What happened
On September 28, 2020, a Tecnam P92 aircraft, registration N562TU, was participating in a flight lesson when the engine began to vibrate severely. The flight, operated by Chesapeake Sport Pilot, was returning to Bay Bridge Airport (W29) in Maryland. During the vibration, the engine RPM dropped from approximately 5,100 to 4,500.
The flight instructor attempted to adjust the throttle and verify the fuel valves and electric fuel pump, but the engine did not respond to these actions. About one minute later, the engine suffered a total loss of power. During the descent, the aircraft's avionics shut down for approximately 30 seconds before restarting. The instructor selected a nearby soybean field for a forced landing and successfully touched down without injury to the two occupants.
As the aircraft decelerated, the nose landing gear dug into the soft soil, causing the gear to separate and the airplane to nose over. The aircraft sustained substantial damage to the fuselage and both wings.
The investigation
Engine examination revealed that the exhaust valve spring retainer for the No. 1 cylinder had broken, causing the exhaust valve to fall into the combustion chamber. This failure resulted in significant internal damage, including a hole in the No. 1 piston crown, a bent and twisted connecting rod, and damage to the No. 3 cylinder.
Investigators noted that this was not an isolated event. Similar failures involving broken valve spring retainers had occurred in other Rotax 900 series engines, including an accident in 2017 involving aircraft N561TU. Metallurgical examinations of components from several engines showed that the fractured surfaces exhibited fatigue with pronounced vibration stripes, even though all parts met manufacturing specifications.
Review of manufacturer guidance indicated that air could enter the lubrication system through several methods, such as exceeding a 40-degree bank angle, insufficient purging of the oil system after maintenance, or spinning the propeller in reverse. Engine testing demonstrated that if air remains trapped in the hydraulic valve tappable, it can take several minutes of operation to vent, potentially overloading the valve train and leading to fatigue cracks.