What happened
On 13 July 2018, a CH 77 Ranabot ultralight helicopter, registration 83ARG, was conducting a private cross-country flight near Saint-Raphaël, France. After departing from Montauroux and picking up a passenger at Fayence aerodrome, the pilot was flying at approximately 500 ft over wooded hills when the engine power unexpectedly reduced. Witnesses observed the main rotor blades slowing down and the aircraft entering a steep descent. The ultralight struck the terrain and vegetation, followed by a post-impact fire. The accident resulted in two fatalities.
The investigation
The BEA examined the wreckage, the engine, and the FlyBox flight computer. Analysis of the engine showed that while the mechanical components and lubrication systems were in good condition, the intake pressure sensor (MAP1) had recorded a sudden, extreme drop in pressure from 22 inHg to 6 inHg in a single second. This rapid variation triggered a "sensor alarm" on the instrument panel.
Data from the FlyBox computer indicated that while the engine's rotation speed decreased, the rotor speed remained within a range consistent with an autorotation. The investigation also reviewed the ergonomics of the engine monitoring system (EMS) and the documentation provided in the Mini EIS user manual regarding how sensor errors are displayed to the pilot.