Accidents
NTSB: Severe Vibration Triggered Bell 525 Breakup
The vibration initiated at 92 percent Nr excited the main rotor scissors mode—the adjacent blades were moving together and apart in a scissors motion.

The fatal July 6, 2016 in-flight break-up of Bell 525 flight test vehicle 1 (FTV-1) was caused by “severe vibration of the helicopter that led to the crew's inability to maintain sufficient rotor rotation speed (Nr), leading to excessive main rotor blade flapping, subsequent main rotor blade contact with the tail boom, and the resultant in-flight breakup,” according to the NTSB final report released on January 16.