What happened
On 19 November 2017, an Esqual VM 1C ultralight aircraft was conducting a flight test near Landegem, Belgium, to determine the optimal blade pitch for a newly installed propeller. The flight was being conducted by an experienced pilot accompanied by a student pilot. While en route, a single blade of the propeller separated from the hub during flight. This sudden failure caused an immediate loss of engine power and forced the pilot to perform an emergency landing in a field. Despite the significant damage to the aircraft, the pilot successfully executed the landing without any injuries to the occupants.
The investigation
Investigators examined the wreckage and the engine components to identify the source of the failure. The investigation focused on the propeller, specifically a Helix H60V 1,75m R-TM-2 that had been shortened to 1.65m for this specific engine/airframe combination. The team analyzed engine parameters and discovered air pressure fluctuations within the air filter box during high RPM operations. Furthermore, the investigation looked into the history of the propeller modifications and the regulatory framework governing ultralight modifications in Belgium. The investigation also noted that the aircraft's weight at the time of the accident exceeded its maximum takeoff weight.
Findings
- The primary cause of the accident was the separation of a propeller blade during flight.
- The investigation identified that the flight test with the new engine and propeller combination was conducted without first verifying their compatibility through adequate endurance testing on a test bench.
- Air pressure fluctuations within the air filter box at high RPM were observed, which may have generated vibrations capable of damaging the propeller.
- The aircraft was operating above its maximum takeoff weight.
- There was a lack of clear guidance from the engine and propeller manufacturers regarding the assessment of compatibility between different propulsion components.
- The modification was performed without obtaining the necessary flight testing permits or notifying the Belgian Civil Aviation Authority.
Safety action
- The investigation highlighted a safety issue regarding the lack of guidance from manufacturers (UL Power and Helix) for operators performing propulsion system modifications.
- The report noted a lack of clear requirements from authorities regarding the validity of flight authorizations following major modifications to ultralight aircraft.