Aircraft systems and instrumentsQuestion 659 of 934

What is the principal fuel contaminant?

  1. Rust and dirt
  2. Lead deposits
  3. Oil residue
  4. Water
Show answer

Correct answer: Water

Suspended water droplets can show as a cloudy fuel, or as water that has settled at the bottom of the tank.

Source: Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25C), Chapter 7: Aircraft Systems — Fuel Systems — Fuel Contamination, page 188

Practice aircraft systems and instruments

Questions last reviewed: 18 October 2026