ch2

relative wind

combining the compressor rotation (RPM) and the inlet airflow

AOA

angle between relative wind and the rotor blades chord line

indications of a compressor stall

abnormal engine noise, increasing ITT, decreasing N1 and torque

causes of compressor stalls and which is most common

airflow distortion (most common) or mechanical malfunctions

what can cause airflow distortion

aircraft attitude and airspeed, high AOA

4 instances where airflow distortions may induce compressor stalls

abrupt change in aircraft attitude

encountering air turbulence

deficiency of air velocity or volume, caused by atmospheric conditions

rapid throttle movement

4 different mechanical malfunctions for a compressor stall

variable inlet guide vane and stator vane

fuel control unit (FCU)

foreign object damage (FOD)

variable exhaust nozzle

variable inlet guide vane and stator vane

failure to change the AOA will cause too much or too little airflow at low engine speed

FCU

too much or too little fuel mixed causing too much burner pressure or a flame out

FOD

caused when an object damages the delicate blades of the compressor

variable exhaust nozzles

fails to open, an excessive back pressure will be produced which could lead to a compressor stall

avoid a compressor stall

avoid erratic or abrupt Power control lever (PCL) movement

avoid severe weather and turbulence

prevention

variable inlet guide vanes and stator vanes

dual/twin/split-spool compressor

bleed valves

variable exhaust nozzle

variable inlet guide vanes and stator vanes

AOA is changed at low speeds and maintains velocity of the air

dual/twin/split-spool compressor

allows the front rotor to turn at a slower rpm than the rear rotor

bleed valves

installed near the middle or rear of the compressor to vent air and increase airflow in the front at low rpm

variable exhaust nozzle

unload pressure during afterburner operation

recover from compressor stall

reduce AOA and retard PCL to just below stall threshold to allow the engine to “catch up” with the inlet airflow. once engine returns to normal the PCL should be advanced slowly to the desired setting