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.

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Original NASA ASRS Text

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

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.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.