Robinson Helicopter CEO Kurt Robinson said the company has been working with the FAA for “some time” to address concerns about “mast-rocking” or “chugging” on its R44 helicopters. On August 22 the NTSB issued several safety recommendations (A-11-82 through -86) related to the condition in the wake of several mast-rocking accidents/incidents dating back to 2006. “We are working with the FAA to address any concerns or recommendations the NTSB has,” Robinson told AIN.
Pilots of the incident/accident aircraft reported yaw and pitch oscillations so severe that they elected to make emergency landings, several of them hard enough to cause substantial aircraft damage. The condition appears to be exacerbated by a forward cg that can still be within the flight envelope and is more readily entered during descending and banking turns of 30 degrees. Flight-test reports dating back to 2006 found that certain combinations of transmission mount and vibration isolator mitigate the problem, as did stopping turns while adding power, especially during practice autorotations.
The NTSB recommends that the FAA require Robinson to resolve the “root cause” of the mast-rocking, create a database of all related incidents, insert a warning in the flight manual and incorporate recognition and mitigation of the vibrations into its pilot training program.
While documented incidents cited by the NTSB date back to 2006, AIN found reports of “chugging” that were more than 10 years old and all seem to have several common characteristics, including loading close to, at or over mtow and a forward cg, often encountered after the R44 had burned off substantial fuel. Several aircraft at/over weight limits were conducting air-tour operations. Pilots reported condition onset after descending left turns, fast straight-and-level flight and aggressive maneuvers. It is sometimes first detected by vibrations felt through the anti-torque pedals that grow more severe until the helicopter enters a “bucking” nose-up/nose-down motion that increases in severity through 30 degrees of pitch until it feels as if the helicopter is “out of control.” Although the condition can successfully be flown out of by raising the collective while adding power, it has prompted some pilots to make hasty emergency landings that in least one case completely destroyed a helicopter.
That accident occurred on May 12, 2009, when an R44 being operated by the Alaska State Troopers made a hard emergency landing after experiencing chugging. The helicopter touched down with 5 to 10 knots of forward speed and the main rotor contacted the tail boom. The pilot and two passengers were not injured. The NTSB determined the probable cause was “the main transmission mount design, which resulted in in-flight vibration/oscillation and damage to the helicopter during the subsequent emergency descent and hard landing. Contributing to the accident was the lack of information from the manufacturer regarding this known flight oscillation, and loading the helicopter beyond the forward center of gravity limit by the pilot.”
During its investigation the NTSB found “at least” three similar incidents: Dec.16, 2006, in Ballymena, Ireland; March 15, 2007, in Opa Locka, Fla.; and Sept. 30, 2007 near Jackson Center, Ohio. None of the accidents/incidents resulted in injury to those aboard. The tail boom separated on landing in the Opa Locka incident.
A 2006 flight-test report prepared by an FAA flight-test engineer found that chugging could be induced under several flight regimes and stopped under certain conditions using an R44 with aft and forward main rotor transmission mounts that are designed to react with upward and downward movement of the transmission. Vibrations stopped when G loading was reduced in turns, but could be induced during straight-in autorotations with a forward cg. The maneuvers were replicated in a second R44 with stiffer aft mounts and did not produce vibrations.
However, according to the FAA test pilot, a standard configuration was not established as each helicopter responded differently during testing. The report stated that Robinson planned to revise its flight-test procedure to have helicopters fly the maneuvers while in forward cg and, if needed, install stiffer main-rotor transmission mounts and retest the aircraft. On Aug. 17, 2008, Robinson informed the UK’s Air Accidents Investigation Branch, which was investigating the Ballymena accident, that it was no longer encountering a vibration problem during test flights, had not received any additional reports of it from customers, and would therefore not issue a service letter about the condition.
The NTSB notes that helicopters in service before the revised flight-test procedure “continue to exhibit this condition.” The Board also noted that “it is not known if the condition will manifest as the helicopters age, even with stiffer transmission mounts installed. It is also not known how frequently mast rocking may occur in the fleet because Robinson Helicopter does not track reports of this condition. The NTSB concludes that the lack of a specific solution for the mast-rocking vibration in all affected R44 helicopters suggests that the manufacturer has not identified the underlying cause of the vibration.”
Kurt Robinson declined to speculate on what specific actions the FAA is likely to take.