Houston, TX, USA
N435AE
BELL HELICOPTER TEXTRON CANADA 206 L4
The commercial pilot was departing for a positioning flight from a helipad bordered on three sides by buildings and parking structures. The pilot reported that, after lifting off, as he translated the helicopter from behind one of the buildings and into the prevailing wind, the nose began yawing right. He applied full left pedal, but the helicopter may have fully rotated once while moving back toward the helipad before the rotation stopped. The low rotor speed warning sounded, and the helicopter then began rotating rapidly right. The pilot lowered the collective and maneuvered toward the helipad. He subsequently raised the collective while at 25 ft above ground level, but the helicopter landed hard. A postaccident examination of the helicopter revealed no preimpact mechanical failures or malfunctions that would have precluded normal operation. A loss of tail rotor effectiveness can be encountered while hovering under certain wind conditions, which may be encountered unexpectedly near buildings due to rapidly changing wind conditions. However, the pilot's report that the low rotor speed warning sounded and engine data provided by the operator indicated that the main rotor speed decayed during the takeoff with a corresponding decrease in the tail rotor speed. A significant reduction in the tail rotor speed could result in an uncommanded yaw and a loss of directional control. The investigation was unable to determine if the pilot's loss of directional control was due solely to the decrease in rotor speed during takeoff, the varying wind conditions, or a combination of both.
On April 25, 2016, about 1845 central daylight time, a Bell Helicopter 206L4, N435AE, was substantially damaged during an emergency landing following a loss of directional control shortly after takeoff from the Memorial City General Hospital Heliport (8TS4), Houston, Texas. The pilot and two medical crewmembers onboard were not injured. The aircraft was registered to Helifleet 2015, LLC, and operated by Air Evac EMS, Inc. doing business as Methodist Air Care under the provisions of 14 Code of Federal Regulations Part 135 as a positioning flight. Day visual meteorological conditions prevailed for the flight, which was not operated on a flight plan. The flight was originating at the time of the accident. The intended destination was the Victoria Regional Airport (VCT), Victoria, Texas.The pilot stated that the helicopter was initially oriented on a west heading. After lifting off, he turned to a south heading so that the helicopter would be oriented into the prevailing wind as he departed the area. About 75 feet above ground level, as the pilot translated toward the east from behind the south building and into the prevailing wind, the nose of the helicopter began yawing to the right. He applied full left pedal; the helicopter may have made one full rotation at that time while moving back toward the helipad. The rotation stopped with the helicopter on a west heading; however, the low rotor speed warning subsequently sounded and the helicopter began rotating rapidly to the right. The pilot lowered the collective and attempted to maneuver back to the helipad. About 25 feet agl, he applied collective but the helicopter landed hard. The paved helipad (30 feet by 30 feet, concrete) was surrounded by a grass area and a paved access driveway. The area outside of the perimeter driveway was bordered by buildings and parking structures to the north, west and south. A small parking area, power lines and an eight-lane roadway were located immediately east of the helipad. The helicopter came to rest upright on the grass area surrounding the helipad. It was oriented on a south heading about 15 feet southeast of the helipad. The landing skids had collapsed. The aft end of the tail boom was partially separated approximately halfway between the stabilizers and the tail rotor. A postaccident examination conducted by a Federal Aviation Administration (FAA) inspector did not reveal any anomalies consistent with a preimpact failure or malfunction. After release of the helicopter by the NTSB, the operator performed an engine test run. No anomalies were reported. Engine torque and main rotor speed data was recovered by the operator from a Turbine Tracker unit on-board the helicopter; no other parameters were available. Thirty seconds of data were provided. During the initial 20 seconds, the engine torque peaked from about 20% to nearly 40% consistent with a pretakeoff hydraulic systems check. Over the final approximately 10 seconds of data, the torque increased as the main rotor speed decreased. The torque increased to a maximum of about 121% as the rotor speed decreased to minimum of about 92% before both recovered toward 100%. The engine control system incorporated a fuel control and governor to provide fuel metering. With the throttle in the full open position, the fuel control unit would meter the fuel flow to maintain the desired engine speed set by the pilot. In addition, the fuel control unit restricted the maximum fuel flow to limit the maximum engine speed. The helicopter was also equipped with a low rotor speed warning system that provided a "low rotor RPM" caution light and an audible tone when the rotor speed decreased to 90% +/- 3%. The engine run-up checklist noted that the GOV RPM switch is to be set at 100% in preparation for takeoff. The before takeoff checklist noted that the throttle is to be full open, and the rotor (Nr) and engine (N2) speeds are to be verified at 100%. The flight manual stated that the engine torque may not exceed 100% for takeoff (5 minutes), with a transient of 105% permitted for no more than 5 seconds. The maximum continuous torque limitation is 75%. The rotor speed limitation (power on) was 99% to 101%, with the minimum transient (5 seconds) of 95% and the maximum transient (5 minutes) 104%. Loss of tail rotor effectiveness (LTE) is caused by the tail rotor not providing adequate thrust to maintain directional control and is usually caused by either certain wind azimuths while hovering or by an insufficient tail rotor thrust for a given power setting at higher altitudes. The result is an uncommanded yaw; to the right in helicopters with counterclockwise rotating rotor systems. Pilots are cautioned to be alert to changing wind conditions, which may be experienced when flying along ridge lines and around buildings. LTE is not related to an equipment or maintenance malfunction and may occur in all single-rotor helicopters at airspeeds less than 30 knots.
The pilot's failure to maintain directional control during takeoff in varying wind conditions, which resulted in a hard landing.
Source: NTSB Aviation Accident Database
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