The U.S. National Transportation Safety Board (NTSB), citing a 2009 accident involving severe vibration known as “mast rocking” in a Robinson R44, says the manufacturer should be required to identify the cause of the phenomenon and develop steps to avoid it.
Robinson Helicopter told NTSB accident investigators that, even before the agency issued its recommendations, it has begun flight tests to evaluate the problem, sometimes called “chugging.”
The pilot of the accident helicopter — operated by the state of Alaska and being flown in visual meteorological conditions on May 12, 2009, by the Alaska State Troopers–Fish and Wildlife Protection on a game-management patrol — said that about 90 seconds after departure from a site 57 nm (106 km) northwest of Iliamna, Alaska, he felt an unusual vibration, mostly in the pedals, followed by a slight yaw.
“The pilot said the vibrations became oscillations, in both yaw and pitch, to the point he felt the helicopter was going to co…
