ARDMORE, OK, USA
N1774
Bell 47G2
The flight instructor heard 'a banging noise accompanied with an increase of engine rpm and a decrease of rotor rpm' during takeoff from a high hover. The helicopter entered a left turn and the pilot rolled the throttle off to enter an autorotation. The helicopter landed hard with low rotor rpm. The main rotor blades contacted the tailboom, and the skid landing gear collapsed. Examination of the engine, main transmission, and free wheeling clutch revealed no discrepancies that would have prevented normal flight operations. Prior to commencing the planned dual cross country flight, the helicopter's fuel system had been filled. The density altitude was calculated at 3,040 feet. The operator stated that the accident could have been prevented by 'not hovering out of ground effect at maximum gross weight with a density altitude of 3,000 foot, and better RPM control.'
On August 16, 1999, at 2000 central daylight time, a Bell 47G2 helicopter, N1774, was substantially damaged during a hard landing at the Ardmore Downtown Executive Airport, near Ardmore, Oklahoma. The flight instructor and the commercial pilot receiving instruction were not injured. The helicopter was owned and operated by Versatile Helicopters, Inc., of Ardmore, Oklahoma. Visual meteorological conditions prevailed for the Title 14 CFR Part 91 instructional flight for which an IFR flight plan was filed. The planned destination was the Will Rogers Airport, near Oklahoma City. The flight was preparing to depart when the accident occurred. The operator reported that the instructional flight was scheduled as a dual night instrument cross country flight to satisfy the requirements for a helicopter instrument rating. The operator added that prior to commencing the flight, the flight instructor, who held an A & P mechanic certificate, elected to adjust the trim tabs on the main rotor blades to "smooth out a slight vertical vibration in the rotor system." After adjusting a trim tab, the flight instructor hovered the helicopter at an estimated height of 5 to 8 feet above the ground to check if the vertical vibration had been reduced or eliminated. The 870-hour flight instructor told the FAA inspector that upon completion of the high hover check, he elected to take the helicopter around the pattern to check the smoothness of the ride in forward flight. During the takeoff sequence, at an estimated height of 25 to 30 feet, the flight instructor heard "a banging noise accompanied with an increase of engine rpm and a decrease of rotor rpm." The helicopter entered a left turn and the pilot rolled the throttle off to enter an autorotation. The helicopter descended and landed hard with low rotor rpm. The main rotor blades contacted the tailboom, and the skid landing gear collapsed. Examination of the helicopter by the operator revealed that the tailboom was severed and both main rotor blades were destroyed. The operator added that the engine continued to run after the hard landing. The helicopter's fuel system had been serviced to 43 gallons of 100LL fuel for the anticipated 78-nautical mile cross country flight. An FAA inspector and the operator conducted a detailed inspection of the engine, main transmission, and free wheeling clutch. No discrepancies were found that could have prevented normal flight operations. In the recommendation block of the enclosed NTSB Form 6120.1/2, the operator recommends "not hovering out of ground effect at maximum gross weight with a density altitude of 3,000 foot, and better RPM control." The winds at the airport were reported from 130 degrees at 3 knots. The temperature and dew point were reported as 91 and 55 degrees Fahrenheit respectively. The NTSB investigator-in-charge calculated the density altitude at 3,040 feet.
The pilot's failure to maintain rotor rpm which resulted in a hard landing. A factor was the high density altitude.
Source: NTSB Aviation Accident Database
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