Detonation During Climb in a Turbocharged Piston Engine
A twin-engine aircraft is powered by turbocharged, fuel-injected six-cylinder piston engines. During a climb at 16,000 ft, the pilot sets 34 inHg manifold pressure and 2,500 rpm. Fuel flow is 19 gph, but the POH climb chart requires 23 gph at that setting. Within 2 minutes, the No. 3 cylinder head temperature reaches 460°F; the limit is 435°F. The No. 3 EGT falls about 80°F while the other EGTs remain steady. OAT is -5°C. The pilot reduces manifold pressure and enriches the mixture; CHT falls to 420°F. Maintenance later finds the No. 3 fuel injector partially blocked and the wastegate controller rigged 0.050 in out of adjustment. The engine has 620 hours since overhaul, and ignition timing was checked 40 hours ago.
1. Explain how the injector and wastegate findings together produced the CHT rise and the EGT drop.
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The two findings act together.
- The partially blocked No. 3 injector reduces fuel delivery to that cylinder. At 34 inHg and 2,500 rpm the cylinder is already at high load, so the lean mixture raises peak combustion temperature and pressure and moves that cylinder toward detonation.
- The wastegate rigging error allows manifold pressure to be higher than the POH setting for the same throttle position. Higher MAP means a larger trapped air mass and higher compression pressure, which further reduces the detonation margin.
- The No. 3 EGT falls because the cylinder has gone lean of peak: beyond peak EGT, further leaning reduces exhaust gas temperature even though cylinder head temperature and peak pressure can continue to rise. The steady EGTs on the other cylinders confirm this is a single-cylinder mixture problem, not a general fuel-flow or timing problem.
So the EGT drop is a lean-mixture clue, not evidence that the cylinder is safe. The CHT rise to 460°F is the more serious detonation/pre-ignition warning.
2. Why is EGT alone an unreliable indicator of detonation, and what parameters should be used to confirm it?
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EGT measures the average temperature of the exhaust gas leaving the cylinder; detonation is a high-frequency pressure oscillation. Depending on mixture and timing, detonation can raise EGT, lower it, or leave it almost unchanged, so EGT cannot be used as a detonation gauge.
Better evidence:
- CHT rise toward or beyond the limit, especially a single-cylinder divergence.
- High manifold pressure combined with a lean mixture or low fuel flow.
- Audible knock or roughness.
- Borescope evidence of piston crown erosion, broken ring lands, or valve damage.
- In instrumented test engines, cylinder-pressure traces.
In this event the CHT of 460°F against a 435°F limit, the 34 inHg MAP, and the 19 gph against a required 23 gph are stronger evidence than the EGT drop.
3. List the corrective actions and inspections required before return to service.
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Before return to service:
- Replace or clean the No. 3 injector and flow-test all injectors to equal delivery.
- Rig the wastegate controller to the AMM setting and verify manifold pressure against the POH throughout the range.
- Check fuel pump output and the fuel-pressure gauge for accuracy.
- Verify ignition timing.
- Borescope all cylinders for piston, valve and ring-land damage; pay particular attention to No. 3.
- Perform a compression test and check the oil filter for metal.
- Calibrate or check the No. 3 CHT probe, because an inaccurate probe could hide a repeat event.
- Ground-run, then flight-test in the climb configuration while monitoring CHT, EGT and fuel flow.
If borescope or compression findings show detonation damage, the cylinder must be repaired before flight.