FAA Aviation Mechanic Powerplant (AMP)Turbine EnginesHard
A high-bypass turbofan engine incorporates an active clearance control (ACC) system on the turbine case. How does this system improve engine performance?
- ABy injecting extra fuel into the turbine section to increase thrust during cruise
- BBy directing fan or compressor bleed air onto the turbine case to control its thermal expansion and minimize blade tip clearance
- CBy increasing oil flow to the turbine bearings to reduce friction
- DBy opening variable stator vanes further during cruise to increase airflow
Show answer & explanationAnswer & explanation
Correct answer: B. By directing fan or compressor bleed air onto the turbine case to control its thermal expansion and minimize blade tip clearance
Active clearance control directs cooling air (from the fan or compressor) onto the turbine case to precisely control its thermal growth relative to the turbine rotor, minimizing blade tip clearance during cruise for improved efficiency without risking rubs during transient conditions.
Why the other options are wrong
- A. Fuel injection into the turbine case is not how ACC works and would be damaging.
- C. Oil flow to bearings is unrelated to case thermal clearance control.
- D. VSVs affect compressor airflow angle, not turbine case clearance.
Active Clearance Control (ACC)
A system that directs fan or compressor bleed air onto the turbine case to control its thermal expansion, minimizing blade tip clearance and improving efficiency during cruise.
- Uses fan/compressor air to cool turbine case
- Controls case thermal growth vs rotor
- Minimizes tip clearance in cruise
- Reduces tip leakage, improves fuel efficiency
Memory trick: ACC = a cooling blanket that shrinks the case to hug the blades