Narrative:

This flight was a cross country instructional flight with my student. We left for ZZZ expecting to return to ZZZ1 back to the flight school. At ZZZ1 we did the normal checklist procedures. In the run up prior to takeoff from ZZZ1 I verified all engine instruments were within limits. This includes engine temperature; engine oil pressure; ammeter; fuel flow; fuel gauges; egt; annunciator panel and the magneto drops. Everything in this flight going to ZZZ went as planned with no indication of engine trouble or engine suspicion.upon arriving to ZZZ we received our clearance for a touch and go on runway xx; and then expected to proceed on course back to ZZZ1. My student setup and configured according to procedures for the touch and go. We touched down on runway xx; he began to retract flaps; then increased power back to 100%. We began accelerating down runway xx; and at 60 knots he rotated the aircraft and we lifted off the pavement. At approximately 25 feet above the pavement I heard and felt a decrease in RPM and thrust. I immediately told him to keep full power in because we are taking off and climbing. As I said this I looked down at his right hand and the throttle was still fully pushed in for open throttle. I then looked over at the engine gauges and all were still in their respective green arcs. The annunciator panel also was not illuminating any indications. At this point; approximately 50 feet above the pavement; and approximately 3/4 down the 7;000 foot runway; the engine RPM's started to 'heave' back and forth going as low as 25% power; and as high as 90% power. This 'heaving' or 'waah-ing' noise was rather quick. I would estimate 2 seconds in between reaching 25% power and back up to 90% power. This is the same point in time that I took control of the aircraft and my student affirmed and repeated verbally 'your aircraft.'I then called ZZZ tower and told them 'we have lost a significant amount of engine power and need to return immediately.' the controller asked me to make right traffic for runway xx. However; after identifying my current altitude; and location in respect to the pattern; I determined that the lack of climb power; as well as the very possible event of completely losing the engine; I was better off taking advantage of the available power to assist me in a 180 degree turn for a landing on runway xy. The opposite runway to xx. At this point when I made that decision we were approximately 300 feet AGL; and less than a tenth of a mile from the beginning of runway xy. Essentially we were over the beginning of the pavement for runway xy; the opposite runway we took off from.at this point I told the ZZZ tower controller that I am unable to continue into the pattern for runway xx and that I am taking runway xy immediately. The controller acknowledged my response. It is worth noting at this point that the 'lows' and the 'highs' of the RPM 'heaving' was decreasing from 25%-90% to now at that point 25%-50%. I fully expected the engine to stop at any moment. However; the lack of power still assisted me in maintaining altitude as I made my 180 degree turn. Also; there were no aircraft on final for runway xx; nor were there aircraft waiting to takeoff of runway xx. This further affirmed my decision that a turn back to runway xy was the most logical decision with the greatest chance of success.when I visualized and identified that I had runway xy made; I powered back completely on the throttle; I dropped all the flaps to 30 degrees to slow ground speed and help descend. I pitched the nose forward to maintain a healthy airspeed at an idle power setting with full flaps. I then proceeded to do a slipping s-turn over the beginning of runway Y in order to descend. I estimate that I landed somewhere near 2;000 feet into runway xy; leaving well over half of the remaining runway. I will admit that I would never have tried this at an airport with a shorter runway; but I was very confident in my skills with a 7;000 foot runway in a ce-172-south. I successfully landed the aircraft before the middle of the runway; on the centerline; and touching down softly. I understand that this is referred to as the 'impossible turn.' I would have never tried something like this with no engine power available to assist in a 180 degree turn below 500-700 feet AGL. I determined that this 'impossible turn' had a better chance for success with assisted low-RPM's; as opposed to continuing a almost negligible climb and flying away from the runway at 300 feet AGL. After landing and turning towards the assigned taxiway to exit on; I keyed the mic and apologized to the controller. I told her that I will need to taxi to the FBO to shut down. I did not determine that emergency fire personnel was required; after she asked. However; the airport police did meet me at the FBO upon shut down. He took my license for copies as well as my students license for copies. I also explained the situation to him. After this interaction; I contacted the flight school owner and informed him of the situation. I also left the keys to the aircraft with the FBO front desk for them to hold until the flight school owner and flight school mechanic are able to arrive and address the aircraft.the flight school owner initially responded with the idea that the magnetos were off-timing. I admit that I am not a mechanic; but I do find it hard to believe that after a successful run up and magneto check less than an hour prior to this event; as well as no engine trouble or engine suspicion from the initial startup; all the way to the point of this event; it does not seem likely that both magnetos would suddenly become 'off-timed' at the same time and cause such drastic power changes so abruptly.the s-model of this 172 is equipped with a fuel injected engine; not carbureted. I must also note that I watch all my students to see how quickly they adjust the throttle from fully closed to fully open. I am a firm advocate on easily adjusting the power; in order for the engine to catch up to the new power setting and pressure changes; instead of a rapid adjustment which becomes a catalyst for future engine troubles.

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Original NASA ASRS Text

Title: C172 instructor pilot reported engine malfunctioning and having to make a 180 turn back to the airport to land safely.

