Fire Test and Normal Operations

Fire protection is silent for 99.99 % of the time — in normal flight you cannot tell it is there (overview). That is exactly why confirming it still works is a daily task. This article does not repeat the system mechanics (articles 01–07); it settles the reading logic of the three TEST pushbuttons — which lights should appear, what they prove, what their absence means — and closes out the two reverse-intuitions hidden in testing ("test the ground APU past three seconds and it really stops", "the battery-only test half-lights").

1. The three TEST pushbuttons

A test is not "light a lamp to look" — it drives the processing unit (FDU / SDCU / CIDS-SDF) through an end-to-end self-check, from detector to circuit continuity to warning output to firing circuit, and then "plays" the result back on the panel lights. A test uses a staged fake fire to verify that the real fire path is intact.

2. ENG FIRE TEST — a white SQUIB means both firing filaments have power

Pressing ENG FIRE TEST plays a full fire warning:

"When pressed: - A CRC sounds - The MASTER WARNING lights flash - The ENG FIRE warning appears on ECAM - On the ENG FIRE panel: The ENG FIRE pb-sws light up red / The SQUIB lights come on white if discharge supplies are available / The DISCH lights come on amber - On the ENG MASTER panel: The FIRE lights come on red."

The key phrase is "SQUIB lights come on white if discharge supplies are available". The AMM gives the self-check behind it:

"The FDU ensures (per channel) the electrical continuity of: - the firing circuitry of the fire extinguisher bottle, - the low pressure warning circuitry... During the test, the FDU checks that there is: - a ground signal through filament 1... - a ground signal through filament 2... NOTE: Although only one filament is necessary to enable firing of the cartridge, the test result is positive if the two filaments are power supplied. The test current is limited to 80 mA... although the firing current is approximately 5A."

[!warning]- A white SQUIB in test does not mean "about to discharge" — it means "both filaments have power" It verifies the twin-filament redundancy of article 03 — that both legs are good. The test demands both powered (though firing needs only one) precisely to check the redundancy while nothing is wrong. So:

• SQUIB should light but does not = a break in the firing circuit (a lost filament/supply) → the agent might not fire → defer / report.
• The test current is only 80 mA (against 5 A to fire) — it exercises the circuit without igniting the cartridge, so the check discharges nothing.

The amber DISCH in test means the FDU simulated a low-pressure bottle, verifying the warning path. Response is < 1 s. A good ENG FIRE test: red, SQUIB white, DISCH amber, ENG MASTER red, CRC — none missing.

3. APU FIRE TEST — closing out two reverse-intuitions

The APU annunciation mirrors the engine (CRC, MASTER WARN, red pb, SQUIB white, DISCH amber), but it carries two peculiarities flagged in article 04.

[!warning]- On the ground, do not hold the APU test past three seconds — it will actually stop the APU

"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 the ground auto-extinguishing AFECU: a test "fire signal" over 3 s is indistinguishable from a real fire and the AFECU shuts the APU down. Test with a quick press, or perform it at the SOP-defined APU FIRE TEST / APU START moment.

[!warning]- Battery-only half-light is normal, not a fault

"If the fire test is performed on ground with only batteries... The SQUIB light and DISCH light come on, without ECAM warnings. The red APU FIRE pb-sw light partially comes on, as one of the sets of bulbs is not electrically available." The half-light confirms the red light is dual-fed across buses (one set on the battery still lights, the other on AC which is unpowered here). Expect it — do not read it as a failed bulb.

4. CARGO SMOKE TEST — a sequential self-check, tens of seconds, "at least once per compartment"

The cargo test is different — it tests several holds in sequence and takes time:

"When pressed for at least 3 s , and until it is released: - The smoke detectors in the FWD, AFT, and BULK CARGO compartment and the avionics bay are tested in sequence - The isolation valves of the cargo ventilation system close - BTL1(2) lights come on white - SQUIB lights come on white, provided that one of the two squib filaments is serviceable - SMOKE lights (AFT, FWD CARGO, and AVNCS) come on red..."

