The pilot noticed what sounded like a slight misfiring noise coming from the engine after start up, however, all other indications, including a magneto check, were normal. The pilot flew the aircraft to a one metre hover, carried out a power and handling check, which was normal, and entered translational flight for a DEPARTURE from the heli-pad. Shortly after entering translational flight the aircraft began to descend. The pilot opened the throttle and raised the collective lever, however, this did not correct the descent. The pilot observed that both engine and rotor rpm were decreasing. At this point the aircraft was approaching a one metre high post and wire fence and the pilot assessed that the aircraft would clear the fence and touchdown in the open area on the other side of the fence. Both skids collided with the top of the fence causing the pilot to lose control of the helicopter which fell to the ground on the far side of the fence. A post accident engineering inspection disclosed that the movement of the exhaust valve in the No. 2 cylinder was restricted by a build up of carbon deposits. The top end of the engine had been overhauled, in Canada, 442 hours prior to the accident. It is likely that the sticking exhaust valve caused the loss of power reported by the pilot. Sticking valves is a known problem with the engine model fitted to the accident aircraft and the manufacturer, the Civil Aviation Authority and the operator had already taken action to correct it.
