What happened
On 6 September 2016, during post-assembly flight testing at Rzeszów-Jasionka aerodrome (EPRZ), a Hummingbird 300 LS helicopter, registration SP-YLN, experienced an engine compartment fire. The aircraft, which had been recently assembled by a private operator, was undergoing a series of ground and flight maneuvers.
During the testing, the pilot observed rising engine oil temperatures. After an initial attempt to cool the engine by landing, the pilot resumed hovering maneuvers at a height of 1.5 to 3 meters. After approximately twelve minutes of continued operation, the oil temperature rose again, prompting the pilot to land once more to suspend testing.
While the engine was being cooled on the ground, the pilot heard a loud thud from the rear of the fuselage. Shortly thereafter, the engine stopped, and a fire was confirmed in the aft section of the aircraft. Although maintenance personnel and the airport fire brigade attempted to extinguish the flames, the helicopter sustained significant damage. There were no fatalities and no injuries to the pilot or ground personnel.
The investigation
The PKBWL investigation focused on the fuel and oil systems, as well as the thermal stability of aircraft components. Investigators examined the wreckage and found a disconnected quick connector in the fuel line near the injection rail, as well as a discontinuity in the oil line between the oil filter and the engine block.
Thermal testing was conducted on the fuel system's quick connectors. The tests revealed that at temperatures reaching approximately 167°C, the plastic inserts designed to prevent disconnection began to soften. By 192.6°C, the connectors were prone to complete disconnection. The investigation also found that the discontinuity in the oil line was likely a secondary effect caused by the fire or firefighting efforts, as the pilot had not reported a drop in oil pressure prior to the engine failure.
Findings
- The primary cause of the accident was the unsealing of a quick connector in the fuel system, which allowed pressurized fuel to leak and ignite.
- High temperatures in the engine compartment caused the plastic inserts in the fuel system connectors to melt, facilitating the disconnection.
- Vibrations and fuel system pressure acted as contributing factors in the disconnection process.
- The engine stop was triggered by the fire following the fuel leak.
Safety action
- The Commission recommends that the manufacturer implement measures to replace fuel system quick connectors with versions resistant to disconnection.
- The Commission recommends that the manufacturer consider the installation of a fire alarm system within the engine compartment of the Hummingbird 300 LS.