Enstrom Helicopter has directed operators to remove new elastomeric dampers from the main rotor of its Model 480B after an accident believed to have been caused by ground resonance. “We are taking the precaution of removing the dampers until we understand what happened,” says chief executive Jerry Mullins.
The accident occurred on 5 August at Goshen airport near Chicago, when a trainee pilot was seriously injured after being thrown from a 480 after the rotor reportedly malfunctioned, causing vibration that tore the airframe apart. “We have never had ground resonance before in the history of the helicopter,” says Mullins. “The only change is the elastomeric dampers.” The dampers, made by Lord, were certificated earlier this year.
Ground resonance affects fully articulated rotors and occurs when the blades “bunch up” on one side of the rotor disc, generating an unbalanced force in phase with the natural frequency of the aircraft rocking on its landing gear. It is overcome by installing dampers. Affected operators of the single-turbine 480 have been instructed to re-install the original hydraulic dampers while Enstrom works with Lord to understand the problem.
“It has not been proved,” says Mullins. “But when we started certification of elastomeric dampers on the piston helicopters [the 280FX and F-28F], we got an indication of ground resonance.”
Source: Flight International
