IAD99LA015

The pilot reported that while in cruise at 700 feet AGL the helicopter yawed left, followed by a low rotor caution light being illuminated. The pilot executed an autorotation landing under dark night conditions to a flat, wet, muddy surface. During the landing the helicopter's skids began to dig in, and the helicopter began to nose over. The pilot applied aft cyclic in an effort to prevent the nose over during which the rotor blades impacted and severed the tailboom. Post-crash examination revealed 51 gallons of fuel aboard and no spreading deformation to the helicopter's skids. Further examination revealed that the 'B' nut on the PC air line from the power turbine governor to the fuel control unit was backed off about 2-3 'flats.' The engine was successfully operated in a test cell in an 'as received' condition. During subsequent engine runs, the 'B' nut was tightened and loosened to one flat with no variation in engine performance. The 'B' nut was also completely removed resulting in a stagnant start. According to the manufacturer, a breach in the PC air line will allow air pressure to bleed out and result in the fuel control unit automatically shifting to a minimum flow condition (less than idle). The test cell conditions may or may not have duplicated the vibratory conditions in the aircraft's airframe at the time of the power loss.

On November 12, 1998, at 1815 eastern standard time, a Bell 206B, N32KY, registered to Helicopters Inc., and being flown by a commercial pilot, was substantially damaged during an autorotation landing following a total loss of power in cruise near Louisville, Kentucky. The pilot and one passenger were uninjured and a second passenger sustained minor injuries. Visual meteorological conditions dark night conditions prevailed, and a company flight plan was in effect. The local, aerial observation flight was to have been conducted under 14 CFR Part 91, and originated in Louisville approximately 1700.

The pilot reported that the aircraft was in cruise flight at 90 knots, 1,200 feet above mean sea level (700 feet above ground level) and about seven miles west of Louisville International airport (refer to CHART I) headed northeast when "...the aircraft yawed left followed by the low rotor caution horn, and caution light. The aircraft was immediately put into autorotation. A clear landing area was spotted to the right of the flight path and the aircraft was turned into that direction (south east heading). The landing area was flat, however the ground was wet and muddy [and] when the landing gear tried to skid across the mud they dug in. This caused the aircraft to roll forward as though it were going to cartwheel. Aft cyclic was applied to level the aircraft, this action resulted in the trail [sic] boom being separated from the aircraft aft of the horizontal stabiltor [sic]" (refer to attached NTSB Form 6120.1/2).

An inspector from the Federal Aviation Administration's (FAA) Louisville Flight Standards District Office (FSDO) examined the aircraft subsequent to the accident. He reported 51 gallons of fuel in the aircraft's fuel tank. He also reported that the tailboom had been severed and the horizontal tailplane had been destroyed (refer to photograph 1). He observed that the although the forward portion of the left skid (ahead of the forward cross tube) was broken off, the skids displayed no evidence of "spreading" or deformation (refer to photograph 2). Inspection of the engine by the inspector revealed that the "B" nut on the PC air line from the power turbine governor to the fuel control unit was backed off about 2-3 "flats" at the governor end of the line (refer to photograph 3).

The FAA inspector reviewed aircraft documentation and reported that the total airframe time at the time of the occurrence was 3238.3 hours.

The engine was shipped to Rolls Royce Allison where an engine test run in an "as received" condition was conducted on December 7, 1998. According to the Rolls Royce Allison report "The engine started and was run with the loose B nut in the same position as found at the accident site. During subsequent engine runs, the B nut was tightened and loosened to one flat. There was no variation in engine performance. The B nut was also completely removed. This resulted in a stagnant start."

According to the manufacturer, a breach in the PC air line will allow air pressure to bleed out and result in the fuel control unit automatically shifting to a minimum flow condition (less than idle).

The test cell conditions may or may not have duplicated the vibratory conditions in the aircraft's airframe at the time of the power loss.