Compressor Stalls

I'd heard the A320 that landed in the Hudson River (notice I said landed, not crashed; as it was definitely a forced landing, but a controlled one!), might have had compressor stalls two days prior. When Mom asked me about that I sat down with Mike to see if I could explain it in an email. Then I figured why not post it for others?

So, in case you're wondering whether a compressor stall would have anything to do with a bird strike - the short answer is no.

And while I'm on the Airbus incident, the pilots certainly did do a great job and it is nice to see that they are reacting to the attention as professionals. It reflects well upon the aviation industry as a whole. Good to get positive media attention. But make no mistake, airline pilots (and charter and corporate, etc.) receive training, called "recurrent" every year. Airline pilots also take a check ride every six months. So their job is on the line every six months with an FAA-designated examiner. Anyway, back to the bird strike and the whispers going around about earlier compressor stalls

It seems the media is reporting "previous compression problems" which is something I have not heard was being investigated. I have heard however that this particular airplane had experienced compressor stalls two days prior to the bird strikes. But the two are not related and a compressor stall does not bring down an airplane. Here's a little bit about jet engines:

Compressor stalls happen in low power settings or going in reverse (reverse thrust is used when landing).

First of all, anytime you hear the word "stall" related to aviation, it is most likely going to refer to aerodynamic stalls. An aerodynamic stall is disruption in air flow that affects lift. We practice stalls all the time. But that refers to the airflow over the wings. We slow the airplane down to idle and as the speed slows, at some point lift is no longer generated because air is not flowing over the wings at the speed necessary to generate lift. To recover we lower the nose, add power and get lift back.

In a jet engine, compressors are used to compress the air prior to mixing with the fuel. It is regulated by computers. Air goes through various "chambers" or "stages" reducing the space, compressing more and more as it is moved through. Picture a shaft that turns little fan blades. Several sets of fan blades lined up down the shaft, descending in circumference so that as air passes from one to the next, the circle of fan blades is smaller and smaller. These are called axial compressors. After the air has compressed through this series of axial compressors, it then enters a centrifugal compressor. Centrifugal force. Spinning so that the air goes out to the sides, making room for more air, thus compressing even more just before mixing with fuel.

Now, at the point where the air is just at the end of axial compressors and just about to enter the centrifugal compressor, at low speed or at reverse thrust - and remember these are run by computers - sometimes the computer doesn't keep up with the exact air flow. Now there is a relief valve there to let out air that just doesn't fit. (Picture filling up a trash bag. There's only so much room. Fill, fill, fill, and pretty soon it's gonna pop.) Well, the air is forced through compressors, more, more, more, and a computer is supposed to open the relief valve when there is too much air. However, if the air gets to be too much before the computer can get the valve open (could be just a split-second), you will hear a "bang" noise. Basically, its a sonic boom; a shock wave is created. Nothing more.

Once the computer catches up, the valve opens, everything goes back to normal. Normally no damage done, just need to adjust the computer and possibly an adjustment to the valve.

When they hit the birds (about 90 seconds after take-off) the engine was turning at about 20,000-30,000 RPMs and when the birds hit those same axial compressor fan blades, the blades disintegrated. Compressor stalls don't happen at those speeds. They happen at low speeds when the computer is trying to regulate the valve in a quick-changing environment.

Let me know if that explanation is clear enough.

Mike assisted me with this explanation, but I have studied jet engines a little bit, some in my classes and some with my private tutor. When we visited the American Airlines museum in Dallas I was able to see a cut-away of a jet engine. That was really helpful to my understanding.

I suspect that non-aviation media has misunderstood "compressor stall" and has translated that to "compression problem". I haven't heard a thing about that airplane having any compression problems.