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The NTSB’s Final Report concerning the Bell 525 crash that occurred during its flight test has been issued.

On 6th July 2016 a Bell 525 Relentless Prototype (N525TA) broke up in flight and impacted terrain - both test pilots on board were killed. At the time, the aircraft was flying in the Arlington Initial Experimental Test Area in Texas on a developmental test flight and was performing the last of a planned sequence of one-engine-inoperative (OEI) tests at increasing airspeeds with a heavy, forward center-of-gravity configuration.

The NTSB report advises that the accident was probably caused by unanticipated severe vibrations as the aircraft attempted to recover rotor rotation speed following one of the OEI tests, this one at 185 knots.

Specifically, the report states:

“The NTSB determined that the probable cause of the accident was a 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. Contributing to the severity and sustainment of the vibration, which was not predicted during development, were (1) the collective biomechanical feedback and (2) the attitude and heading reference system response, both of which occurred due to the lack of protections in the flight control laws against the sustainment and growth of adverse feedback loops when the 6-hertz (Hz) airframe vibration initiated. Contributing to the crew’s inability to maintain sufficient Nr in the severe vibration environment were (1) the lack of an automated safeguard in the modified one-engine-inoperative (OEI) software used during flight testing to exit at a critical Nr threshold and (2) the lack of distinct and unambiguous cues for low Nr.”

As the helicopter was an experimental research and development aircraft, it was not required to be equipped with either a flight data recorder (FDR) or cockpit voice recorder (CVR) under the provisions of 14 CFR 91.609.3. Although there was a combination CVR and FDR (CVFDR) installed, it was not operational at that time. The NTSB report highlighted the difficulty this presented them, stating that “a properly functioning CVFDR would have recorded any discussions between the accident pilots that could have offered more information” and that “cockpit image recording capability would have recorded any pilot actions and interactions with the aircraft systems including avionics button presses, warning acknowledgements, and any other physical response to the aircraft”.

In response, Bell Helicopter independently addressed this deficiency before the 525 testing program recommenced on 7th July 2017. Cockpit audio and communications to and from the ground monitoring station by means of an onboard CVFDR were added, cockpit video is now also being recorded and a company-wide business directive was issued to ensure that cockpit audio is recorded during all telemetered flight test activities across Bell’s flight test sites.

As a result of the investigation, the NTSB issued one safety recommendation to the FTSC and one safety recommendation to Bell Helicopter Textron. The NTSB accident report can be found here.