What happened
On April 28, 2009, a Dynamic WT-9 ultralight aircraft, registration OH-U474, experienced an engine failure during the initial climb following a departure from Torbacka airfield. The flight, which was a cross-country mission from Nummela to Helsinki-Malmi, was underway in the evening twilight.
Approximately 47 seconds after takeoff, the engine ceased operation at an altitude of roughly 60 meters. The pilot attempted an emergency forced landing in a field adjacent to the runway threshold. During the maneuver, the aircraft executed a steep right-hand turn with a bank angle exceeding 60 degrees. In the process of descending, the aircraft's nose gear struck a 20 kV electrical line located at the edge of the field, severing the upper circuit and causing a small, self-extinguishing fire on the fuselage. The aircraft subsequently struck the ground, resulting in the total destruction of the airframe. The pilot and the passenger, an ultralight student, escaped the wreckage without injury.
The investigation
The investigation focused on the engine's failure, the aircraft's weight and balance, and the pilot's pre-flight procedures. Investigators examined the engine, which was found to be in good working order during post-accident testing; however, it was noted that the engine stopped after 45 seconds of operation when the fuel valves were closed, suggesting a fuel delivery issue.
Data from the aircraft's GPS was used to reconstruct the flight path, confirming the low altitude and airspeed during the engine failure. The investigation also reviewed the aircraft's weight, finding that the takeoff mass was approximately 495 kg, which exceeded the maximum authorized takeoff mass of 472.5 kg by about 22.5 kg. Furthermore, the investigation looked into the pilot's use of checklists and the adequacy of the aircraft's documentation regarding weight allowances.
Findings
- The primary cause of the engine failure was likely that the fuel valves were inadvertently turned to the OFF position during pre-flight or during the takeoff roll.
- The forced landing was unsuccessful due to the pilot's choice of landing site, the low altitude, and the low airspeed at the time of the failure.
- The aircraft was operating in an overweight condition, exceeding the maximum allowable takeoff mass by 22.5 kg.
- The pilot's limited flight experience, combined with time pressure due to approaching darkness, contributed to the unsafe flight profile.
- The pilot failed to use the official checklists provided in the flight manual, relying instead on incomplete, non-standardized plastic checklists.
- The pilot initiated a steep turn immediately after takeoff at a low altitude and low airspeed, which degraded the aircraft's glide performance and prevented the avoidance of the power lines.