Date: 201810
Local time of day: 0601-1200
Location: ZZZ.Airport, US
Flight conditions: VMC
Weather elements / visibility: 10
Light: Daylight
Aircraft: Skyhawk 172/Cutlass 172
Operator: FBO
Operating under: Part 91
Flight plan: VFR
Mission: Training
Flight phase: Cruise
Airspace: Class E ZZZ
ATC / advisory: TRACON ZZZ
Aircraft component: Cylinder
Component problem: Failed
Reporter organization: FBO
Reporter function: Pilot Flying; Trainee
Reporter qualification: Flight Crew Private
Human factors: Situational Awareness; Training / Qualification
Second reporter function: Instructor; Pilot Flying
Anomaly: Aircraft Equipment Problem Critical
Detector: Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Provided Assistance; Aircraft Aircraft Damaged; Flight Crew Diverted; Flight Crew Landed As Precaution; Flight Crew Landed in Emergency Condition; Flight Crew Requested ATC Assistance / Clarification
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: The event began as a training flight. The intended flight was to be a dual VFR cross country from ZZZ using pilotage and dead reckoning to ZZZ1; then GPS navigation from ZZZ1 to ZZZ2 and then ZZZ2 back to ZZZ. We completed a thorough run-up at ZZZ; with no engine issues. Takeoff; climb; and cruise to ZZZ1 was normal. After a full stop and taxi back; we continued to ZZZ2. During the cruise; I noticed that the number four CHT was a few bars higher than the rest of the cylinders. I inquired my instructor about it; and he believed it was nothing to worry about; since the number four cylinder is usually the hottest of all the cylinders. Landing was normal at ZZZ2; and we stopped at the FBO for approximately forty-five minutes. My instructor and I completed an engine run-up; and there were no abnormal indications. We then took off on runway XXL; and I noticed the engine seemed to be making a deeper and somewhat louder noise than usual. Once we began the after-takeoff checklist; my instructor checked the CHTs and noticed that the number four CHT was at 515 degrees and rising (normal operating range for the C172 being 200 - 500 degrees; and [guidance] calling for no more than 400 degrees). We immediately contacted tower and informed them that we needed to land back on runway XXL due to an engine issue. They asked my instructor if we wanted to declare an emergency; and he elected to not; since we did not have any other abnormal conditions. During the precautionary landing; my instructor noticed the number four CHT reach over 550 degrees. Upon landing; we taxied back to the FBO and informed [the rental FBO] about the situation. A mechanic from [a nearby field] was contacted and he tested the CHT temperature probes and was unable to find anything definite due to not having the necessary tools. Afterwards; a mechanic and another flight instructor flew in a PA-44 from [ZZZ] to attempt to fix the engine. The mechanic performed multiple tests throughout the day and was unable to fix the problem. He discussed the problem with his superiors and they all had strong reason to believe that the issue was a faulty temperature probe. We all ended up staying overnight in ZZZ2 since neither instructor was night-current in the PA-44. In the morning; the mechanic conducted several more tests and my instructor explained to me that the mechanic found that some of the protective wiring on one of the wires was frayed; so he wrapped it in electrical tape and it seemed to stabilize the number four CHT from rising as much during the tests. After contacting several personal; we all believed that the aircraft was safe to fly and began to brief for takeoff. My instructor and I were advised to keep the mixture full rich during the entire flight to assist in CHT cooling in the small chance that the issue was not a faulty CHT probe. We planned to increase all speeds of the takeoff and climb as well to ensure optimal engine cooling. The mechanic explained to us that we should monitor the EGT and use that as our reference instead of the CHT since the issue was likely the faulty CHT probe. We then discussed what airports we could use in the event of an emergency along the route and found that there were a significant number available for the majority of the trip. We then conducted an engine run-up and did not notice any anomalies. We then took off on runway XY and again noticed the CHT climbing; but the EGT remained normal. I rotated at 63 KIAS (55 KIAS was what [guidance] called for); initially climbed at 85 KIAS (we normally used 74 KIAS for this phase); and then cruise climbed at 90 KIAS [FBO] normally uses 85 KIAS for this); and kept the mixture full rich the entire flight; thus; we were taking multiple precautions to avoid an engine overheat. During the climb; I noticed the same seemingly louder and deeper engine noise but continued flying since we were told that the issue should be the CHT probe; and not a cylinder issue. The CHT rose again toover 500 degrees; but the EGT was reading normal. We initially planned to fly 2500 MSL and then to 6500 MSL but stayed at 3000 MSL due to some scattered clouds. Once we were cruising; the engine noise slowly grew louder and more noticeable. We then were about to climb to 3500 MSL; approximately fifteen to twenty minutes into the flight; I noticed a significantly louder noise from the engine; somewhat like a deep; metallic clanging noise. I informed my instructor; and immediately he also noticed the engine was vibrating substantially. We then realized that we were having hazardous engine trouble and [notified ATC]. Until this point; I had been the pilot flying. I gave the controls to my instructor and he began coordinating with ATC the nearest airport; what heading to fly; and answered the standard emergency questions. We received a heading to ZZZ3; and my instructor increased the throttle to full power in an attempt to gain as much power as possible. We only had approximately 2100 RPM (C172 normally provides 2700 RPM at full power); and the RPM was slowly decreasing. I then noticed that the EGT had massively spiked on the number four cylinder; and it was reading over 1460 degrees. My instructor kept us between 3000 and 3200 feet MSL until we were within a few miles of the airport and made an emergency descent to ZZZ3. The CHT had cooled during the descent due to the increased airflow; but the engine was still violently vibrating and the number four EGT was still massively higher than the other cylinders. We side-slipped down to runway YY and landed without incident. We had the PA-44 with the mechanic follow us out of ZZZ2 in the case of an emergency; so they landed shortly thereafter and inspected the engine. The mechanic immediately noticed a large crack around the number four cylinder; oil splattered across the lower cowling and several exhaust pipes; and a burn mark behind the number four cylinder.


