The catastrophic failure of a third-party rod end caused the Nov. 8 crash of Composite Helicopters’ experimental helicopter during flight testing in New Zealand, the manufacturer revealed this week.
In an exclusive interview with Vertical, Composite Helicopters International chief designer and test pilot Peter Maloney explained that the aircraft was nearing the conclusion of a test flight when a rod end component failed in the single scissor link assembly of the upper swash plate, resulting in a loss of phase control to the main rotor.
Maloney and fellow test pilot Norbert Idelon were in the helicopter, 500 feet above the ground, when the incident occurred. Dramatic video footage from cameras mounted on the tailboom and in the cockpit clearly depicts the failure of the scissor link assembly, as well as the helicopter’s extreme pitching and rolling moments as Maloney fought to guide the aircraft to an emergency landing.
“The initial failure came as an upwards thump in the helicopter followed by a downwards thump,” Maloney recalled. This violent vertical vibration — so severe that Idelon had to brace himself with a hand grip to stay in his seat — would continue all the way to the ground. Later investigation revealed that the 1-per-rev vibration resulted from the broken scissor link limiting the movement of the helicopter’s rotating swashplate, resulting in a pitching moment up and down as the swashplate lagged and advanced.
With the vibration too extreme to sustain autorotation, Maloney entered a gentle right-hand spiral to lose altitude. “Then,” he said, “all hell broke loose.”
Main rotor r.p.m. decayed, due to the drag associated with the rapidly fluctuating pitch changes and the inability of the engine governing system to cope with them. The helicopter began to roll left and right, and pitch up and down, in a way that most pilots have only experienced in out-of-control simulators, if at all. “It felt like a rollercoaster swooping up vertical then tipping over nose down and rolling right, and then it began to rotate about the rotor mast nose right,” Maloney said. That’s when he realized that the incident could get serious.
According to Maloney, the investigation into the incident conducted by Composite Helicopters International, Metlab, and the University of Auckland found that the failed component — a standard M81935-1-5 scissor link rod end — suffered a crack across the thread between the lock nut and the ball end. The cause of the failure was determined to be high cycle fatigue. However, a design review found it unlikely that the part was subjected to operational loads that exceeded its design endurance limit, suggesting the possibility of a manufacturing defect.
Maloney declined to identify the manufacturer of the rod end, as that manufacturer has not yet had the opportunity to conduct an independent review of the findings. He noted, however, “This is a standard aviation part used widely across the helicopter industry. Its use was similar to other certified commercial helicopters.”
As a result of the crash, Composite Helicopters will be accelerating the addition of a double scissor link assembly on both the upper rotating swashplate and lower non-rotating swashplate, with a swaged bearing in each arm rather than a rod end.
“Our original swashplate configuration incorporated a single upper scissor link assembly with an opposing balance weight,” Maloney explained. “We had earlier decided that as the balance weight was non-functional, we would delete the balance weight at the next design review and replace it with a double scissor link. . . . Having said this, we had no cause to suspect any elevated risk of the single scissor assembly, considering such mechanisms are almost standard practice in light helicopters.”
Maloney said that the flight testing program will likely resume within the next six months, once a new helicopter — serial number 004 — is completed, and the company has had the opportunity to complete a thorough design review. Whereas previous design reviews have focused on Composite Helicopters’ proprietary technology, he said, going forward, the company will look more closely at third-party components in critical roles.
This is the second accident to occur in the flight-testing program for Composite Helicopters’ new, five- to six-seat, light turbine-engine helicopter. In May 2013, an earlier prototype ditched in Auckland Harbour, with Maloney again at the controls. Neither he nor his crewmember were injured in the accident.
According to Maloney, that event was attributed to a hydraulic lock out plunger seal leak on the main rotor servos, which compromised the integrity of the main rotor gearbox (MRGB) aft dampers, allowing the MRGB/engine drive shaft to exceed its range of movement. As a result of the 2013 accident, Composite Helicopters modified the servo design, deleted the dampers, and increased the range of movement limits on the drive shaft from 1.5 to 3.5 degrees.