APU Fire — Detection, Extinguishing and Ground Automation

APU fire detection shares its principle with the engines (dual loop + FDU + hydrogen/helium element, article 02), but the APU has a capability nothing else on the aircraft has: on the ground it detects, shuts down and discharges on its own, unattended. This is the aircraft's only automatic extinguishing of a running machine. This article develops that ground automation — the controller (AFECU), the on-ground gate (LGCIU), the three-second timer, the belly light for ground crew, and the "test longer than three seconds and the APU really stops" trap.

1. Detection — dual loop, a single 6.8 m element per loop

"It has two fire detection loops and a Fire Detection Unit (FDU) 13WG. Each detection loop has an electro/pneumatic fire detector... They are installed in the APU compartment."

Where each engine loop carries five short detectors, each APU loop carries one long element that snakes past every ignition source in the bay:

"The overall length of each sensing element is 268 in (6800 mm)... The sensing elements are installed adjacent to critical components, such as: - The fuel lines - Oil tubes - The APU generator - The APU Fuel Control Unit (FCU) - The APU ignition exciter - The APU turbine plenum."

[!warning]- Engines post multiple sentries; the APU runs one continuous tripwire Five short detectors guard five engine zones; a single 6.8 m tube wraps the whole tight APU bay past fuel lines, oil, generator and turbine. Same objective — cover every ignition source — but a small crowded bay is better served by one long tube than five points.

The element is the same hydrogen/helium construction as the engine, with the APU helium pressure specified:

"The inner core contains a hydrogen-charged material. The gap between the inner core and the outer tube is filled with helium. The helium is pressurized to approximately 29 psia (2 bar)."

2. Four conditions, two named thresholds — helium for overheat, hydrogen for fire

The APU detector states its logic more explicitly than the engine, in four conditions with two named thresholds:

"The fire detectors have four detection conditions: - The correct condition... - The overheat condition (increase in temperature along the length of the sensing element) - The fire condition (increase in temperature at one point on the sensing element) - the failure condition..."

"(2) Overheat Condition... the helium in the sensing element expands... the 'alarm' switch closes. The 'overheat' threshold is 352 - 396 deg.F (178 - 202 deg.C). (3) Fire Condition... the hydrogen core material releases a large volume of gas... The 'fire' threshold is 845 - 953 deg.F (452 - 512 deg.C)."

[!warning]- One tube, two gases, two thresholds — mapped cleanly Helium (filling the whole gap) responds to the whole element being baked — large area, lower temperature (178–202 °C) = overheat, thermally reversible. Hydrogen (locked in the core, released by the material) responds to one point directly burned — local, high temperature (452–512 °C) = fire. This is the engine's "average versus discrete" (article 02) named in plain words: a hot section is overheat, a hot point is fire. Both alarm outputs are identical — the crew sees APU FIRE either way (there is no separate APU-overheat caution; red in one step). The failure state: element damaged → helium escapes → pressure falls → integrity switch opens → fault.

3. Fire and fault logic — AND→OR, 16 s, 5 s

The FDU 13WG structure and fire criteria match the engine:

"The FDU 13WG generates a fire warning signal if one of the subsequent conditions occurs: - Fire loop A and fire loop B, - Fire loop A and failure loop B, - Fire loop B and failure loop A, - Failure loop A and failure loop B within 5 seconds."

But the single-loop fault timer differs:

"The FDU 13WG also generates a failure signal if: - Only one loop sends a fire warning signal for more than 16 seconds while the other loop is in the correct condition..."

[!warning]- Lone-report timer: engine 20 s, APU 16 s Same meaning (give a lone report a window to confirm or be faulted), different number — a favourite exam trap. The 5-second flame effect (both loops breaking together = fire; one after another = DET FAULT) is the same on engine and APU. Remember: lone report — engine 20 / APU 16; broke together — both 5.

Single- and dual-loop faults mirror the engine: APU FIRE LOOP A(B) FAULT (single) and APU FIRE DET FAULT (both, with MASTER CAUT + single chime).

4. The bottle — one, 2.27 kg, and a belly disc that means overpressure

The APU has one bottle to the engine's two:

"The APU fire-extinguishing system has one fire extinguisher bottle... pressurized with nitrogen... The container has a volume of 224 in.3. It is charged with 5.00 lb (2.27 kg) of extinguisher agent and pressurized to 600 psi. ... An electrically-operated explosive cartridge is installed in the discharge head. The cartridge has two squibs (squib A and squib B)... If one squib has a failure the other squib makes sure that the cartridge fires correctly."

The discharge indicator on the belly reports overpressure only, not normal discharge:

"The red disc is only ejected if the pressure in the bottle is too high. If the cartridge is fired and the agent is released into the APU compartment the red disc stays in the discharge indicator housing."

[!warning]- A blown belly disc means the bottle over-pressured, not that you fired it Two distinct paths: normal discharge — the cartridge ruptures the internal diaphragm, agent goes into the APU bay, the red disc stays, DISCH lights. Overpressure relief — the bottle over-heats and over-pressures, the relief disc bursts, agent dumps overboard, the belly disc ejects and the bottle is dead. On the walk-round the ejected red disc is a "this bottle over-pressured overboard" flag, akin to the pneumatic pressure-relief doors. DISCH-amber follows bottle pressure falling to 200 psi.

5. Airborne manual, ground automatic — AFECU, 3 s, the LGCIU gate

The central fact: the same APU fire is handled differently airborne and on the ground.

"In flight, the crew must operate the system manually from the cockpit. On the ground, the Automatic Fire-Extinguishing Control-Unit (AFECU) 36WF operates the extinguishing system automatically, if there is no manual operation from the cockpit."

