Big Creek, KY, USA
N36HF
Bell 206B
The pilot of the helicopter was en route to pick up passengers, when approximately 16 miles prior to his intended destination, he landed in a field due to deteriorating weather conditions. During a cellular telephone conversation with his employer, it was decided that the pilot should reverse course, land at a nearby airport, or return to the helicopter's base. The pilot took off from the field and attempted to fly directly to an airport, but was again unable to follow his desired course due to weather. As he maneuvered, the weather improved, the ridgelines were visible, and the pilot was able to fly "about 100 feet above the trees, and 200 feet below the clouds" as he approached a field that was crossed by a power line. The pilot said he chose to land in the field because of a "klunking" noise behind him. "As I entered the approach the noise got worse and the aircraft started to vibrate. The vibrations got very bad as I tried to arrest my descent, so I decided that touching down hard was better than shaking apart." Examination of the global positioning system ground track revealed that the helicopter descended at a rate of 1,000 feet per minute during an s-turn across the face of a mountain before it leveled over flat terrain, and slowed below effective translational lift, in the vicinity of the crash site. The crash site was surrounded by rising terrain, in the lee of a mountain with strong winds, and an AIRMET for turbulence was in effect along the route of flight. The helicopter experienced a high sink rate and high levels of vibration prior to ground contact. According to The Good Aviation Practices publication Helicopter Performance, published by the Civil Aviation Authority of New Zealand (November 2002), "Local terrain, trees, and buildings all influence the flow of wind near them. The mechanical turbulence resulting from this disturbed airflow may become very marked in the lee of the obstruction. A gusting wind situation where windshear is likely to be present will require a greater power margin to deal with any unexpected loss of airspeed and sink." Examination of the wreckage and a post accident engine run revealed no preimpact anomalies.
HISTORY OF FLIGHT On February 17, 2006, at 0950 eastern standard time, a Bell 206B helicopter, N36HF, was substantially damaged during collision with terrain while maneuvering near Big Creek, Kentucky. The certificated commercial pilot was seriously injured. Instrument meteorological conditions prevailed for the flight that originated at Yeager Airport (CRW), Charleston, West Virginia, and was destined for Middlesboro-Bell County Airport (1A6), Middlesboro, Kentucky. A company visual flight rules (VFR) flight plan was filed for the positioning flight conducted under 14 Code of Federal Regulations Part 91. The pilot was interviewed by telephone and provided a written statement. According to the pilot, the purpose of the flight was to pick up passengers in Middlesboro, and fly them to Bluefield, West Virginia. Approximately 16 miles prior to his intended destination, the pilot landed in a field due to deteriorating weather conditions, and called his employer by cellular telephone. During the telephone conversation, it was decided that the pilot should reverse course, and either land at Hazard, Kentucky, to wait for improved weather conditions, or return to Charleston, West Virginia. The pilot took off from the field and attempted to fly directly to Hazard, but was again unable to follow his desired course due to weather. He then attempted to backtrack along his original course using the "breadcrumbs" feature on the helicopter's Global Positioning System (GPS) receiver. The pilot said that as he maneuvered towards Hazard, the weather improved. The ridgelines were visible, and he was able to fly "about 100 feet above the trees, and 200 feet below the clouds" as he approached a field that was crossed by a power line. The pilot was unclear of the details after that point in the flight. He only recalled that he closed the throttle because the helicopter was rotating, and was later evacuated from the field by MEDEVAC helicopter. In a subsequent written statement, the pilot said he underwent hypno-therapy in an effort to recall more details from the flight. He stated, "I remembered that I started the approach to the crash [site] because there was a 'klunking' noise coming from behind me. As I entered the approach the noise got worse and the aircraft started to vibrate. The vibrations got very bad as I tried to arrest my descent, so I decided that touching down hard was better than shaking apart. I remembered hitting the ground with the front part of the right skid first and chopping the throttle sometime soon after that as the aircraft spun." In a telephone interview with a Federal Aviation Administration (FAA) inspector, a witness stated he saw the helicopter descending to land but thought that it seemed to be descending "too fast." The helicopter came down to the ground then pitched nose down (the tail went upward) and the main rotor struck the ground and the helicopter bounced back upward as it crashed. The witness added that the helicopter's engine "might not have been running properly." Data from the GPS receiver was downloaded under the supervision of the FAA, and forwarded to the National Transportation Safety Board. Examination