Automatic Shutdown — The Phase-Dependent Table and the Backup-Source Logic

This is the central table of the whole APU chapter. On a fault the APU may automatically shut down — but which faults shut it down depends on where the aircraft is. On the ground with engines off the APU is expendable and quits for almost anything; in flight (or on the ground with an engine running) it tolerates nearly every fault, because it may be the critical backup source.

In the case of an APU failure or misbehavior, the APU may automatically shut down, without a cooling period, in order to avoid subsequent damage... The automatic shutdown is associated with the APU FAULT alert. — FCOM DSC-49-30

1. The phase-dependent shutdown table

Per FCOM DSC-49-30, the automatic-shutdown conditions split by flight phase:

Shutdown condition	On ground, engines not running	On ground with ≥ 1 engine running, or in flight
APU fire	Yes	Yes (on ground only)
Overspeed: N > 107 %	Yes	Yes
Certain critical ECB failure	Yes	Yes
Certain ECB internal failure	Yes	No
DC power interruption	Yes	No
Generator high oil temperature	Yes	No
High oil temperature	Yes	No
Load compressor overtemperature	Yes	No
Low oil pressure	Yes	No
Overtemperature	Yes	No
Start abort	Yes	No
N underspeed	Yes	No

[!warning]- In flight, only THREE things auto-shut the APU Read the right-hand column: in flight the APU auto-shuts only for overspeed > 107 % and certain critical ECB failure — and APU fire shuts it automatically on the ground only, never in flight (FCOM DSC-49-30). Every oil, temperature, start and DC fault that quits the APU on the ground is tolerated in flight. The machine is deliberately kept running through faults that would otherwise stop it, because it may be the last source of electrical and pneumatic power.

2. Why these three survive the filter

The two conditions that shut the APU in every phase are the ones where keeping it running is more dangerous than losing the source:

Overspeed > 107 % — an uncontained overspeed threatens to burst the turbine (01 turbine containment); no backup value justifies that.
Certain critical ECB failure — if the full-authority controller itself is critically lost, the APU can no longer be governed safely; there is no hydromechanical backup beneath it.

And APU fire shuts down automatically on the ground only because on the ground the APU is unattended and expendable, while in flight the crew must weigh the fire against the APU's role and command the emergency shutdown themselves.

[!note]- "If not inhibited" finally has a precise meaning (integrative synthesis) The oil-system protections each read "auto shutdown if it is not inhibited." This table is that inhibit: low oil pressure, high oil/generator temperature, load-compressor overtemperature and overtemperature all show Yes on ground / No in flight (FCOM DSC-49-30). The inhibit is not a vague caveat — it is exactly the right-hand column.

3. The APU FAULT alert and no cooling

Per FCOM DSC-49-30, every automatic shutdown is without a cooling period and is annunciated by the APU FAULT ECAM alert (with its procedure). After more than three consecutive start attempts, the crew waits 60 min before another attempt (06).

[!note]- Auto shutdown and emergency shutdown share the no-cool sequence Both the automatic shutdown (FCOM DSC-49-30) and the emergency shutdown stop the APU without cooldown; the difference is the trigger (a monitored fault vs a fire/overheat or manual command), not the stop itself. A normal shutdown is the only one that gets the 85 s cool-down.

4. Counterintuitive points

[!warning]- The same fault has opposite outcomes by phase Low oil pressure (or high oil temp, overtemperature, start abort…) auto-shuts the APU on the ground but is tolerated in flight (FCOM DSC-49-30). Where the aircraft is changes whether a fault is fatal to the APU.

[!warning]- APU fire does not auto-shut the in-flight APU Fire auto-shutdown is ground-only; in flight the crew commands it (FCOM DSC-49-30 / 12).

Self-test

[!note]- Q1. In flight, which conditions automatically shut the APU down? Only overspeed > 107 % and certain critical ECB failure. (APU fire is auto ground-only.)

[!note]- Q2. Why is the APU kept running through oil/temperature faults in flight? Because it may be the critical backup electrical/pneumatic source — losing it can be worse than running degraded.

[!note]- Q3. What does "auto shutdown if not inhibited" refer to? The phase-dependent table — the inhibit is the in-flight / engine-running column (those faults are No in flight).

[!note]- Q4. Cooling and alert on an automatic shutdown? No cooling period; annunciated by APU FAULT. Three attempts → 60-min wait.

Key takeaways

Point	Detail
Always (any phase)	overspeed > 107 %, certain critical ECB failure
Ground-only	APU fire, ECB internal failure, DC interruption, generator/high oil temp, load-comp overtemp, low oil press, overtemp, start abort, N underspeed
In flight	tolerates all the ground-only faults — backup-source logic
Sequence	no cooling period; ECAM APU FAULT; 3 attempts → 60 min
"If not inhibited"	= the in-flight / engine-running column of this table

References

FCOM DSC-49-30 (APU Automatic Shutdown / Abnormal Operations) — automatic shutdown without a cooling period, APU FAULT alert; phase-dependent conditions: always (ground and in-flight/engine-running) = APU fire (ground only), overspeed N > 107 %, certain critical ECB failure; ground-only (engines not running) also = certain ECB internal failure, DC power interruption, generator high oil temperature, high oil temperature, load compressor overtemperature, low oil pressure, overtemperature, start abort, N underspeed; after more than three consecutive start attempts wait 60 min.
AMM 49-94-00 — low oil pressure / high oil temperature start an automatic shutdown "if it is not inhibited" (the inhibit = the in-flight column of the DSC-49-30 table).

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.