Skyhawk 172/Cutlass 172
Flight Crew Private; Flight Crew Instrument
Flight Crew Last 90 Days 137.3; Flight Crew Total 255
Situational Awareness; Training / Qualification; Troubleshooting
Aircraft Equipment Problem Critical
Air Traffic Control Provided Assistance; Flight Crew Returned To Departure Airport; Flight Crew Took Evasive Action; Flight Crew Landed in Emergency Condition
Aircraft
The aircraft involved had just had its engine overhauled; and I was conducting the engine break-in flight with a (less experienced) co-pilot. We departed ZZZ and coordinated with ZZZ Approach to get a Class B clearance VFR at 5;000 feet; and then circled the airport using power settings in accordance with the engine break-in manual for the particular engine. After we were done with our break-in; I decided to test a gentle power reduction prior to descent. I knew that engine troubles were most likely to present themselves during power changes and I would rather have a problem at 5;000 feet than 500 feet. As I reduced power; the engine RPM jumped to 2100 RPM. I noticed this jump and applied full throttle; this did nothing. I also tried a gentle reduction - this also did nothing. At that point; I did not mess with the throttle further. 2100 RPM was better than 0; I figured. I then decided that the best course of action was to [advise ATC] and prepare to do a power-off 180 at ZZZ. I [advised] with Approach; and they let ZZZ Tower know that we were coming. I circled the airport and set up for a downwind. Once I was on downwind; I experimented further with the throttle and my suspicions were confirmed; we did not have any control of the engine power. So; just like the commercial maneuver; as we passed abeam the thousand foot markers on the runway on downwind; I pulled the mixture to idle cutoff; and we proceeded to safely execute a power-off 180 and exit the runway using our forward momentum.Our mechanic looked at the engine after we brought the plane back and it turned out that there was a pin of some sort missing; and it was this that allowed the throttle to stop controlling engine power.Two things contributed to our successful safe landing in this event. Most importantly; both myself and my co-pilot remained calm. I definitely felt the stress; especially as I realized that I was actually [advising ATC]. Nobody wants to ever need to do that. However; I kept calm and focused on the task at hand. Second; as a Commercial Pilot student; I had done dozens of power-off 180s in the preceding weeks. Once we were established on the downwind; most of my stress was gone; I had full confidence that I would make the runway; even if the engine quit on us.
C172 pilot reported an engine malfunction while on a break-in flight.