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SESK: Stuff Everyone Should Know · sesk.info/comm-overboost-waste-gate

A manual-waste-gate turbo airplane descends with the waste gate left closed. What can happen?

An overboost that can cause severe detonation.

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  1. Prop overspeed only
  2. Overboost and severe detonation ✔
  3. Immediate fuel starvation

Why: Descending air is denser, so a gate left closed can push manifold pressure past the engine's limit. The denser air also leans the mixture, which can cause severe detonation.

Huh, didn't know that: An automatic waste gate is less likely to overboost, but applying takeoff power with cold oil can still cause one.

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References

Answer: Overboost and severe detonation. Descending air is denser, so a gate left closed can push manifold pressure past the engine's limit. The denser air also leans the mixture, which can cause severe detonation.

  1. PHAK ch 7, Aircraft Systems (FAA-H-8083-25C) www.faa.gov/sites/faa.gov/files/09_phak_ch7.pdf
    may produce severe detonation because of the leaning effect
    Checked 2026-10-11.
  2. PHAK ch 7, Aircraft Systems (FAA-H-8083-25C) www.faa.gov/sites/faa.gov/files/09_phak_ch7.pdf
    waste gate, essentially an adjustable butterfly valve installed
    Checked 2026-10-11.
  3. PHAK ch 7, Aircraft Systems (FAA-H-8083-25C) www.faa.gov/sites/faa.gov/files/09_phak_ch7.pdf
    experience an overboost condition, it can still occur.
    Checked 2026-10-11.

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