The pass criterion:

"Note: Each SDCU channel or SDF controller generates its own warning, which lasts approximately 25 s , with a delay of approximately 30 s between both. The test is successful if an ECAM smoke warning is triggered at least once for each compartment (avionics bay, forward, aft, and bulk cargo compartments)."

[!warning]- The cargo test takes tens of seconds and cycles hold-by-hold Each channel warns for ~25 s with ~30 s between, so do not judge it failed because everything did not light at once — wait for each hold to be called. The criterion is not "all lights together" but "each compartment named at least once". Note the SQUIB threshold here is looser than the engine: the engine test needs both filaments powered (section 2), the cargo test lights SQUIB if one is serviceable — do not confuse them. The test really closes the isolation valves (coupling into ATA-21), so confirm they reset afterwards.

5. When to test — a fixed item in the cockpit preparation

Fire testing is a standard cockpit-preparation item, not something done ad hoc:

"ENG FIRE ... CHECK / TEST"

APU fire testing is often tied to APU start, sequenced by SOP to avoid a ground nuisance shutdown:

"APU FIRE TEST pb ... PRESS and MAINTAIN CM2 ... APU FIRE TEST ... PERFORM"

[!warning]- Fire testing is done on the ground, pre-flight — not in the air Because a test raises real warnings (CRC / MASTER WARN), lights SQUIB/DISCH, actually closes the cargo isolation valves, and — on the ground — can shut the APU down past 3 s. Test once well pre-flight and trust it in flight: the operating philosophy of a dark-cockpit system is leave it alone in normal operation, but confirm it is alive before every flight.

6. Reading the result and the normal panel state

A good test — the "all-green" standard:

The normal fire panel is dark — ENG/APU FIRE pb no light, AGENT no light, DISCH out, SMOKE out. Any lit lamp is a signal: an amber DISCH on (not a test) is a bottle that has lost pressure (03/04); a LOOP/DET FAULT is a demoted detection loop (02/04); a red SMOKE (not a test) is a real smoke warning. What the test surfaces maps to the MEL article — testing is the health check behind a dispatch decision.

Self-test

[!note]- Q1. Why is fire testing done only on the ground pre-flight? Give three reasons. A test raises real warnings (CRC/MASTER WARN); it lights SQUIB/DISCH and closes cargo isolation valves; and a ground APU test past 3 s can shut the APU down.

[!note]- Q2. What does a white SQUIB in the ENG FIRE test prove? Why does firing need one filament but the test wants both? Why 80 mA? Both firing filaments have power (redundancy verified). The test checks both while nothing is wrong. 80 mA exercises the circuit without igniting the 5 A cartridge.

[!note]- Q3. Why must a ground APU test not exceed 3 s? Is battery half-light a fault? The AFECU cannot tell a > 3 s test signal from a real fire and shuts the APU down. Half-light is normal — it confirms the dual-fed red light.

[!note]- Q4. Why does the cargo test take tens of seconds? What is the pass criterion? How does its SQUIB threshold differ from the engine's? It cycles holds ~25 s each, ~30 s apart. Pass = each compartment warns at least once. Cargo lights SQUIB if one filament is serviceable; the engine test needs both.

[!note]- Q5. What should the fire panel look like normally? What does a steady amber DISCH mean? Dark. A steady amber DISCH means a bottle has lost pressure.

Key takeaways

References

• FCOM DSC-26-20-20 — ENG/APU FIRE TEST annunciation, SQUIB "if supplies available", APU > 3 s, battery half-light.
• FCOM DSC-26-50-20 — CARGO SMOKE TEST sequence, 25 s/30 s, "at least once per compartment", SQUIB on one filament.
• AMM 26-21-00 — ENG test logic: FDU checks twin-filament continuity, SQUIB = both powered, 80 mA vs 5 A, < 1 s.
• QRH Cockpit Preparation — ENG FIRE CHECK/TEST as a preparation item.
• FCOM PRO-NOR-SOP — APU FIRE TEST / APU START timing.

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.