Narrative: This flight was a cross country instructional flight with my student. We left for ZZZ expecting to return to ZZZ1 back to the flight school. At ZZZ1 we did the normal checklist procedures. In the run up prior to takeoff from ZZZ1 I verified all engine instruments were within limits. This includes engine temperature; engine oil pressure; ammeter; fuel flow; fuel gauges; EGT; annunciator panel and the magneto drops. Everything in this flight going to ZZZ went as planned with no indication of engine trouble or engine suspicion.Upon arriving to ZZZ we received our clearance for a touch and go on Runway XX; and then expected to proceed on course back to ZZZ1. My student setup and configured according to procedures for the touch and go. We touched down on Runway XX; he began to retract flaps; then increased power back to 100%. We began accelerating down Runway XX; and at 60 knots he rotated the aircraft and we lifted off the pavement. At approximately 25 feet above the pavement I heard and felt a decrease in RPM and thrust. I immediately told him to keep full power in because we are taking off and climbing. As I said this I looked down at his right hand and the throttle was still fully pushed in for open throttle. I then looked over at the engine gauges and all were still in their respective green arcs. The annunciator panel also was not illuminating any indications. At this point; approximately 50 feet above the pavement; and approximately 3/4 down the 7;000 foot runway; the engine RPM's started to 'heave' back and forth going as low as 25% power; and as high as 90% power. This 'heaving' or 'waah-ing' noise was rather quick. I would estimate 2 seconds in between reaching 25% power and back up to 90% power. This is the same point in time that I took control of the aircraft and my student affirmed and repeated verbally 'your aircraft.'I then called ZZZ Tower and told them 'We have lost a significant amount of engine power and need to return immediately.' The Controller asked me to make right traffic for Runway XX. However; after identifying my current altitude; and location in respect to the pattern; I determined that the lack of climb power; as well as the very possible event of completely losing the engine; I was better off taking advantage of the available power to assist me in a 180 degree turn for a landing on Runway XY. The opposite runway to XX. At this point when I made that decision we were approximately 300 feet AGL; and less than a tenth of a mile from the beginning of Runway XY. Essentially we were over the beginning of the pavement for Runway XY; the opposite runway we took off from.At this point I told the ZZZ Tower Controller that I am unable to continue into the pattern for Runway XX and that I am taking Runway XY immediately. The Controller acknowledged my response. It is worth noting at this point that the 'lows' and the 'highs' of the RPM 'heaving' was decreasing from 25%-90% to now at that point 25%-50%. I fully expected the engine to stop at any moment. However; the lack of power still assisted me in maintaining altitude as I made my 180 degree turn. Also; there were no aircraft on final for Runway XX; nor were there aircraft waiting to takeoff of runway XX. This further affirmed my decision that a turn back to Runway XY was the most logical decision with the greatest chance of success.When I visualized and identified that I had Runway XY made; I powered back completely on the throttle; I dropped all the flaps to 30 degrees to slow ground speed and help descend. I pitched the nose forward to maintain a healthy airspeed at an idle power setting with full flaps. I then proceeded to do a slipping S-turn over the beginning of runway Y in order to descend. I estimate that I landed somewhere near 2;000 feet into Runway XY; leaving well over half of the remaining runway. I will admit that I would never have tried this at an airport with a shorter runway; but I was very confident in my skills with a 7;000 foot runway in a CE-172-S. I successfully landed the aircraft before the middle of the runway; on the centerline; and touching down softly. I understand that this is referred to as the 'impossible turn.' I would have never tried something like this with no engine power available to assist in a 180 degree turn below 500-700 feet AGL. I determined that this 'impossible turn' had a better chance for success with assisted low-RPM's; as opposed to continuing a almost negligible climb and flying away from the runway at 300 feet AGL. After landing and turning towards the assigned taxiway to exit on; I keyed the mic and apologized to the Controller. I told her that I will need to taxi to the FBO to shut down. I did not determine that emergency fire personnel was required; after she asked. However; the airport police did meet me at the FBO upon shut down. He took my license for copies as well as my students license for copies. I also explained the situation to him. After this interaction; I contacted the flight school owner and informed him of the situation. I also left the keys to the aircraft with the FBO front desk for them to hold until the flight school owner and flight school mechanic are able to arrive and address the aircraft.The flight school owner initially responded with the idea that the magnetos were off-timing. I admit that I am not a mechanic; but I do find it hard to believe that after a successful run up and magneto check less than an hour prior to this event; as well as no engine trouble or engine suspicion from the initial startup; all the way to the point of this event; it does not seem likely that both magnetos would suddenly become 'off-timed' at the same time and cause such drastic power changes so abruptly.The S-model of this 172 is equipped with a fuel injected engine; not carbureted. I must also note that I watch all my students to see how quickly they adjust the throttle from fully closed to fully open. I am a firm advocate on easily adjusting the power; in order for the engine to catch up to the new power setting and pressure changes; instead of a rapid adjustment which becomes a catalyst for future engine troubles.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.