What does a variable bleed valve (VBV) do in compressor stability control?

• A. Bleeds off air from the compressor at certain speeds to maintain stable operation and prevent surge.
• B. Increases fuel flow during high demand.
• C. Provides lubrication to compressor bearings.
• D. Adjusts compressor exit guide vane angle to optimize efficiency.

Answer: (A)

The main idea is that a variable bleed valve keeps the compressor operating in a stable range by removing some of the air from the compressor at certain speeds. By diverting a portion of the compressed air to the bypass or inlet, it lowers the effective pressure rise across the compressor, which helps prevent flow reversal and stall—i.e., surge. This bleed action smooths operation during changing conditions like acceleration or light-load, maintaining stable, surge-free flow.

It isn’t about increasing fuel flow, providing lubrication, or changing the vane angle. Those functions belong to other systems and controls.