C172 Flight Instructor and student reported large fluctuations in engine RPM during cruise. Flight crew diverted and landed safely.
Synopsis
C172 Flight Instructor and student reported large fluctuations in engine RPM during cruise. Flight crew diverted and landed safely.
Narrative
The flight was a part 141 Instrument cross country flight. Our flight plan was ZZZ ZZZ1 ZZZ2 ZZZ. The plane passed all ground checks. Temperature was 12 at ZZZ and dew point was 02. Winds were variable from the northeast. Visibility was in excess of 50 miles. Skies were clear. We picked up our CRAFT (Clearance Limit; Route; Altitude; Frequency; Transponder Code) clearance about XA04; and we were released to depart from RWY XX about 15 minutes later. We departed IFR and were allowed to proceed directly on course ZZZ1 at 6;000. Climb was uneventful. We maintained sterile cockpit until we entered the cruise portion of the flight and after we completed the cruise checklist. Afterwards we monitored the airplane instruments and engaged in small talk for the long cruise portion. We were passed off from ZZZ departure and checked in with ZZZ Center on XXX.XX uneventfully. The plane was configured at 2200rpms and mixture slightly rich of peak. Fuel selector was on both; and we had full tanks. Oil pressure and temperature were in the green. Shortly after being passed off to ZZZ Center on XXX.XY; about 60nm from ZZZ1 airport (we were heading southwest directly to the airport on about a 240 heading; and groundspeed was about 104kts); we experienced a sudden loss of RPMs. The RPMs started to sound sluggish and reduced from 2200 to 2000; tried to catch itself; and then kept dropping. After processing what was happening; I called 'my controls' from my student who was under the hood. I started to pitch for glide and ran through quick memory items and began looking for a place to land. My student went visual and grabbed the emergency checklist. During this time the engine kept sounding like it was going to quit and remained between 1000-1500 RPMs; but was not producing steady power to maintain altitude. I quickly verified fuel was on both; and pushed the mixture to rich. The alternator was showing a charge and the oil temperature and pressure were both in the green arcs. I requested priority handling and explained I was losing the engine and that we needed vectors to the nearest airport. I turned the airplane left and right trying to decide whether I should try to glide for a road or field or if I could glide to an airport. The nearest airport was 15-16nm from our position and my student identified it and started to help me get oriented to that and was running through checklists to try and troubleshoot while I flew. We reported 5hrs of fuel on board and 2 souls. Center asked if we could make the airport and at this time the RPMs were hanging around 1000-1100 and I said not likely. I was expecting to land off site. I did say that I was going to fly toward the airport; but that I didn't think I would have the altitude to glide 15miles. We were at this time descending in the 4;000s ft. At this time the engine began surging from 1000-2500rpms in and out quickly. I did my best to keep around 70kts for the glide. I tried turning the mag from 'both' to 'lefT' and the engine about died outright; so I quickly went back to both. A few moments later Center asked how we were doing; and I said we had a surging engine but that we were descending. He gave me the distance to a couple of airports but we decided on ZZZ3 since it was closest and inside of 10miles. I tried switching to the right mag and also turned on the carb heat. When I did; the engine gained power and held it more steadily. Center also chimed in checklist items for carb heat and mixture and fuel selector. I verified that we had carb heat out and mixture full rich; and that we were running on one mag and had some power back; but it still felt a little shaky. He told me airport was 12 o'clock in 7 miles; and that's when we got the field in sight. We switched over to local frequency. The descent was arrested with having power back and we flew a circuit in the pattern to lose altitude safely before landing. We landed Runway XY at ZZZ3; and performed a further ground check of the mags with and without carb heat just in case it was heavy carb icing. The engine seemed shaky at higher power settings and the left mag barely showed an rpm loss and was rough even with carb heat engaged. In any event; we decided to shut down and call maintenance and get further direction. We shut down and while I was calling Center to verify we were safe; my student noticed white smoke coming from the engine inlets. Landing and taxi was uneventful and we never fully loss engine power; but it was close for a while. I said my prayers to my guardian angels which helped me calm and focus on flying it safely.
Second reporter narrative
Routine cross-country training for instrument rating; cruising at 6000 feet about 60 miles from ZZZ1. No initial indicators of malfunctions until we started to have roughness of engine loss of RPMs. The engine continued to gain RPM and then rapidly lose RPM. It was not an It was not a complete engine failure; but the engine was failing to produce power to continue flight. It was unclear if the magneto's were the main issue however we did check the magneto's in the air and both seem to be low or rough. The aircraft started to lose altitude and we requested priority handling. ZZZ Center directed us to the newest Airport. Find it safely no injury to persons or aircraft.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.