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Aircraft systems and instrumentsQuestion 608 of 934

Why does a pilot lean the mixture at high altitude?

  1. To raise the oil temperature at altitude
  2. To compensate for the lower air density
  3. To raise the manifold pressure at altitude
  4. To lower the cylinder pressure at altitude
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Correct answer: To compensate for the lower air density

Leaning decreases fuel flow, which matches the lower air density at altitude.

Source: Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25C), Chapter 7: Aircraft Systems — Induction Systems — Mixture Control, page 169

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Questions last reviewed: 18 October 2026