Compressor Stall

A compressor stall in a jet engine is an abnormal airflow condition caused on by the aerodynamic stall of the compressor's airfoils (compressor blades). This happens when the compressor's blades' critical angle of attack is exceeded, which prevents one or more stages of rotor blades from smoothly transferring air to subsequent stages. a situation in which one or more turbine engine axial-flow compressor blades have excessive angles of attack, disrupting the compressor's normally smooth airflow.

Compressor stall Indication

A compressor stall may be momentary and self-correcting depending on the cause, or it may be constant and call for pilot intervention in line with the Quick Reference Handbook (QRH) or other manufacturer instructions. Increases in engine temperature and variations in engine RPM are flight deck indicators. These can be seen on any of the gauges that have been installed in the aircraft, including:

Exhaust Gas Temperature (EGT)
Turbine Inlet Temperature (TIT)
Interstage Turbine Temperature (ITT)
N1 Indicator
N2 Indicator
N3 Indicator

Due to the reverse airflow, a compressor stall will cause a loss of thrust and is likely to sound like a "backfire." Flame from the exhaust or one or both of the engine's inlets may also be present. An extended compressor stall can harm the engine and even cause it to fail.

Compressor stall factors

There are various things that can cause a compressor to stall, such as:

Damage from foreign objects (FOD), such as bird strikes or worn, filthy, or contaminated compressor parts that cause in-flight icing.

Aircraft operation includes excessive flight manoeuvres and incorrect engine management outside the engine design envelope.

All types of gas turbine compressors have the ability to stall under specific operating conditions. There are numerous ways and circumstances in which a compressor can stall. Because no two engines will exhibit the same stall characteristics, stall is neither simple to explain nor comprehend. Stall typically happens when a compressor tries to produce pressure ratios that are higher than it is capable of.

When an engine is operating at low thrust on the ground, a situation known as "chugging" occasionally occurs. This is a milder type of compressor stall. In flight, stall can develop severe enough to produce loud bangs and engine vibration in extreme circumstances such slam accelerations, while slipping or skidding during evasive manoeuvres, or when flying in extremely turbulent air. The majority of the time, this problem is transient and may be resolved by reversing the throttle's advance to idle and then advancing it again.

The blade would stall if a physical event occurred that significantly raised the blade's angle of attack. Similarly, if the flow rate were decreased, the blade's angle of attack would increase and stall might happen. If the fuel scheduling to the combustor is incorrect, the same problem may take place during rapid rotor acceleration. If the fuel flow rate is too high during acceleration, the combustor's high temperature and pressure will result in excessive back pressure, which will raise the blade angle of attack and, if it is high enough, cause stall.

The most frequent causes of compressor stalls are afterburner start and engine acceleration. "Rotating stall" is another aspect of stall. The stall zone moves from one blade to the next, and the stall cell that results rotates at 0.4 to 0.5 the speed of the rotor and in the same direction as the rotor. The adjacent blade's angle of attack increases due to the flow separation on the stalled blade. It then stops, and the process moves on to the following blade and so forth.

Because the airflow rate lowers quickly and the fuel-to-air ratio in the combustor rises when rotating stall occurs, the combustor gas temperature likewise rises quickly. The risk of rotating stall operation in a gas turbine is that either the rotor speed will drop below the self-sustaining level or the combustor gas temperature will go above the turbine's permissible limits. The engine will then need to be stopped and given some time to cool before being restarted. The design and operation of the compressor should prevent all but the tiniest amounts of compressor stall.

Other factors can also contribute to compressor stall, such as high altitude operation, which results in a minor reduction in the compressor stall pressure ratio due to the reduction in compressor inlet Reynolds number. Pressure gradients that could be present across the compressor face could diminish the stall margin by significantly shortening the stall line to produce stall. These pressure distortions may be caused by inadequate inlet duct design, insufficient removal of the inlet duct boundary layer, flying at high angles of attack or sideslip, ingesting exhaust gases from guns or rockets, moisture, ice, turbulence, and other factors. A well-designed airframe can manage several of these factors.

Increasing Stall Margin

Today's engines use a variety of techniques to either lower the running line or raise the stall line for a larger stall margin. Compressor bleed is one of these strategies. In order to increase airflow and thereby decrease the angle of attack on the rotor blades, air is bled off the compressor in this instance between two stages. But this is a needless procedure.

Utilizing variable stator vanes in between two rotors is another technique. This is advantageous for regulating the airflow velocity and angle of attack, particularly during part-power operation. Variable vanes are more difficult to operate and manage mechanically, and they do result in more air escaping from the compressor flow channel. Splitting the compressor into two or more mechanically separate rotor systems will more effectively result in greater flexibility for beginning and part-throttle settings. Typically referred to as low-pressure and high-pressure spools, respectively, each is propelled at its optimal speed by a different turbine. Thus, the two rotor spool speeds can be matched for maximum efficiency and stall margin.

The high-pressure compressor is typically smaller in weight and has shorter blades than the low-pressure compressor. Higher top speeds are attainable before the blade tips reach their limiting Mach number because the high-pressure compressor's job of compression causes the air's temperature to increase to higher temperatures than those that occur in the low-pressure compressor. The high-pressure compressor can therefore operate at a faster pace than the low-pressure compressor as a result.

The compressor can be divided into two spools to aid with engine starting. The high-pressure compressor is turned first to start the engine because it is lighter. On the negative side, the need for two separate shafts to connect the two spools and their corresponding turbines results in a large penalty in engine weight and complexity.

In more recent engines, the fan operating line is additionally adjusted according to the amount of distortion the inlet is producing and/or whether a throttle transient is required. The expanding usage of digital flight and propulsion system controls has made these methods for maximising stall margin or performance more practical.