Turbocharged Engine Overboost and High Cylinder Head Temperature
You are flying a turbocharged single-engine aircraft at 16,000 ft in a climb. The manifold pressure gauge has reached the red line and continues slightly above it despite the throttle being eased back. Cylinder head temperature is rising through 240 °C, oil temperature is 105 °C, and the engine sounds rough. The wastegate is controlled automatically, but you suspect it has stuck closed. The aircraft is 45 nm from a suitable aerodrome with a 2,000 m asphalt runway; the nearest alternate is 90 nm away. Weather is VMC, OAT -12 °C, and you have 2 h 20 min fuel remaining. The aircraft is not pressurised, and you and the passenger are using supplemental oxygen. You must decide whether to continue the climb, level off, or divert immediately.
1. Diagnose the most likely fault and state the immediate engine and flight actions.
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The most likely fault is a wastegate stuck closed or a failed automatic boost control, allowing exhaust gas to bypass the turbine and overboost the engine. The manifold pressure above the red line, rough running and rapidly rising CHT support this, rather than a simple gauge error. Immediately reduce throttle further, reduce propeller RPM if the propeller is controllable, enrich the mixture, open cowl flaps and lower the nose to increase cooling airflow. Do not shock-cool: avoid a rapid power reduction that could crack cylinders, but do reduce power enough to bring manifold pressure within limits. Level off or descend to reduce the turbocharger load. If manifold pressure remains high despite throttle closure, treat it as an engine emergency and shut down or prepare for shutdown. Declare an emergency and divert to the 45 nm aerodrome, not the 90 nm alternate.
2. Explain the engineering consequences of continuing the climb with the wastegate stuck closed.
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Continuing the climb would keep the turbocharger at maximum boost, raising manifold pressure and therefore peak combustion pressure and temperature. The mixture, even if enriched, may not be enough to suppress detonation, which can crack pistons and damage valves. Sustained CHT above the manufacturer's limit (often around 238–240 °C for this class) reduces cylinder strength and can lead to pre-ignition, where the charge ignites before the spark and rapidly destroys the piston. High oil temperature reduces oil viscosity, lowering bearing protection and increasing the risk of seizure. At 16,000 ft, the engine is also operating at high continuous power with reduced cooling because of thinner air. The rough running may indicate detonation already occurring. The correct judgement is to stop the climb, reduce power and land before a catastrophic failure, not to 'nurse' the engine to destination.
3. What operational factors should determine your choice between the 45 nm aerodrome and the 90 nm alternate?
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Choose the 45 nm aerodrome because it minimises exposure to a possible engine failure and gives the shortest time to a runway. With 2 h 20 min fuel, fuel is not the deciding factor; the deciding factor is the probability of power loss and the availability of a landing site. The 45 nm aerodrome has a 2,000 m asphalt runway, which is long enough for a single-engine aircraft and gives margin for a flapless or fast landing. The 90 nm alternate adds about 45 nm, perhaps 25–30 min, during which the engine could fail over less suitable terrain. Weather is VMC, so a visual approach is likely; oxygen is available, so a descent is not urgently required for hypoxia. Declare an emergency, request priority, and plan a normal approach but brief for an engine-failure glide. Do not continue the original climb.