FTW00LA175

During a simulated emergency approach to a landing area, at 75 feet agl, the flight instructor told the student pilot to go-around. The student attempted to add full power; however, he could not fully turn off the carburetor heat. The instructor stated that the engine would not produce full power. The student retracted the flaps to 1/4-flap setting once the airplane's descent was arrested. The instructor noticed that the airplane was not climbing and took control of the airplane. The airplane impacted trees at a speed of approximately 50 mph. A week prior to the accident flight the student experienced a similar situation during a go-around following a simulated loss of engine power. The carburetor heat would not shut off and the airplane would not climb. The stuck carburetor heat control could not be duplicated by maintenance personnel. 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 the accident. The carburetor heat control cable was fractured in two, approximately 8 inches from the carburetor heat deflection door attach point. Examination of the carburetor heat control cable revealed that u-shaped indentions had been worn into the inner wire by its metal spiral sleeve. Examination of the inner wire revealed that it could not carry its design load due to the wear marks, and had failed as a result of tensile overload. Examination of the aircraft maintenance records revealed that the carburetor cable had not been replaced since the airplane was manufactured in 1968.

On June 8, 2000, at 1530 central daylight time, a Cessna 177 single-engine airplane, N29383, was substantially damaged during a forced landing following a loss of engine power while maneuvering near Nacogdoches, Texas. The flight instructor, student pilot, and rear seat passenger were seriously injured. The airplane was registered to the student pilot's father. Visual meteorological conditions prevailed and a flight plan was not filed for the 14 Code of Federal Regulations Part 91 instructional flight. The local flight departed the A.L. Mangham Regional Airport, Nacogdoches, Texas, 15 minutes prior to the accident.

According to written statements provided by the flight instructor and the student pilot, the flight departed the A.L. Mangham Regional Airport for a training flight. At 2,000 feet agl, the flight instructor pulled the throttle to 12 inches of manifold pressure and pulled the carburetor heat to the on position to simulate a loss of engine power. The student pilot set up for a simulated emergency landing to a private paved landing strip with a 3/4-flap setting. Upon realizing that the student would be able to make the emergency landing, the instructor told the student to go-around. The airplane was 75 feet agl at the time of the go-around. The student applied full throttle, rich mixture, and maximum propeller RPM, and attempted to turn off the carburetor heat. He then pitched the airplane to a higher angle of attack and set the flaps to a 1/2-flap setting. After the descent was arrested, the student retracted the flaps to a 1/4-flap setting. The flight instructor noticed that the airplane was not obtaining a "good rate of climb," and that the carburetor heat control had been pushed in, but was not in the full off position. The instructor added that the engine was not running rough, but it was not producing full power. He attempted to turn the carburetor heat off; however, "the cable felt jammed." At this time the airplane was "too far down the landing strip" to make a safe landing, so the instructor took control of the airplane. The instructor told the student to turn on the fuel pump and attempt to turn off the carburetor heat. The instructor noted that the airspeed was slow and that the terrain was rising. He elected to maintain a straight and level flight attitude instead of attempting to return to the landing strip. The airplane eventually impacted trees at a speed of approximately 50 mph.

According to the FAA inspector, who responded to the accident site, the aircraft was separated in three sections, and the engine was separated from the fuselage.

At 1555, the weather observation facility at the A.L. Mangham Regional Airport reported the wind from 120 degrees at 5 knots, visibility 10 statute miles, sky clear, temperature 84 degrees Fahrenheit, dew point 64 degrees Fahrenheit, and altimeter setting 30.02 inches of mercury.

According to the student pilot's father, the student flew the accident airplane with a different flight instructor a week prior to the accident flight. The student flew the airplane to New Orleans, Louisiana, and the instructor simulated a loss of engine power while in the traffic pattern. The student made the simulated emergency approach a "little high," and the instructor told the student to go-around. The student attempted to apply full power; however, the carburetor heat would not move forward (to the off position). The engine did not produce full power and the airplane would not climb, so the flight instructor flew the airplane around to the active runway and landed. After the landing, the carburetor heat control was "easily" placed in the off position.

The student and the flight instructor, who was involved in the accident flight, flew the airplane from New Orleans, back to Nacogdoches, where they requested a mechanic examine the carburetor heat control. The mechanic could not find any problems with the carburetor heat control. The student pilot's father flew the airplane with the flight instructor, who was involved in the accident, two days prior to the accident flight and did not experience any problems with the engine or the carburetor heat control.

On July 18, 2000, the NTSB investigator-in-charge (IIC) examined the airplane with the assistance of a representative from the airplane manufacturer. The engine cowling had been removed in order to remove the engine from the airframe and place the engine on a test stand. When the cowling was removed, an 8-inch section of the carburetor heat control cable fell out of the carburetor heat control sleeve. The engine was run for seven minutes at various power settings up to 2,500 RPM. The magnetos were tested at 2,500 RPM and found to have a 125-RPM drop on the left magneto and a 150-RPM drop on the right magneto. The carburetor heat air box was disassembled and examined. Impact marks on the carburetor air box aligned with the carburetor heat deflection door when the door was placed in the carburetor heat on position.

The carburetor heat cockpit control and cable were removed from the airplane and examined along with the section of cable that fell out of the cowling. The carburetor heat control cable consisted of an inner control wire, which slid inside a metal spiral sleeve as the cockpit control was positioned in the on or off position. The metal spiral sleeve supported the inner wire through the firewall and support clamps. The inner wire was removed from its spiral sleeve. Examination of the inner wire revealed that it was fractured in two, approximately 8 inches from where the cable attached to the carburetor heat deflection door. The 8-inch section of cable was also separated from the carburetor heat deflection door linkage. The inner wire fracture area was matched to an area in the sleeve where it had been kinked nearly 90 degrees. The inner wire displayed u-shaped grooves near the carburetor attachment end. The u-shaped grooves appeared to be worn into the inner wire and were equal in distance apart as the windings in the spiral sleeve. The carburetor heat control cable and the carburetor were kept for further examination.

The carburetor heat control cable was examined by the aircraft manufacturer's facility under the supervision of an FAA inspector. The fracture surfaces of the inner wire displayed ductile dimples, which were consistent with an overload failure. Examination of the inner wire revealed that it was "significantly worn from rubbing back and forth inside of the cable sheath, to the point that the load bearing cross-section of the cable was reduced at the location of the fracture." Cracks were also found in a few of the u-shaped wear areas when examined under a scanning electron microscope. According to the manufacturer, the cable's cross-section "became worn down," which "resulted in a stress concentration." The cable fractured "due to tensile overload, i.e. [it] could no longer carry its design load."

The carburetor was disassembled and examined under the supervision of the NTSB IIC on December 14, 2000. No anomalies were noted that would have prevented its operation prior to the accident.

The aircraft was manufactured in 1968, by the Cessna Aircraft Company. On March 19, 1969, the aircraft's original engine was replaced with a Lycoming O-360-A1A, 180-horsepower engine (serial number L-13884-36A). According to the aircraft maintenance records, the airplane underwent its last annual inspection on March 2, 2000, at an aircraft total time of 1,791.4 hours. An "inspection report" for the annual inspection listed two discrepancies for the carburetor heat; "carburetor heat valve lever loose on shaft" and "carburetor air inlet box missing piece and cracked." On March 2, 2000, the carburetor heat valve and carburetor heat box were "repaired." There was no indication in the aircraft maintenance records of the carburetor heat control cable being replaced sinc