ANC25LA025 · Cessna 172A

The pilot, who was also the owner of the airplane, was conducting a familiarization flight with the pilot-rated passenger, who was a prospective buyer of the airplane. The pilot reported having flown the airplane for a total of 35 hours after the engine had been removed, disassembled, and reinstalled just over 3 years before the accident; however, the airframe and engine logbooks show no hours accrued since the engine reinstallation. The pilot reported that, during engine run-up, the engine speed did not decrease when he pulled the carburetor heat knob. He stated that, after he shut down the engine, he “had the pilot actuate the carb heat push/pull knob several times while I observed and felt the mechanism on the air box move in a way that seemed normal.” The pilot then elected to continue the flight. During the initial climb out, the pilot pulled the carburetor heat knob to recheck the functionality of the carburetor heat and immediately experienced a large drop in engine speed. He kept the carburetor heat on, “expecting it to clear quickly if it was ice, as it has always done in the past.” After observing that power was not being restored, he turned back toward the airport. The engine continued to run at low power; however, the airplane was unable to maintain altitude and the pilot elected to land on a frozen river south of the airport. During the landing, the airplane nosed over, which resulted in substantial damage to the wings. Postaccident examination of the engine revealed that the carburetor heat cable was too long, had a visible kink where it attached to the heat control mechanism, and did not properly engage the heat control mechanism in either the fully open or closed position. Due to the excessive length of the carburetor heat control cable, the carburetor heat was always partially on, and this condition could not be changed when actuated by the control knob in the cockpit. The excessive cable length most likely occurred due to inadvertent repositioning of the clamp attaching the carburetor heat control cable sheath to the carburetor airbox during a previous engine removal and reinstall about three and a half years before the accident flight. The environmental conditions at the time of the accident were not conducive to the accumulation of carburetor icing. The carburetor heat control cable’s excessive length and incorrect positioning likely prevented the proper operation of the carburetor heat, resulting in a partial application of carburetor heat and partial loss of engine power during climb out.

Verbatim NTSB analysis and probable cause from the NTSB dataset