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Water extinguishers and Class D fires

Aviation Mechanic General (AMG) practice question 6 of 26 on Ground Operations and Servicing. ACS element AM.I.F.K6 · difficulty: standard. Want to answer it in a timed quiz? Use the Aviation Mechanic General (AMG) practice mode.

Question bank updated 2026-09-22

QUESTIONWhy must a water-type fire extinguisher never be used on a Class D fire?

  1. AWater reacts with the burning metal and releases hydrogen gas.
  2. BBurning metal floats on the water and spreads the fire.
  3. CWater cooling causes explosive expansion of the metal.
Show the answer and explanation

Correct answer: C. Water cooling causes explosive expansion of the metal.

  • A — incorrect The handbook attributes the hazard to the explosive expansion of the hot metal when cooled, not to a hydrogen-releasing reaction.
  • B — incorrect Floating on water is the reason water is not recommended for Class B petroleum fires, not the reason it is prohibited on Class D metal fires.
  • C — correct Burning metals are at extremely high temperatures, and the cooling effect of water causes an explosive expansion of the metal.

BACKGROUND

The National Fire Protection Association classifies fires as Class A (ordinary combustibles), Class B (flammable liquids), and Class C (energized electrical equipment); Class D fires involve flammable metals such as magnesium in aircraft wheels and brakes. Water extinguishers are best for Class A fires because water removes oxygen and cools the material. They are not recommended for Class B fires because petroleum products float on water, and they demand extreme caution on Class C fires because of shock hazard. Water is never used on Class D fires: metals burn at extremely high temperatures, and the sudden cooling causes an explosive expansion of the metal. Carbon dioxide is prohibited on Class D fires for the same reason, and dry powder is the preferred agent.

STUDY TIP

Link each extinguisher type to the fire class it should never be used on; water and CO2 on metal fires is the classic trap.

KEY POINTS

  • Class D fires involve flammable metals such as magnesium.
  • Water or CO2 on a Class D fire causes explosive expansion of the hot metal.
  • Dry powder extinguishers are best for Class D fires.

SOURCES

  • FAA-H-8083-30B ch01 "Types and Operation of Shop and Flight Line Fire Extinguishers"
Practice the Aviation Mechanic General (AMG) test →Timed 60-question exam, by ACS area, or review your misses. Free.
MORE ON GROUND OPERATIONS AND SERVICING

SkyDrill is not affiliated with the FAA. This is an original practice item based on FAA-H-8083-30B Aviation Maintenance Technician Handbook – General, 14 CFR part 43 and part 65. Regulations and handbooks are revised; confirm details against the current edition.