High CHT and Magneto Differential on a Constant-Speed Single
A four-cylinder horizontally opposed engine (O-360 type, 2,000 h since overhaul, 400 h since top overhaul) in a constant-speed single is cruising at 2,400 rpm and 24 inHg. #3 CHT is 425°F, oil temperature 205°F, oil pressure 55 psi, and oil consumption is 1 qt per 5 h. During the run-up, the left magneto drop was 150 rpm and the right drop 70 rpm; the engine is smooth on both. In cruise, leaning causes roughness that was not present 20 h ago. The propeller cycles 200 rpm during the run-up. The pilot suspects a fouled spark plug and proposes to lean aggressively in climb and continue the 2-hour flight. OAT at altitude is 5°C; the aircraft is normally aspirated. The engine has a carburettor.
1. Interpret the magneto drop, the CHT and the new lean-roughness together.
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Use the magneto limits, not just the absolute drop. For this engine the usual limits are 175 rpm maximum drop and 50 rpm maximum differential. The left drop of 150 rpm is within the absolute limit but the differential is 80 rpm, so the left magneto circuit is suspect-spark plug, ignition lead, distributor or magneto timing. The #3 CHT of 425°F is high and the new lean-roughness points to a mixture distribution or induction problem, a partially blocked nozzle, an induction leak, a baffle defect or a failing plug/valve. Oil consumption of 1 qt/5 h is not alarming, and a 200 rpm propeller cycle is normal. The combination should stop the flight: one hot cylinder plus an ignition fault can progress to detonation or a stuck valve.
2. Should the pilot continue the 2-hour flight or lean aggressively to smooth the engine? Explain.
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Do not continue the 2-hour flight and do not “lean aggressively” to mask roughness. Aggressive leaning raises CHT further and can cause detonation, especially with #3 already at 425°F. The aircraft should return to base or land at the nearest suitable aerodrome using a conservative power setting, rich enough to keep temperatures stable, and with the ignition discrepancy treated as a known fault. If the roughness worsens, land. A fouled plug is possible but is not the only explanation; the differential magneto drop and the lean roughness must be investigated before further flight. The pilot’s suspicion is a hypothesis, not a diagnosis.
3. What ground checks should be done before the aircraft is released?
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On the ground, the engineer should re-check the magneto drop hot, inspect the left harness and plugs, and replace/rotate plugs as needed; borescope #3 to look for a stuck valve, scored cylinder or detonation damage; check baffles and seals; and check the induction system and carburettor for leaks or blockage. A cylinder compression check and oil filter inspection are warranted. The aircraft should not be released until the magneto differential is within limits and the #3 CHT cause is identified. If a plug is found fouled, the cause of fouling-rich mixture, worn rings, or lead fouling-still needs addressing.