On the fateful morning of July 6, 2016, Bell Textron Inc. test pilots Jason Grogan and Erik Boyce took off from Arlington Municipal Airport in the striking, orange-painted Bell 525 Relentless test aircraft at 10:38 a.m. and flew to the company’s flight test area south of Midlothian.

Just 70 minutes later catastrophe struck. The experimental helicopter tore itself apart in midair, and both pilots were killed.

A detailed account of the timeline and events leading up to the accident, as well as possible contributing factors, was produced by the National Transportation Safety Board and released in January 2018, more than 18 months later. The NTSB report cast no specific blame for the accident, but chronicles in great detail the chain of events and contributing factors.

NTSB experts examined volumes of aircraft performance data, Bell’s manufacturing, maintenance, and testing records, and flight test planning and documentation. Investigators also interviewed numerous Bell personnel including engineers and other 525 test pilots. Bell submitted a detailed report of its own investigation that it submitted to the NTSB.

Test pilots Grogan and Boyce had arrived about 6:30 a.m., along with the rest of the Bell flight test team to begin preparation for the day’s work. They had an intense, but highly planned and scripted morning ahead. About 50 individual test points were to be flown to determine the aircraft’s response to a variety of maneuvers and scenarios.

The flight had been delayed a couple of hours because of gusty winds in the area, which could impact aircraft handling and completion of the specific tests. In addition to the Relentless test aircraft, another flight crew followed behind in a Bell 429 chase helicopter to observe the tests.

The final eight tests of the day were to determine if the aircraft could be flown to a safe landing if one of its two jet turbine engines failed in midflight while loaded with passengers or cargo. A series of similar flights had been flown in recent days, apparently without serious complications or incidents.

For each test, the pilots would ascend to 4,000 feet altitude, reach the assigned airspeed for the test point, then push a button signaling the digital engine controls to reduce power by 50 percent – simulating the sudden loss of one engine.

As power dropped, the helicopter’s five-bladed rotor – spinning at hundreds of revolutions per minute – would quickly slow and the aircraft’s speed would shortly follow. Sensors aboard the aircraft would transmit thousands of data points to Bell’s test center in Arlington, where a team of engineers monitored aircraft performance and strain on its structures, rotor and other components. 

At 11:48 a.m., Grogan and Boyce initiated the day’s final test point, this one starting at the helicopter’s maximum speed of 185 knots (212 mph). Engine power was cut. After about a one-second delay to simulate normal human response time, the pilots began lowering the collective – a floor-mounted, vertical movement lever on the left side of each man’s seat that controls the angle of rotor blades – to reduce drag on the blades, regain lost rotor and aircraft speed.

About six to seven seconds into the test, unexpected vibrations began to buffet the helicopter, according to the NTSB report. The pilots, strapped in their seats, were repeatedly jolted vertically.  Powerful forces were bending the 54-foot-long helicopter up and down at each end. 

The vibration intensity increased greatly over the next three to four seconds, according to the NTSB. The pilots were likely quickly confused, trying to analyze what was happening, and unable to read the instrument data displayed on their digital display screens. Their hands imparted involuntary movements to the flight controls, compounding the vibrations.

Outside the cockpit, the helicopter’s 25-foot-long rotor blades, under tremendous forces, were “scissoring” — moving back and forth horizontally, a movement that looks like it sounds. They were also flapping up and down severely. 

At 12 seconds into the test, Bell flight test engineers in Arlington monitoring the digital data stream recognized the danger. They radioed “knock it off,” ordering Grogan and Boyce to terminate the test. There was no response from the pilots.

Nine seconds later — now 21 seconds into the test — the wildly flapping rotor blades sliced through the tail boom and nose cone of the aircraft, dismembering the helicopter. The wreckage plunged to the ground and portions burned.

Bell, in its court filings and the 32-page document submitted to the NTSB, contends that in the stress of the moment the pilots did not react as trained and briefed. If they had responded to the onset of the vibrations by immediately lowering the collective further, the rotor speed would have increased, stress on the rotor blades would have been relieved, the extreme vibrations controlled, and the helicopter safely recovered.

This chart reflects the rotor speed and vibration intensity over the 21 second duration of the accident flight test.

That’s asking a lot of the pilots given the few seconds they had to respond to the unanticipated and worsening vibrations, says William Lawrence, a retired Marine colonel, test pilot, and an expert witness from Fort Worth who served as a consultant for the pilot’s families lawyers in a lawsuit.

“They’re busy trying to figure out what’s going on and what to do about it,” Lawrence said. “By that point (10-12 seconds into the test), they were shaking so bad they were just holding on for dear life.” 

In its report, the NTSB said that Bell personnel had indicated that the Relentless pilots, based on the previous flight tests, may have reacted conservatively to the onset of the vibrations, assessing the problem which spiraled out of control in three or four seconds.

“(Bell) Test pilot interviews suggest that, in an abnormal situation, stabilizing the aircraft would be the first priority,” the report said.

Other than instrument data transmitted to the Bell test facility, no one knows how the pilots reacted to the emergency. Although a cockpit voice-recorder/flight-data-recorder was installed on the test aircraft, it was not operating during the accident test flight, a shortcoming cited by the NTSB. There was no cockpit video recorder, and the chase helicopter was not equipped with a video camera.

“It’s just inconceivable to me that in this day and age they would have been conducting a flight test like this without a video and voice recorder in the cockpit,” said Lawrence.

Other critical factors that contributed to the accident were cited by the NTSB in its report. GE, on orders from Bell, had removed a safety feature from the engine control software that would have been triggered by low rotor speed and automatically increased engine power and possibly enabled the pilots to regain control.

 Bell officials told the NTSB they had the software code altered  to prevent the safety feature  from kicking in too soon and disrupting the one-engine-inoperative tests and forcing additional, costly tests.  Also, components of the digital flight control system enhanced the effects of the already severe vibrations.

In response to the NTSB investigation, Bell agreed to make a number of changes to the remaining three Relentless test aircraft and to its flight testing processes to avert the circumstances that led to the accident. These included reinstating the safety features of the engine controls and updating fight control software to negate vibrations.

Bell, after extensive study of the crash test flight data and negotiations with the NTSB and the Federal Aviation Administration, resumed flight test of the Relentless about a year after the accident. The FAA has required significant additional data about how the aircraft’s fly-by-wire controls react in unusual situations.

Bob Cox is a freelance reporter.

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