High Cylinder Head Temperature on Climb-Out After Maintenance
A four-cylinder horizontally opposed engine with 1,200 hours since overhaul has just left a 100-hour inspection. On the first climb after maintenance, the pilot reports cylinder head temperature on the number 3 cylinder reaching 245 °C (about 470 °F) while the other three sit near 190 °C (375 °F). Oil temperature is normal at 85 °C. The baffle seals were replaced during the inspection, and the mechanic notices the number 3 upper spark plug lead is routed differently than the others. The engine runs smoothly, mag check is normal, and the mixture responds normally. Outside air temperature is 30 °C at field elevation 600 m. The aircraft is away from base with a passenger waiting and a 40-minute return flight planned.
1. Identify the most probable causes of the single-cylinder temperature anomaly and rank them, explaining the reasoning that supports each ranking.
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A single hot cylinder with the other three normal points to a localised problem, not a system-wide one such as a lean mixture or overall cooling deficiency. Ranked causes:
- Cooling airflow disruption around cylinder 3 - the baffle seals were just replaced, and a mis-seated or folded seal, or a gap at the cylinder 3 opening, would starve that cylinder of air while the others stay cool. This is the highest-probability cause because it is the only item changed in the work package and it explains the single-cylinder pattern.
- Ignition lead routing or chafing - the differently routed spark plug lead may be resting against a hot surface or partially shorting, causing late or weak combustion on that cylinder. Lower probability because the mag check is normal and the engine runs smoothly, but still worth inspecting.
- Fuel distribution or induction leak local to cylinder 3 - a leaking intake pipe flange gasket can lean that cylinder only. Possible, but a significant leak usually produces rough running or a larger temperature rise.
- Cylinder-specific mechanical issue (sticking valve, carbon buildup, failing injector) - the engine is 1,200 hours since overhaul, so wear is plausible, but these usually present with roughness or abnormal mag drop.
A blocked or partially restricted cooling path is far more consistent with the evidence than an internal defect, and the recent maintenance history makes it the first thing to verify.
2. Decide whether the aircraft may be flown back to base. State the decision, the conditions that would change it, and the specific inspection steps you would take before any flight.
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Decision: do not fly until the cause is identified. 245 °C is above the typical continuous limit for this class of engine (commonly around 230–240 °C; the type-specific limit in the flight manual governs), and an unexplained single-cylinder exceedance after maintenance is exactly the scenario where the next climb could overheat a cylinder to the point of detonation damage or valve distress. A 40-minute flight with a passenger is not a reason to defer diagnosis.
Inspection before flight:
- Remove the cowling and check every baffle seal around cylinder 3 for folding, gaps, or contact with the cylinder fins.
- Confirm the cylinder 3 fins are clean and unobstructed and that no rag or tape was left in the plenum.
- Trace the number 3 spark plug lead for chafing, correct support, and full seating on the plug.
- Check intake pipe flange nuts and gasket for leaks; look for staining.
- Verify the cylinder 3 injector nozzle is clean and correctly seated.
Conditions that would change the decision:
- If a folded baffle seal or misrouted lead is found and corrected, a short ground run and a cautious climb with reduced power may be acceptable at the mechanic's discretion, within the flight manual.
- If nothing is found, the aircraft stays grounded until a borescope inspection and, if needed, a maintenance facility evaluation.
- Any roughness, high oil temperature, or rising CHT trend on a test run confirms the grounding.
3. Explain how the environmental conditions at the field affect the interpretation of the reported temperatures and what you would record.
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At 30 °C ambient and 600 m elevation, the density altitude is well above field elevation, so the engine produces less power at a given throttle setting and cooling airflow is reduced relative to the mass of air the engine is trying to burn. High ambient temperature raises all cylinder head temperatures, so the 190 °C readings on the other cylinders are already elevated relative to a cold-day baseline. This means the 245 °C on cylinder 3 is more significant, not less, because it occurred in conditions where a healthy cooling system should still keep all cylinders within limits.
The 55 °C spread between cylinder 3 and the others is the key number. A spread of that size is diagnostic of a local fault regardless of ambient temperature; ambient temperature shifts all cylinders together. I would record the ambient temperature, field elevation, power setting, and time at which the readings were taken, along with the spread. This documentation supports the maintenance finding and protects the pilot and operator if the exceedance is later questioned. If the flight manual has a maximum CHT that varies with ambient or power setting, I would compare the reading against the applicable limit rather than a generic figure.