A compressor stall sounds pretty bad from inside the a/c, especially to the pax who may travel for 20 years and never get one. Speaking as pax and not technical or air crew, I understand that it happens like this: [FC + techs pls correct!!]
Air is rushing into the front of the engine, where it is compressed by a series of whirling fan blades and then enters the combustion chamber. The first blade is what you can see when the a/c is on the ground.
Sometimes, the airflow into the running engine is disrupted and this can be for any number of reasons, such as a sudden change in air pressure and of course the engines are designed to accept this and are tested to see that they continue to operate smoothly. But, once in a while, the change in airflow is very great and the result is that the flow of air into the engine is broken.
One example that was given to me was this: Think of water flowing out of a dam, through a pipe. Sometimes the pipe cannot take all of the water at once and the flow is broken and the water is said to 'back up'. Shortly, the overwhelming pressure of water in the dam will overcome this and flow will be re-established.
For water into a pipe - think air into an engine! This means that the flow may actually stop for a moment and it is said that the air has 'stalled' (stopped moving) and it happens at the compressor, so it is a compressor stall.
Now the engine finds that it has no air but is still pumping fuel into the combustion chamber! So then there is a loud BANG! and you get a small flame out of the back. This is why the term 'back-fire' is sometimes used to explain the situation to pax.
Usually the engine will recover itself as the airflow returns to normal a few seconds later and sometimes the flight crew will need to make adjustments to it. If the engine does not return to normal function, it will be shut down. (The topic of what happens then is for another thread!!)
I experienced a compressor stall when departing HKG (the old Kai Tak) in a fully loaded 747-400 bound for the UK and the whole machine shuddered. It was just a couple of seconds after rotation but the other three engines were more than capable of taking us up.
So it sounds bad but it is nothing to worry about.
