C-152 private pilot and accompanying safety pilot reported fluctuating RPM and a rough running engine during cruise. RPM reductions correlated to a reduction in aircraft speed so pilots elected to divert and land as a precaution.
Synopsis
C-152 private pilot and accompanying safety pilot reported fluctuating RPM and a rough running engine during cruise. RPM reductions correlated to a reduction in aircraft speed so pilots elected to divert and land as a precaution.
Narrative
I am an ICAO CPL holder. I came to the United States to build hours and I have a temporary FAA PPL License. I am renting a Cessna 152 from Company that is based in ZZZ1. Yesterday; I was flying in Aircraft X (C152) for some cross country flights. I was the pilot flying and I had a safety pilot beside me who is also a ICAO CPL pilot with a temporary FAA PPL license. We both had above 180 hours. During my last leg which was from ZZZ2 to ZZZ1; I experienced an engine roughness. I was cruising at 5500 ft. on my way back to ZZZ1. I was cruising with 2300-2400 (recommended during cruise) and never above and the indicated airspeed was near 85. We don't know the exact indicated airspeed as we were told by the instructors to maintain RPM during cruise; not speed; because the horsepower on the 152 isn't big. As per the mixture; we set to towards the rich side of peak so that the engine would not be overheated. In addition; during the climb phase; we tried to cool the engine by climbing at speeds above Vy; Vx.Shortly after our aircraft's rpm dropped about 300 and fluctuated. The fluctuation became bigger and we know it wasn't just the indicator's problem because the airspeed was dropping and we felt the engine losing power. We instinctively had the mixture at full rich at full throttle. The engine power loss worsened so we conducted the engine failure checklist and looked for a nearby airport. The engine's rpm dropped to about 1400; which is idle during its windmilling state. The engine's rpm fluctuated from idle to 1800; and this fluctuation cycled a couple of times. With the rpm fluctuation being unstable; we were losing altitude and airspeed rapidly. In addition we weren't able to visually have contact with the nearest field; so we [requested priority handling]. We were barely in the glide range; however; because the engine was not reliable and we were approaching the runway with headwind; we felt that there was not enough safety margin to land without priority. We were given clearance to spiral down to ZZZ; Runway XX. We had to lose around 4000 ft. to land in ZZZ so we spiraled down. The engine came back on when we were descending to the runway so we advised the Tower the engine's back on but we'll make a precautionary landing just to be safe.
Second reporter narrative
I am a ICAO CPL holder who has a temporary FAA PPL license. I came to the United States to build some hours. I am renting a Cessna 152 from Company based in ZZZ1. Yesterday during our first run up check; the right magneto dropped more than 200 RPM - which is not normal. As a result; we headed back to maintenance to get it fixed. The mechanic fixed it and had no problems during the next run up check. That is why I decided to continue with the flight. In my last leg which was from ZZZ2 to ZZZ1; I was cruising at 5;500 ft. MSL and receiving radar flight following to ZZZ1. When I was near another nearby airport; I had engine roughness issues which the engine RPM dropping to around 1400 RPM. That is when I decided that it would be best to divert to the nearest field. I advised the Approach that we would like to divert due to engine roughness problems. I was handed off to the Tower and I advised them of our situation and informed them we would make the landing from a left downwind entry. The ATC controller asked if we were [requesting priority handling]; however; since our engine wasn't completely dead; we did not [request priority handling]. During our final approach phase; the engine's RPM was fluctuating from idle to 1500 RPM even with the throttles at closed position. We landed safely and taxied to the parking spot.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.