C172 student pilot and flight instructor reported that during cruise flight they experienced a loss of engine RPMs resulting in the flight diverting to the nearest airport.
Synopsis
C172 student pilot and flight instructor reported that during cruise flight they experienced a loss of engine RPMs resulting in the flight diverting to the nearest airport.
Narrative
I was the Student Pilot in command of this flight receiving dual instruction from a CFI. I was on the controls for the entirety of the flight. We took off out of ZZZ1 at XA:15 and departed west towards ZZZ. The lesson was a cross country flight with an instrument approach. We got VFR flight following from ZZZ1 Approach then ZZZ Approach. We executed the RNAV XX at ZZZ. On decent into ZZZ; I observed a low thumping feeling at low RPM. I asked my instructor if she felt or heard anything unusual and described what I experienced. After roughly 10 seconds it went away. We continued our descent and executed a touch and go at ZZZ at XB:45. Upon reaching cruise altitude of 3500 at XB:50 we attempted to run the cruise checklist. When leaning the mixture we did not see any rise in RPM. We went back to full rich as we started to slowly and progressively lose 10-30 RPM at a time. We each checked the throttle friction and attempted to lock it at a particular RPM as we observed it fall below each RPM setting we tried; we decided to navigate to a nearby airport; ZZZ2. We notified ATC that we would like to cancel flight following and land early at ZZZ2. We started a descent and flew past the airport to have appropriate time to descend. We noted that the throttle was further in than it normally was for the RPM we needed to maintain and we had to continuously adjust for greater RPM as it lowered. We entered the pattern and reduced throttle to descend for landing. We decided to maintain 1700 RPM rather than 1500 (what we would normally use). I noted on base leg that I was having to add RPM to arrest descent rate. We continued in for Runway; XY; flaps 10 degrees; floated slightly; and made a safe landing. Upon clearing the runway we attempted an after landing checklist and observed no RPM rise when attempting to lean the mixture. We taxied to the ramp and made contact with the maintenance team at the ramp who provided us with chocks for the tires. We immediately called Dispatch upon shutdown and described the incident. The maintenance team at ZZZ2 stated that they would move the aircraft to a different parking spot and tie it down until company could come get it.
Second reporter narrative
We left ZZZ1 for a cross country flight to ZZZ and back to ZZZ1. On the approach course for RNAV XX; during descent into ZZZ; student asked if instructor felt engine roughness; later reported as a low thumping noise at low RPM; instructor did not hear anything or felt anything; engine instruments did not show any changes. We did a touch and go at ZZZ; engine instruments were verified in the green on the takeoff roll by both student and instructor. At cruise altitude; after completing the cruise checklist and leaning the mixture; we noticed that the RPM kept slowly dropping 20-30 RPM at a time every few seconds. Student increased throttle to maintain airspeed. Engine instruments were checked; no large changes were noted. Instructor ran floor to door; setting mixture back to full rich; fuel pump on to check if that helped. Pitot heat on in case of icing. RPM still maintained slow and steady drop. We did a slow climb to 4000 ft at about 85- 90 kts for more altitude as we searched for nearest airport on our path to ZZZ1 while monitoring the RPM and engine instruments. Cruising at 4000 ft; while maintaining altitude; given the RPM continued the slow drop; and we were about 1NM from ZZZ2 at that point; we decided to divert there. We told ZZZ1 Approach that we were cutting our flight short and landing at ZZZ2; we did not declare an emergency. We landed XY at ZZZ2; maintained higher RPM than SOP (about 1700-1800 rpm) once we started the descent from pattern altitude since we were not sure we would be able to get more RPM back if we reduced it to 1500. When landing was assured; we added flaps 10; and slowly reduce the power on final. Student noted later that he had to increase the throttle setting on base to maintain airspeed. After we cleared the runway; we completed the after landing checklist; noted that there were no RPM rise when we leaned the mixture. We taxied and secured the aircraft and called school Dispatch for notification.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.