On November 10, 2009, about 1545 central standard time, a Younkin Mullicoupe experimental airplane, N273X, sustained substantial damage following a forced landing near Smith Field Airport (SLG), Siloam Springs, Arkansas. The commercial pilot received serious injuries. The personal flight was being conducted under provisions of Title 14 Code of Federal Regulations Part 91, without a flight plan. The local flight originated at SLG about 1530. Visual meteorological conditions prevailed at the time of the accident.

According to the owner, the pilot told him he was practicing aerobatic maneuvers and was performing a reverse Cuban 8 maneuver when the engine quit. The pilot confirmed he was performing aerobatics, but without a lot of G-loading on the airplane during the maneuvers. After the engine quit he performed an emergency landing to a field. During the approach to land he struck an unseen power line and the airplane cart-wheeled to a stop in the field

A Federal Aviation Administration inspector examined the airplane on the day of the accident and found three of the four wings and the empennage separated from the airplane. Both left and right wing fuel tanks were found intact. Fuel was found in the left wing tank, the right wing tank was empty, and the fuel selector was found in the right position. The inspector could not examine the entire fuel system on the day of the accident due to the position of the wreckage. Permission was given to the owner to move the airplane, but told not to conduct any maintenance on the airplane until after the FAA inspector had another opportunity to look at the airplane after it had been moved. When the inspector arrived to examine the airplane the day after the accident he found the airplane had been partially disassembled, switches and controls had been manipulated, and the fuel tanks had been drained.

The owner/builder of the airplane said there was a header tank installed in the airplane, which was designed by the owner to maintain fuel to the engine during aerobatics. In addition to the header tank, there were left and right wing tanks, front and aft body tanks, and an auxiliary tank. Both the pilot and the owner thought a vacuum had been created within the header tank, and that there was inadequate venting, which caused fuel starvation, and that was why the engine quite running. The pilot said during post accident examination there was fuel in both body tanks. He thought there were about 15 gallons in the front tank and about 25 gallons in the rear tank. Requests for a copy of the fuel system schematic were unanswered by the owner and the investigator was unable to verify the proper fuel burn sequence or the configuration of the fuel system at the time of the accident due to the post accident manipulation of the wreckage.

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