Second narrative: I was conducting a cross-country with my student. We landed at ZZZ1; taxied back to the active runway and proceeded directly to ZZZ2. After reaching cruise altitude to ZZZ2; my student and I notice the CHT of the #4 Cylinder was indicating hotter than the other three. The #4 cylinder was stable and within limitations. We landed at ZZZ2; refueled; and filed [our flight plan]. We then taxied to runway XXL at ZZZ2; completed the run-up and all engine indications were normal and within limitations. The #4 CHT was still indicating hotter than the other three cylinders. After being cleared to take-off runway XXL; we were about 600 feet AGL; at 85 KIAS (to keep the CHT cool). I checked the CHTs on the MFD and the #4 Cylinder was flashing red. We had a positive exchange of flight controls and I was flying the aircraft immediately after this occurred. I immediately contacted tower and requested to return back to land. I was then cleared to enter the left downwind for runway XXL and cleared to land. After landing we shut the aircraft down at [the FBO] and I contacted [the rental FBO] management and maintenance. Maintenance contacted a nearby aircraft mechanic. The aircraft mechanic could not find a solution to the #4 high-temperature indication. [They] flew an instructor in a Piper Seminole and one   aircraft mechanic to work on the aircraft. The mechanic could not find the solution to the #4 Cylinder high-temperature indication. We stayed the night because it was getting very late at night and the instructor who flew the Seminole was not night current to carry passengers. After spending the night at a hotel; we arrived at the [FBO] at ZZZ2; preflighted the Seminole to return back to ZZZ. The Seminole instructor filed an IFR flight plan back to ZZZ. After taxiing out and holding short behind [another aircraft] at runway XC; I received a call from the [Operations Director]. He requested that we cancel the takeoff and taxi back to the airplane because [he] wants our A&P Mechanic to conduct more tests on aircraft. After arriving back to [the FBO] support; the A&P mechanic conducted more tests but was still unable to lower the #4 CHT indication. The A&P mechanic completed one last test and was able to lower the #4 CHT to the normal operating range. After the A&P mechanic discussed the situation with [the rental FBO]; and they concluded that the aircraft is airworthy. Based on the previous tests and the competent advice from the maintenance department; they all came to the consensus that the aircraft was safe to fly. I agreed with their decision and the A&P signed and dated all previous maintenance discrepancies. My student and I conducted a normal preflight of the aircraft; completed a normal run-up and the A&P mechanic was able to see the #4 CHT temperature gauge from the outside (He walked around and checked while my student and I were in the aircraft on the ramp). He gave us the thumbs up and I agreed because all engine indications (i.e. EGT and CHT indications) were within normal operating ranges and stable. During the taxi out to the active runway XY at ZZZ2; I was closely monitoring the #4 CHT indication and EGT indication as well. All indications were in the normal operating range. I did one more close check of the CHT and EGT before the taxiing onto the runway; I saw all engine indications were in the normal operating range before advancing the throttles. The A&P mechanic told us that we need to have the mixture rich during the whole entire flight to aid in cooling the CHT. After taking off of runway XY at ZZZ2; I had my student rotate at 55 KIAS (flaps up Vr) and climb out at 90 KIAS cruise climb to help keep the CHTs cool. After leveling off at 3000 feet before a 4000 scattered cloud layer; I told Departure that I cannot climb to the assigned 6500 cruising altitude back to ZZZ. We were cleared at or below 3500 feet until we were able to do our VFR climb to 6500 feet. About 10-15 minutes after departure from ZZZ and around2900-3100 feet; my student and I were told to contact Approach. After contacting Approach; my student and I were monitoring the CHT and EGTs and we both saw a rising EGT temperature on the #4 cylinder; but the CHT was not rising... We both heard a strange noise from the engine; felt a shaking sensation; and saw the engine cowling shaking as well. Immediately after this; the EGT indication rose rapidly to 1460 degrees. I immediately heard and felt engine roughness and a decrease in engine RPM. I immediately contact Approach and [notified them of the situation]. I told the controller that I needed vectors to the closest airport and distance. The controller told me to turn to a heading of 180. I turned to the heading of 180 and saw [another] airport (ZZZ3) in front of me. I told my student to run the checklist while I fly the airplane and communicate with ATC. The controller during the whole entire time was very professional and calm. The controller wanted us to write down a frequency and number to contact them when we landed. I told my student to write the number and frequency down; which he did. I saw runway YY insight at ZZZ3 airport and knew I was going to make the runway. We experience the engine failure/roughness around 7-8 miles North East of ZZZ3 airport around 3100 feet. My student and the controller were very helpful and we all conducted great Crew Resource Management during the emergency. I landed on runway YY for the straight in approach. After landing I taxied off the runway and shut the aircraft down at the nearest ramp off the taxiway. After securing the aircraft I called the TRACON and told them we landed safely at ZZZ3 airport. The lady on the phone asked me a couple questions to see if anyone was hurt and the rest I cannot remember; due to the rough situation I was in. There was no one hurt! The other instructor who took off behind me on an IFR flight plan was on the same frequency. He then diverted to ZZZ3 airport to be with us. After the instructor flying the Seminole landed; the A&P mechanic opened up the cowling of the single-engine airplane I was flying and saw that the #4 cylinder cracked. We came into contact with management and they told us to stay at ZZZ3 and relax for a little bit before we return home in the Seminole.


Synopsis: C172 flight instructor and student reported high Cylinder Head Temperature with a possible resolution. The ensuing flight diverted with a partial engine failure.