Sunday, December 11, 2005

Southwest flight: thrust reversers delayed deployment?

Some reports are coming in that federal investigators have found that the thrust reversers on Southwest flight 1248 that crashed in Chicago did not immediately deploy. While they state that they haven't determined if such a delay played a part in the crash it would be, if true, a serious problem. Modern jets land at a pretty good clip. They have to because if the plane slows too much on approach, it simple drops out of the sky. The minimum speed to maintain flight on many of these aircraft is over 125 miles per hour. Doing the math, that's about 2 miles per every 60 seconds and on a runway the size of Midway's longest one, that gives the crew about 30 seconds before running right off the end. The physics of that dynamic with a loaded aircraft simply do not allow the plane to stop by itself in that space.

Planes, of course, have brakes on their tires and those can do quite a bit in bringing the aircraft to a halt. They can't do it alone, however, and trying to do so can wind up setting the brakes on fire. Then, of course, there's the problem with the conditions on that particular day. Locking up your wheels on a surface that has compacted snow and ice is a bad idea every one of us who drives a car is well familiar with.

Thrust reversers were invented and installed on jet aircraft as the planes got bigger as a way to slow the plane down and get a larger aircraft into a given airport's runway. Whether they're clamshell types or "cascades", the job is the same: to direct the jet's thrust forward and cut the plane's forward momentum. The pilot has considerable control over how much and how long the thrust is applied and they can actually drop from pretty large planes into some relatively small runways. Midway's, for example, are plenty long enough for normal operation and have been just fine for literally thousands and thousands of flights. The reversers must, however, work and work on command.

If this finding pans out, the obvious question will be what kept them from deploying? Was it that the pilots simply didn't activate them in time? Did they try to activate them and they simply didn't work? If that's the case, what kept them from working? Was it the conditions (meaning: did ice form somewhere that blocked the deployment)? Was it mechanical? Was there an electrical/software problem? Assuming we get the answers to all that, was there any reasonable way that the crew could have registered the problem in time to simply retract the reversers, hit the gas and take back off again? Going around for another approach would have been inconvenient, to be sure, but it beats the alternative. (Note: I am asking if there was a reasonable chance of that. It's possible that the problem simply appeared with no time left to touch-and-go.)

Again, I'm waiting for the findings. Will advise.