APU Overview — GTCP 331-350C Single-Shaft Gas Turbine, ECB Full Authority

This is the chapter's mental model and map. The APU (Auxiliary Power Unit) is a self-contained gas turbine that makes the aircraft independent of external pneumatic and electrical power — and, in flight, the backup to the main engines for both. It is a Garrett GTCP 331-350C, managed in full authority by an ECB (Electronic Control Box). This article sets up its three roles, the single-shaft/three-module architecture, the ECB, the shutdown logic, and points each subsystem to its deep-dive article.

1. Three roles — ground, take-off, in-flight

The Auxiliary Power Unit (APU) is a self contained unit which makes the aircraft independent of external pneumatic and electrical power supply.

Per DSC-49-10-10:

• On ground — supplies bleed air (for main-engine start + air conditioning) and electrical power.
• During take-off — supplies bleed air for air conditioning, so engine bleed isn't taken for that, avoiding the thrust reduction it would cause when optimum performance is needed.
• In flight — backs up the electrical system and the air conditioning, and can start the engines.

[!note]- The APU is fundamentally a backup in flight — remember this for the shutdown logic On the ground the APU is the primary independent source; in flight the main engines are primary and the APU is the backup (DSC-49-10-10). This single fact shapes the whole chapter — most visibly the automatic-shutdown logic, where the APU tolerates far more faults without shutting down once in flight / with an engine running, precisely because it may be the last electrical source.

APU start power comes from the APU-dedicated batteries, external power, or normal aircraft supply; start is permitted throughout the normal flight envelope, except when on APU battery only (see Starting / LIM-APU).

2. GTCP 331-350C — single-shaft, three modules

The basic element of the APU is a single shaft gas turbine which delivers mechanical shaft power for driving the accessory gearbox (electrical generator) and produces bleed air.

Per DSC-49-10-20 / AMM 49-00-00, the APU is a single-shaft gas turbine (type GTCP — Gas-Turbine Compressor Power-unit) in three modules:

[!warning]- The APU runs at one constant speed — 41730 rpm = 100 % N Unlike a main engine that varies speed with thrust, the APU power section runs at a constant 41730 rpm (indicated 100 % N) (AMM 49-21-00). Its outputs (electrical + bleed) are regulated not by shaft speed but by the load compressor's variable IGVs (02) and the generator — the shaft just turns at 100 %. This is the defining difference from the main engine's three variable spools.

It is installed in the tailcone; the APU compartment is a fireproof fire zone, and the ECB sits in a rack in the (pressurised) bulk-cargo area (AMM 49-00-00).

3. The ECB — full-authority control

The Electronic Control Box (ECB) is primarily a full authority digital electronic controller that performs the APU system logic for all modes of APU operation such as : ‐ Sequence the start and monitors it ‐ Monitors speed and temperature ‐ Monitors bleed air (IGV) ‐ Sequence the shut down (manual, protective or inhibited).

Per DSC-49-10-20, the ECB is to the APU what the FADEC is to the main engine — full authority over start sequencing, speed/temperature monitoring, bleed (IGV) control, and shutdown sequencing (manual / protective / inhibited). Detail in ECB Control.

4. Shutdown — automatic vs emergency

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.

Per DSC-49-30, there are two shutdown families:

• Automatic shutdown (ECB-driven, on a fault) — but the conditions differ by phase (13).
• Emergency shutdown (immediate, no cooling) — APU FIRE pb, or the ground-only APU EMER SHUT DOWN / APU SHUT OFF switches (12).

[!warning]- The "tolerate faults in flight" rule (the backup-source logic in action) Per DSC-49-30, on the ground with engines not running the APU auto-shuts-down on a long list (low oil pressure, overtemperature, start abort, DC interruption, generator high oil temp…). But on the ground with an engine running, or in flight, only APU fire, overspeed > 107 %, and certain critical ECB failures force an auto-shutdown — the rest are tolerated. The reason is §1: in flight the APU may be the critical backup electrical source, so the design keeps it running through faults it would shut down for on the ground. After three consecutive start attempts, wait 60 min.

5. The subsystem map

6. Counterintuitive points

[!warning]- Three things that surprise pilots ① The APU is one constant speed (100 % N = 41730 rpm) — bleed/power are regulated by IGV + generator, not shaft speed. ② With APU BLEED ON, the bleed valve closes at 25000 ft climbing / reopens at 23000 ft descending (DSC-49-10-20). ③ In flight the APU tolerates most faults rather than auto-shutting-down — because it may be the critical backup source.

Self-test

[!note]- Q1. The APU's three roles? Ground: bleed (engine start + a/c) + electrical. Take-off: bleed for a/c (avoid the thrust loss of engine bleed). In flight: backup electrical + a/c + can start engines.

[!note]- Q2. What type of machine, what speed, what three modules? A single-shaft gas turbine (GTCP 331-350C) at constant 41730 rpm = 100 % N; power section + load compressor + accessory gearbox.

[!note]- Q3. What is the ECB? The full-authority digital electronic controller — APU's equivalent of the FADEC (start/shutdown sequencing, speed/temp/bleed monitoring).

[!note]- Q4. Why does the APU tolerate more faults in flight than on the ground? In flight it may be the critical backup electrical source; only fire / overspeed > 107 % / certain critical ECB failures force an auto-shutdown.

Key takeaways

References

• FCOM DSC-49-10-10 — APU self-contained; ground / take-off / in-flight roles; start power sources & envelope.
• FCOM DSC-49-10-20 — single-shaft gas turbine; ECB full authority; IGV / bleed (25000 ft close / 23000 ft reopen); fuel via trim-tank line; independent lubrication.
• FCOM DSC-49-30 — automatic shutdown (phase-dependent conditions) vs emergency shutdown; 3-attempt 60-min cooldown.
• AMM 49-00-00 / 49-21-00 — GTCP 331-350C, three modules, constant 41730 rpm, tailcone fire zone.

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.