Compressor stall occurs whenever the airflow in the compressor becomes unstable. Instability occurs when the effective angle of attack of one or more compressor blades is exceeded, just as an airplane wing will stall when its angle of attack is too high. In a compressor, the blades effective angle of attack will vary whenever the compressor's operating conditions, such as the inlet air pressure or rotational speed, are changed. Conditions which cause the onset of instabilities are referred to as unstable, or stall, operating conditions.
Because of its destructive nature, compressor stall is one of the most critical concerns in axial flow compressor design. Different types of stall can occur, ranging from mild to severe. In mild cases, stall can manifest itself by abnormal engine noise. In more severe cases, stall can produce loud banging noises and flame or smoke at the engine inlet and/or exhaust. Compressor stall causes excessive engine wear, degradation of performance, and a reduction in engine reliability and durability.
the rest of the article is at:-. .<a href="http://hpcc.grc.nasa.gov/catsslm/StallLine/chapter1.shtml" target="_blank">http://hpcc.grc.nasa.gov/catsslm/StallLine/chapter1.shtml</a>