Turbocharged twin: creeping overboost at FL160 over high terrain

Most likely cause: the left turbocharger wastegate is not opening - a creeping overboost.

Key reasoning chain:

• MAP has risen without throttle movement. On a turbocharged engine the throttle plate sits downstream of the compressor, so MAP higher than the commanded value can only come from excess deck pressure - the wastegate is failing to bypass exhaust gas around the turbine.
• Fuel flow tracks MAP (24 vs 17.5 gph). The fuel servo meters according to air mass/boost, so this confirms that real extra air is entering the engine, not a false gauge reading. Three independent indications (MAP, fuel flow, TIT/CHT) agree, which eliminates an indicator error.
• TIT and CHT rise together with the overboost. The fuel servo does not lean sufficiently at this boost, so the extra air raises combustion temperature.

Alternatives excluded:

• Induction leak - gives high TIT at low power and a low or unstable MAP, not an overboost.
• Turbo bearing failure or exhaust leak upstream of the turbine - gives low MAP, not high.
• Normal operation - retarding the throttle masks the fault, but boost returns when power is reset, so the wastegate is sticking and the engine is being repeatedly overboosted. The turbo is at risk of overspeed and the cylinder head of detonation damage.

Immediate actions, in order:

1. Reduce power with the left throttle until MAP is at or below the POH cruise-climb limit and TIT/CHT are inside limits. Because the throttle is downstream of the compressor, it can still regulate MAP, but it cannot repair a wastegate stuck closed.
2. Enrich the mixture and open the cowl flaps fully to assist cooling; select a lower power setting rather than chasing a target MAP.
3. Check for secondary damage: oil pressure/temperature, any vibration, unusual noise, fuel flow consistency.
4. Tell ATC you have an engine problem and may need to divert, and start drift-down planning.

Shutdown criteria - any one of these is decisive:

• MAP cannot be held below the limit without losing usable power, or TIT/CHT stay above the limit at the lowest safe power.
• Abnormal oil pressure/temperature, metal in the oil, vibration, smoke or a fire warning.
• Uncommanded power loss or a runaway that the throttle cannot restrain.

If the engine can be held at, say, 26 inHg with TIT 1,570 °F and CHT below the limit, it is producing usable thrust. That is not a reason to press on to Leh, but it is a very different situation from shutting down over 12,500 ft terrain. The decision of whether to shut it down is driven by the third task, not by the fault name alone.

The numbers decide it.

• MSA on route is 14,000 ft and terrain rises to 12,500 ft.
• Single-engine service ceiling at present weight is 11,200 ft - below both the terrain and the MSA.

Consequences:

• Shut down the engine: you must drift down to about 11,200 ft (or lower in the climb-limited case). You cannot maintain MSA over 12,500 ft terrain. You would need an immediate track change to lower ground, a diversion south rather than to the planned 95 NM airport, and a step-down profile with a large safety margin. Drift-down rate and available oxygen (pressurisation/altitude) also matter.
• Keep it running at 26 inHg: you can hold FL160, clear all terrain, and keep full navigation and systems capability. The risk is that TIT/CHT or MAP deteriorates and you are forced into a shutdown later, possibly over worse terrain or while already committed.

Decision: keep the engine running at the minimum power that holds the temperatures inside limits, declare a PAN, divert to the nearest airport with a suitable runway reachable without crossing the high ground, and plan the descent so that if the engine must be shut down you are already over terrain below 11,200 ft. Set a firm trigger - for example, a TIT at the limit for more than two minutes, or any oil-pressure abnormality - that converts the plan into an immediate shutdown and diversion. Not continuing to Leh is part of the decision.