On June 29, 1997, at 1245 central daylight time, an Engineering & Research Ercoupe 415-C airplane, N2308H, was substantially damaged when it collided with a power line and then impacted the ground during a forced landing near Ardmore, Oklahoma. The student pilot, the sole occupant and owner of the airplane, was seriously injured. No flight plan was filed and visual meteorological conditions prevailed for the Title 14 CFR Part 91 personal cross country flight that departed Pauls Valley, Oklahoma, at 1200 with an intended destination of Roanoke, Texas.

During a personal interview conducted by the NTSB investigator-in-charge and on the enclosed Pilot/Operator Report, the pilot stated that he noticed the engine was not producing full power during the initial takeoff climb. He elected to continue the flight, and after "about 30 minutes," he noted "some loss of power." By the time he reached the Ardmore area, the engine was no longer producing sufficient power to maintain altitude. The tachometer was indicating a lower than normal rpm, and the oil pressure and temperature were normal. He applied carburetor heat, but this "had no effect." During his attempt to land in the median of Interstate 35, the pilot did not see the power line crossing the interstate.

According to the FAA inspector who responded to the scene of the accident, the airplane struck and severed the power line. It then entered an uncontrolled descent, impacted a bridge railing, and came to rest on the guard rail on the west side of the northbound lanes of the interstate. Both wings and the fuselage sustained structural damage.

On July 2, 1997, the airplane was examined by a FAA inspector and a representative of the engine manufacturer at a storage facility in Ardmore, Oklahoma. The engine, a Continental C-75-12, S/N 3045-6-12, was intact with the exception of the carburetor which was separated at the throat. Impact damage was noted to the pushrods of cylinder three, which were bent. The crankshaft was rotated, and continuity was confirmed to the accessory drive gears, the valves of cylinders one, two, and four, and the piston of cylinder three. The throttle and carburetor heat control cables were attached to their respective arms on the carburetor and the air box and were free to move. The mixture control cable was not attached to the control arm on the carburetor, and the control arm was free to move. A label reading "NO OP" was found on the mixture control knob in the cockpit.

On August 5, 1997, the airplane was examined by the NTSB investigator-in-charge and the engine manufacturer's representative at the owner's hangar at Northwest Regional Airport, Roanoke, Texas. The carburetor, a Stromberg NA-S3A1, was disassembled and found to be equipped with a metal float and a rubber tipped needle assembly, P/N 390077, which was intact and free to move. Both magnetos (Eisemann LA-4, P/N 27-709, S/N 6-6462 and V27884) sparked at all terminals when hand rotated. The magnetos were disassembled, and no evidence of internal damage or arcing was noted within either magneto. The muffler was examined and no damage was noted.

According to the pilot/owner, in the weeks prior to the accident, he had been placing the fuel selector in the "OFF" position when the airplane was parked, because if he did not, fuel leaked from the engine compartment. He stated that he was servicing the airplane with automotive gasoline. He further stated that the mixture control had been inoperative since the airplane was returned to service in November 1996 following extensive maintenance.

Review of the airplane's maintenance records indicated that the last annual inspection was completed on November 1, 1996. According to the recording tachometer, at the time of the accident, the airplane had accumulated 58 hours since this inspection. The logbook entry for the inspection stated, in part, that the airplane was "disassembled, cleaned and inspected, and reassembled." No reference to the mixture control was made in the logbook entry.

Review of the carburetor manufacturer's service instructions revealed that the mixture control system on the NA-S3A1 does not incorporate an idle cutoff.

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