History of the Flight
On 16 December 2006, at 1115 UTC, a Robinson R44 Raven II (registration G-CEFR) was on approach to Ballymena, Northern Ireland, after a private flight. The pilot had just completed a 15-minute sightseeing flight with three children and then departed on a second flight with three adults. While on downwind leg at approximately 700 ft agl and 75-80 KIAS, the aircraft suddenly started oscillating in pitch, with high vibration felt through the cyclic control. The pilot could not arrest the oscillation or vibration with normal inputs, and the magnitude increased to the point of concern about structural integrity. He decided to land immediately and performed a run-on landing. The aircraft came to rest without apparent damage or injury. During engine shutdown, vibration persisted though temperatures and pressures were normal. The aircraft had 18.1 hours total time since new.
Examination and Rectification
The maintenance organisation conducted inspections as recommended by the manufacturer. The only damage found was a distorted aluminium lower rib within the mast fairing assembly, suspected to be caused by excessive rocking of the main rotor gearbox impacting the rear hydraulic servo. The main rotor hub teeter friction was measured at 22 lbf, exceeding the 19 lbf limit, and was adjusted to 6.2 lbf. The main rotor blade coning hinge frictions were uneven (4.5 and 2 lbf) and adjusted to 2.2 lbf. The two forward main rotor gearbox rubber isolation mounts (p/n A653-1) were replaced with newer stiffer mounts (p/n A653-2) following manufacturer advice. A subsequent flight test at various weight and CG configurations showed no recurrence.
Manufacturer's Assessment
The manufacturer stated that the vibration problem was first experienced during flight test in 1993, manifesting at forward CG when CG was forward of the main rotor gearbox. The vibration was caused by the gearbox rocking fore and aft on its mounts, felt as a 0.6 per main rotor revolution vertical vibration. The pilot could cure the problem by increasing power. To eliminate excessive vibration, the manufacturer had replaced forward gearbox mounts (p/n C653-4) with stiffer mounts (p/n A653-1). After the G-CEFR incident, the manufacturer measured the stiffness of new A653-1 mounts and found them softer than those manufactured in 2000. They believe this softening led to recurrence. They have not found evidence that hub teeter friction or coning hinge friction contributes to the problem. Since June 2007, the manufacturer conditions mounts by heating at 200°F under 1,000 lbf load for 12-24 hours to give them compression set. They install new stiffer mounts (p/n A653-2) on new aircraft and retrofitted G-CEFR. As of August 2007, no further vibration reports had been received.
Previous Incident
In November 2006, the same aircraft experienced a heavy vibration incident at 10.1 hours. At that time, the aircraft was loaded to 2,566 lb (66 lb above MTOW) with a CG of 93.64 inches (near forward limit of 93 inches). The maintenance organisation inspected and flight tested at 2,350 lb, finding no vibration. They recommended not flying above MTOW and avoiding forward CG until manufacturer consultation.