Wastegate Failure and Overboost in the Climb
You are climbing a turbocharged twin (TSIO-520, 285 hp per side) through FL170 towards FL210 at 32 in Hg and 2,400 rpm, which is 75% power; CHT is 420 °F, EGT 1,540 °F, oil temperature 205 °F, OAT -14 °C. Passing FL180, without any throttle movement, the right manifold pressure rises to 38 in Hg and keeps creeping up; the right CHT climbs to 465 °F in two minutes while the EGT begins to oscillate ±60 °F. Retarding the right throttle has little effect on manifold pressure until the throttle is nearly closed, and right fuel flow fluctuates. The left engine is unaffected. MEA on the route is FL190; the destination is 55 minutes away and an airport with a 2,000 m runway is 20 minutes behind you.
1. Diagnose the failure and explain what the manifold pressure, CHT and EGT readings are telling you.
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In a turbocharged engine the wastegate bypasses exhaust around the turbine to hold manifold pressure at the pilot's selected value; above critical altitude the controller modulates the gate to keep MP constant. Here the gate is stuck closed, or its controller has failed, so the turbo keeps charging as the aircraft climbs and MP rises instead of being capped. The throttle being almost ineffective is diagnostic: the throttle is downstream of the compressor and the wastegate is not modulating, so only a nearly closed throttle can choke the flow. A rising CHT with an oscillating EGT is the signature of detonation and possible pre-ignition: the charge is over-boosted, too hot and the mixture is unstable. If it continues, expect burnt valves, a holed piston or turbo overspeed and turbine failure.
2. State your immediate actions to protect the engine, and the decision that follows.
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Protect the engine first: throttle back, reduce rpm with the propeller control, enrich the mixture towards full rich to use fuel evaporation for cooling, open cowl flaps if fitted, and lower the nose to increase cooling airflow and reduce the climb. Do not chase the manifold pressure with the throttle indefinitely; if MP cannot be held at or below the top of the green (about 32 in Hg) and CHT keeps rising past 435 °F, shut the engine down. Once it is stopped, feather the propeller and plan the drift-down. At FL190 on one engine the aeroplane will not hold level, so I would brief the cabin, declare an urgency call, and turn towards the airport 20 minutes behind while descending to a level where single-engine level flight is possible.
3. If fully closing the throttle brings manifold pressure back under control, can you continue the flight?
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If closing the throttle brings MP to a controllable value and CHT stabilises below 435 °F, the engine is still making some power, but that does not make the flight safe. Single-engine performance at FL190 is likely below the single-engine service ceiling, so the aeroplane will descend anyway, and the route MEA is FL190. A wastegate stuck closed can over-boost again the moment the throttle is opened, and an engine that has been detonating may fail completely later. So I would not continue at FL190. I would descend to a level where the aeroplane holds altitude, turn back to the airport 20 minutes behind, keep that engine at low power and treat it as a precautionary diversion. If roughness or high CHT persists, shut it down and land single-engine.