One Cylinder Hot and Lean in Cruise
A turbocharged, fuel-injected six-cylinder engine in a single-engine aircraft is cruising at 11,500 ft: 29 in Hg, 2,300 rpm, about 65% power, mixture set 30 °F lean of peak on the #3 cylinder. Ten minutes after leaning, the #3 CHT climbs from 195 °C to 232 °C and is still rising about 1 °C per minute; the other five sit between 185 °C and 192 °C. The #3 EGT is about 25 °C above its previously stable lean-of-peak value and drifting up; the other EGTs are stable. Fuel flow is normal for the power setting, manifold pressure is steady, oil temperature is 92 °C and oil pressure 65 psi. The engine is slightly rough. The CHT redline is 238 °C. There is an A&P at an airport 20 minutes ahead, and a larger field with a full workshop 35 minutes behind. Cloud tops are at 14,000 ft and there is no icing.
1. Interpret the CHT and EGT pattern. What do the two together rule in and rule out?
Read the worked answer
EGT and CHT moving the same way on one cylinder is the key. A single-cylinder EGT rise with a simultaneous CHT rise points to that cylinder running leaner than the rest: less fuel for the air it is getting. Fuel flow to the engine is normal and the other five cylinders are stable, so the cause is local. Rule in: an induction leak at that cylinder's intake pipe or flange gasket (unmetered air), or a restricted fuel nozzle or injector line (less fuel). Rule out: an ignition fault, which raises EGT (unburnt fuel burning in the exhaust) but normally lowers CHT and makes the engine rougher than this; a cooling baffle problem, which raises CHT but leaves EGT alone; and a probe fault, which moves the indication only, not CHT and roughness together. Two independent indications agreeing is what makes it real.
2. State your in-flight actions in order, with the numbers that trigger the decision.
Read the worked answer
Act on the numbers, not on the schedule. Go full rich, open the cowl flaps and reduce power to about 55–60% (roughly 26 in Hg, 2,200 rpm) to cut heat input; the mixture is the fastest lever. If the #3 CHT falls below about 200 °C within two minutes and stays there, continue at reduced power to the field 20 minutes ahead. If it keeps climbing past the 238 °C redline, or roughness increases, or the EGT is still drifting up, stop diagnosing and land - an induction-leak lean condition can progress to detonation and pre-ignition and destroy a piston in minutes. In that case tell ATC it is a priority and take the 20-minute field, not the 35-minute one. Do not re-lean, and do not 'try it again' in the air.
3. What must the maintenance inspection cover, and what would stop you returning the aircraft to service?
Read the worked answer
Leak-test the induction system at that cylinder (intake pipe gaskets, flange torque, hose clamps), flow-check the nozzle and injector line, and read the plug colour - a lean cylinder reads white. Borescope the #3 cylinder for detonation damage (pitted piston crown, burnt exhaust valve) and do a compression check; if compression is down or the borescope shows damage, the cylinder comes off. Replace the EGT probe only if everything else checks out, then re-verify. Return to service only when the cause is found; then a run-up and a test flight, comparing EGT/CHT spread at full power against book values. 'CHT within limits on the ground' is not evidence, because the fault only shows at cruise power.