What happened
On August 30, 2011, a training flight involving an instructor and a student pilot was underway near Olomouc, Czech Republic. The flight was being conducted in a Schweizer HU269C-1 helicopter, registration OK-PIH, for the purpose of pilot training. During the approach to the left circuit for runway 28 at Olomouc Airport, the instructor noted a sudden drop in engine RPM to zero.
The instructor immediately took control and transitioned the aircraft into autorotation. Seeking a suitable landing site, the instructor targeted a nearby plowed field. While the touchdown itself was described as smooth and level, the helicopter's skids dug into the soft, freshly plowed soil during its forward momentum. This sudden deceleration caused the aircraft to pitch forward and roll onto its left side, resulting in significant damage to the main rotor blades, the tail boom, and the fuselage.
The investigation
The ÚZPLN investigation focused on the cause of the engine failure and the subsequent loss of control. Investigators examined the fuel system and found that the fuel tank was completely empty at the time of the accident. A technical inspection of the fuel gauge and the "FUEL LOW" warning light revealed that while the fuel quantity sensor was functioning correctly, the warning light itself was unreliable. The bulb socket had loosened due to vibrations, preventing the light from illuminating when fuel levels were low.
Furthermore, the investigation noted a discrepancy in fuel gauge scales. The instructor had previously operated a different Schweizer HU26 and was accustomed to a different scale graduation. On the aircraft involved in the accident, the first graduation mark represented zero gallons, whereas on the previous aircraft, it represented a higher quantity. This led the instructor to misinterpret the remaining fuel as approximately 5 gallons when it was actually zero.
Findings
- The engine failure was caused by complete fuel exhaustion.
- The instructor performed the flight with unverified fuel reserves.
- The "FUEL LOW" warning light failed to activate due to a loose electrical connection in the bulb socket.
- The instructor misread the fuel gauge due to differences in scale markings between this aircraft and a previously operated model.
- The aircraft overturned because the landing skids became embedded in the soft, plowed ground during the landing roll.