A Bell Helicopter 525 test aircraft crashed, killing two flight test pilots, 18 months ago after a series of system design flaws in the fly-by-wire helicopter aggravated severe vibrations caused by the crew’s unusually slow recovery from a test using a low rotor speed, the US National Transportation Safety Board says in a final investigation report released on 17 January.
As the vibrations worsened, with vertical oscillations up 3g’s up to six times a second, the main rotor blades slowed even more, then started flapping until one blade sliced through the tail boom and ripped the helicopter apart, the NTSB report says.
Lessons from the fatal crash already prompted Bell to make several changes to the 525 Relentless helicopter’s flight control systems, including the biomechanical feedback filters for the collective and the attitude and heading reference system (AHRS). The 525 test fleet returned to flight last July after a one-year hiatus.
“These enhancements are being carefully tested to ensure that our corrective actions have fully addressed the unique problem encountered on July 6, 2016,” Bell says in response to the NTSB report on 17 January.
The aircraft crashed within 30s after the flight test crew began the last in a series of simulated one-engine inoperative tests. Each test measured how the aircraft performed at progressively higher speeds with one engine shut down and a forward centre of gravity. The final test was set up to examine the 525 at 180kt, the twin-engined helicopter’s fastest speed in level fleet.
The test was designed to have the crew pull up on the collective to reduce the rotation speed of the main rotor with a simulated engine failure, then quickly recover to full rotor speed. For a reason still not fully understood, the test pilot flying the aircraft returned to about 92% of the main rotor’s maximum speed and remained there for several seconds. The US Federal Aviation Administration does not require a test aircraft to carry a cockpit voice recorder and flight data recorder, so the NTSB was unable to determine why the crew did not recover the rotor speed to 100% as planned.
In any event, the extended flight operations at 92% main rotor speed caused a sequence of events that exposed unforeseen gaps in the flight control systems, the NTSB report says. First, the main rotor blade entered into a “scissors mode”, with the lead and lagging blades on either side converging. That effect created a severe vibration that was felt all over the aircraft. In the cockpit, the vibration forced the pilot to inadvertently push on the collective, which, in turn, increased the severity of the vibrations in what engineers called a “biomechanical feedback loop”, the NTSB report says.
Bell designed the 525 with a system that dampens such a closed-loop effect on inputs into the cyclic control, the NTSB says. The test aircraft’s collective, however, lacked the same software filter, so the vibration level continued to rise, prompting the pilot involuntarily to push on the collective harder, further increasing the vibration, according to the NTSB.
Bell also designed the AHRS to dampen vibrations by sending commands to the swashplate. But the NTSB investigation found that Bell’s flight control design had not anticipated severe oscillations up to six times each second. Instead of dampening the rotor vibrations, the AHRS sent command signals to the swashplate that worsened the vibrations, the NTSB says.