of the ground track of the helicopter in the last minute of flight revealed the helicopter was on a northerly track as it crossed a saddle of about 1,300 feet elevation, at an altitude of about 1,700 feet mean sea level (msl). The helicopter then turned left, across the northern face of the western peak of the saddle. The helicopter followed along the 1,100-foot contour line, and descended 300 feet during the 18-second turn to the west. The track then made a sharp, right-hand, descending turn to the north, and leveled about 100 feet above flat terrain about 900 feet msl, and surrounded by rising terrain on all sides. The track continued in a turn to the west for approximately 15 seconds before it ended in the vicinity of the crash site. Interpolation of the track data revealed that after crossing the saddle, the helicopter descended at a rate of 1,000 feet per minute during the s-turn across the face of the mountain before it leveled over the flat terrain in the vicinity of the crash site. The helicopter's ground speed slowed below 20 knots as it leveled. The accident occurred during the hours of daylight approximately 37 degrees, 05 minutes north latitude, and 83 degrees, 32 minutes west longitude. PERSONNEL INFORMATION The pilot held a commercial pilot certificate with a rating for rotorcraft-helicopter and instrument helicopter. The pilot's most recent FAA second-class medical certificate was issued January 16, 2006. The pilot reported 2,069 total hours of flight experience, all of which was in helicopters. He reported 1,153 hours of experience in make and model. AIRCRAFT INFORMATION The helicopter was manufactured in 1975, and had accrued 10,800 total aircraft hours. The helicopter was on a manufacturer's inspection program, and it's most recent annual inspection was completed February 10, 2006, at 10,796 aircraft hours. During his initial interview, the pilot described the performance and handling of the helicopter as "flawless." He said there were no unusual noises or indications from the helicopter during the flight. METEOROLOGICAL INFORMATION At 0712, the weather reported at Yeager Airport, Charleston, West Virginia, included broken cloud layers at 1,900 feet, 2,600 feet, and 5,000 feet. The wind was from 300 degrees at 18 knots, gusting to 30 knots. At 0753, the weather reported at Julian Carroll Airport (JKL), Jackson, Kentucky, 47 miles southwest of CRW, included an overcast ceiling at 900 feet and the wind was from 270 degrees at 9 knots, gusting to 18 knots. At 0753, the weather reported at London-Corbin/Magee Airport (LOZ), 25 miles west of the crash site, included an overcast ceiling at 800 feet, visibility 7 miles, and wind from 330 degrees at 9 knots gusting to 19 knots. At 0953, the weather reported at London-Corbin/Magee Airport (LOZ), included a broken ceiling at 1,500 feet, an overcast ceiling at 2,000 feet, visibility 10 miles, and wind from 330 degrees at 6 knots gusting to 14 knots. At 1058, the weather reported at Wise, Virginia (LNP), 53 miles east of the crash site included an overcast ceiling of 100 feet and visibility of 1/4 mile. At the time of the accident, Airman's Meteorological Information (AIRMET) Sierra was in effect for mountain obscuration, and AIRMET Tango was in effect for turbulence, in the area that included the route of flight. The pilot reported that the winds at the time of the accident were from 270 degrees at 20 knots, gusting to 25 knots. According to a Safety Board meteorologist, a review of the weather around the time of the accident revealed, "A strong cold front passed through the area a little earlier in the morning. Strong, gusty northwesterly low-level winds were occurring behind the front. Weather observations and visible satellite imagery indicated overcast clouds (wave clouds under the strong NW flow). There were AIRMETs out for mountain obscuration and moderate low-level turbulence." According to FAA-P-8740-2, "When the wind speed is above about 25 knots, and flowing perpendicular to the ridge lines, the air flow can form waves, much like water flowing over rocks in a stream bed. The waves form down wind from the ridgeline and will be composed of very strong up and down drafts, plus dangerous rotor action under the crests of the waves. If enough moisture is present, lenticular clouds can form to give a visual indication of the wave action. These clouds are reported in the remarks section of hourly sequence reports as ACSL (altocumulus standing lenticular) or CCSL (cirrocumulus standing lenticular)." WRECKAGE AND IMPACT INFORMATION The helicopter was examined in the field by an FAA inspector, and all major components of the helicopter were accounted for at the scene. The helicopter came to rest on its right side, and the main rotor, 90-degree gearbox, tail rotor, and vertical fin were separated and scattered about within an approximate 100-foot radius. The main transmission was broken free of its mounts. The windscreen, chin bubbles, and the instrument panel were separated from the cockpit, leaving the flight controls and cockpit seats exposed. The anti-torque pedals, the floor, and the seats in the cockpit appeared intact. The helicopter was removed from the scene, and a detailed examination was performed in Charleston, West Virginia, under the supervision of an FAA inspector on February 22, 2006. The examination revealed no preimpact