C-172 Flight Instructor reported an engine failure during training in the traffic pattern. Once safely at the airport the Instructor determined the student had improperly operated the carburetor mixture which simulated an engine failure.
Synopsis
C-172 Flight Instructor reported an engine failure during training in the traffic pattern. Once safely at the airport the Instructor determined the student had improperly operated the carburetor mixture which simulated an engine failure.
Narrative
The flight was for take-off and landings in the traffic pattern at ZZZ. We were on about the 5th lap in the pattern on the downwind leg. Abeam of our landing point the student adjusted power; put in 10 degrees of flaps; and turned carb heat on. I noticed the student then added throttle a few seconds later; but I observed no change in engine sound; felt power; or RPM. I took control of the aircraft and added power and observed no change. I immediately turned towards the runway and checked the ignition switch; the carb heat; and fuel selector valve. At this point I was convinced my engine had failed and I [requested priority handling] to ZZZ Tower. I was given landing clearance for Runway XX. I continued my short approach to the run way and figured out the by pulsing the power; that the engine would give me very small bursts of power. I continued to do this in order to extend my glide. The landing was normal. By continuing to pulse the power; I was able to taxi to the edge of the ramp; where the engine then shut off. After the engine shut off I removed my hand from the throttle and observed the knob was out several inches towards 'lean'. We checked our fuel quantity and observed the plane still had 14 gallons on board. I pushed the mixture knob in to 'rich' and attempted to restart the engine. It restarted and ran normally. My conclusion is that the student accidentally leaned the mixture while attempting to adjust carb heat; power; and flaps while abeam the landing point. Since I reached for the throttle when I observed the issue; my view of the mixture knob was obscured; and I failed to notice it in the lean position. Due to this and the limited time to trouble shoot from 1000 AGL; I treated it as an engine failure and executed an [immediate] landing. In any future event I will be sure to double check the mixture in the event of an abnormal engine event. Additionally; I will stress the importance of adjusting the correct thing while preforming the before landing checklist. Also; I will stress the importance of verbalizing any abnormal that the student may observe so that the maximum about of time possible is available to troubleshoot.
Second reporter narrative
I am a student who has 10 hours of total flight time. I was working touch and goes at the ZZZ airport. At the beginning of my base turn I opened up 10 degree of flaps; put on carb heat; and reduced power. I believe that by accident instead of pulling the carb heat I leaned the fuel mixture to the engine. This led to the engine to turn off and my CFI had to make an immediate landing. My CFI did a great job of taking over the controls and getting us safely down on the runway. Nobody was injured due to this mistake.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.