On May 12, 2009, a Robinson R-44 helicopter was damaged during a hard landing about 57 miles northwest of Iliamna, Alaska. The purpose of the flight was game management patrol for the Alaska State Troopers, Fish and Wildlife Service. After take-off from a ridge, about 300 feet above the ground, the helicopter was flying about 90 knots when the pilot felt an unusual medium-frequency vibration in the controls. The pilot told the NTSB that the vibrations turned to oscillations in both yaw and pitch to the point he felt the helicopter was going to come apart. He decided to make an immediate precautionary landing. During the descent the vibration increased and the helicopter landed hard causing the main rotor blades struck the tail boom.
The NTSB discovered that operators of the Robinson R44 helicopter were aware of similar events and that the condition had been dubbed “chugging.” According to Robinson Helicopter, tests determined that a mast rocking oscillation may develop during operation of the helicopter at high gross weight and about 90 to 100 knots. The oscillation was more of a “bucking” motion due to the fore-and-aft movement of the rotor mast. Tests also showed the tendency to enter the oscillation was exacerbated by a forward CG (within the CG envelope) and a 30 degree banked turn to the left. The oscillation is not divergent (that is, the main rotor blades do not diverge from their normal plane of rotation) and can be reduced by adding power. The oscillation is due to the firmness, or lack of firmness, of the transmission mounts. At the time there were no service alerts/bulletins referencing the phenomena or the remedies to resolve it.
On August 22, 2011 the NTSB issued the following safety recommendations to the FAA:
- Require Robinson Helicopter to resolve the root cause of the mast-rocking vibration in the main rotor assembly to ensure that all applicable R44 helicopters are free of excessive vibrations in all flight regimes, as required by 14 Code of Federal Regulations Section 27.251, “Vibration.” (A-11-82)
- Require Robinson Helicopter to maintain a database of all reported incidents of mast rocking in the main rotor assembly of R44 helicopters. (A-11-83)
- Require Robinson Helicopter to issue a service letter to all approved service centers describing the mast-rocking vibration that can occur in the main rotor assembly of R44 helicopters and instructing service centers to report all incidents of mast rocking to the manufacturer. (A-11-84)
- Require Robinson Helicopter to amend the R44 helicopter flight manual to inform pilots of the potential for mast-rocking vibration in the main rotor assembly and how to safely exit the condition. (A-11-85)
- Require that the Robinson Helicopter R44 pilot training program be revised to provide pilot instruction in the recognition and mitigation of in-flight mast-rocking vibrations in the main rotor assembly. (A-11-86)
Most helicopter pilots are aware of mast bumping in semi-rigid (two-bladed) rotor systems, but this issue is new and not as well-known and raising awareness is important to safe operations.