What happened
On April 29, 2011, a Dynali H2S helicopter, registration F-PJMI, departed from a private helisurface in Meynes, France, for a series of low-altitude maneuvers. The flight, which included several takeoffs and landings, was part of a 15-hour endurance program intended to obtain a special airworthiness certificate for a kit-built aircraft.
While flying at an altitude of approximately 300 feet, the engine and main rotor RPM began to decrease simultaneously. As the power decayed, the aircraft lost altitude rapidly and entered a left-hand turn. The helicopter struck the ground on its left side with a high vertical velocity. The accident resulted in two fatalities, involving the pilot and a passenger, and the total destruction of the aircraft.
The investigation
Investigators analyzed data from the aircraft's EFIS, which revealed that the engine and rotor RPM decreased at a constant rate while the aircraft was traveling at approximately 60 knots. Although the engine governor attempted to compensate by increasing power for six seconds, the pilot did not manually intervene with the power lever until nine seconds after the initial decay began. At the time of the pilot's manual intervention, the rotor speed had dropped to approximately 420 RPM.
An examination of the engine revealed that the timing belt was loose, and the timing on cylinders 1 and 3 had jumped by four teeth. While the tensioner's trigger threshold appeared functional, it was unable to maintain sufficient tension on the belt. Physical damage to the pistons in cylinders 1 and 3 indicated contact with the exhaust valves, a condition that occurs when timing is lost. The investigation also noted that while visual low-RPM alarms were functional, the aircraft was not equipped with a standard audible warning.
Findings
- The primary cause of the accident was a loss of engine and rotor RPM caused by a timing belt failure.
- The timing belt became slack because the tensioner failed to maintain adequate tension, leading to a timing shift in the engine.
- The pilot's reaction to the power loss was delayed, likely due to the absence of an audible low-RPM alarm.
- The aircraft's low altitude and the significant decay in rotor speed made the situation unrecoverable by the time manual corrective actions were taken.