What happened
On June 4, 2016, during the "XXVI Slet letadel Nymburk" airshow at LKNY, a Zenair CH-601XL, registration OK-OUL 46, was involved in a fatal accident. The aircraft, operated by a private owner, had arrived at the airfield as a visitor following a flight from LKCE.
After performing a left-hand circuit at approximately 100 meters altitude, the pilot executed a low pass over runway 09. Following this pass, the pilot increased power and initiated a steep climb. Video evidence and witness accounts indicate that after leveling the aircraft, the pilot entered a sharp right-hand turn. During this maneuver, the aircraft's airspeed decreased significantly. As the bank angle increased, the aircraft exceeded its critical angle of attack, leading to a stall and a subsequent transition into a right-hand spin. The aircraft struck the ground at a steep angle in a field south of the runway, resulting in two fatalities and the total destruction of the aircraft.
The investigation
The ÚZPLN investigation examined video recordings, witness statements, and the wreckage of the aircraft. The investigation established that the aircraft was in a maintained technical state and the engine was producing power at the moment of impact. Investigators also reviewed the pilot's flight experience, noting he had approximately 346 hours of total flight time, with 400 hours specifically on the Zenair CH-601XL type.
Technical analysis of the cockpit instruments revealed that the airspeed indicator, which was originally intended for a different category of aircraft, featured color-coded markings that did not correspond to the actual performance parameters of the Zenair CH-601XL. Specifically, the red warning arc began at 83 km/h, whereas the aircraft's stall speed was documented at 60–68 km/h.
Findings
- The primary cause of the accident was pilot error during a sharp turn at low airspeed, which led to an exceedance of the critical angle of attack and an unrecoverable spin at an altitude too low to allow for recovery.
- The aircraft's takeoff weight exceeded the maximum allowable takeoff mass (MTOW) by at least 33 kg.
- The airspeed indicator's color markings were inaccurate for this specific aircraft type, potentially misleading the pilot regarding stall margins.
- The aircraft was not equipped with a stall warning device.
- The type of stay belts used on the aircraft did not meet the required regulatory standards for this category of ultralight aircraft.