Three prerequisites and six automatic actions:

"The Landing Gear Control and Interface Unit (LGCIU) transmits a signal to the AFECU to show that the aircraft is on the ground... If there is no manual operation from the cockpit, three seconds after the fire warning the AFECU automatically: - Energizes the horn supply relay... ground crew horn - Sends a signal to the APU FIRE light 38WF on the external power panel 925VU - Sends a signal to the light in the APU FIRE pushbutton switch 12WG - Starts the cockpit aural warning - Energizes the fire emergency-stop relays (the APU shuts down and the APU LP fuel-shutoff-valve closes) - Fires the extinguisher cartridge (the agent flows into the APU compartment)."

The prerequisites are on-ground (LGCIU gate) + fire warning + no crew action. The three seconds are a human-machine hand-over window: if ground personnel see it and want to act, three seconds lets them take over; if nothing happens, the AFECU judges "unattended" and does everything itself.

[!warning]- Airborne, an APU fire does not auto-extinguish — the crew must act Airborne the APU is non-critical (the engines supply power and air), and an unwanted automatic shutdown-plus-discharge would add confusion; on the ground the APU often runs unattended (turnround, crew away), so extinguishing itself is safest. Hence ground automatic (nobody watching), airborne manual (someone watching, avoid nuisance) — design following the situation.

Airborne the crew presses APU FIRE first, then AGENT. The APU FIRE pushbutton's six linkages (FCOM):

"When the flight crew pushes it... performs the following for the APU: - Shuts down the APU - Silences the aural warning - Arms the squib on the APU fire extinguisher - Closes the low-pressure fuel valve - Shuts off the APU fuel pump - Closes the APU bleed valve and X bleed valve and deactivates the APU generator."

The sixth item closes the cross-bleed valve — cutting the APU cleanly out of the pneumatic network so bay fire/smoke cannot travel to the wing or cabin.

The survival wiring confirms the "hardest buses" claim from the overview: squib A on the HOT BAT BUS 701PP, squib B on BUS 301PP, the AFECU on HOT BUS 702PP — so ground automatic extinguishing works on batteries alone.

6. Operations — the drills, the reset, and the test traps

Airborne APU FIRE (ECAM, LAND ASAP):

  APU FIRE (red) + CRC + MASTER WARN + SD APU page + LAND ASAP
   1  APU FIRE P/B ... PUSH   shut down + close fuel + close bleed/X-bleed + deactivate APU GEN + arm AGENT
   2  MASTER SW ...... OFF
   3  AGENT .......... DISCH  fire the one bottle (DISCH amber confirms)

Ground APU FIRE — essentially "you do not fire the AGENT"; the AFECU does it in three seconds. The crew confirms:

"When there is an APU fire and this button is pressed, it confirms that the APU has shut down automatically and silences the external warning horn."

The reset:

"Pressing this button resets the automatic shutdown function. During the test or if the fire signal is active, the reset is inoperative."

[!warning]- On the ground, do not hold the APU fire test longer than three seconds

"The automatic shutdown of the APU on ground may occur if the flight crew performs this test for more than 3 seconds." This is the same three seconds as ground auto-extinguishing: a test "fire signal" lasting over 3 s is indistinguishable from a real fire, and the AFECU shuts the APU down. Test with a short press. The battery-only "half-light" (article 01) is the other APU-test peculiarity, developed in the test article.

Self-test

[!note]- Q1. How does APU detection differ from engine detection numerically? Why is the element 6.8 m long? One detector per loop (vs five), single-loop timer 16 s (vs 20 s). The 6.8 m element wraps the whole tight APU bay past every ignition source.

[!note]- Q2. What are the APU overheat and fire thresholds, and which gas senses each? Overheat 178–202 °C (helium expands, whole element hot); fire 452–512 °C (hydrogen released, one point hot).

[!note]- Q3. Why is APU extinguishing manual airborne but automatic on the ground? What are the three ground prerequisites? Airborne the APU is non-critical and auto-action risks nuisance; on the ground it often runs unattended so self-extinguishing is safest. Prerequisites: on-ground (LGCIU), fire warning, no crew action.

[!note]- Q4. What does an ejected belly red disc mean? Is that the same as firing the AGENT? It means the bottle over-pressured and dumped overboard (bottle dead) — not a normal discharge. Normal discharge sends agent into the bay and the disc stays.

[!note]- Q5. What six things does the APU FIRE pushbutton do, and why close the cross-bleed valve? Shut down APU; silence; arm squib; close LP fuel valve; shut off fuel pump; close APU bleed + cross-bleed valves + deactivate APU generator. The cross-bleed closure cuts the APU out of the pneumatic network so bay fire/smoke cannot spread.

Key takeaways

References

• FCOM DSC-26-20-10 — ground APU fire auto shutdown + discharge.
• FCOM DSC-26-20-20 — APU FIRE pushbutton six linkages, external power panel (APU FIRE light/horn/SHUT OFF), maintenance panel RESET, > 3 s test, battery half-light.
• AMM 26-13-00 — dual loop + FDU 13WG, 6.8 m element locations, four conditions and thresholds, AND→OR, 16 s, LOOP/DET FAULT.
• AMM 26-22-00 — one bottle (2.27 kg + N₂ 600 psi), twin squib, belly disc overpressure logic, AFECU ground automation (3 s, six actions, LGCIU), HOT bus supplies.

Independent study material, not an Airbus publication and not endorsed by the manufacturer. Always defer to the current operator FCOM, FCTM, and QRH for operational use.