How many stages does the APU turbine have?

How many stages does the APU turbine have?

• A. 1 Stage
• B. 2 Stages
• C. 3 Stages
• D. 4 Stages

Answer: (B)

Two stages of turbine expansion are used in a typical APU. The hot gases flow through two turbine disks in sequence, extracting energy in two steps. The first stage provides the power to drive the compressor, while the second stage delivers additional power to the accessory gearbox and generators. This two-stage arrangement gives enough shaft power for starting and electrical/bleed-air needs without adding unnecessary weight or complexity. Using a single stage wouldn’t supply enough energy for both the compressor and the load, and three or four stages would be excessive for a small APU.

Two stages of turbine expansion are used in a typical APU. The hot gases flow through two turbine disks in sequence, extracting energy in two steps. The first stage provides the power to drive the compressor, while the second stage delivers additional power to the accessory gearbox and generators. This two-stage arrangement gives enough shaft power for starting and electrical/bleed-air needs without adding unnecessary weight or complexity.

Using a single stage wouldn’t supply enough energy for both the compressor and the load, and three or four stages would be excessive for a small APU.