What is the purpose of the isolation valve?

• A. Shuts off bleed air to all systems
• B. Allows air from any source to supply both sides of the manifold
• C. Only blocks cross flow to cabin
• D. Regulates electrical power to valves

Answer: (B)

The isolation valve exists to manage cross-connection between bleed air sources and the distribution manifold. When it’s opened, air from any active source (engine bleed or APU) can feed both sides of the environmental control system manifold, giving redundancy so the systems still receive air even if one source is unavailable. When closed, the two sides are isolated from each other, preventing crossflow. This is not about shutting off all bleed air, regulating electrical power to valves, or merely blocking cross flow to the cabin—the valve’s job is to allow cross-feed between the sources and the manifold to ensure a reliable air supply.