N121CMCessna 208B · 1993

On May 23, 2013, about 1200 central daylight time, a Stinson 108-3, N121CM, sustained substantial damage when it nosed over during a forced landing following a loss of engine power near Beaumont, Texas. The pilot received minor injuries. The airplane sustained damage to the vertical tail surfaces, wings, wing struts, and fuselage. The aircraft was registered to and operated by Aircapital Nevada, Inc., under the provisions of 14 Code of Federal Regulations Part 91 as a business flight. Visual meteorological conditions prevailed for the flight, which was not operated on a flight plan. The flight was originating from the Beaumont Municipal Airport, Beaumont, Texas, and was bound for College Station, Texas.The pilot reported that he had performed a pre-takeoff run-up and all engine indications were normal. He proceeded to take off and made two 90-degree turns prior to the loss of engine power. The pilot said that the engine was still producing some power, but he was not able to maintain altitude. He attempted to land in a field and the airplane nosed over.

Examination of the engine confirmed compression on all cylinders, valve system continuity, and magneto operation. The carburetor appeared new and the fuel strainer was clean with no debris. The fuel within the gascolator did not appear to have any water or sediment. The upper spark plugs were of three different electrode configurations. All of the plugs exhibited a wet-black appearance. The rubber boot that was located between the carburetor heat box and the cowl was very pliable and had cracks on the exterior surfaces. The rear of the boot was attached to the carburetor heat box with a clamp and sealant. The front of the boot did not have a clamp installed and one was not found in the wreckage.

Discussions with a member of the International Stinson Club (ISC) revealed that the front of the boot between the carburetor heat box and the cowl should have had a 2-piece clamp installed. The ISC member said that without the clamp installed it is possible for the boot to be sucked into the inlet of the carburetor heat box thereby restricting airflow, resulting in a rich mixture and reduction of engine power.