Fuel System — FCU, Atomizers, Ecology Drain, the IGV/SCV Fuel Supply
The load compressor and surge valve are fuel-powered — this article shows where that fuel comes from. The Fuel Control Unit (FCU) is the heart: it meters combustion fuel and supplies the hydraulic fuel that drives the IGV and surge-control actuators.
The Fuel Control Unit (FCU) supplies the correct quantity of fuel... It also supplies the fuel which is necessary to operate the Inlet Guide Vane (IGV) actuator and the Surge Control Valve (SCV) actuator. The ECB... controls, through the torque motor metering valve and fuel flow solenoid, the fuel flow. — AMM 49-32-00
1. The FCU — one source, several jobs
The FCU supplies filtered, pressure controlled high-pressure fuel to the fuel distribution system. It also supplies filtered, pressure controlled fuel to the SCV actuator and the IGV actuator (the actuators use the hydraulic energy of the fuel). Filtered high-pressure fuel is also used for the operation of the de-oil valve on the oil pump module.
Per AMM 49-32-00, the FCU (59KF19) takes fuel from the APU fuel pump, raises and controls its pressure, and feeds four destinations: the combustor (via the distribution system), the IGV actuator, the SCV actuator, and the de-oil valve on the oil module.
[!warning]- One FCU fuel source powers combustion AND the IGV/SCV actuators — the link the earlier articles pointed to Articles 02 and 03 said the IGV and surge-control actuators are "fuel-pressure-powered" — this is the source: the FCU's filtered high-pressure fuel is the hydraulic power for both, as well as the combustion fuel (AMM 49-32-00). So a fuel-pressure problem affects not just combustion but bleed control and surge protection too.
Inside: an inlet filter + HP filter + HP gear pump (driven by the accessory gearbox via the oil-pump-module shaft) + a torque-motor metering valve + a fuel shutoff solenoid + four pressure-control valves (HP relief / differential-pressure regulation / pressurising / actuator pressure regulation). A low-fuel-pressure switch sits on the FCU inlet (→ low-pressure warning / AMM 49-34).
[!note]- The ECB meters fuel via a torque-motor metering valve (its FADEC parallel) The ECB sets fuel flow through the torque-motor metering valve + fuel-flow solenoid (AMM 49-32-00) — the same kind of torque-motor metering the main engine FMU uses. Constant-speed control = the ECB trims this fuel to hold 100 % N as loads change.
2. Fuel distribution — 12 dual-orifice atomizers
The APU has 12 dual orifice fuel-atomizers. They are installed in equal distances around the outer case of the combustion chamber. This makes sure that the fuel temperature in the combustion chamber is equally divided.
Per AMM 49-31-00, 12 dual-orifice atomizers (primary + secondary) are evenly spaced around the combustor, fed by primary and secondary manifolds through a flow divider (59KF25) — the flow divider apportions fuel between primary and secondary orifices as flow rises, for a clean light and a wide turn-down. A fuel-temperature sensor (59KF6) monitors the fuel.
3. Ecology drain — burn the residual fuel, don't dump it
The fuel distribution system also has an ecology drain system. The ecology drain system makes sure that the fuel is removed from the fuel atomizers and manifolds during the APU shutdown. The drain fuel is burned in the combustion chamber.
Per AMM 49-31-00, at shutdown the ecology drain (solenoid 59KF28 + air filter + check valves) clears residual fuel from the atomizers/manifolds and burns it in the combustor — not overboard.
[!warning]- Ecology drain burns the residual fuel internally — the APU's "clean" shutdown Where the main engine's dump valve purges burner fuel to a drain tank, the APU's ecology drain burns the residual fuel in the combustor (AMM 49-31-00) — no overboard discharge, preventing both pollution and coking of the atomizers.
4. Feed and pressurisation
The APU is supplied with fuel through the trim tank transfer line... The APU fuel feed line is pressurized when the APU MASTER pb-sw is set to ON. If the initial start attempt fails, the crew may wait 3 min before setting the APU START pb-sw to ON again. This delay ensures the APU fuel feed line is sufficiently pressurized. (DSC-49-10-20)
Per DSC-49-10-20, fuel arrives via the trim-tank transfer line; MASTER ON pressurises the feed line, and after a failed first start the crew waits 3 min to let the line re-pressurise.
[!note]- Why the 3-minute wait after a failed start (integrative synthesis) MASTER ON starts pressurising the feed line; a start needs that pressure. If the first attempt fails, the 3-min wait lets the line re-pressurise (DSC-49-10-20) so the next attempt has adequate fuel pressure — distinct from the starter-motor 60-min cooldown after three attempts (06).
5. Counterintuitive points
[!warning]- The FCU fuel is hydraulic power, not just combustion fuel The same FCU high-pressure fuel drives the IGV actuator + SCV actuator + de-oil valve as well as combustion (AMM 49-32-00). Fuel pressure is the APU's hydraulic system.
[!warning]- Shutdown burns the residual fuel; a failed start needs a 3-min repressurise wait Ecology drain → combustor (no overboard dump); failed start → wait 3 min for feed-line pressure (≠ the 3-attempt 60-min starter cooldown).
Self-test
[!note]- Q1. What four things does the FCU fuel feed? Combustion (distribution), the IGV actuator, the SCV actuator, and the de-oil valve — the FCU is the APU's hydraulic source too.
[!note]- Q2. How does the ECB meter fuel? Via the torque-motor metering valve + fuel-flow solenoid (trims fuel to hold 100 % N).
[!note]- Q3. The atomizers, and the ecology drain? 12 dual-orifice atomizers (primary/secondary manifolds + flow divider); ecology drain burns residual fuel in the combustor at shutdown (not overboard).
[!note]- Q4. Why wait 3 min after a failed start? To let the fuel feed line re-pressurise (MASTER ON pressurises it) — separate from the 60-min starter cooldown.
Key takeaways
| Point | Detail |
|---|---|
| FCU (59KF19) | meters combustion fuel + powers IGV / SCV actuators + de-oil valve; torque-motor metering valve; HP gear pump (gearbox-driven) |
| Distribution | 12 dual-orifice atomizers, primary/secondary manifolds, flow divider (59KF25) |
| Ecology drain | shutdown residual fuel burned in combustor (not overboard) |
| Feed | trim-tank line; MASTER ON pressurises; failed start → wait 3 min |
| Low fuel pressure | switch on FCU inlet → warning |
References
- AMM 49-32-00 (Fuel Control — Description and Operation) — FCU (59KF19): HP gear pump (gearbox-driven), torque-motor metering valve + shutoff solenoid, four pressure-control valves, low-fuel-pressure switch; supplies combustion + IGV actuator + SCV actuator + de-oil valve.
- AMM 49-31-00 (Fuel Distribution) — 12 dual-orifice atomizers, primary/secondary manifolds, flow divider (59KF25), fuel-temp sensor; ecology drain (solenoid 59KF28) burns residual fuel in the combustor.
- FCOM DSC-49-10-20 — trim-tank feed; MASTER-ON pressurisation; 3-min wait after a failed first start.
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.