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N12085 accident description

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Tail numberN12085
Accident dateJuly 10, 1998
Aircraft typeBell 47G
LocationJones, OK
Additional details: None

NTSB description

HISTORY OF FLIGHT

On July 10, 1998, approximately 1222 central daylight time, a Bell 47G helicopter, N12085, collided with trees during cruise flight and then impacted the ground in the backyard of a residence near Jones, Oklahoma. The helicopter, which was registered to and operated by Allied Helicopter Service, Inc., of Tulsa, Oklahoma, was destroyed by impact forces and fire. The two occupants of the helicopter, a private pilot and a flight instructor, received fatal injuries. There were no injuries to persons on the ground, and property damage was confined to a water well pump house that was destroyed by fire and minor impact damage to the roof of the residence. No flight plan was filed and visual meteorological conditions prevailed for the 14 Code of Federal Regulations (CFR) Part 91 instructional flight that departed from Downtown Airpark in Oklahoma City, Oklahoma, approximately 1205. The helicopter was en route to Downtown Airpark in Tulsa, Oklahoma.

According to an FAA inspector, on the morning of July 10, 1998, the private pilot and flight instructor flew the helicopter from Tulsa to Oklahoma City. The private pilot then successfully completed a practical test (check ride), given by the FAA inspector, for the addition of a rotorcraft helicopter rating to his private pilot certificate. The inspector stated that the private pilot "hovered exceptionally well" and had "good control" of the helicopter. The inspector reported that during one of the check ride maneuvers (a hovering autorotation), the helicopter hit on the aft part of the skids and rocked onto the forward part of the skids. The inspector further reported that he did not feel the tail rotor guard make ground contact during the maneuver and does not believe it made contact. Following the check ride, the private pilot and the flight instructor had lunch with the inspector at the airport restaurant. The inspector then observed the two pilots performing a pre-flight inspection of the helicopter as it was being refueled prior to their departure for the 86-nautical mile return flight to Tulsa.

Two witnesses, who were employees of Downtown Airpark in Oklahoma City, reported that they observed the helicopter make a "hard landing" in the grass at the airpark on the morning of July 10. One witness stated that the helicopter "fell out of the sky" from a hover at "about 40 feet agl," hit tail first, bounced, and came back down on the skids. The other witness stated that the helicopter's tail guard hit the ground, and the helicopter then "hit hard" on the skids.

A witness, who was working at a sand pit located about 1/2 mile northwest of the accident site, observed the helicopter south of his position, moving from west to east, in straight and level flight. According to the witness, the helicopter was "flying just fine." He lost sight of the helicopter and then heard a noise "like a whistle," followed by a "clanging [noise], like metal banging." He saw a "puff of smoke" and suspecting that the helicopter had crashed, got into his truck and drove to the accident site. When he reached the site, the fire department was already on scene.

Another witness, who was working at a residence located approximately 1/4 mile west-southwest of the accident site, observed the helicopter north of his position, flying at a "pretty low" altitude heading east-northeast. According to the witness, the helicopter "sounded like it was running ok." He reported that the helicopter's tail was swinging from side to side, which prompted him to say to himself, "this guy can't fly this thing." He further reported that as the helicopter neared a "big tree" in its flight path, the helicopter's "nose went up a little, and the helicopter rose up over the tree." He looked away, heard the noise of the helicopter stop, looked back, and "saw pieces flying through the air." The witness stated that the helicopter "caught fire and burned rapidly," and he immediately called 911 on his cellular phone. The Oklahoma City Police Department received the 911 call at 1222:27.

PERSONNEL INFORMATION

The flight instructor, who occupied the right seat, held a commercial pilot certificate with helicopter, airplane single-engine land and sea, airplane multiengine land, and instrument airplane ratings. He was rated as a flight instructor in helicopters and single-engine land airplanes. His most recent FAA second-class medical certificate was issued on July 9, 1997, with the limitation, "must wear corrective lenses." Additionally, he held a waiver for defective color vision.

According to the flight instructor's logbook, he had accumulated 8,394 total flying hours, of which 6,535 were in helicopters. In the 90 days preceding the accident, he had flown 120 hours, of which 95 hours were in a Bell 47. In the 30 days preceding the accident, he had flown 32 hours, of which 30 hours were in a Bell 47.

The private pilot, who occupied the left seat, was issued a private pilot certificate with a single-engine land airplane rating on November 20, 1995. His most recent FAA first-class medical certificate was issued on March 25, 1997, with the limitation, "must wear corrective lenses."

