Fire, Limits and Interfaces — the Bleed Ceilings, ETOPS Role, and the Fire Zone

The closing article pulls the APU outward — to the operational limits the crew flies to, the ETOPS role it plays as a backup generator, the fire zone it lives in, and the system interfaces (electrical, pneumatic, fire, fuel) that the rest of the chapter has been feeding into.

The APU is a self contained unit which makes the aircraft independent of external pneumatic and electrical power supply... In flight it backs up the Electrical system, it backs up the Air conditioning, it can be used to start the engines. APU start is permitted throughout the normal flight envelope, except when APU battery only is supplying. — FCOM DSC-49-10-10

1. The bleed ceilings (LIM-APU)

Per FCOM LIM-APU, APU bleed is limited by altitude — and these ceilings are well below the APU's own operating altitude:

[!warning]- APU bleed is a low-altitude resource — electrical backup goes higher The bleed ceilings (20 000 / 22 500 / 17 500 ft, FCOM LIM-APU) are far below the cruise levels where an all-engine flameout starts. So at high altitude the APU's usable contribution is electrical, not pneumatic — and an APU-assisted engine bleed start is only available once descended to ≤ 20 000 ft. Wing anti-ice is never an APU-bleed function. Pair this with the APU BLEED valve auto-closing at 25 000 ft / reopening at 23 000 ft.

The start envelope itself is the full normal flight envelope except on APU battery only, which is restricted (06). During refuel/defuel, APU start/shutdown is permitted with restrictions: do not restart after a failed or automatic shutdown, and on a fuel spill perform a normal shutdown.

2. The ETOPS role — APU as a backup main generator

APU: Fuel consumption of 130 kg/h or 287 lb/h (APU GEN ON, APU BLEED OFF)... Diversion is required in the case of: only one generator (either one IDG, APU GEN or EMER GEN) remaining available following a multiple failure...

Per FCOM PRO-SPO-40 (ETOPS), the APU GEN counts as a main generator in the electrical-generation diversion criteria, and APU fuel burn for planning is 130 kg/h (APU GEN ON, BLEED OFF).

[!note]- On ETOPS the APU is dispatched as a real electrical reserve (integrative synthesis) The diversion logic treats the APU generator alongside the IDGs (FCOM PRO-SPO-40): losing down to a single remaining generator (IDG, APU GEN or EMER GEN) requires diversion. This is the operational face of the backup-source logic — the same reason the in-flight APU is kept running through oil/temperature faults is why it is planned as a generator reserve on long over-water legs. Note BLEED is OFF for that role (the 130 kg/h figure assumes GEN-only).

3. The fire zone and APU fire

The APU lives in the tailcone fire zone, bounded by the forward firewall FR95 (behind the air intake) and the rear firewall FR101 (the exhaust firewall seal). Fire detection and extinguishing are ATA-26 functions; their APU-facing behaviour was covered in the emergency shutdown:

• on the ground: APU fire/overheat → automatic emergency shutdown + automatic extinguishing;
• in flight: the crew commands shutdown (APU FIRE pb) and discharges the bottle (APU AGENT pb).

[!note]- Passive and active fire defences meet here (integrative synthesis) The exhaust compartment held below fuel ignition (insulation ≤ 200 °C, ventilation ≤ 115 °C) is the passive defence; the ATA-26 detection/extinguishing is the active one; and the phase-dependent shutdown governs when the machine itself is stopped. The fire zone FR95–FR101 is the volume all three protect.

4. The interfaces — where the chapter connects outward

[!warning]- The take-off bleed role is a performance tool On take-off the APU supplies air-conditioning bleed so engine bleed need not be tapped — avoiding the thrust loss that engine bleed would cause when peak performance is needed (FCOM DSC-49-10-10). This is a distinct, deliberate use of the APU, not just a backup.

5. Counterintuitive points

[!warning]- The APU's bleed is altitude-capped far below its electrical role Bleed ceilings 20 000 / 22 500 / 17 500 ft; no wing anti-ice (FCOM LIM-APU). High up, the APU is an electrical machine.

[!warning]- ETOPS plans the APU as a generator, with BLEED OFF 130 kg/h GEN-on/BLEED-off; APU GEN is a diversion-criteria main generator (FCOM PRO-SPO-40).

Self-test

[!note]- Q1. The three APU-bleed altitude ceilings, and the forbidden use? Engine-start assist 20 000 ft; a/c single pack 22 500 ft, dual pack 17 500 ft; wing anti-ice not permitted.

[!note]- Q2. How does ETOPS treat the APU generator? As a main generator in the diversion criteria (single remaining generator → divert); planning fuel 130 kg/h (GEN on, BLEED off).

[!note]- Q3. The fire-zone boundaries, and ground vs flight fire response? FR95 (forward) to FR101 (rear); ground = auto shutdown + auto extinguishing; flight = APU FIRE pb + APU AGENT pb.

[!note]- Q4. Why supply air-conditioning bleed from the APU on take-off? To avoid tapping engine bleed, preventing the thrust loss when peak performance is needed.

Key takeaways

References

• FCOM DSC-49-10-10 — APU roles (ground bleed + electrical; take-off a/c bleed to avoid engine-bleed thrust loss; in-flight electrical/air-conditioning backup, engine start); start permitted throughout the normal flight envelope except APU battery-only.
• FCOM LIM-APU — APU bleed max altitude: engine-start assist 20 000 ft, a/c & pressurisation single pack 22 500 ft / dual pack 17 500 ft; APU bleed for wing anti-ice not permitted; refuel/defuel start/shutdown restrictions (no restart after failed/auto shutdown; normal shutdown on fuel spill).
• FCOM PRO-SPO-40 (ETOPS) — APU GEN treated as a main generator in diversion criteria; APU fuel consumption 130 kg/h (287 lb/h) GEN on / BLEED off.
• ATA-26 (Fire Protection) — APU tailcone fire-zone (FR95–FR101) detection and extinguishing; ground automatic, in-flight crew-commanded (APU FIRE pb + APU AGENT pb).

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.