What aircraft was involved and where did it happen?

The accident on 2015-09-24 involved a Xenon 2RST, at EPMO, PL.

What was the probable cause of the accident?

The engine failure was caused by a broken exhaust valve in cylinder number 2 that jammed the piston, a condition exacerbated by high combustion temperatures from using Avgas 100LL and increased cylinder pressure resulting from the manufacturer's power increase modification.

Engine Failure During Training Flight at EPMO Aerodrome

An engine malfunction involving a broken exhaust valve led to an emergency landing of a Xenon 2RST autogyro during a student pilot's training flight.

What happened

On September 24, 2015, a Xenon 2RST autogyro was conducting VFR training flights at EPMO aerodrome. During the fourth circuit of the airfield, while the student pilot was in a climb at approximately 600 ft, the engine experienced a sudden and uncontrolled drop in RPM to about 3550 rpm, accompanied by significant vibrations and irregular operation. Although the engine did not shut down, the instructor took control of the aircraft. After notifying the tower controller and receiving clearance, the instructor performed a precautionary landing on the runway and taxied to taxiway E.

Following the flight, the crew noted that the propeller could not be rotated through its full 36 and 360-degree range, suggesting internal engine damage. No fluid leaks were observed during the initial inspection.

The investigation

An inspection of the CA 912 ULT engine revealed significant internal damage. Investigators found visible damage to two spark plugs in cylinder number 2 and a broken exhaust valve in the same cylinder, where a fragment of approximately 7 mm had detached from the valve seat. Additionally, cracks were found on the exhaust valves of two other cylinders. A fragment of the broken valve had struck the piston crown in cylinder number 2, causing mechanical damage that prevented the piston from reaching top dead center.

Further analysis showed excessive carbon buildup on the surfaces of all four pistons and valves, which likely hindered heat dissipation and reduced valve clearance. The investigation also noted that the engine, originally designed by Rotax for 80 HP, had been modified by the aircraft manufacturer to produce 122 HP, leading to higher combustion pressures due to increased airflow from the turbine.

Findings

The primary cause of the engine malfunction was the fracture and detachment of an exhaust valve fragment in cylinder number 2, which physically blocked the piston's movement.
The use of Avgas 100LL fuel, which has a higher lead content than automotive fuel, contributed to excessively high combustion temperatures.
Excessive carbon deposits on engine components likely contributed to increased operating temperatures and reduced valve clearance.
The modification of the engine to increase power output beyond its original design specifications increased combustion chamber pressure.