NTSB Report Bell 525 N525TA July 06, 2016

NTSB, January 16, 2018 - The National Transportation Safety Board (NTSB) issued report DCA16FA199 about accident occurred Wednesday, July 06, 2016 in Italy, Texas by the Bell 525 registration N525TA resulting in 2 fatalities. The full report can be located on the NTSB website

Probable cause(s) of this accident

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 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 software used during flight testing to exit at a critical Nr threshold and (2) the lack of distinct and unambiguous cues for low Nr.

Since the accident, the helicopter manufacturer has

- designed a software filter for the collective control law to dampen biomechanical feedback due to oscillatory control inputs as the frequency of control input increases;

- adjusted the aero-servo-elastic model with a correlation factor to incorporate the aerodynamic effects observed during flight test and the accident test to preclude such occurrences seen in the accident flight's telemetry data;

- performed shake tests with pilots using a side-stick collective to determine and incorporate the transfer function for human biomechanical feedback;

- modified the Attitude and Heading Reference System (AHRS) software filters to further reduce the AHRS response to a 6-Hz airframe vibration;

- indicated that, for the accident helicopter model, cockpit audio is now being recorded by an onboard CVFDR (A combination of Cockpit Voice Recorder and Flight Data Recorder), and communications to and from the ground monitoring station are recorded by the CVFDR and the telemetry system during all flights (cockpit video is also being recorded by the instrumentation system and archived at the ground station);

- issued a company-wide business directive to ensure that cockpit audio is recorded during all telemetered flight test activities across all flight test sites;

- plans to conduct flight testing in the 95% to 100% range of Nr in an OEI condition;

- plans to implement, for the accident helicopter model, the unique low Nr aural tone in their test aircraft, and a software update that includes a larger font size for the Nr numeric display on the PSI;

- plans to implement a separate PBA specifically for low Nr and is incorporating more salient cues into the tactile cueing system;

- plans to incorporate the automatic termination of OEI training mode should Nr fall below a certain limit; and

- incorporated a safety officer for the accident helicopter model test program who will have dedicated safety-related responsibilities.