NTSB debrief details 2016 Bell 525 prototype break-up in Texas test flight

The first prototype of the Bell 525 Relentless broke up in clear weather about 38 miles (62km) south of Bell Helicopter's XworX facility in Arlington, Texas, on July 6, 2016. Both test pilots died. Erik Boyce and Jason Grogan were former US Marine Corps officers and graduates of the US Naval Test Pilot School. The aircraft was the lead example of a programme meant to produce the first fly-by-wire commercial helicopter.

The crew was finishing the last point in a series of tests on how the helicopter recovers from a simulated engine failure as airspeed increases. That point was flown at the aircraft's redline speed of 213mph (343km/h), at approximately 2,000ft.

The test uses a software mode that reproduces an engine failure by limiting the power available from both engines. The main rotor dropped to roughly 91% of normal speed. The pilot lowered the collective, the lever controlling how hard the blades bite into the air. The rotor held at 92% instead of rising back through 100%, which a correct recovery requires.

At that combination of high forward speed and reduced rotor speed, the five-bladed main rotor entered a condition engineers call scissors mode, in which the mast rocks back and forth at about six cycles per second. The motion bent the airframe and reached the pilots' seats, building to peaks of plus and minus 3g. According to the National Transportation Safety Board account, that was nearly 40 times the vibration level normally seen in the experimental aircraft.