Cessna 172 pilot and instructor reported safely diverting to an alternate airport after experiencing severe engine roughness.
Synopsis
Cessna 172 pilot and instructor reported safely diverting to an alternate airport after experiencing severe engine roughness.
Narrative
The aircraft was Aircraft X; a 2012 Cessna 172S G1000 equipped aircraft. The flight was conducted under Part 91 and an IFR flight plan; but in VMC conditions. The purpose of the flight was to log simulated instrument time under safety pilot conditions. I and a CFI were the two pilots on board. We departed ZZZ with an IFR clearance of the ZZZZZ SID for ZZZ1. During the run-up checks; all engine instruments were green; and both magnetos dropped 110 RPM; which was within standards on the published checklist. On the takeoff roll before the airspeed was alive; we noticed a short burst of engine roughness; likely attributed to my rapid input of throttle power. Still; it immediately smoothed out; and we proceeded with the roll. The flight was uneventful until approximately 60 miles from ZZZ; near ZZZ2. At that point; we began noticing engine roughness; and both turned our heads to look at each other. We immediately started looking below us and at a VFR sectional chart for areas where an emergency landing could occur. We notified ZZZ TRACON of our situation and that we would land on a road if we had lost our engine; as we were not within gliding distance of ZZZ2 or any suitable airports. We then proceeded to troubleshoot the issue and requested a higher altitude of 5000 instead of our assigned altitude of 4000. I flew the aircraft while the CFI troubleshooted. While troubleshooting; ATC gave us priority handling and gave us XXL or XXR as needed. We told them we had sufficiently troubleshooted the issue and would still proceed to ZZZ1. Upon hearing that; ATC provided us with a direct to ZZZ1 clearance. Moments later; the engine's roughness returned; and further troubleshooting was unsuccessful. All engine instruments and temperatures were within standards. The engine monitoring systems showed no abnormalities either. We then decided to divert to ZZZ3 and were cleared for the visual approach into ZZZ3. At this point; the CFI assumed control of the aircraft and pulled the power to descend into ZZZ3. Upon reducing the power to around 1700 RPM; violent and intense shaking of the engine occurred. This lasted for the remainder of the descent and landing into ZZZ3. At moments; the engine also sputtered and popped with bangs; and I felt it would give out at any moment. Upon landing; we taxied to the ramp; and ZZZ TRACON contacted us for details about the situation.We had not decided to divert to ZZZ2 since the issue was successfully troubleshooting; and we were not within gliding distance. We also had ZZZ3 on our right side in case the engine began to act up again. In this case; it did; and we diverted to ZZZ3. We did not decide to proceed any further as the nearest suitable airport would have been ZZZ4 and ZZZ5; which was another 15-20 miles away; both of which would have been troublesome in their ways. We believed that we had another plan B option besides ZZZ2; and given the circumstances of our ability to troubleshoot initially; we objected to ZZZ2 and proceeded on the flight. However; after experiencing the issue again; we diverted to ZZZ3.
Second reporter narrative
My name is Person A; a Commercial rated pilot with ASEL and AMEL ratings; with a Certified Flight Instructor certificate (CFI). On the evening of Day 0; I conducted a cross-country flight with my friend and colleague. I acted as PIC and was operating under an IFR flight plan in VMC conditions. My buddy is a Private Pilot (ASEL) currently in instrument training; was on board as a safety pilot. He is not yet instrument rated and did not act as PIC during this flight unless I was under the hood. The aircraft was a Cessna 172S with a Garmin G1000. It is a leaseback aircraft owned by a Part 61 flight school based at ZZZ1. This flight was operated under Part 91. This cross-country flight was from ZZZ1 to ZZZ; then back to ZZZ1. We departed ZZZ1 at approximately XA36Z and arrived at ZZZ at XC18Z. The flight had no adverse weather; and systems operated normally. We conducted a visual approach to Runway XXR and refueled at the FBO. The aircraft was fueled to full tanks on both sides. I filed an IFR flight plan for the return flight [to ZZZ1].During pre-flight; everything was normal. Systems were checked and were within limits. During takeoff before rotation at 55 knots; we both heard a loud bang from the engine. I immediately assumed control and stated; 'My controls.' I opted to extend the takeoff roll to assess engine performance. At Vr; I rotated the aircraft and noted no abnormal engine behavior. The aircraft performed normally during the climbout; and all indications were within the green. Based on my experience as a flight instructor; I believe the noise may have resulted from an abrupt application of full throttle; a habit I've seen in students; which can occasionally produce a sharp detonation like sound. I always teach gradual throttle application to avoid engine stress and surging. We climbed via the ZZZZZ Departure to 3;000 feet; then expected 6;000 feet in 10 minutes on the SID. Cruise performance remained stable; and we encountered no weather issues.1 hour into the flight and while in contact with ZZZ TRACON on frequency XXX.X; in the vicinity of ZZZ2 and ZZZ3; we began experiencing noticeable engine roughness. It felt like we were on a roller coaster. I immediately began troubleshooting per the engine roughness checklist. I leaned and enriched the mixture incrementally to determine if the issue was fuel-mixture related. I also cycled between the left and right magnetos. Both magnetos dropped approximately 100 RPM; but the roughness reduced slightly during the check. EGT readings were normal. Oil temperature and pressure remained in the green. I verified the fuel selector was set to both; and the fuel shutoff valve was fully in. Both fuel gauges indicated 4 hours of fuel remaining. I also manipulated the throttle to see if the roughness correlated with RPM changes.I notified ATC of the rough engine condition and continued to work the checklist. ATC advised that ZZZ2 was located at our 10 o'clock and ZZZ3 at our 2 o'clock. I responded that we were still troubleshooting and would advise further. A few minutes later; the engine's roughness got better. I again contacted ATC and advised them of the engine performance. At this point; ATC gave us priority handling to be safe rather than sorry. ATC advised that ZZZ2 Runway XYR was available and equipped with full lighting; should we decide to divert. After assessing the situation; I advised that we would continue toward our destination; but I remained fully aware that ZZZ3 was nearby and within gliding range. I treated ZZZ3 as our contingency landing site. 2 minutes later; engine roughness started again. The aircraft began to shake violently; and managed the throttle inputs. I told ATC that we were diverting to ZZZ3. ATC asked if we wanted to cancel IFR and if we were okay; and if we needed assistance; I said yes and switched to CTAF. My buddy took over radio communications while I continued flying the aircraft. I kept the throttle at idle; as any increase caused additional engine roughness. We touched down safely on Runway XY. We taxied to a tie down spot early in the morning and shut down. Soon after landing; I received a call from ZZZ TRACON checking if we were okay.After thinking about my actions; I feel like landing sooner at a towered airport with more comprehensive resources (ZZZ2) may have been a better; more conservative course of action; while flying single engine at night is an increased risk operation. While ZZZ3 is a legitimate airport and was within gliding distance; if I did not make it; nobody would have known. I canceled IFR; and there is no tower there. This is probably why ZZZ [TRACON] called me; and they were concerned. There are also no emergency equipment resources at ZZZ3 if I have landed short. This flight was late at night; outside my normal circadian rhythm.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.