Incident Description
During a simulated emergency approach to a landing area, at an altitude of 75 feet above ground level, the flight instructor instructed the student pilot to execute a go-around. The student attempted to apply full power but was unable to fully shut off the carburetor heat. According to the instructor, the engine did not produce full power. After arresting the descent, the student retracted the flaps to a quarter-flap setting. When the instructor observed that the airplane was not climbing, he took control. The airplane subsequently struck trees at a speed of approximately 50 miles per hour.
Background
One week before the accident, the student had experienced a comparable situation during a go-around following a simulated loss of engine power. On that occasion, the carburetor heat also would not disengage, and the airplane failed to climb. Maintenance personnel were unable to reproduce the stuck carburetor heat control.
Examination Findings
Post-accident examination of the airplane revealed impact marks indicating that the carburetor heat deflection door was in the "on" position at the time of impact. The carburetor heat control cable was found fractured into two pieces, approximately 8 inches from the attachment point of the deflection door. Inspection of the cable showed that u-shaped indentations had been worn into the inner wire by its metal spiral sleeve. The inner wire could not support its design load due to the wear marks and had failed as a result of tensile overload. Review of the aircraft maintenance records showed that the carburetor cable had not been replaced since the airplane was manufactured in 1968.