Cessna 152
Flight Crew Commercial; Flight Crew Flight Instructor
Situational Awareness; Human-Machine Interface; Training / Qualification
Aircraft Equipment Problem Critical; Inflight Event / Encounter Loss Of Aircraft Control
Flight Crew Overcame Equipment Problem; General Maintenance Action
Aircraft
My student and I departed southwest from ZZZ on a mini cross country. When I took the controls so my student could get his charts out (approximately 7 miles away from the airport); I noticed that the trim wheel could only be progressively moved in the nose down position. At some point I had to apply quite a lot of yoke aft pressure to keep the plane in straight and level flight. I made a 180; and let ATC know the situation. On the way back for landing; my student appeared to have forced the nose trim wheel downwards in order to get some nose up trim action and we heard a loud snapping or cracking noise. After the noise; the trim wheel operated normally; and we were able to fly the plane no issues and land uneventfully. We landed and downed the plane so our mechanic could look at it. I can't imagine a student solo or a low time private pilot dealing with this situation well. Not sure if this was a factor; but we were the first flight after maintenance was done to repair the carb heat control.
Instructor Pilot reported pitch trim wheel would only trim nose down; not up and required strong force to free it during flight.