Justice, IL, USA
N741LL
Bell 206L-3
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.
HISTORY OF FLIGHT On July 26, 2005, about 1010 central daylight time, a Bell 206L-3 helicopter, N741LL, piloted by an airline transport pilot, was destroyed when it impacted a sign, a privacy/guard fence, and the terrain adjacent to an interstate highway near Justice, Illinois. The helicopter was owned by the State of Illinois and was operating as a public use flight surveying the Illinois Tollway. The flight was conducted under 14 CFR Part 91, and was operating in visual meteorological conditions without a flight plan. The pilot and one passenger received serious injuries. Two passengers received minor injuries. The local flight originated about 0910 from a heliport near Downers Grove, Illinois. In a written statement, the pilot reported that he had flown the helicopter from the Springfield Capitol Airport, Springfield, Illinois, earlier in the day. He reported that this flight was "routine" and that no mechanical difficulties were detected during that flight. The pilot reported that after picking up 3 passengers, he departed from the Tollways commission heliport and proceeded to a construction site near the junction of interstate highways I-57 and I-80. He stated that he then proceeded to the next site and then flew past the toll plaza and began a left turn. He stated that his intent was to fly back northbound toward the plaza at 200 to 300 feet above ground level at 40 to 50 knots indicated airspeed (KIAS). He stated "this northbound cruise appeared to be normal until there was a sudden right yaw with possibly a raising of the tail. The aircraft then yawed left, possibly in response to pedal input, I don't remember. Then started what seemed to be a slower right rotation." According to the pilot and passenger statements, the helicopter continued to rotate to the right. The helicopter then impacted the structure of an overhead road sign, a privacy/guard fence and then came to rest on the ground adjacent to the fence. PERSONNEL INFORMATION The pilot held an airline transport pilot certificate with ratings for multiengine land airplanes and helicopters. The certificate listed commercial privileges for single engine land airplanes. The pilot's first class medical certificate was issued on April 27, 2005, and listed no limitations. According to a report filed by the operator, the pilot had accumulated a total of 9,682 hours of flight experience. The operator listed the pilot's recent flight experience as 125 hours in the last 90 days, which included 53 hours in the same make and model as the accident helicopter. AIRCRAFT INFORMATION The helicopter was a Bell 206L-3, serial number 51165. A Rolls Royce model 250 C30P engine rated at 650 shaft horsepower powered it. The helicopter had a semi-rigid single main rotor and a tail mounted anti-torque rotor. The helicopter's rotor rotated in a counter-clockwise direction when viewed from above. Maintenance records showed that the helicopter had undergone an annual inspection on May 5, 2005. According to the maintenance records, the helicopter had accumulated a total of 8,026 hours time in service and 88 hours since the annual inspection. METEOROLOGICAL INFORMATION The weather report for the Chicago Midway International Airport (MDW), Chicago, Illinois, located about 5 nautical miles northeast of the accident site, was recorded at 0853 as: Winds 270 degrees at 10 knots gusting to 19 knots; Visibility 10 statute miles; Sky condition, few clouds at 3,100 feet above ground level (agl), scattered clouds at 4,900 feet agl, broken clouds at 15,000 feet agl, broken clouds at 25,000 feet agl; Temperature 28 degrees Celsius; 19 degrees Celsius; Altimeter setting 29.79 inches of Mercury. The MDW weather was recorded at 0953 as: Winds 290 degrees at 15 knots; Visibility 10 statute miles; Sky condition, broken clouds at 3,000 feet agl, broken clouds at 30,000 feet agl; Temperature 28 degrees Celsius; 20 degrees Celsius; Altimeter setting 29.79 inches of Mercury. The MDW weather was recorded at 1053 as: Winds 280 degrees at 10 knots gusting to 18 knots; Visibility 10 statute miles; Sky condition, broken clouds at 3,300 feet agl, broken clouds at 15,000 feet agl, broken clouds at 30,000 feet agl; Temperature 28 degrees Celsius; 20 degrees Celsius; Altimeter setting 29.80 inches of Mercury. WRECKAGE AND IMPACT INFORMATION The helicopter came to rest adjacent to the northbound lanes of interstate I-94 between the guardrail and the privacy/guard fence. The location was determined using a global positioning system receiver to be 41 degrees 44.592 minutes north latitude, 87 degrees 50.064 minutes west longitude, at an elevation of 992 feet above mean sea level (msl). Tubular debris collected in the area of the wreckage was consistent with the tubular structure of an overhead road sign. The overhead sign structure had several pieces of cross-brace tubing missing. The overhead sign also had indentations along with black and white colored paint transfer marks. The helicopter's main rotor blades were fragmented and had a black lower surface and a white upper surface. The inboard portions of both rotor blades remained attached to the blade grips and the main rotor hub. The main rotor hub and mast remained attached to the helicopter. The helicopter's fuselage came to rest on its right side. The right landing skid was separated from the helicopter. Portions of the right landing skid were found on the opposite side of the privacy fence. The left landing skid was bent upward relative to the fuselage. The rear landing gear brackets were separated from the fuselage. The fuel bladder was exposed and was leaking a fluid. A lateral upward bend accompanied by parallel impressions was found