You must have heard of, or even experienced, engine stalls in a car. But did you know that jet engines also suffer from stalls? These stalls, or, more specifically, engine compressor stalls, are caused by airflow instability inside the engine.
What causes a compressor stall
The compressor’s job in a jet engine is to compress the air from the intake. This compression causes the pressure of the air to increase. At the same time, it reduces the velocity of the air. The compressor assembly consists of rotor blades and stators. A stator follows each rotor. The rotors are the moving bodies, while the stators remain static.
When the airflow passes through the rotors, there is a net increase in airflow velocity, and, as it is passed onto the stators, the kinetic energy in it is converted to pressure energy. This is done by creating divergent passages between the rotor blades and the stators.
In a typical jet engine, the pressure rise in each rotor and stator couple is quite small at about 1.1 to 1.2:1. This means that, to achieve a compression ratio of, say, 20:1, several rotors and stators will be required. In early-generation engines, this was done on one single compressor turbine assembly or one single spool. This was one of the main reasons why they were often subject to stalls.
How does the stall occur? The rotor blades of the compressor are essentially small airfoils like the wings. Due to this, they require air to flow over them at an optimum angle of attack. If this angle is too small or too big, the blades can no longer maintain the smooth airflow inside the engine. The angle of attack on a rotor is generated by the compressor’s RPM and the airflow’s axial velocity.
Image: Oxford ATPL
Jet engines (particularly those with a single spool) have rotor blades that are fixed to give the best performance at a very high RPM. When the RPM is low, the angle of attack over the blade gets messed up, and the airflow inside the engine breaks down. It was not uncommon for early-generation engines to stall while taxiing on the ground, as engines run below the optimum RPM during this phase.
To give you an example of how it really works, you can think of an engine compressor running at an RPM of 100% (its optimum RPM). In this condition, it can compress the air at a compression ratio of 20:1.
This means that, as the air flows through the compressor assembly, it gets smaller and smaller in volume. As it comes out of it, it is compressed to 20:1. In this scenario, because the blades are optimized to run at 100% RPM, the rotors can gain an optimum angle of attack and the airflow remains smooth inside the engine.
Now, let’s say that we reduce the engine’s RPM to about 50%. This reduces the speed of the compressor assembly, and the air that enters it is no longer compressed to 20:1, but, say, it is compressed to about 8:1. In this case, the higher volume of air travels through the compressor at a higher speed or a higher axial velocity.
This messes up the angle of attack on the blades and causes the airflow to break down inside the engine. Similarly, if the engine is allowed to run at a higher RPM than its design RPM, say 110%, the airflow is more compressed.
This makes it travel through the compressor drum at a higher axial velocity, causing the angle of attack on the rotors to go above optimum, causing airflow to break down and resulting in a compressor stall.
The stall by itself is a partial breakdown of airflow. When the airflow completely breaks down inside the engine, it is called an engine surge. The pilots must react to a stall event promptly to prevent a surge from occurring.
Image: Oxford ATPL
Image: Oxford ATPL
Which conditions can cause a compressor stall?
As established, any time airflow over the blades occurs below or above the optimum angle, there is a breakdown of airflow which causes the compressor to stall. Here we will look at some conditions that may lead to a compressor stall.
Incorrect throttle or thrust lever handling
If the pilot opens the throttles too quickly, there is an increased mass flow rate through the engine. The faster flow rate may not allow the compressor to compress the air at the required rate, causing airflow to bunch up between the combustion chamber and the back of the compressor drum, increasing the combustion chamber back pressure. This can reduce the axial velocity of the incoming air, causing the blades and, eventually, the compressor to stall.
Distortion of airflow through the engine
It is now quite clear how important it is for the airflow to pass through the compressor at a perfect angle. If this is somehow disrupted, the compressor can enter a stall. Crosswinds can lead to this, and it can also be caused by operating the aircraft at very high angles of attack. This can also distort the airflow entering the engine.
Damaged compressor blades
If the compressor blades are eroded or damaged, they cannot compress the airflow that efficiently. This can lead to an engine stall as well. This can be caused by Foreign Object Damage (FOD), bird strikes, or even poor maintenance.
Design features in engines that reduce the risk of compressor stalls
Variable inlet guide vanes and variable stator vanes
The variable inlet guide vanes are found right at the beginning of the compressor assembly. They can move in relation to the engine’s speed to ensure the air flows correctly onto the rotor blades and stators.
When engine speed is low, the vanes are positioned at a steep downward angle, which puts in a swirl to optimize airflow. When the engine is spun at a higher speed, the angle is lessened to reduce the swirl. This reduces the risk of a stall.
Similar to the variable inlet guide vanes, the stator vanes can also be designed to move to ensure that air flows at the correct angle to the rotor blade that follows the respective stator blade.
Image: Oxford ATPL
The compressor bleeds are valves in the engine’s compressor section that open when the engine is operating at low RPM or when a sudden acceleration is demanded. The valves, when open, allow built-up air to escape, reducing the pressure at the rear of the compressor. This reduces the risk of a compressor stall.
Image: Oxford ATPL
Use of Multi spools in engines
This is something that is taken for granted these days, as most engines today have at least two spools. Previously, a single spool was used, and, as it is optimized to operate best at one single RPM, they tend to stall when the engine speed deviates from this optimum level.
Multi-spool engines consist of several spools. This means that each compressor is allowed to be run by its own turbine assembly and has no connection to the other compressors. For example, a twin-spool engine consists of a Low-Pressure compressor (LP) and a High-Pressure compressor (HP).
The LP compressor is rotated by its LP turbine, and the HP turbine rotates the HP compressor. In this configuration, the LP compressor blades are optimized to rotate at a lower RPM than the HP compressor blades. Thus, when a pilot reduces the throttles, the LP compressor RPM falls off much quicker than the HP compressor. Hence, each compressor maintains its optimum RPM at all times.
Some large modern engines are designed with three spools. They contain an LP, HP, and an Intermediate Pressure (IP) compressor, further improving the stall characteristics of the engine.
Image: Oxford ATPL
What procedures should be followed in the event of an inflight compressor stall?
An encounter with an engine stall is associated with one or more loud bangs. The bangs are similar to that of a shotgun being fired.
It may also cause fluctuations in engine parameters, cause abnormally high vibrations, and can even prompt the Exhaust Gas Temperature (EGT) to rise. From outside the aircraft, flames may also be visible from the exhaust, and, at times, from the engine’s intakes.
As soon as the engine stalls, the pilot must reduce the thrust to idle by moving the thrust lever(s) of the affected engine(s) back. This should reduce the back pressure and help to stabilize the airflow. Once the engine(s) is/are at idle, the engine parameters must be checked to see if they are sort of stable. Depending on the aircraft type, a very interesting thing can be done at this point.
Specifically, in aircraft with anti-icing systems powered by engine bleed air, the pilots can turn on the anti-ice system. This removes air from the engine compressor allowing air to flow stably.
If things look stable (engine parameters), the pilot can then slowly advance the thrust lever(s). The thrust lever should be pushed until the pilot sees a stall recurring. This establishes the engine RPM at which the stall had occurred.
The engine must then be operated between the idle point and the RPM at which the stall reoccurs. There is a chance that the engine might even fully recover from the stall. In this case, the pilots can resume normal engine operations.
In the event, the stall continues to occur or recur, or the engine parameters keep fluctuating, the engine(s) must be immediately shut down to prevent further damage to the engine, and to avoid the possibility of another surge.