Turbocharged twin: overboost, high TIT, and driftdown
On a Cessna 421C at FL190, the right TSIO-520-NB begins surging: manifold pressure oscillates between 27 and 32 inHg, fuel flow between 24 and 30 gph, and TIT between 1,680 and 1,720 °F. CHT is 410 °F, oil pressure 55 psi, and a light airframe vibration is present. The left engine shows 30 inHg, 26 gph, and 1,600 °F TIT. The TIT limit is 1,650 °F. The destination is 45 minutes ahead; the nearest alternate is 20 minutes behind. The single-engine service ceiling at the current weight is 11,800 ft, and the MEA on the route is 16,000 ft. The airplane has known-icing equipment, and the freezing level is 14,000 ft. You have 2,100 lb of fuel remaining and about 45 minutes of daylight.
1. Interpret the indications. What is the most likely failure, and what immediate power management do you choose?
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High and fluctuating manifold pressure, TIT above the 1,650 °F limit, fluctuating fuel flow, and vibration point to an overboost condition, most likely a wastegate stuck closed or a turbocharger controller/actuator failure. A failed turbo bearing can produce the same vibration and erratic boost. The fuel control may be reacting to the changing airflow, which explains the fuel-flow swings. I would reduce the right throttle until MP and TIT are within limits; if TIT cannot be brought below the redline or the MP remains uncontrollable, I would shut the engine down. Continuing at 1,700 °F risks preignition, detonation, and exhaust-valve damage. I would also enrich as required by the procedure and monitor CHT and oil pressure. The decision point is control: if a power reduction restores stable, in-limit operation, I can consider continuing at reduced power; if not, the engine is secured.
2. Should you continue to destination or divert? Use the single-engine ceiling and terrain in your reasoning.
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The single-engine service ceiling of 11,800 ft is below the 16,000-ft MEA, so after a shutdown I cannot maintain the route MEA. I would advise ATC immediately, request a lower altitude, and plan a driftdown. Terrain and the 14,000-ft freezing level make a continued 45-minute flight over high ground a poor choice, especially with known icing. The alternate 20 minutes behind is more attractive if it has a suitable approach and lower terrain. I would declare an emergency if unable to maintain terrain clearance. Fuel is not the limiting factor-2,100 lb gives adequate range-but performance is. The practical decision is to divert to the nearest suitable airport with a lower MEA, not to press toward destination while drifting down through mountainous terrain.
3. What crew coordination and checklist discipline should follow the shutdown?
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I would divide the workload explicitly: one pilot flies and calls headings/altitudes, the other runs the engine-secure and single-engine checklists and talks to ATC. The flying pilot should not be digging through a checklist while drifting down in terrain. I would confirm the failed engine is identified, fuel and ignition are off, the propeller is feathered if applicable, and the fire warning is clear. Then I would brief the approach, the missed approach, and the cabin. If the vibration or high TIT was severe, I would consider an emergency descent only if smoke or fire appears. The coordination point is that the abnormal engine is secured first, then the airplane is re-planned; trying to do both at once is how altitude and terrain clearance get lost.