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Aircraft systems and instrumentsQuestion 652 of 934

What can happen if a pilot runs a fuel tank completely dry?

  1. The fuel in the other tank becomes contaminated
  2. Air can enter the fuel lines and cause vapor lock
  3. The mixture becomes too rich after the tank change
  4. The engine-driven fuel pump gets more pressure
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Correct answer: Air can enter the fuel lines and cause vapor lock

Vapor lock makes it difficult to restart the engine. Running a tank dry also stops the engine.

Source: Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25C), Chapter 7: Aircraft Systems — Fuel Systems — Fuel Selectors, page 186

Practice aircraft systems and instruments

Questions last reviewed: 18 October 2026