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DESCRIBE the angle of attack of compressor blades
- The relative wind is formed by combining the compressor rotation (RPM) and the inlet airflow.
- The angle between the relative wind and the rotor blade's chordline make up the AOA.
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DESCRIBE a compressor stall
- Too high of an AOA yields a possible stall.
- Changing the rotation speed of the rotors (RPM) during engine operation and/or the velocity of the inlet airflow will cause the compressor AOA to change.
- Can be caused by airflow distortions or mechanical malfunctions.
- Results in a decrease of thrust
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DESCRIBE four mechanical malfunctions that can lead to a compressor stall
- Variable inlet guide vanes (IVG) and stator vanes
- Failure to change the AOA will cause too much or too little airflow at low engine speed.
- Fuel Control Unit (FCU)
- If the FCU fails, too much or too little fuel is added.
- Too much fuel causes excessive burner pressure and a back-flow of air into the compressor.
- Too little fuel may cause the engine to flame out.
- Foreign object damage (FOD)
- An object damages the compressor.
- Variable Exhaust Nozzles
- Used for afterburner operations for supersonic airflow.
- If it fails to open, it will create excessive back pressure to lead to a stall.
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DESCRIBE appropriate actions a pilot can take regarding compressor stalls
- Avoidance
- Avoid erratic or abrupt Power Control Lever (PCL) movements, especially at high AOA and low airspeeds.
- Maintain at least the prescribed minimum airspeed.
- Avoid abrupt changes in attitude.
- Avoid severe weather and turbulence.
- Remedy and Recovery
- Reduce attitude to reduce inlet's AOA.
- PCL retarded to just below stall threshold to allow engine to "catch up" with inlet airflow.
- Many aircraft will have automatic bleed valves or a procedure to open a valve to allow airflow through the compressor.
- Once engine indications return to normal, slowly advance the PLC
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DESCRIBE four engine design features that can be incorporated into a gas turbine engine design to minimize the potential for a compressor stall
- Variable inlet guide vane and stator vane
- Installed so the AOA is changed at low engine speed.
- Automatically positioned by the stator vane actuator (SVA) using fuel pressure via the FCU.
- Dual/twin/split-spool compressor
- This design allows the front rotor to turn at a slower RPM than the rear rotor.
- Bleed Valves
- "Bleed" air (vent to the atmosphere) and increase airflow in the front of the compressor at low engine RPMs.
- Variable Exhaust Nozzle
- Used to unload the pressure during afterburner operation.
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