Accident Overview
On August 16, 2014, about 1830 central daylight time, a Bellanca 7GCBC, registration N36236, operated by a private individual, was substantially damaged during a forced landing after a total loss of engine power while maneuvering near Itta Bena, Mississippi. The flight instructor and a student pilot were seriously injured. Visual meteorological conditions prevailed, and no flight plan had been filed for the local flight that originated at Greenwood-Leflore Airport (GWO), Greenwood, Mississippi, about 1810. The instructional flight was conducted under 14 Code of Federal Regulations Part 91.
Sequence of Events
According to the flight instructor, the student pilot was practicing simulated forced landings and had performed two without incident. During a third simulated forced landing, when the student pilot applied engine power to go-around, the engine began to sputter and lost power. The flight instructor assumed control and executed a forced landing to a corn field, resulting in substantial damage to both wings and the fuselage.
Aircraft and Engine Information
The airplane was purchased by its current owner on February 15, 2014, and was equipped with a Lycoming O-320-A2B, 150-horsepower engine. At the time of the accident, the airplane had accumulated about 1,265 total hours, including about 13 hours since its most recent annual inspection on December 2, 2013. The annual inspection was "owner assisted" by the previous owner and included replacement of the induction filter. The No. 4 cylinder was replaced on August 9, 2001, about 70 hours prior to the accident, and the "Bracket air filter" was replaced on October 12, 2011, about 25 hours prior. There was no indication that the engine had been overhauled since manufacture in 1973. Lycoming Service Instruction No. 1009AW recommended that engines not accumulating recommended hourly time be overhauled in the 12th year since last overhaul. The Bracket Aero Filters polyurethane foam information manual stated that filter elements are required to be replaced each 12 calendar months regardless of flight time.
On-Site and Engine Examination
Examination by an FAA inspector revealed no evidence of catastrophic engine failure. Fuel sampled from the tanks was consistent with 100-low-lead aviation gasoline and free of contamination. Subsequent examination by NTSB and the engine manufacturer found the No. 4 intake valve inner spring fractured in three locations; the outer spring remained intact. When the propeller was rotated by hand, camshaft, crankshaft, and valve train continuity were confirmed, and thumb compression was attained on all cylinders. Both magnetos produced spark from all leads. The carburetor fuel inlet screen was absent of debris. Foam consistent with a portion of air intake filter material was observed obstructing more than half of the carburetor venturi opening. The carburetor heat air box was crushed, and the air intake filter was not located among the recovered wreckage.
Materials Analysis
Examination of the fractured spring and the material from the carburetor by an NTSB Materials Engineer showed fracture features consistent with fatigue and pitting on the inner surfaces of the spring. Fourier-transform infrared (FTIR) spectrometry of the material found in the carburetor was consistent with polyurethane. Detailed findings are in the NTSB Materials Laboratory Factual Report.