Load Compressor — Single-Stage Centrifugal, Variable IGV Bleed Source

The power section makes the shaft power; the load compressor is the mid module that turns it into the APU's bleed air. This is where the APU's output is regulated — at constant shaft speed, the bleed is controlled by the variable inlet guide vanes, not by rpm.

The APU bleed air is supplied from the load compressor to the pneumatic system... for the Environmental Control System (ECS) and to do the Main Engine Start (MES)... The load compressor is of the single-stage centrifugal-compressor design. The APU power section drives the load compressor impeller directly... at the constant APU speed of 41730 rpm. — AMM 49-23-00

1. The bleed-air source — and a power pass-through

The load compressor transmits the necessary shaft power to drive the accessory drive gearbox... The load compressor thrust bearing assy is of the ball bearing type and has good operating characteristics for an APU start after high altitude, extended cold-soak conditions.

Per AMM 49-23-00, the load compressor (mid module, between power section and gearbox) does two jobs: it produces the bleed air (for ECS + MES), and it is the power pass-through — the power section drives it, and it in turn drives the accessory gearbox. Its ball thrust bearing is chosen for good behaviour on a high-altitude, cold-soak start.

[!note]- The load compressor sits in the power-transmission chain (integrative synthesis) Per AMM 49-23-00 the impeller is driven by the power section through a quillshaft, and it drives the gearbox input gear through a quillshaft — so the load compressor is mechanically in series in the drive train (power section → load compressor → gearbox). A quillshaft is a slender torsion shaft that tolerates slight misalignment and damps torsional shock — useful between modules turning at 41730 rpm.

2. The impeller

The load compressor has a single stage impeller, which is splitter bladed. The 11 main and the 11 splitter blades are curved rearward. The APU power section drives the impeller directly through a quillshaft. The impeller turns with the constant APU speed of 41730 rpm.

Per AMM 49-23-00: a single splitter-bladed impeller — 11 main + 11 splitter blades, curved rearward — at the constant 41730 rpm, with a forward and aft hold. (Rearward-curved + splitter blades = stable, efficient flow at high tip speed.)

3. The variable IGV — the APU's "output throttle"

There are 24 IGVs, which all move at the same time. The IGV assembly geartrain makes a synchronized movement. The IGV actuator... is a hydraulic actuator which operates with high pressure fuel supplied from the Fuel Control Unit (FCU). The ECB 59KD controls and monitors the IGV actuator rod-extension and thus it controls the opening angle of the IGVs... The opening angle depends on the different bleed air demands from: ‐ the Main Engine Start (MES) system, ‐ the Environmental Control System (ECS).

Per AMM 49-23-00, 24 IGVs move together (synchronised geartrain); the IGV actuator is hydraulic, powered by high-pressure fuel from the FCU, and the ECB controls the actuator rod extension → the IGV opening angle, computed from the MES + ECS demand.

[!warning]- At constant speed, the IGVs are how the APU varies its output — fuel-powered, like the main engine's VSV The APU shaft is fixed at 41730 rpm, so it cannot vary bleed by changing speed. Instead the ECB opens/closes the 24 IGVs to meter the air into the impeller to match ECS + MES demand (AMM 49-23-00) — the IGV is the APU's bleed "throttle." And like the main engine's VSV, the IGV actuator is fuel-pressure-powered (04), not electric or pneumatic. Bleed control + surge protection downstream are in 03.

4. Diffuser and sensors

The diffuser is downstream of the load compressor impeller. It is of the radial design with 19 guide vanes in the shape of cambered blades.

Per AMM 49-23-00, a radial diffuser with 19 cambered guide vanes recovers pressure downstream of the impeller, with a static-pressure channel sampling the bleed pressure. The ECB monitors the load compressor via LCIT (inlet temp), LCOT (outlet temp) and an inlet-pressure transducer — the data behind bleed control and surge protection.

5. Counterintuitive points

[!warning]- The IGV — not shaft speed — is the APU's bleed control Constant 41730 rpm means the APU regulates bleed by IGV opening angle, ECB-scheduled to ECS + MES demand (AMM 49-23-00). When MES asks for max bleed, the IGVs open; for low ECS demand, they close. Watch this in Bleed & Surge.

[!warning]- The load compressor both makes bleed AND passes power to the gearbox Via quillshafts it sits in series: power section → load compressor → gearbox (AMM 49-23-00). It is not a dead-end accessory — the generator's drive runs through it.

Self-test

[!note]- Q1. What does the load compressor do, and at what speed? Produces the APU bleed air (ECS + MES) as a single-stage centrifugal compressor at constant 41730 rpm, and passes shaft power through to the gearbox.

[!note]- Q2. How does the APU vary its bleed output at constant speed? Via the 24 variable IGVs, ECB-controlled to MES + ECS demand — a fuel-pressure-powered hydraulic actuator sets the opening angle.

[!note]- Q3. The impeller blading? Single splitter-bladed impeller, 11 main + 11 splitter blades, rearward-curved.

[!note]- Q4. Why a ball thrust bearing? Good behaviour on a high-altitude, cold-soak start.

Key takeaways

References

• AMM 49-23-00 (Load Compressor — Description and Operation) — single-stage centrifugal, direct quillshaft drive at 41730 rpm, power pass-through to gearbox; splitter impeller (11+11 rearward-curved); 24 synchronised IGVs with fuel-pressure-powered actuator, ECB-scheduled to MES+ECS; 19-vane radial diffuser; LCIT/LCOT/inlet-pressure sensors; ball thrust bearing.
• FCOM DSC-49-10-20 — IGVs control bleed flow, fuel-pressure-powered actuator, ECB-controlled.

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.