Power Section — Single Shaft, Two-Stage Compressor, Reverse-Flow Combustor, Three-Stage Turbine
The overview gave the APU as a single-shaft GTCP 331-350C. This article opens its aft module — the power section, the gas-turbine engine that produces the shaft power. It runs at one constant speed and drives everything else.
The power section is a single-shaft gas-turbine engine. It provides the shaft power which is necessary to drive: ‐ the load compressor, ‐ the accessory drive gearbox with the APU accessories, ‐ the APU generator... It has: ‐ a two-stage centrifugal compressor, ‐ a reverse-flow annular combustion-chamber, ‐ a three-stage axial-flow turbine... The speed is 41730rpm, indicated as 100 % N. — AMM 49-21-00
1. The single main shaft
The power section has a single shaft, which is referred to as the main shaft... A tie-shaft holds the different components of the main shaft together... There are two bearings, which hold the main shaft: ‐ a duplex ball bearing at the compressor end of the main shaft (the forward bearing), ‐ a roller bearing at the turbine end of the main shaft (the aft bearing).
Per AMM 49-21-00, one main shaft carries the compressor impellers + turbine rotors, held together by a tie-shaft, on two bearings: a duplex ball bearing forward (compressor end) and a roller bearing aft (turbine end).
[!note]- One shaft, one speed — the opposite of the three-spool main engine Where the main engine splits compression across three independently-speeding shafts, the APU puts compressor + turbine on one shaft at one constant speed (41730 rpm = 100 % N). The APU doesn't need variable speed because its job is steady electrical + bleed output, regulated by IGV and generator, not by spool speed.
2. Two-stage centrifugal compressor
The power section compressor... is a two-stage radial compressor-design. It increases the pressure of the air from the ambient with an approximately 11.5 : 1 ratio. The primary components... ‐ the first stage impeller (which is splitter bladed), ‐ the second stage impeller, ‐ the first stage diffuser, ‐ the second stage diffuser, ‐ the inlet bellmouth, ‐ the compressor housing.
Per AMM 49-21-00, two centrifugal (radial) stages raise pressure ~11.5 : 1; the first-stage impeller is splitter-bladed (alternating long/short blades for flow efficiency), each impeller followed by a diffuser.
[!note]- Centrifugal, not axial — and why (integrative synthesis) The APU compressor is centrifugal/radial (two stages to ~11.5:1), unlike the main engine's many axial stages. Centrifugal stages give a high pressure ratio per stage in a short, robust, compact package — ideal for a tailcone-mounted APU where length and simplicity matter more than the last percent of efficiency. The splitter blades on stage 1 improve the flow at the high tip speeds of a centrifugal wheel. (Per AMM; the "why compact" is engineering reading.)
3. Reverse-flow annular combustor
The combustion chamber of the GTCP 331-350C power section is of the reverse-flow annular design. It makes a ring around the turbine submodule. The forward part of the heat shield is the cover of the combustion chamber (it is also called the turbine plenum)... The turbine containment makes the inner case of the combustion chamber.
Per AMM 49-21-00, the combustor is reverse-flow annular, a ring around the turbine; the heat-shield front is its cover ("turbine plenum"), and the turbine containment forms its inner case. It carries the igniter plugs (05), fuel nozzles (04) and a drain valve.
[!warning]- "Reverse-flow" wraps the combustor around the turbine to save length A reverse-flow annular combustor folds the gas path back on itself — air enters, reverses direction through the liner, and exits forward into the turbine. Wrapping it as a ring around the turbine (AMM 49-21-00) shortens the engine dramatically vs a straight-through combustor — again the compact-tailcone logic. Doubling the turbine containment as the combustor inner case is a neat structural economy.
4. Three-stage axial turbine
The turbine changes the energy of the hot gas flow into mechanical energy. It drives the power section compressor-impellers, the load compressor impeller and the geartrain of the accessory drive gearbox. The turbine... is of the three-stage axial design. The turbine rotors are at the rear part of the main shaft.
Per AMM 49-21-00, a three-stage axial turbine at the shaft's rear extracts the gas energy to drive all three loads at once: the power-section compressor, the load-compressor impeller, and the accessory-gearbox geartrain — all on the one shaft at 41730 rpm.
5. Heat shield — insulation, noise, and the exhaust cone
The heat shield is the cover around the combustor and the turbine. It has a thermal insulation which keeps the skin temperature within the permitted limits. It also decreases the noise... Together with the turbine support it also makes the exhaust cone of the engine... The surge air flows through the surge air duct to the heat shield, this sends the exhaust gas and the surge air to the exhaust muffler.
Per AMM 49-21-00, the heat shield wraps the combustor/turbine: thermal insulation (skin-temperature limit) + noise reduction, and with the turbine support it forms the exhaust cone. Note the surge air (03) routes through it, joining the exhaust gas to the muffler — the power section is where bleed-surge air rejoins the exhaust.
6. Counterintuitive points
[!warning]- Compact-by-design: centrifugal compressor + reverse-flow combustor + one shaft Every architecture choice serves the tailcone package: centrifugal stages (high ratio per stage, short), a reverse-flow combustor (folded around the turbine to save length), and one shaft at constant speed (no need for variable spools). It is a different design philosophy from the main engine — optimised for compactness and steady output, not variable thrust.
[!warning]- One turbine drives three loads simultaneously The 3-stage turbine drives the power-section compressor and the load-compressor impeller and the gearbox — all on one shaft (AMM 49-21-00). A load change on any (e.g. bleed demand) is felt by the whole shaft, which the ECB holds at 100 % N by modulating fuel.
Self-test
[!note]- Q1. The power section's three gas-path elements? Two-stage centrifugal compressor (~11.5:1) + reverse-flow annular combustor + three-stage axial turbine, all on one main shaft at 41730 rpm.
[!note]- Q2. How is the main shaft supported? A tie-shaft holds the components; duplex ball bearing forward (compressor end) + roller bearing aft (turbine end).
[!note]- Q3. Why centrifugal compressor + reverse-flow combustor? Compactness for the tailcone — high pressure ratio per centrifugal stage, and a reverse-flow combustor folded around the turbine to save length.
[!note]- Q4. What does the turbine drive? Simultaneously the power-section compressor, the load-compressor impeller, and the accessory-gearbox geartrain — one shaft.
Key takeaways
| Point | Detail |
|---|---|
| Architecture | single shaft: 2-stage centrifugal compressor (~11.5:1) + reverse-flow annular combustor + 3-stage axial turbine |
| Shaft | tie-shaft; duplex ball bearing fwd + roller bearing aft; 41730 rpm = 100 % N |
| Combustor | reverse-flow annular, rings the turbine; turbine containment = inner case |
| Turbine | drives power-section compressor + load compressor + gearbox simultaneously |
| Heat shield | insulation + noise + forms exhaust cone; surge air rejoins exhaust here |
References
- AMM 49-21-00 (Power Section — Description and Operation) — single-shaft gas turbine; main shaft + tie-shaft + duplex ball / roller bearings; two-stage centrifugal compressor (~11.5:1, splitter-bladed stage 1, diffusers); reverse-flow annular combustor (rings the turbine, turbine containment = inner case); three-stage axial turbine; heat shield (insulation/noise/exhaust cone, surge-air routing).
- FCOM DSC-49-10-20 — single-shaft gas turbine delivering shaft power + bleed (overview).
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.