Skyhawk 172/Cutlass 172
Flight Crew Instrument; Flight Crew Flight Instructor; Flight Crew Commercial; Flight Crew Multiengine
Flight Crew Last 90 Days 50; Flight Crew Total 9800; Flight Crew Type 500
Distraction; Situational Awareness
Aircraft Equipment Problem Critical
Aircraft Equipment Problem Dissipated; Flight Crew Landed As Precaution; General Maintenance Action
Aircraft
My instrument student is in the final stages of preparing for his test. In order to do all the required approaches in a time-efficient manner; he has elected to do the flight portion at night; at a nearby Class B airport. That means conducting the test between 2 and 4 am. We decided to conduct a practice flight in the training aircraft which he rented.During our last approach; we both heard an unusual; subtle undertone to the engine noise. It sounded like we were in a twin with poorly synchronized propellers. A rhythmic 'thrumming'. I thought that perhaps one of our wheels was spinning.  When my student looked up; he deployed full flaps; and began to adjust his speed and match his glide path to the PAPI. A few seconds later; as we were on approximately 1/2 mile final; he announced 'I have no power'. I checked things. Fuel on BOTH; carburetor heat ON; throttle MAX; Mixture full RICH. The engine was running; but producing only idle thrust. I played with the mixture control for about 3 seconds before looking outside again and seeing that we had enough energy to make the runway. I told Tower that we would be full stop; with a request for progressive taxi to park the plane overnight. The time elapsed from the onset of the issue to being on the runway was less than a minute. After we landed; the engine behaved normally. On final; I thought I recognized the symptom as indicating fuel starvation. If we hadn't had the runway made I would have pulled the throttle back and tried a lower power setting. I've experienced something similar in old; carburetor cars. A few hours later; when it was light out; another instructor went to where we had parked; accompanied by two mechanics. They checked everything they could. Cleaned the plugs; checked the air filter; etc. They ran the engine up at various power settings for 30 minutes. They found no fault. The engine was normal.The instructor and one mechanic flew the airplane back to the home airport. Takeoff; climb; cruise; descent were all normal. But when they were on final approach; they experienced the same power loss as we had. They landed successfully; and proceeded to conduct a days-long thorough examination of the engine during which they found a significant fuel leak in the carburetor.
C172 Flight instructor reported a loss of engine power on final approach.