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The in-flight breakup of the first Bell 525 prototype was caused by unanticipated severe vibrations as the aircraft attempted to recover rotor rotation speed following a one-engine inoperative (OEI) test at 185 knots, the U.S. National Transportation Safety Board (NTSB) has found in its final report on the accident.
The report also highlighted the role played by “feedback loops” from unintentional control inputs on the collective and attempts at corrective actions from the aircraft’s attitude and heading reference system (AHRS) that were caused by the vibration, and then served to sustain and amplify it.
As the main rotor continued to lose rotation speed, excessive blade flapping caused the main rotor to strike and sever the aircraft’s tail boom. Bell test pilots Jason Grogan and Erik Boyce were killed in the resulting crash.
The accident took place on July 6, 2016, as the aircraft performed the last of a planned sequence of OEI tests at increasing airspeeds in the Arlington Initial Experimental Test Area, about 30 miles south of Arlington Municipal Airport near Italy, Texas.
The purpose of the day’s flight was to evaluate engine loads at maximum continuous power, the simulated failure of one engine, longitudinal roll osciallations, and run-on landings in a heavy, forward center-of-gravity configuration.
The test card for the simulated engine failure utilized the aircraft’s OEI training mode, which reduced the power output of both engines to represent the maximum power available from one engine.
With the loss of power simulated, the pilot was to monitor the rotor’s rotation speed, and delay his response by about one second before recovering from the maneuver by lowering the collective. This would reduce the power required by the rotor, and therefore increase its rotation speed.
The lowest allowable limit for rotation speed was 86 percent — below that, the test would be halted, the crew would recover rotation speed to 103 percent (to have more energy available in the rotor in the event of a single-engine failure), exit OEI training mode, and return to steady level flight.
The OEI tests began shortly after 11 a.m. local time, with the test team completing simulated engine failures and recoveries at 102, 131, 145, 155, 160, 165, and 175 knots true airspeed. The latter tests required the aircraft to be in a shallow descent.
The tests slowed the rotor’s rotation speed by five to 13 percent, and to allow the rotor speed to recover to 97 percent or greater, the crew lowered the collective input to near or below 50 percent. During most of the OEI transitions, the pilot responded by lowering the collective between one and two seconds after the simulated loss of engine power. But with each increase in airspeed, it took the crew longer to recover the aircraft’s rotor rotation speed to the target of 103 percent.
At 11:48 a.m., the crew performed the final scheduled OEI test, which was at 185 knots — the aircraft’s never-to-exceed speed at the time of the flight. The crew engaged the OEI training mode, and the rotor speed dropped to about 91 percent within 1.5 seconds, where it was stopped by the pilot’s lowering of the collective. The rotor’s rotation speed began to recover, but about 5.5 seconds into the test, the crew stopped lowering the collective at the 58 percent stick position — and the rotor speed levelled out at 92 percent.
While at 92 percent rotor speed, the main rotor had excited “scissors mode” — where the lead lag motions of the blades act in such a way that adjacent blades move together and apart in a scissoring motion.
This main rotor scissors mode caused a six-hertz vertical vibration of the airframe — which was transmitted to the pilot seats.
About seven seconds after the crew had begun to counter the reducing rotor speed (and 12 seconds into the test), the test flight’s structural dynamics engineer noticed increased engine vibrations, and issued the “knock-it-off” call to end the test. The test director radioed the message to the pilots, while other engineers in the test’s telemetry room received warnings and alerts.
The vibrations were so severe in the cockpit (peaking at 3 G) that they caused unintentional control inputs on the collective that further amplified the problem — causing, in the NTSB’s words, a “biomechanical feedback loop.”
Compounding this was the aircraft’s attitude and heading reference system, which, in attempting to correct the airframe’s vertical vibration with a “cyclic stir” input to the main rotor swashplate, served to exacerbate the rotor blades’ scissoring motion — creating a secondary feedback loop.
Eighteen seconds into the test, the rotor speed dropped below 80 percent.
The crew of the test flight’s chase helicopter — a Bell 429 flown by two pilots from the program’s flight test team — reported hearing the test director call “knock-it-off” about the same time they observed the 525’s rotor blades flying high and the rotor looking “wobbly and slow.”
They radioed the 525 crew, but they got no response. About 21 seconds into the test, the 525’s main rotor severed the tail boom. The chase helicopter crew watched the helicopter’s tail and fuselage jack-knife and debris separate from the helicopter.
Completing the investigation
In attempting to understand the crash, the NTSB highlighted the difficulty presented by the lack of data from a flight data recorder (FDR) or cockpit voice recorder (CVR). The accident aircraft had a combination CVR and FDR (CVFDR), but it was not being used at the time of the accident, and as an experimental research and development helicopter, the 525 was not required to be equipped with either.
