Bell 525 Probe Points to Vibration, Frequency Response in Fatal Crash

Investigators have found that Bell Helicopter’s 525 prototype experienced rotor system vibration and frequency resonance in its airframe and flight control system seconds before the aircraft broke up in flight last July, killing its two test pilots.

Three people briefed on the investigation by the National Transportation Safety Board have told R&WI data analysis of a recovered flight-test recorder, telemetry from the accident aircraft and simulations conducted by Bell for the safety board indicate the onset of the vibration and the subsequent response by the No. 1 Relentless prototype.

Analysis of the aircraft’s cockpit voice recorder was not possible because the device was not powered during the July 6, 2016, test flight. One official said FAA guidance permits a CVR to be unpowered during a test flight.

The NTSB has refused to answer questions about factual findings of the investigation to date. It has provided no updates since July 29, 2016, when it posted a 71-word summary of the fly-by-wire helicopter’s crash, whose evidence includes the obscuration of two chase pilots in a Bell 429 in addition to the telemetry and flight-recorder information and wreckage examinations.

The 525 crashed about 30 nm south-southwest of the flight’s launch point, Bell’s Xworx research center at Arlington Municipal Airport, to which it was transmitting telemetry. The three individuals who spoke with R&WI gave the following description of the accident sequence.

The twin-engine helicopter was flying at 183 kt with rotor power at 92% during a single-engine, never-exceed-speed test when a six-hertz appeared on recorded flight data that appeared to originate in the tail rotor system. (Flightradar24 had reported July 6 that its last data set on the 525 flight put the aircraft at 199 kt at 1,975 ft.)

Frequency resonance developed in the tail boom and then in the 525’s flight control system. Rotor rpm dropped; the NTSB’s investigator in charge, John Lovell, told R&WI in late July that retrieve data indicated “that main rotor rpm dropped significantly.” Lovell leads a team of six investigators from NTSB’s Washington headquarters. Bell is a party to the NTSB’s investigation as is General Electric, maker of 525’s CT7-2F1 engines. The FAA by law is a party to NTSB probes.

The aircraft did not emerge from that flight condition, the three individuals said. The aircraft’s breakup followed.

The NTSB’s Lovell in late July said that the pilots in the chase 429 reported that the 525’s main rotor blades “appeared to have dropped from their normal plane of rotation.” He added that examination of the wreckage indicated that main rotor blades sliced through the tailbone and the aircraft’s nose.

The 525 broke up over Italy, Texas.

The three individuals said that Bell had not seen the vibration frequency resonance condition on any previous 525 flight. The company’s prototypes of that “super medium” twin have been grounded since the accident, and no ground tests of them have been conducted either. Its personnel are working with the NTSB to resolve open questions on the aircraft’s design and performance to clear the way for resumption of flight and ground tests.

In regard to its 525 investigation, the NTSB told R&WI Feb. 22 that “we hope to complete it this summer.” Courtesy of Rotor & Wing International.

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