Which stage of bleed air is specifically used for balance piston air?

The use of bleed air from different stages in a gas turbine engine serves various purposes, and each stage's air pressure and temperature characteristics make them suitable for specific applications. In gas turbine engines, the balance piston is a component utilized to maintain rotor stability by counteracting forces acting on the rotor shaft.

The 9th stage bleed air is utilized specifically for the balance piston air. This choice is based on the operational characteristics of the bleed air extracted from the 9th stage, which is typically at an appropriate pressure and temperature that provides effective support for the balance piston. This allows for a stable and efficient performance of the engine by managing the rotor dynamics.

Other stages, like the 5th, 7th, and 11th stages, while they have their own respective uses for bleed air—such as engine starting, cabin pressurization, and anti-icing systems—are not specifically designated for the balance piston. The characteristics of the air from these stages may not be optimized for the demands of the balance piston application, making the 9th stage the correct choice in this context.