Production models of the 525 will have a CVR and FDR when certified, and as a result of the crash and investigation, Bell has implemented new procedures to mandate the recording of cockpit audio during all telemetered flight test activities.
While telemetry data was available to the NTSB, the lack of CVR or FDR presented a particular challenge in trying to understand why the crew stopped their recovery from the initial dropping of the rotor speed.
“A properly functioning CVFDR would have recorded any discussions between the accident pilots that could have offered more information about potential abnormal conditions, distractions, or reasons for their stop in recovery after initiation of the OEI test,” the report states. “Additionally, cockpit image recording capability would have recorded any pilot actions and interactions with the aircraft systems including avionics button presses, warning acknowledgements, and any other physical response to the aircraft. Cockpit audio and imagery could have provided insight into when the crewmembers first felt or detected the 6-Hz vibration, how they may have verbalized their assessment of an observed anomaly, and whether they attempted any specific corrective action because of the vibration.”
The report states that other pilots suggested that the lack of further collective input may have simply been a conservative response due to the high airspeed to avoid recovering too fast and overspeeding the rotor or damaging the transmission.
Test pilots and engineers told the NTSB there were two ways the pilots could have exited the low rotor speed condition — and therefore correct the vibration. The first would have been to lower the collective to increase rotor speed; the second, to exit the OEI training mode to increase the power available from the engines.
But in order to perform any corrective action, the pilots would have needed to be aware that the low rotor rotation speed was the problem.
With regards to visual cues, the NTSB found that the crew was unlikely to have been able to read the aircraft’s displays due to the extreme vibration levels. A master aural tone would have sounded, but it was associated with 21 other warning messages. The master tone was chosen for the test flight as Bell had not yet developed separate audio files for different systems, and the test team decided a distinct tone for low rotor speed was not immediately needed for flight tests.
“Without an unambiguous cue for low Nr [rotor rotation speed], it was unlikely that the pilots had properly distinguishable awareness of the low Nr condition for them to appropriately respond,” the report surmised.
With regards to the second solution — exiting the OEI mode — the NTSB said telemetry data indicated that the crew had not done so, but due to the lack of CVFDR data, could not say why. Post-accident shake tests of the Garmin touch control panel showed it remained intact during the vibration profile, but whether the crews failure to exit OEI mode was because the vibration physically prevented them from doing so, or simply that they did not try, remains unknown.
However, the NTSB found that Bell had modified the OEI training mode software to prevent it from automatically exiting the OEI mode when rotor speed falls below 90 percent — as it was originally designed to do by the engine manufacturer.
Bell told the NTSB that an automatic disengagement of the OEI mode at 90 percent rotor speed is not low enough to allow for the development and demonstration of OEI recovery across the flight envelope during testing. Further, Bell said a lower value for automatic disengagement was deemed unnecessary due to the “highly controlled test environment.”
Addressing the causes
In a statement issued following the release of the NTSB’s report, Bell said it had devoted a small team of flight technology engineers, pilots, and flight test specialists to work with the NTSB’s investigators to determine the cause of the breakup.
“Bell and the NTSB have carefully studied the cause of the vibration, which had never been encountered before,” the company 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.”
Among these are changes to address the feedback loops from the pilot control inputs and the aircraft’s AHRS.
In the NTSB’s investigation into the accident, it found Bell had included software filters in the cyclic control laws to reduce certain types of cyclic control inputs by the pilot — but no filter was designed for the collective. In addition, the gain between the pilot’s movement and the collective movement in the vertical axis was never tested on a shake table.
In the aftermath of the accident, Bell has enhanced the filtering system on the pilot’s side-stick controller, so that the vibrations of the pilot stick are not passed onto the rotor system. It has also performed shake tests with pilots using a side-stick collective to determine and incorporate the transfer function from pilots.
The company has also modified the 525’s AHRS software filters to reduce the system’s response to a six-hertz airframe vibration.
“These enhancements are being carefully tested to ensure that our corrective actions have fully addressed the unique problem encountered on July 6, 2016,” Bell said in its statement.
Finally, the NTSB report states that Bell plans to conduct flight testing of an OEI condition between 95- to 100-percent rotor speed, incorporate an automatic termination of OEI training mode if rotor speed falls below a certain limit, and implement a unique low rotor speed aural tone in the 525 test aircraft.
“We remain committed to the 525 program,” a Bell spokesperson told Vertical when contacted by email. “The continued work of the program team will result in a reliable, innovative helicopter with advanced rotorcraft safety features when it comes to market.”
The aircraft resumed flight tests on July 7, 2017, and the manufacturer said “a carefully planned approach” is underway to complete the remaining certification flight testing.
The 20,000-pound gross weight 525 will become the first commercial fly-by-wire helicopter when certified. As Bell resumed flight testing, it was aiming for certification in 2018.