Aircraft and Crew
The aircraft involved was a Hughes 369HS (registration G-NIPY), manufactured in 1975 and powered by one Allison 250-C20 turboshaft engine. The commander, aged 50, held an Airline Transport Pilot's Licence with 2,898 total flying hours, of which 137 were on type. His recent flying included 38 hours in the last 90 days and 17 hours in the last 28 days.
Flight Details
On 23 March 2003 at 1124 UTC, an instructor pilot was conducting engine-off landing training with a student pilot, who was the helicopter owner and in the final stages of his Private Pilot's Licence training. The flight took place at Blackpool Airport on Area 'B', a hard grass area designated for helicopter training and engine-off landings. Weather was good with a surface wind of 100° at 9 kt, visibility 9 km, and clear sky. Prior to the flight, a full briefing covered wind conditions, run-on landings, flare recovery, and zero-speed hover landings. Air traffic control cleared the instructor to Area 'B' not above 800 feet.
Accident Sequence
The instructor demonstrated four autorotations, including three power recoveries and one run-on landing. During the run-on landing, the aircraft showed a marked tendency to tip forward, prompting the instructor to elect only zero-speed landings. For the next demonstration, he rolled the throttle to idle and entered autorotation. The helicopter was flared at the normal height, and the cabin floor levelled at approximately six feet with low forward speed as intended. However, a high rate of descent developed that could not be arrested by collective lever application. As the helicopter touched down, it began to tip forward. The instructor applied aft cyclic control in an attempt to contain the forward movement. This caused the main rotor blades to strike the tail boom, severing it. The instructor shut down the engine and requested emergency services, which attended immediately.
Post-Accident Inspection
After the accident, the instructor inspected the area at the front of the helicopter and noted that the forward 2.5 feet of the skids were overhanging a hollow in the ground approximately 8 inches deep. This depression was not visible from the air and had increased the forward pitching motion of the helicopter.
Probable Cause
The instructor assessed the cause as the front of the skid landing gear entering a depression in the grass surface, which allowed the aircraft to tip forward. In attempting to prevent further forward pitch, he made a large, rapid, aft cyclic control input, which caused the main rotor blades to contact and sever the tail boom.