C-GYZA, a Cessna 172N aircraft operated by Springbank Air Training College, was conducting a local instructional flight at Springbank Airport, AB (CYBW) with one flight instructor and one student pilot on board. Due to the student pilot having difficulties controlling the altitude of the aircraft during the downwind leg of the circuit, the flight instructor assumed control of the aircraft.
The flight instructor reduced power to arrest the ascent rate. When the aircraft had descended to the appropriate altitude, the instructor applied power to the aircraft, the engine did not respond as was expected. Believing there was an issue with the engine, the flight instructor declared an emergency with ATS.
A successful forced approach and landing was carried out on RWY 26. After landing, the
flight instructor observed that the engine mixture control was pulled to a “lean” condition.
When the mixture control was manipulated to the full rich condition, the engine began to operate normally. Ground taxi clearances were obtained and the aircraft taxied back to company facility.