**This report was modified on July 25, 2017. Please see the public docket for this accident to view the original report.** On August 16, 2014, about 1830 central daylight time, a Bellanca 7GCBC, N36236, operated by a private individual, was substantially damaged during a forced landing, after it experienced 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 the Greenwood-Leflore Airport (GWO), Greenwood, Mississippi, about 1810. The instructional flight was conducted under the provisions of 14 Code of Federal Regulations Part 91. According to the flight instructor, the student pilot was practicing simulated forced landings, and had performed two simulated forced landings 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 of the airplane and performed a forced landing to a corn field, which resulted in substantial damage to both wings and the fuselage. According to Federal Aviation Administration (FAA) records, the airplane was purchased by its current owner on February 15, 2014. It was equipped with a Lycoming O-320-A2B, 150-horsepower engine. On site examination of the airplane by an FAA inspector did not reveal any evidence of a catastrophic engine failure. Fuel sampled from the airplane's fuel tanks was consistent with 100-low-lead aviation gasoline and absent of contamination. Subsequent examination of the engine by an investigator from the NTSB and the engine manufacturer revealed that the No. 4 intake valve inner spring was 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 when rotated by hand. The carburetor remained attached to the engine, and the 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 amongst the recovered wreckage. Examination of the fractured No. 4 intake valve inner spring and the material recovered from the carburetor was performed by an NTSB Materials Engineer. Fracture features consistent with fatigue, and pitting were present through the inner surfaces of the spring. In addition, Fourier-transform infrared (FTIR) spectrometer examination of the material found in the carburetor was consistent with polyurethane. [Additional information can be found in the NTSB Materials Laboratory Factual Report located in the public docket] According to maintenance records, at the time of the accident, the airplane had been operated for about 1,265 total hours, which included about 13 hours since its most recent annual inspection, which was performed on December 2, 2013. According to an entry in the airframe maintenance logbook, the annual inspection was "owner assisted" by the previous owner and included "Replaced induction filter." According to engine logbook entries, 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 to the accident. In addition, there was no indication that the engine had been overhauled since it was manufactured in 1973. Lycoming Service Instruction No. 1009AW recommended that engines which have not accumulated the recommended hourly time in service be overhauled in the 12th year since the last overhaul. According to the Bracket Aero Filters polyurethane foam information manual, "…All Brackett Air Filter elements are required to be replaced each 12 calendar months regardless of actual flight time…."