NTSB Factual Report

On September 15, 2002, about 1445 eastern daylight time, a Hughes 369A helicopter, N336SP, was substantially damaged during a forced landing to the Rocky Gorge Reservoir, Burtonsville, Maryland. The certificated private pilot and passenger received minor injuries. Visual meteorological conditions prevailed, and no flight plan was filed for the personal flight conducted under 14 CFR Part 91.

According to the pilot, he had alerted a mechanic that he had felt a shutter on the previous flight, and requested that work be performed on the tailrotor assembly during an upcoming annual inspection. When the pilot examined the helicopter after the annual inspection was completed on September 13, 2002, he noticed that the rudder pedals "were a little stiff." The mechanic balanced the tailrotor, and subsequently signed the maintenance logs.

On the day of the accident, the pilot preflighted the helicopter and departed on the helicopters first flight after the annual inspection. About a minute after liftoff, the pilot experienced a sudden shuttering of the fuselage, followed by a chip detector light and a loss of directional control. The helicopter began to turn to the left, and would not respond to anti-torque pedal input. The pilot maintained control of the helicopter by utilizing the collective. As the helicopter neared the heliport, "everything in the cockpit lit up," and the pilot observed the rotor tachometer decrease below 96 percent. The engine then lost total power, and the pilot performed a forced landing to a reservoir. As the helicopter neared the water, the engine "ignited" again, and the helicopter began to rotate to the right. The pilot set the helicopter down on the water, where it rolled to the right. The main rotor blades impacted the water and sheared from the rotor mast. The helicopter came to rest on its right side, where the occupants egressed before it sunk.

The helicopter and its engine were retained for further examination under the supervision of a Federal Aviation Administration (FAA) inspector.

Examination of the wreckage revealed that one of the main rotor blades was wrapped around the main mast several times. Two of the other main rotor blades were bent opposite the direction of rotation, and the fourth main rotor blade, which was not located during the recovery of the wreckage, was separated at the hub. The tailrotor driveshaft exhibited torsional twisting. The forward coupling on the tailrotor driveshaft was separated. The input bearing cover assembly on the tailrotor gearbox was broken and pushed outwards. The tailrotor gearbox magnetic chip detector exhibited significant metal particles when examined.

According to a manufacturer's report of the examination, no abnormalities were noted with the engine or its related components. Damage to the tailrotor drive system was consistent with sudden stoppage or gradual binding of the tailrotor gearbox. Disassembly of the tailrotor gearbox revealed evidence that the gear teeth of one gear had "jumped" over the gear teeth of the other gear resulting in damage to several gear teeth.

Review of the helicopters maintenance records by the FAA inspector revealed that the tailrotor swash-plate assembly, the tailrotor pitch bearings on both blades, and the tailrotor bellcrank bearing at the swash-plate, were replaced. The inspector also noted that the helicopter was produced for military use, and had several owners since. The maintenance records reviewed were inconsistent and inaccurately recorded. The component history of the tailrotor gearbox could not be determined.

The helicopter had accumulated about 4,408 hours of total operation.

NTSB Probable Cause

The failure of the tailrotor gearbox for undetermined reasons.