BROWNSBORO, TX, USA
N575PF
Hughes 269A
Prior to the accident, the helicopter was flying on an easterly heading in level flight at approximately 20 knots while making shallow turns following a creek bed. The pilot reported that the helicopter lost all of its lift and power and started descending toward the trees. The pilot added that the anti-torque pedals were ineffective and the helicopter was spinning to the right prior to impact with the trees. Minor repairs were made to the engine in order to run the engine on the helicopter. The engine was started and operated twice within the 1300 to 2500 RPM range for 18 minutes. A representative of the engine manufacturer reported 'no pre-existing engine deficiencies were noted and the investigation did not produce any evidence that the engine was not capable of operating and producing power at the time of the accident.' The conditions at the time of the accident were found to be conducive to the formation of a tail rotor vortex ring state.
On December 18, 1999, at 1330 central standard time, a Hughes 269A helicopter, N575PF, was substantially damaged during a hard landing following a loss of engine power while maneuvering near Brownsboro, Texas. The instrument rated commercial pilot sustained minor injuries and his passenger was seriously injured. The helicopter was owned and operated by Brown Helicopter Services, Inc., of Flint, Texas. Visual meteorological conditions prevailed for the Title 14 Code of Federal Regulations Part 91 aerial survey flight for which a flight plan was not filed. The flight originated from the operator's private heliport near Flint, Texas, approximately 1250. In a written statement submitted by the pilot, he stated that just prior to the accident, the helicopter was flying about 100 feet above the ground on an easterly heading at an estimated airspeed of 20 knots, while making shallow turns following a dry-creek bed. The pilot reported that "suddenly the helicopter lost all of its lift and power and started descending toward the trees below." The pilot further stated that "he applied forward cyclic in an attempt to fly out of it" to no avail. He added that the "anti-torque pedals felt totally ineffective and just prior to impact with the trees the helicopter was in a flat spin to the right." Subsequently, the helicopter fell through the thick canopy of trees and rolled 30 to 45 degrees to the left prior to coming to rest on a northerly heading in the heavily wooded area. The FAA inspector, who traveled to the accident site, reported that the left side of the airframe sustained structural damage, the tailboom was severed approximately 4 feet aft of the main transmission, and the main and tail rotor blades were structurally damaged. The helicopter was released and recovered to the maintenance facilities of Reynolds Aviation at the Wills Point Municipal Airport, near Wills Point, Texas, for further examination and testing. On February 3, 2000, the Lycoming HIO-360-B1A engine, serial number L-1720-51A, was examined under the supervision of an FAA inspector. Minor repairs and adjustments were made in order to run the engine while still mounted on the helicopter. A temporary fuel supply line was made to provide fuel from an external source. The engine was started and operated twice at within the 1300 to 2500 RPM range for a cumulative total of 18 minutes. A representative of the engine manufacturer, who witnessed the engine run, reported "no pre-existing engine deficiencies were noted and the investigation did not produce any evidence that the engine was not capable of operating and producing power at the time of the accident." The loss of tail rotor effectiveness reported by the pilot and the resulting unanticipated right yaw (spin) also reported by the pilot are signatures consistent with an aerodynamic rotorcraft phenomena known as "tail rotor vortex ring state," for which certain relative wind velocities and azimuth (direction of the relative wind) must be present. These characteristics are present only at airspeeds below 30 knots, and with relative wind directions of 210 to 330 degrees. The pilot reported that, at the time of the occurrence, the winds were from 340 degrees at 10 knots. Given the easterly heading of the helicopter, the relative wind direction was approximately 250 degrees.
The loss of tail rotor effectiveness as a result of tail rotor vortex ring state.
Source: NTSB Aviation Accident Database
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