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What fires a discharge cartridge (squib)

Aviation Mechanic Powerplant (AMP) practice question 2 of 7 on Engine Fire Protection Systems. ACS element AM.III.E.R2 · difficulty: recall. Want to answer it in a timed quiz? Use the Aviation Mechanic Powerplant (AMP) practice mode.

Question bank updated 2026-09-22

QUESTIONWhat causes the discharge cartridge (squib) in a fire extinguisher container to fire?

  1. AElectric current from the fire extinguishing circuit
  2. BA rise in bottle pressure above the relief setting
  3. CHeat from the fire melting a fusible plug in the valve
Show the answer and explanation

Correct answer: A. Electric current from the fire extinguishing circuit

  • A — correct The squib is an electrically operated explosive device, and an electric current from the fire extinguishing circuit fires it.
  • B — incorrect Excess bottle pressure is handled by the safety relief and fill port or relief diaphragm, which does not fire the squib.
  • C — incorrect The container has no fusible plug that sets off the cartridge; thermal discharge occurs through the relief fitting, not the squib.

BACKGROUND

Discharge valves on extinguisher containers carry a cartridge, or squib, together with a frangible disc. The squib is installed in the discharge assembly at the bottom of the container, and a container may have two squibs so that agent can be routed to either engine. The squib is an electrically operated explosive device: current from the fire extinguishing circuit fires it, driving a slug through the breakable disc so nitrogen pressure pushes the agent out the discharge port. On the flight deck, that current is commanded when the fire switch is pulled and turned to a discharge position and held against the stop for one second. Because the cartridge is set off electrically, any maintenance on the associated wiring must treat the circuit as capable of firing the cartridge.

STUDY TIP

Remember that the squib is an electrical initiator, which is why work on its circuits is treated as hazardous. Keep the electrical firing path separate in your mind from the purely thermal relief path that dumps agent overboard when a container overheats.

KEY POINTS

  • The squib is an electrically operated explosive device in the container's discharge assembly.
  • Current from the fire extinguishing circuit fires it, driving a slug through a breakable disc.
  • Nitrogen pressure then pushes the agent out through the discharge port.

SOURCES

  • sources/amp-powerplant/ch09.txt "Squib"
  • sources/amp-powerplant/ch09.txt "Discharge Valves"
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MORE ON ENGINE FIRE PROTECTION SYSTEMS

SkyDrill is not affiliated with the FAA. This is an original practice item based on FAA-H-8083-32B Aviation Maintenance Technician Handbook – Powerplant, and 14 CFR part 43. Regulations and handbooks are revised; confirm details against the current edition.