The National Transport Safety Board (NTSB) has attributed last year’s Bell 525 Relentless test flight crash to “severe vibrations”.
The helicopter’s airframe started vibrating severely soon after take-off, slowing the rotation speed of the main rotor. The two pilots attempts to recover the rotation speed resulted in rotor flapping, causing the main rotor to come into contact with the tail boom, which bought the aircraft down.
Vibrations caused the two pilots to unintentionally input controls that exacerbated the rotor flapping. The test pilots Jason Grogan and Erik Boyce died in the crash on 6 July 2016.