MIA05LA129 · Cessna 172S

The certified flight instructor (CFI) and student pilot departed on an instructional flight and when the CFI reduced throttle to demonstrate a power-off stall to the student, the engine experienced a loss of power, but the propeller continued to windmill. Attempts to restore engine power were unsuccessful and the CFI maneuvered the airplane towards a nearby field. The airplane was landed in the field before a row of trees and collided with them during the landing roll. Two weeks before the accident the engine "...cut out on round out at Fernandina Airport." The corrective action performed by maintenance personnel included adjusting the idle speed and the idle mixture. The CFI reported that on several previous occasions when flying the accident airplane, the engine experienced a loss of power when the throttle was reduced. . Postaccident examination of the airplane revealed a sufficient quantity of fuel. During an engine run in a test cell at the manufacturer's facility the engine lost power when the throttle was reduced below 1,500 rpm. The engine was only able to sustain operation at low power settings by manually leaning the fuel to air ratio. Examination of the servo fuel injector (servo) at the manufacturer's facility following the engine run revealed a plastic sleeve was missing/separated from the idle stop. Bench testing of the servo at the manufacturer's facility revealed with the throttle at the idle stop and the mixture control in the full rich position, the airflow in terms of pounds-per-hour was 6.0 instead of 100.0, and the fuel flow was 1.2 pph instead of 6.0 to 7.0 pph. Further bench testing of the servo revealed the fuel flow at 4 of the test points exceeded specified limits, while the fuel flow was within limits at 2 of the test points. No determination could be made as to when the plastic sleeve separated from the idle stop.

Verbatim NTSB analysis and probable cause from the NTSB dataset