separation. Although the private use airport was charted, the local municipality had not approved the airport, and the airport had been erroneously activated; thus the airport should not have been charted. The airport owner later requested its deactivation, which became effective about 3 months after the accident.

Verbatim NTSB analysis and probable cause from the NTSB dataset