On the application form submitted for his rotorcraft helicopter rating, the private pilot reported a total flight time of 122 hours, of which 36.5 hours were in helicopters (36.1 hours in a Bell 47). The application form indicated that the pilot had graduated on July 8, 1998, from Allied Helicopter's private pilot helicopter course, which was approved under 14 CFR Part 141. According to the private pilot's logbook, in the 90 and 30 days preceding the accident, he had flown 30 and 6 hours, respectively, all in a Bell 47.

The investigation could not determine which pilot was at the controls of the helicopter at the time of the accident.

AIRCRAFT INFORMATION

A review of the airframe and engine logbooks, conducted by the NTSB investigator-in-charge (IIC), revealed no record of any uncorrected maintenance discrepancies. According to Bell Helicopter's records, the helicopter was manufactured on December 12, 1952, as a military model H-13E. The maintenance records indicated that the helicopter was converted to a civilian model 47G on September 22, 1960.

The most recent annual inspection was completed on May 1, 1998, at a total time of 5,981 hours. At the time of the accident, the airframe had accumulated 6,026 hours of flight time. The engine, a 200-horsepower Franklin 6V4-200-C32, had accumulated 339 hours since its most recent major overhaul, which was completed on December 14, 1990.

METEOROLOGICAL INFORMATION

At 1153, the reported weather conditions at Wiley Post Airport (PWA) in Oklahoma City, located 16 nautical miles west of the accident site, were wind from 200 degrees at 13 knots, visibility 10 statute miles, sky clear, temperature 36 degrees C (97 degrees F), dewpoint 21 degrees C (70 degrees F), and altimeter setting 30.03 inches of mercury.

At 1156, the reported weather conditions at Will Rogers World Airport (OKC) in Oklahoma City, located 18 nautical miles southwest of the accident sire, were wind from 230 degrees at 12 knots, visibility 10 statute miles, sky clear, temperature 37 degrees C (99 degrees F), dewpoint 21 degrees C (70 degrees F), and altimeter setting 30.02 inches of mercury.

WRECKAGE AND IMPACT INFORMATION

The accident site was located in a rural area approximately 13 nm northwest of Oklahoma City's Downtown Airpark and along the direct route of flight to Tulsa (magnetic course 052 degrees). Examination of the site revealed a linear wreckage path, including tree strikes, extending for a total distance of approximately 175 feet on a measured magnetic heading of 050 degrees. Evidence of a tree that was freshly topped at a height of about 40 feet above the ground marked the beginning of the wreckage path. A piece of the tail rotor guard and fragments of one main rotor blade were found at the base of the tree. Numerous fragments of plexiglas and another piece of the tail rotor guard were found at the base of a second topped tree, located 115 feet along the wreckage path from the first tree.

The main wreckage, consisting of the fuselage, engine, transmission, and the main rotor head, was located 60 feet along the wreckage path from the second tree, in the backyard of the residence at 11167 East Hefner Road. Impact marks were found on the northwest corner of the roof of the residence, which was located between the second tree and the main wreckage. Additionally, a piece of the landing gear (part of the aft cross tube with the left leg attached) and several pieces of plexiglas were found on the roof.

The fuselage came to rest inverted on a measured magnetic heading of 115 degrees, adjacent to the south side of a water well pump house. Fresh gouges and scrape marks were found on the southwest corner of the pump house. The tail boom was fractured into numerous pieces from approximately station 62 aft. All three of the bolts that attach the tail boom to the center frame remained secure. The majority of the tail boom pieces, the tail rotor and the ventral fin were found scattered to the north of the pump house. The synchronized elevator separated from the tail boom and was split at its midpoint. The two elevator sections were found south of the main wreckage. Both synchronized elevator control cables were separated, and the separated ends were unraveled.

The white-tip main rotor blade appeared to have disintegrated on contact with the first tree. Only the metal spar of this blade remained attached to the rotor head. Two sections of the blade's trailing edge, each measuring about 6 feet long, were found at the base of a freshly topped tree located about 63 feet north of the first tree. The metal leading edge of the blade was found an additional 130 feet to the north. Wood fragments from the tip of the blade were found about 90 feet south of the first tree.

The red-tip main rotor blade remained intact and attached to the rotor head. Blue and orange paint transfers, which corresponded to the paint colors of the helicopter, were found on the outboard upper surface of this blade.

