NTSB Approves Probable Cause of Fatal Bell Helicopter 525 Crash

By S.L. Fuller | January 17, 2018
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Image courtesy of Bell Helicopter

NTSB has determined the probable cause of the fatal Bell Helicopter 525 crash that killed both test pilots in July 2016. As R&WI was told in March 2017 by three people briefed on the investigation by the NTSB, investigators found vibration to be the probable cause.

This probable cause was approved Tuesday:


“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.”

Bell said investigation activities were completed in 2017. The manufacturer and the NTSB studies the cause of the vibration, "which had never been encountered before," Bell said.

"The vibration was the result of an unanticipated combination of very high airspeed with a sustained low rotor rpm condition. The in-depth analysis of the flight data resulted in a thorough understanding of the corrective actions necessary, and appropriate changes to the aircraft have been implemented," Bell said. "Changes include a further enhancement of the filtering system on the pilot’s side-stick controller so that vibrations of the pilot stick are not passed to the rotor system. Additional control system filtering was also applied to the system that is used to stabilize the helicopter in gusts and maneuvers."

In December 2017, the FAA proposed special conditions for the 525. It believed the model should feature mode annunciation because of the fly-by-wire technology.

“This helicopter will have a novel or unusual design feature associated with fly-by-wire flight control system functions that affect the pilot awareness of the flight control modes while operating the helicopter,” the Federal Register stated. “The applicable airworthiness regulations do not contain adequate or appropriate safety standards for this design feature. These proposed special conditions contain the additional safety standards that the administrator considers necessary to establish a level of safety equivalent to that established by the existing airworthiness standards.”

Aural cues were available to the crew regarding low rotor rotation, including the master warning annunciation and the sound of decreasing rotation. The NTSB said the master warning aural tone would have annunciated at 12.5 seconds and 16.8 seconds, continuing until acknowledged by the crew. But the same tone was used with at least 21 other warning messages, the NTSB said, because audio files had not yet been developed — the flight test team had decided this type of aural annunciation would be sufficient and a distinct tone for low rotor rotation speed was not immediately needed to conduct flight tests.

The 525 impacted terrain near Italy, Texas, July 16, 2016, after the tail boom separated from the fuselage in flight. The aircraft was involved in a series of one-engine-inoperative tests, according to the NTSB. On the final test, rotor rotation slowed from 100% to some 91%. In response, the crew began lowering the collective recover. The NTSB said that after the crew stopped lowering the collective, rotation only recovered to about 92%. Then the helicopter began vibrating at a frequency of 6 Hz.

The NTSB said vibration was evident in both rotor systems, the airframe, the pilot seats and the control inputs. After rotor rotation remained somewhat constant for a few seconds, it began to fluctuate, consistent with collective control inputs. The NTSB said subsequent collective control input increases led to further decay in rotation. As the collective was raised, it decreased to some 80% and the main rotor blades began to flap. NTSB said that the flapping then occurred low enough to impact the tail boom, severing it.

According to the investigation body, the main rotor, tail rotor, flight controls, powerplants and rotor drive systems showed no evidence of preexisting malfunction before the vibrations began. Structural wreckage did not show evidence that the oscillations alone resulted in a structural failure leading to the in-flight breakup. NTSB also said examination of the wreckage revealed no indications that the helicopter had been improperly maintained.

Bell designed software filters in the cyclic control laws in consideration to biomechanical feedback. (NTSB defines biomechanical feedback as “unintentional control inputs resulting from involuntary pilot limb motions caused by vehicle accelerations.”) However, no filter was designed for the collective. This means that the 6-Hz oscillatory collective inputs by the pilot were not filtered, NTSB said. The result was a control feedback loop when the pilot-held collective stick commanded an oscillatory collective pitch input into the main rotor, increasing the 6-Hz vibration, which in turn increased the magnitude of the oscillatory collective pitch input.

In addition, the NTSB continued, the gain between the pilot movement and the collective control stick movement in the vertical axis had not been tested on a shake table before the accident. For the cyclic control, though, lateral vibration was introduced on a shake table.

According to the NTSB, since the accident, Bell 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 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, 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.

Bell resumed flight testing July 2017 after receiving an experimental certificate renewal from the FAA. It said at that time it still hopes to certify the 525 this year.

This article has been updated to include statements from Bell Helicopter.

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