A compressor stall in a jet engine is a circumstance of abnormal airflow resulting from the aerodynamic stall of aerofoils (compressor blades) within the compressor. This occurs when the angle of attack of the blades of the compressor exceed their critical angle of attack resulting in one or more stages of rotor blades failing to pass air smoothly to the succeeding stages.
A compressor blade is an aerofoil and is subject to the same aerodynamic principals that apply to other aerofoils such as a wing or a propeller. Compressor blades are set at a fixed angle on each stage of the compressor. However, the blades have an effective angle of attack which is the vector sum of the inlet air velocity and the compressor rotational speed. A compressor stall occurs when there is an imbalance between the air flow supply and the airflow demand; in other words, a pressure ratio that is incompatible with the engine RPM. When this occurs, smooth airflow is interrupted and turbulence and pressure fluctuations are created within the turbine. Compressor stalls cause the air flowing through the compressor to slow down or stagnate and sometimes result in reverse flow.
Compressor stalls can be caused by a number of factors including:
- Foreign Object Damage (FOD) including bird strike
- worn, dirty or contaminated compressor components
- in-flight icing
- aircraft operation outside of the engine design envelope including extreme flight manoeuvres
- improper engine handling
Depending upon the cause, a compressor stall can be transient and self correcting or steady and require pilot intervention in accordance with Quick Reference Handbook (QRH) or other manufacturer directives. Flight deck indications include an increase in engine temperature and fluctuations in engine RPM. These may be observed on any of the following gauges as fitted to the aircraft:
- Exhaust Gas Temperature (EGT)
- Turbine Inlet Temperature (TIT)
- Interstage Turbine Temperature (ITT)
- N1 Indicator
- N2 Indicator
- N3 Indicator
A compressor stall will result in a loss of thrust and is likely to produce a "backfire" like sound due to reverse airflow. It may also be accompanied by flame from either or both of the engine inlet and engine exhaust. A sustained compressor stall can result in engine damage and can lead to engine failure.