Cylinder Head Temperature (CHT) is one of the most important indicators of reciprocating engine health available to a pilot or aviation maintenance technician. Unlike oil temperature, which reflects a broad average of engine thermal conditions, CHT measures heat precisely where combustion energy is transferred to metal — at the cylinder head itself. Because the cylinder head is exposed to the most intense thermal stress of any engine component, monitoring its temperature gives immediate, actionable information about whether an engine is operating safely within design limits.
For AMT powerplant students, understanding CHT means understanding the interplay between combustion, fuel-air mixture, airflow, and material limits. This knowledge is tested on the FAA AMT Powerplant Knowledge Test and is equally essential on the shop floor and in advisory conversations with pilots performing engine run-ups and cruise power settings.
How CHT Monitoring Works
Cylinder head temperature is measured by a thermocouple — typically a bayonet-style probe or a gasket-style probe — installed directly in or on the cylinder head, usually near the hottest zone: the exhaust valve area or the spark plug boss. Two dissimilar metals form a junction; when heated, they generate a small electromotive force (voltage) that is proportional to temperature. This signal travels through dedicated thermocouple extension wire (matched to the thermocouple alloy type, commonly chromel-alumel for Type K) to a cockpit gauge or digital engine monitor.
On multi-cylinder engines, a simple CHT gauge may monitor only one cylinder — usually the cylinder identified by the manufacturer as most likely to run hottest under normal conditions. More sophisticated digital engine monitors can display CHT for every cylinder simultaneously, which is far more useful for diagnosing cooling problems, identifying detonation-prone cylinders, or catching a failing injector nozzle that leans one cylinder relative to the rest.
Thermocouple Types and Installation
Two common installation styles are used in light aircraft:
- Bayonet probe: A spring-loaded probe inserted into a threaded boss in the cylinder head. This design allows relatively easy replacement and gives a direct metal-to-metal temperature reading. It is important that the spring tension keeps the probe firmly seated; a loose bayonet probe will read lower than actual head temperature and give false assurance.
- Spark plug gasket probe: A thin metallic washer thermocouple that replaces one of the standard spark plug crush washers. While convenient to install, it may read slightly lower than a bayonet probe because the measurement point is not as deep into the head material. AMTs must use only the gasket size and type specified by the engine manufacturer to ensure correct seating torque and heat transfer.
Thermocouple wire must never be substituted with ordinary copper wire. Doing so introduces a spurious junction at the substitution point, corrupting the temperature signal and potentially causing dangerously false readings. All extension wire must match the thermocouple alloy, as specified in the aircraft maintenance manual and the instrument manufacturer's documentation.
What Affects CHT — The Cooling System Connection
Air-cooled reciprocating engines (the vast majority of certificated light aircraft powerplants) rely on ram air flowing over finned cylinder barrels and heads to carry away combustion heat. Several factors directly control how hot the cylinder head gets:
- Power setting: Higher manifold pressure and RPM produce more combustion heat per unit time. CHT rises with increasing power output.
- Mixture: This is the most commonly tested factor. Peak CHT typically occurs at a mixture slightly rich of peak EGT (stoichiometric), because combustion is most complete and cylinder pressures are highest near that point, generating maximum heat before additional fuel enrichment begins carrying heat out of the cylinder through evaporative cooling. Leaning beyond peak EGT while operating at high power can still risk detonation if done improperly, so manufacturer guidance must always be followed. A rich mixture beyond peak lowers CHT because excess fuel carries heat out of the cylinder as it vaporizes.
- Airspeed and cowl flap position: More airspeed drives more ram air through the cowling, increasing cooling airflow over the fins. Cowl flaps, where installed, open to increase cooling airflow during high-power, low-airspeed operations (climb) and close during cruise to reduce drag.
- Ambient temperature and density altitude: Hot days and high density altitudes reduce air density, reducing the mass of cooling air passing over the fins per unit volume of airflow.
- Fin condition: Damaged, corroded, or clogged cooling fins dramatically reduce heat transfer. An AMT must inspect fins carefully during maintenance and replace cylinders whose fins are damaged beyond the limits published in the overhaul manual.
