What is bleed air and where is it used in gas turbine systems?

• A. Fresh air from environment used for cooling
• B. Air from sump used for lubrication
• C. Compressed air bled from the compressor used for starting, anti-icing, environmental control, and system actuation
• D. Air from turbine exhaust used for propulsion

Answer: (C)

Bleed air is compressed air taken from the engine’s compressor stages, before it reaches the combustor. That air is hot and at a higher pressure, and it’s used to run a variety of pneumatic systems on the engine and aircraft.

This air powers the pneumatic starter to spin up the engine until it runs on its own, provides heating for anti-icing of inlet lips and other surfaces, supplies the environmental control system for cabin pressurization and air conditioning, and drives various pneumatic actuators and system controls (valves, dampers, and similar components). Because it comes directly from the compressor, it’s clean air that doesn’t require combustion to be generated at that moment.

It isn’t fresh air pulled from the outside environment, it isn’t air from the lubrication sump, and it isn’t exhaust from the turbine. Those sources serve different purposes and have different properties.