anomalies. The engine was removed for further examination at a later date. On March 10, 2006, the engine was examined at the Rolls-Royce Factory, Indianapolis, Indiana under FAA supervision. There was damage to the compressor first stage wheel, with blade material missing and bent opposite the direction of rotation, both as a result of impact and foreign object damage (FOD). The N1 section rotated freely by hand and continuity was established to the starter/generator. The N2 section rotated freely by hand and continuity was established to the power takeoff. The engine was mounted in a test stand, but the starter generator required replacement due to a broken wire. A Pc (compressor discharge pressure) air tube leak was noted, but the test was continued. The engine started and ran without interruption, but the full test was not completed due to excessive vibration from the FOD-damaged compressor. Extrapolation of the run data revealed that the engine met the manufacturer's specifications for an overhauled engine. The Pc air tube leak did not affect engine performance. ADDITIONAL INFORMATION The Rotorcraft Flying Handbook, FAA-H-8083-21, Chapter 11, Helicopter Emergencies, Vortex Ring State (Settling With Power), stated: "Vortex ring state describes an aerodynamic condition where a helicopter may be in a vertical descent with up to maximum power applied, and little or no cyclic authority. The term 'settling with power' comes from the fact that the helicopter keeps settling even though full engine power is applied. In a normal out-of-ground-effect hover, the helicopter is able to remain stationary by propelling a large mass of air down through the main rotor. Some of the air is recirculated near the tips of the blades, curling up from the bottom of the rotor system and rejoining the air entering the rotor from the top. This phenomenon is common to all airfoils and is known as tip vortices. Tip vortices consume engine power but produce no useful lift. As long as the tip vortices are small, their only effect is a small loss in rotor efficiency. However, when the helicopter begins to descend vertically, it settles into its own downwash, which greatly enlarges the tip vortices. In this vortex ring state, most of the power developed by the engine is wasted in accelerating the air in a doughnut pattern around the rotor. In addition, the helicopter may descend at a rate that exceeds the normal downward induced-flow rate of the inner blade sections. As a result, the airflow of the inner blade sections is upward relative to the disc. This produces a secondary vortex ring in addition to the normal tip-vortices. The secondary vortex ring is generated about the point on the blade where the airflow changes from up to down. The result is an unsteady turbulent flow over a large area of the disc. Rotor efficiency is lost even though power is still being supplied from the engine. A fully developed vortex ring state is characterized by an unstable condition where the helicopter experiences uncommanded pitch and roll oscillations, has little or no cyclic authority, and achieves a descent rate, which, if allowed to develop, may approach 6,000 feet per minute. It is accompanied by increased levels of vibration. A vortex ring state may be entered during any maneuver that places the main rotor in a condition of high upflow and low forward airspeed. This condition is sometimes seen during quick-stop type maneuvers or during recoveries from autorotations. The following combination of conditions are likely to cause settling in a vortex ring state: 1. A vertical or nearly vertical descent of at least 300 feet per minute. (Actual critical rate depends on the gross weight, r.p.m, density altitude, and other pertinent factors.) 2. The rotor system must be using some of the available engine power (from 20 to 100 percent). 3. The horizontal velocity must be slower than effective translational lift." The Good Aviation Practices publication Helicopter Performance, published by the Civil Aviation Authority of New Zealand (November 2002), stated: "Turbulence and Windshear: The possibility of turbulence and windshear should be considered when determining takeoff and landing performance. (Windshear is a change in wind speed and or direction over a very short distance.) The presence of windshear can cause the sudden loss of translational lift and increase the power required to that of OGE hover and beyond - particularly accompanied by a downdraught. Local terrain, trees, and buildings all influence the flow of wind near them. The mechanical turbulence resulting from this disturbed airflow may become very marked in the lee of the obstruction. In winds below 15 knots, the turbulence in the lee may extend vertically to about one third higher than the obstruction. In winds above 20 knots, eddies can occur on the leeward side to a distance of about 10 to 15 times the obstruction height, and up to twice the obstruction height above the ground. A gust wind situation where windshear is likely to be present...will require a greater power margin to deal with any unexpected loss of airspeed and sink."
The pilot's inadvertent settling with power during a descent. Contributing to the accident was mechanical turbulence in the lee of an obstruction in windy conditions.
Source: NTSB Aviation Accident Database
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