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SESK: Stuff Everyone Should Know · sesk.info/comm-turbo-critical-altitude

What is the critical altitude of a turbocharged engine?

The highest altitude where it can still make rated horsepower.

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  1. The altitude where the turbo first spools up
  2. The altitude where oxygen is needed
  3. The maximum altitude at which it can produce rated horsepower ✔

Why: A turbocharged engine can hold sea-level power up to its critical altitude. Above that, power output drops like a normally aspirated engine. Many turbo engines use an intercooler to cut the risk of detonation.

Huh, didn't know that: A normally aspirated engine at 8,000 feet makes about 75 percent of its sea-level power.

The dad joke
Why did the turbo engine tell its pilot to go no higher? It had reached its critical stage.
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References

Answer: The maximum altitude at which it can produce rated horsepower. A turbocharged engine can hold sea-level power up to its critical altitude. Above that, power output drops like a normally aspirated engine. Many turbo engines use an intercooler to cut the risk of detonation.

  1. PHAK ch 7, Aircraft Systems (FAA-H-8083-25C) www.faa.gov/sites/faa.gov/files/09_phak_ch7.pdf
    a turbocharged engine can produce its rated horsepower.
    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
    many turbocharged engines use an intercooler.
    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
    produce 75 percent of the power it could produce at mean sea level
    Checked 2026-10-11.

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