FAA Aviation Mechanic Powerplant (AMP)Turbine EnginesHard
During acceleration from idle to takeoff power, a turbofan engine's control system automatically closes the compressor bleed (anti-surge) valve as N2 increases. What is the primary purpose of this bleed valve during low-speed operation?
- ATo dump excess rear-stage compressor air and maintain adequate surge margin at low speeds
- BTo provide cabin pressurization air only during ground operations
- CTo supply hot air to the wing anti-ice system only
- DTo increase turbine inlet temperature during acceleration
Show answer & explanationAnswer & explanation
Correct answer: A. To dump excess rear-stage compressor air and maintain adequate surge margin at low speeds
At low compressor speeds, the front and rear stages of a multi-stage axial compressor are mismatched, causing rear stages to be prone to stall. Bleed (surge) valves open at low N2 to dump excess air from the middle/rear stages, restoring the pressure balance and preventing compressor stall/surge; they close as speed increases and the stages become better matched.
Why the other options are wrong
- B. Cabin pressurization uses separate bleed taps, not the surge control valve.
- C. Anti-ice is a separate bleed application, not the surge valve's primary function.
- D. Bleed valves reduce compressor loading, not turbine temperature directly.
Anti-Surge/Compressor Bleed Valve
A valve that opens at low compressor speeds to dump excess air from mid/rear compressor stages, preventing stall/surge caused by front-to-rear stage mismatch, then closes as speed increases.
- Open at low N2, closed at high N2
- Prevents compressor stall/surge
- Improves stage matching during acceleration
- Also called surge bleed or interstage bleed valve
Memory trick: Bleed valve = a pressure relief door that shuts as speed climbs