in the lower fuselage just aft of the forward edge of the rear door opening (approximately fuselage station 100). The width of the bent portion and the impressions were consistent to the thickness of the top edge of the privacy fence. The privacy fence had damage to its top edge directly above the main wreckage. The tail boom of the helicopter was separated at the aft end of the fuselage and came to rest adjacent to the fuselage. The tail boom was folded to the left at a point about 18 inches forward of the tail rotor. The tail rotor gearbox remained attached to the tail boom. The tail rotor hub and the inboard portions of the tail rotor blades remained attached to the gearbox. The tail rotor drive shaft was separated at the forward end of the tail boom adjacent to the location of the boom separation. It was also fractured at the location of the bend in the tail boom forward of the tail rotor. Examination of the control system revealed that one of the pitch links from the rotating swash plate to the rotor blade pitch horn was broken about mid-length. The break was consistent with bending and overload. Linkages on the cyclic and collective system hydraulic servo actuators were bent and fractured. The fractures had signatures consistent with overload. Movement of the cyclic and collective controls within the cockpit resulted in corresponding movement of the main rotor swash plate. Movement of the anti-torque pedals resulted in corresponding movement of the tail rotor push-pull rod at the aft end of the fuselage. This push pull rod was separated adjacent to the tail boom separation at its forward end and was bent at the location of the bend in the tail boom forward of the tail rotor. The helicopter's engine was visually examined and no anomalies were noted. The engine gearbox magnetic indicating plugs were removed and no anomalies detected. The N1 and N2 sections of the engine were rotated by hand and were free to rotate. All pneumatic lines were examined for security. The aircraft fuel filter was examined and was clean and free of contaminants. The engine fuel nozzle was examined and no anomalies were noted. No pre-impact anomalies were found with respect to the helicopter. TESTS AND RESEARCH Radar track data was obtained from the Federal Aviation Administration (FAA). The data was plotted on aerial photographs depicting the accident area. The plotted data showed that the helicopter traveled in a northwesterly direction parallel to Interstate 294 in northeastern Illinois. The data showed that upon passing a toll plaza the helicopter began a left turn at approximately 1013:33 (hhmm:ss). The helicopter continued the left turn and at 1013:47 the helicopter's ground track was 143 degrees relative to true north. At this time, the helicopter's groundspeed was 20 knots, and the calibrated airspeed was 13 knots. The wind direction relative to the helicopter's ground track was determined to be 147 degrees to the right of the ground track. The helicopter continued to turn to the left and at 1013:52 the ground track was 126 degrees, the groundspeed was 18 knots, the true airspeed was 5 knots, and the wind direction relative to the ground track was 164 degrees to the right. The last radar return was received at 1014:24. The radar data showed that the wind direction relative to the aircraft ground track was between 147 degrees and 223 degrees from 1013:47 to the end of the data. During this same time period, the calibrated airspeed was between 4 and 20 knots. The FAA Rotorcraft Flying Handbook (FAA-H-8083-21) states: "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 and to the left in helicopters with a clockwise main rotor rotation. Again, this discussion covers a helicopter with a counter-clockwise rotor system and an antitorque rotor. LTE is not related to an equipment or maintenance malfunction and may occur in all single-rotor helicopters at airspeeds less than 30 knots. It is the result of the tail rotor not providing adequate thrust to maintain directional control, and is usually caused by either certain wind azimuths (directions) while hovering, or by an insufficient tail rotor thrust for a given power setting at higher altitudes. For any given main rotor torque setting in perfectly steady air, there is an exact amount of tail rotor thrust required to prevent the helicopter from yawing either left or right. This is known as tail rotor trim thrust. In order to maintain a constant heading while hovering, you should maintain tail rotor thrust equal to trim thrust. The required tail rotor thrust is modified by the effects of the wind. The wind can cause an uncommanded yaw by changing tail rotor effective thrust. Certain relative wind directions are more likely to cause tail rotor thrust variations than others. Flight and wind tunnel tests have identified three relative wind azimuth regions that can either singularly, or in combination, create an LTE conducive environment. These regions can overlap, and thrust variations may be more pronounced. Also, flight testing has determined that the tail rotor does not actually stall during the period. When operating in these areas at less than 30 knots, pilot workload increases dramatically." FAA-H-8083-21 further states: "Weathercock Stability (120-240°) 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. In order to avoid the onset of LTE in this downwind condition, it is imperative to maintain positive control of the yaw rate and devote full attention to flying the helicopter." ADDITIONAL INFORMATION The FAA, Rolls-Royce, Bell Helicopter, and the State of Illinois were parties to the investigation.
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.
Source: NTSB Aviation Accident Database
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