CHT Limits — Why They Exist and What They Are
Every certified reciprocating engine has a maximum allowable CHT published by the engine manufacturer in the Type Certificate Data Sheet (TCDS) and the engine operator's manual. For most Continental and Lycoming engines commonly found in light training aircraft, published maximum CHT limits are generally around 500°F (260°C), with some models as low as approximately 460°F, though the precise limit varies by engine model and must be verified in the applicable approved data. Always consult the specific engine manufacturer's limits — never apply one engine's limit to a different model.
Operating below the maximum limit is not merely a regulatory suggestion; it reflects the thermal properties of the aluminum alloy used in cylinder head construction. Sustained high temperatures accelerate oxidation of valve seats and guides, soften valve springs, reduce the tensile strength of the aluminum, promote lead fouling of spark plugs, and increase the probability of detonation by raising the temperature of the fuel-air charge before ignition. Each of these effects compounds the others.
Equally important, shock cooling — an excessively rapid decrease in CHT — poses its own risk. Aluminum and steel components in the cylinder head assembly have different coefficients of thermal expansion. Rapid cooling can cause differential contraction stresses, leading to micro-cracks in the head or around valve seats. A commonly cited general-practice guideline from engine manufacturer service literature suggests limiting the rate of CHT decrease to approximately 50°F (28°C) per minute or less, though this figure varies by source and is not a specific FAA handbook-published limit. Pilots should avoid rapid power reductions from high-power settings, particularly during descent, to protect cylinder heads from shock cooling.
Key Numbers and Rules
- Typical maximum CHT (most Lycoming/Continental engines): generally around 500°F, with some models as low as approximately 460°F; always verify in the specific engine TCDS or operator's manual.
- Recommended cruise CHT target: Some general aviation guidance suggests keeping CHT well below the absolute maximum for extended engine life, though this is not a standardized FAA or manufacturer-mandated figure and varies by engine model.
- Shock cooling rate limit: Approximately 50°F (28°C) per minute is a commonly cited general-practice figure, not a uniform FAA-published limit.
- Mixture and CHT relationship: Peak CHT occurs slightly rich of peak EGT; leaning or enriching further from that peak generally lowers CHT. Full-rich mixture during high-power operations provides a cooling effect from excess fuel.
- Thermocouple wire: Must match thermocouple alloy type; copper substitution is prohibited and will corrupt readings.
- Cooling fin inspection: Cracks, breaks, and missing fin area beyond manufacturer limits require cylinder replacement — not field repair by welding, which is generally not approved.
Maintenance Implications for AMTs
When a cylinder is removed for inspection or overhaul, the AMT must inspect the thermocouple boss or spark plug boss for thread condition and ensure the replacement probe is the correct type, range, and connector configuration. After reinstallation, a ground run and comparison of CHT readings across all cylinders helps verify sensor function. A cylinder reading significantly lower than others at the same power setting may indicate a faulty thermocouple, broken extension wire, or an open circuit — not necessarily a cooler-running cylinder.
Consistently high CHT on one cylinder during engine runs warrants investigation of that cylinder's fuel injector (for fuel-injected engines), its ignition leads, valve clearance, and cooling fin integrity before returning the aircraft to service. A single high-CHT cylinder is a classic signature of a partially clogged injector nozzle leaning that cylinder relative to its neighbors.
Engine logs and trend monitoring are valuable tools. FAA guidance emphasizes the value of ongoing trend monitoring in maintenance decision-making. If CHT readings on a particular cylinder have been climbing gradually over successive flights, this trend — even if still within limits — warrants investigation before a limit exceedance causes in-flight damage.
Common Test Traps
- Assuming one CHT limit fits all engines: The FAA knowledge test may present a specific value and ask whether it exceeds limits for a given engine. Always apply the specific manufacturer's data — do not assume a universal number.
- Confusing EGT and CHT: Exhaust Gas Temperature peaks at a slightly different mixture setting than CHT does. Peak CHT occurs slightly rich of peak EGT on most engines. These are two separate sensors measuring two different things.
- Thinking only overheating is dangerous: Shock cooling from rapid CHT decrease is equally damaging and equally tested. Both excessive heat and excessive cooling rate are failure modes.
- Substituting thermocouple wire with copper wire: This is a maintenance error that produces false readings. The test may present this as a plausible shortcut — it is never acceptable.
- Overlooking fin damage during inspection: Cooling fin condition directly affects CHT. Test questions may describe a cylinder with fin damage and ask what effect it has on engine temperature — the answer is increased CHT due to reduced heat dissipation.