NTSB Identification: CHI05GA198.
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Accident occurred Tuesday, July 26, 2005 in Justice, IL
Probable Cause Approval Date: 12/28/2006
Aircraft: Bell 206L-3, registration: N741LL
Injuries: 2 Serious,2 Minor.
: NTSB investigators either traveled in support of this investigation or conducted a significant amount of investigative work without any travel, and used data obtained from various sources to prepare this public aircraft accident report.
The helicopter was destroyed when it impacted a sign, a privacy/guard fence, and the terrain adjacent to an interstate highway. The public use flight was conducted in order to survey a tollway. According to the pilot, the helicopter began, "a sudden right yaw with possibly a raising of the tail. The aircraft then yawed left, possibly in response to pedal input ... Then started what seemed to be a slower right rotation." The helicopter continued to rotate to the right, descended, and impacted the structure of an overhead road sign, a privacy/guard fence and then came to rest on the ground adjacent to the fence. The reported winds in the vicinity of the accident site were 290 degrees at 15 knots. Radar track data showed that the helicopter traveled in a northwesterly direction parallel to an interstate highway, passed a toll plaza, and then began a left turn. The radar data showed that during the left turn, the wind direction relative to the aircraft ground track entered the region defined in the Federal Aviation Administration "Rotorcraft Flying Handbook" (FAA-H-8083-21), as being conducive to weathercock instability. During this same period, the helicopter's calibrated airspeed was between 4 and 20 knots. FAA-H-8083-21 states that, "In this region, the helicopter attempts to weathervane its nose into the relative wind. Unless a resisting pedal input is made, the helicopter starts a slow, uncommanded turn either to the right or left depending upon the wind direction. If the pilot allows a right yaw rate to develop and the tail of the helicopter moves into this region, the yaw rate can accelerate rapidly." FAA-H-8083-21 also states that, "Unanticipated yaw is the occurrence of an uncommanded yaw rate that does not subside of its own accord and, which, if not corrected, can result in the loss of helicopter control. This uncommanded yaw rate is referred to as loss of tail rotor effectiveness (LTE) and occurs to the right in helicopters with a counter-clockwise rotating main rotor." No anomalies were found with respect to the helicopter.
The National Transportation Safety Board determines the probable cause(s) of this accident to be: The pilot's improper in-flight decision while maneuvering which resulted in a tailwind during low airspeed helicopter flight and subsequent loss of tail rotor effectiveness. The struck sign and fence were factors in the accident. Full narrative available
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