NTSB Cites ‘Adverse Feedback Loops’ In Bell Crash

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The NTSB issued its final report on Tuesday following its investigation of the Bell 525 crash in July 2016 that killed two pilots during a test flight in Texas. The board cited a “severe vibration” of the helicopter, “which was not predicted during development,” as the probable cause of the crash. The vibration led to the crew’s inability to maintain sufficient rotor rotation speed, leading to excessive flapping of the main rotor blade. The rotor blade contacted the tail boom, leading to the inflight breakup. The board said its investigation was hampered by the lack of cockpit audio or images, which made it impossible to determine exactly what actions the crew might have taken to try to cope with the vibration. The NTSB also issued a safety recommendation urging the Flight Test Safety Committee to issue guidance for the use of recordings during flight tests, and asking Bell Helicopter Textron to share its “lessons learned” with other manufacturers.

Other factors contributing to the accident, according to the NTSB report, were the collective biomechanical feedback and 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 airframe vibration initiated.” Contributing to the crew's inability to maintain control in the severe vibration environment were the lack of an automated safeguard in the software used during flight testing, and the lack of distinct and unambiguous cues for low rotor rotation speed. The 525 test program was grounded for about a year after the crash, but test flights resumed last July. Bell has said it expects certification sometime this year. The 525 is equipped with fly-by-wire controls and Garmin G5000H fully integrated touchscreen avionics.

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