NTSB: Severe Vibration Triggered Bell 525 Breakup

 - January 17, 2018, 12:22 PM

The fatal July 6, 2016 in-flight break-up of Bell 525 flight test vehicle 1 (FTV-1) was caused by “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,” according to the NTSB final report released on January 16.

The NTSB wrote, "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.”

The vibration initiated at 92 percent Nr excited the main rotor scissors mode, meaning the adjacent blades were moving together and apart in a scissors motion. This resulted in a 6-Hz airframe vibration that, the NTSB concluded, “was transmitted to the crew seats and created a biomechanical feedback loop through the pilot-held collective control. A second feedback system, driven by the attitude and heading reference system (AHRS) inputs to the main rotor swashplate, also continued to drive the scissors mode and its resultant 6-Hz airframe vibration.”

The accident occurred during a simulated OEI (one engine inoperative) test with forward center of gravity at 185 knots. The test used special software to reduce the power in both engines to simulate OEI. At the time of the accident, FTV-1 was equipped with a combination flight data recorder/cockpit voice recorder (CVFDR), but it was not activated, nor was it required to be under FAA rules for flight test/experimental operations.

Neither pilot made radio transmissions during the accident sequence, which was monitored from close range by a chase aircraft and by ground-based telemetry and test team members. They noticed the increased vibration during the fatal test and radioed instructions to “knock it off” during the accident sequence. The chase aircraft also radioed cautions to the FTV-1 pilots during the test about excessive blade flapping.

The NTSB noted, “After the crew engaged OEI special training mode, rotor rotation speed (Nr) decayed from 100 percent to about 91 percent, and the crew began lowering the collective to stop Nr decay and increase Nr to 103 percent (the target Nr for recovery). About 5.5 seconds into the test, the crew stopped lowering the collective, and Nr only recovered to about 92 percent. About 6 to 7 seconds into the test, the helicopter began vibrating at a frequency of 6 Hertz.

“The vibration was evident in both rotor systems, the airframe, the pilot seats, and the control inputs; the vertical vibration amplitude at the pilot seat peaked at about 3 G. Nr remained between about 90 percent and 92 percent until about 12 to 13 seconds into the test, then began fluctuating consistent with collective control inputs; subsequent collective control input increases led to further decay in Nr.

“Nr decayed to about 80 percent as the collective was raised, and the main rotor blades began to flap out of plane. About 21 seconds into the test, the main rotor blades flapped low enough to impact the tail boom, severing it and causing the in-flight breakup of the helicopter,” according to the NTSB report.

Return to Flight Test

Since the crash, Bell has made numerous changes to the 525 and its test program. These include:

  • designing a software filter for the collective control law to dampen biomechanical feedback due to oscillatory control inputs as the frequency of control input increases;
  • adjusting 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;
  • performing shake tests with pilots using a side-stick collective to determine and incorporate the transfer function for human biomechanical feedback;
  • modifying the AHRS software filters to further reduce the AHRS response to a 6-Hz airframe vibration;
  • indicating that, for the 525, 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);
  • issuing a company-wide business directive to ensure that cockpit audio is recorded during all telemetered flight test activities across all flight-test sites;
  • developing plans to conduct flight testing in the 95 percent to 100 percent range of Nr in an OEI condition;
  • developing plans to implement, for the 525, 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; planning to implement a separate PBA specifically for low Nr and is incorporating more salient cues into the tactile cueing system;
  • planning to incorporate the automatic termination of OEI training mode should Nr fall below a certain limit; and
  • incorporating a safety officer for the accident helicopter model test program who will have dedicated safety-related responsibilities.

Bell resumed flight testing of the 525 on July 7, 2017. In a prepared statement, Bell said, “An 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. A carefully planned approach is under way to complete the remaining certification flight testing. We remain committed to the 525 program–the continued work of the program team will result in a reliable, innovative helicopter with advanced rotorcraft safety features when it comes to market.”

Further, Bell said the changes/enhancements made to the 525 post-crash “are being carefully tested to ensure that our corrective actions have fully addressed the unique problem encountered on July 6, 2016.” It said the vibrations encountered on the fatal flight were "the result of an unanticipated combination of very high airspeed with a sustained low rotor rpm condition.”

Due to the lack of a functioning CVR in the accident aircraft, investigators could only theorize why it took the test pilots so long to recover from the low Nr condition that soon became fatal. On previous OEI tests at slower speeds the crew had lowered the collective to 50 percent to recover, during the accident sequence it was lowered only to 58 percent. Recovery times required increased with airspeed.

“Investigators explored possible reasons why the crewmembers stopped their recovery from the initial Nr drop, including a reaction to an abnormal condition on the helicopter, distraction from the recovery task, or a conservative response due to the high airspeed. Telemetry data does not indicate the existence of an abnormal condition when the crewmembers stopped their recovery,” the NTSB noted.

Based on interviews with Bell flight-test members, the NTSB noted, “Helicopter manufacturer test pilots indicated that they interpreted this trend as the tendency of the crew to be more judicious while applying collective at successively higher airspeeds to avoid recovering too fast and overspeeding the rotor or damaging the transmission. Thus, the crew may have been more conservative during recovery at the helicopter's high speed during the final test. The chief test pilot also stated that if Nr had stabilized, the pilot would not have been in a rush and was possibly initiating a slow recovery.”