What happened
On 19 November 2017, an Esqual VM 1C ultralight aircraft was conducting a flight test near Landegem, Belgium, to determine the optimal propeller pitch. The flight was being conducted by an experienced pilot accompanied by an owner to record engine and flight parameters. While flying in a descent near the city of Gent, one blade of the propeller separated from the hub. This sudden failure caused the engine to lose power and resulted in the aircraft making a forced landing in a field. While the aircraft sustained substantial damage, there were no injuries to the occupants.
The investigation
Investigators from AAIU(Be) examined the wreckage and the history of the aircraft's recent modifications. It was established that the aircraft had recently undergone a propeller change, moving from a standard three-blade propeller to a shortened two-blade Helix H60V 1,75m R-TM-2 propeller. The investigation focused on the mechanical failure of the propeller and the circumstances surrounding the flight test. The team analyzed engine parameters, which revealed air pressure fluctuations within the air filter box during high RPM operations. Furthermore, the investigation looked into the regulatory compliance of the modification and the authorization for the flight test.
Findings
- The primary cause of the accident was the separation of a propeller blade during flight.
- The investigation identified air pressure fluctuations inside the air filter box at high engine speeds, which may have contributed to abnormal vibrations.
- The flight test was conducted with a new engine and propeller combination that had not been sufficiently verified for compatibility through endurance testing.
- The aircraft's weight during the flight exceeded its maximum takeoff weight.
- The engine mount and rear plate sustained significant damage, leaving the engine attached to the airframe by only one of its four mounting points.
- The modification was performed without obtaining the necessary flight permit or authorization from the Belgian Civil Aviation Authority.
- There was a lack of specific guidance from the engine and propeller manufacturers regarding the compatibility of these specific components.