Emergency Shutdown — Immediate, No Cooling, and Why It Is Crew-Commanded in Flight

The emergency shutdown is the one path that stops the APU instantly, with no cooling period. Its defining operational facts are where it auto-activates (ground only) and who commands it in flight (the crew) — both expressions of the APU's backup-source role.

The APU emergency shutdown-system stops the APU if an emergency occurs. The system is automatically activated on the ground if the APU fire and overheat detection system finds an overtemperature in the APU compartment... If the ECB receives the emergency shutdown signal during the operation of the APU, it stops the APU immediately without a cooldown cycle. — AMM 49-62-00

1. Immediate — no cooldown

Per AMM 49-62-00, an emergency shutdown stops the APU immediately, without a cooldown cycle — whether it arrives during normal operation or during a shutdown's cooling cycle. The sequence is the same as an automatic shutdown but with the cooldown removed.

[!warning]- This is the only "no cooling" stop (integrative synthesis) A normal shutdown gives the APU an 85 s cooling period at 82 % if APU BLEED was on (11); a protective shutdown even leaves the air intake flap open 15 min. The emergency shutdown does none of that — it cuts fuel and stops the APU at once (AMM 49-62-00). It is reserved for fire/overheat precisely because the cooling that protects the turbine is sacrificed to remove the hazard faster.

2. Auto-activation — on the ground only

Per AMM 49-62-00, an emergency shutdown occurs automatically on the ground if the APU fire and overheat detection finds an overtemperature in the compartment (ATA-26). The ECB receives the signal from the fire emergency-stop relays 1 and 2 and runs the shutdown.

[!warning]- Automatic only on the ground — in flight it is the crew's decision (integrative synthesis) The automatic fire shutdown is ground-only (AMM 49-62-00) — matching the automatic-shutdown table where APU fire → auto shutdown "on ground only." In flight the APU may be the critical backup source (00), so the system will not kill it automatically even for fire; the crew commands the shutdown via the APU FIRE pb after weighing the situation. This is the backup-source logic at its sharpest.

3. Manual emergency shutdown — three switches

Per AMM 49-62-00, the APU can be emergency-stopped manually from:

• the APU FIRE PUSH pb (overhead panel, cockpit) — available in flight and on the ground;
• the APU SHUT OFF pb (external power panel, nose landing gear) — ground only;
• the APU EMER SHUT DOWN pb (refuel/defuel panel, belly fairing) — ground only.

All three send the ECB an emergency-shutdown signal; the ECB then runs the immediate sequence. Per FCOM DSC-49-30, the emergency shutdown is annunciated by the APU FAULT ECAM alert.

[!note]- Two of the three switches are ground servicing-safety controls The NLG and belly-fairing switches (AMM 49-62-00) exist so ground crew at the external-power or refuel panels can stop the APU instantly without entering the cockpit — they are deliberately disabled in flight.

4. Fire extinguishing — automatic on ground, manual in flight

On the ground the APU fire extinguishing system operates automatically. During the flight you must push the APU AGENT pushbutton switch 10WF on the APU panel to operate the fire extinguishing system.

Per AMM 49-62-00, on an APU fire the extinguisher discharges automatically on the ground, but in flight the crew must push the APU AGENT pb to discharge it (ATA-26).

[!warning]- Same ground/flight split as the shutdown Extinguishing follows the same rule as the shutdown: automatic on the ground, crew-commanded in flight (AMM 49-62-00). Both reflect that an unattended ground APU should self-protect, while an in-flight APU stays under crew control as a potential lifeline.

5. Counterintuitive points

[!warning]- Emergency shutdown sacrifices the cooling cycle on purpose No 85 s cool-down, no 15-min flap-open — fuel is cut and the APU stops at once (AMM 49-62-00), trading turbine cooling for hazard removal.

[!warning]- In flight, nothing shuts the APU for fire automatically Auto fire shutdown and auto extinguishing are ground-only; in flight the crew uses APU FIRE pb + APU AGENT (AMM 49-62-00) — preserving the backup source.

Self-test

[!note]- Q1. How does an emergency shutdown differ from a normal/protective shutdown? It is immediate with no cooldown (even interrupts a cooling cycle) — vs an 85 s cool-down or a 15-min flap-open.

[!note]- Q2. When does an emergency shutdown happen automatically? On the ground only, on APU fire/overheat detection (via fire emergency-stop relays 1 & 2).

[!note]- Q3. The three manual emergency-shutdown switches? APU FIRE pb (cockpit, any time); APU SHUT OFF (NLG external power, ground only); APU EMER SHUT DOWN (refuel/defuel, ground only).

[!note]- Q4. How is APU fire extinguishing triggered on the ground vs in flight? Automatic on the ground; in flight the crew must push APU AGENT.

Key takeaways

References

• AMM 49-62-00 (Emergency Shutdown — Description and Operation) — stops the APU immediately without a cooldown cycle (even during a cooling cycle); auto-activated on the ground only on APU fire/overheat (ECB via fire emergency-stop relays 1 [5WF] and 2 [6WF]); manual switches APU FIRE pb (cockpit), APU SHUT OFF (NLG external power panel, ground only), APU EMER SHUT DOWN (refuel/defuel panel, ground only); fire extinguishing automatic on ground, APU AGENT pb required in flight.
• FCOM DSC-49-30 — APU emergency shutdown immediate without cooling; associated ECAM alert APU FAULT.

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.