What happened
On July 16, 2005, a Ka-4 "Šumák" ultralight aircraft, registration OK-DUX 31, departed from Tábor airport for a flight to Bechyně. During the initial climb, the pilot performed a right turn at an altitude of approximately 70 meters. While passing over the end of runway 30, the pilot reported an engine failure via radio and declared the intention to perform an emergency landing.
In an attempt to return to the airport for a landing on runway 12, the pilot entered a right-hand turn. During this maneuver, the aircraft lost airspeed and entered a steep, right-hand spiral. Due to the low altitude, the pilot was unable to recover the aircraft before it struck the ground in a rapeseed field near Svatá Anna hill. The impact destroyed the aircraft, resulting in one fatality (the pilot) and one serious injury (the passenger).
The investigation
The ÚZPLN investigation focused on the mechanical failure of the engine and the flight dynamics of the aircraft following the loss of power. Investigators examined the Rotax 503UL engine and found significant internal damage. The failure was traced to the release of piston pin retaining rings in both cylinders, which caused extensive damage to the piston surfaces and cylinder walls, ultimately leading to the engine seizure.
Technical analysis revealed that non-original retaining rings had been used to secure the piston pins. Furthermore, the fuel-to-oil mixture ratio was found to be 67:1, which deviated from the manufacturer's required 50:1 ratio. The investigation also considered the aerodynamic characteristics of the Ka-4, specifically how the pylon-mounted engine configuration affects stability and creates a destabilizing aerodynamic moment when power is lost.
Findings
- The engine failure was caused by the failure of piston pin retaining rings in both cylinders.
- The use of non-original retaining rings contributed to the mechanical failure.
- The fuel-oil mixture ratio was incorrect and did not comply with the Rotax engine manual.
- The pilot was under the influence of alcohol at the time of the accident.
- The pilot's decision to attempt a return to the airport and the subsequent flight technique following the engine failure led to the aircraft exceeding its minimum stall speed.