Compressor stall is a reduction or disturbance to the airflow in the compressor resulting in a reduced axial flow component.
As the axial flow component reduces the angle of attack of the compressor blades increases. Compressor stall is more prevalent in axial compressors.
Once a compressor stage stalls this will normally have downstream effects of subsequent stages, and cause abnormal pressure gradients (the pressure gradient should try and put air towards the exhaust). The reduction in the pressure gradient can lead to a reduction in the axial flow to a point where the flow can stagnate or even reverse in very severe cases.
After the engine is stable other causes of compressor stall are :
Poor air flow coming into the inlet :
Turbulence
High angle of attack
High sideslip angles
Poor fuel or fuel control management
Excessive fuel flow
Rapid opening of the throttles
Malfunctioning fuel control unit or governor
High altitude flight
At very high altitudes the air can become thin that at a given RPM the compressor blades will stall
What you see/hear
At the lower end you may get some slight vibration and little if any cockpit instrumentation changes. More spectacular stalls (and generally more expensive) will show up as fluctuation in RPM, EGT, or loud bangs or metal coming out the rear.
How to fix it …
Reduce throttle, angle of attack, sideslip (may not be possible in a cross wind), or height.
Other ways to over come stalling include …
Multiple spool design so that each spool is operating near design RPM, the RB-211 is an example of a triple spool engine
Bleed valves to reduce any excessive pressure build up.
On a PT-6 the compressor consists of axial and a centrifugal compressor stages, and during a start at low RPM the axial compressor is more efficient than the centrifugal stage and can cause a back pressure on the axial stage. To relieve this they have a compressor bleed valve which bleeds off P2.5 air (air after the axial compressor) until the engine RPM increases and slowly closes between 70-90% RPM.
Airflow conditioning devices like variable stator or guide vanes to improve the airflow direction onto the compressor blades.
Cheers
Z