The tail rotor gearbox separated from the tail boom and was found on the north side of the pump house. The hub and blade assembly remained attached to the gearbox, and the pitch change links remained connected. The gears in the gearbox rotated freely when the tail rotor was turned by hand, and the pitch control mechanism operated freely. One tail rotor blade was intact and displayed leading edge damage and chordwise scratches. The other tail rotor blade was separated about 4 inches outboard of the root. The separated section of the blade was found a few feet from the tail rotor assembly. Both tail rotor pitch control cables were separated, and the separated ends were unraveled.

Fire damage to the helicopter was confined to the main wreckage. Except for the steel tubular frame, the stainless steel firewall, and other steel parts, the fuselage was consumed by fire. The engine's accessory case, cooling fan, baffling, and both magnetos were destroyed by fire. Black soot coated the transmission, swashplate, main rotor mast, and rotor head. Both aluminum control rods connecting the swashplate to the stabilizer bar assembly were partially consumed by fire. One stabilizer bar was bent, and the other stabilizer bar had separated at the root end. The separated stabilizer bar was found with the main wreckage. The aluminum components of the flight control systems and the throttle control system, including bellcranks and support structures, were destroyed by fire. Examination of the flight control system verified that dual controls were installed.

Continuity of the mixture control cable was verified from the cockpit control to the yoke attached to the carburetor end of the cable. The yoke was fractured in the area where it bolted to the carburetor mixture arm. Continuity of the carburetor heat control cable was verified from the cockpit control to the flapper valve.

MEDICAL AND PATHOLOGICAL INFORMATION

Autopsies of both pilots were performed by Fred B. Jordan, M.D., at the Office of the Chief Medical Examiner in Oklahoma City, Oklahoma. Toxicological tests performed by the FAA's Civil Aeromedical Institute were negative.

FIRE

Fire damage to the helicopter wreckage was consistent with a fuel-fed fire erupting on impact.

TESTS AND RESEARCH

On July 21 and 22, 1998, a wreckage reconstruction, supervised by the IIC, was performed at the facilities of Air Salvage of Dallas in Lancaster, Texas. Examination of the helicopter revealed no evidence of an in-flight structural failure. According to a Bell Helicopter metallurgist, who examined the fragmented tail boom and tail rotor guard, all of the fractures in these structures appeared to be the result of overload related to impact.

All steel components of the cyclic control system were identified, and the security of all bolted connections between the cyclic sticks and the swashplate was verified. All steel components of the synchronized elevator control system were identified, and the security of all bolted connections between the bellcrank and the fore-and-aft cyclic control system was verified. All steel components of the collective pitch control system were identified, with the exception of the lower push-pull tube, part number (P/N) 47-727-110-1. The security of bolted connections in the collective pitch control system was verified from the base of the upper push-pull tube to the swashplate. All steel components of the tail rotor pitch control system were identified, and the security of all bolted connections between the pedals and the jackshaft was verified. All steel components of the throttle control system were identified, and the security of all bolted connections between the cockpit controls and the carburetor was verified.

The security of the bolted connections at the upper and lower ends of the control rods connecting the swashplate to the stabilizer bar was verified. Both control links connecting the stabilizer bar assembly to the blade pitch horns were bent but remained intact. One of four equalizer links, P/N 47-120-025-1, was fractured through the banjo section of the lower rod end. The bolt securing the upper rod end of this equalizer link was fractured through the threaded section. The fractured equalizer link and bolt were retained for metallurgical examination.

Transmission continuity was confirmed by hand rotating the main rotor mast and observing the rotation of the tail rotor drive and engine cooling fan drive assemblies. The transmission was disassembled, and a visual examination of the gears and clutch revealed no evidence of any abnormalities.

On July 27, 1998, the engine was examined under the supervision of an NTSB investigator at the facilities of Air Salvage of Dallas. After molten metal was removed from the starter ring gear, the crankshaft turned freely. The #2 and #3 cylinder exhaust valves were found fixed in the open position. The engine was disassembled, and a visual examination of the components revealed no evidence of any abnormalities.

On July 29, 1998, the fractured equalizer link and bolt were examined under the supervision of the IIC at the facilities of Bell Helicopter in Fort Worth, Texas. According to the Bell metallurgist, who examined the parts, the equalizer link and the bolt fractured as a result of overstress.

ADDITIONAL INFORMATION

The helicopter wreckage was released to Allied Helicopter Service on November 17, 1998.

(c) 2009-2011 Lee C. Baker. For informational purposes only.