Saturday, January 28, 2006

NTSB suggests banning specific landing procedure

When a Southwest Airlines flight overran the runway at Chicago Midway I cautioned that everyone should wait for the NTSB to finish its investigation before concluding what happened. In the meantime, the investigation began to focus on the thrust reversers of the aircraft and whether they had deployed as expected. Today, we have a report that offers more detail and a suggestion from the NTSB.

::::::::The National Transportation Safety Board said the pilots should not have factored in the plane's thrust reversers _ which help slow the plane _ when they estimated how long it would take to stop during a December snowstorm at Chicago's Midway Airport.

The agency said the Southwest Airlines jet touched down with about 4,500 feet of runway remaining, but snowy conditions and other factors meant the plane needed about 5,300 feet of runway to stop.

According to flight recorder data, the thrust reversers did not deploy until 18 seconds after landing, the report also said. That's more than 10 seconds beyond normal deployment, according to aviation experts.
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Calculating in the effect of the thrust reversers is actually not a common procedure. Many aircraft manufacturers simply don't allow it when they publish the performance tables for their planes. There are some models, however, that do still permit it and this specific model of the Boeing 737 was one of those. The critical factor here is in the time it took the reversers to deploy. If you've read my other posts, you know that the speed the plane was moving at touchdown would have allowed roughly 30 seconds to get the plane to a halt. If the reversers didn't deploy until 18 seconds into the landing, then you've got 12 seconds of reverse thrust available to bring the plane to a stop. That clearly wasn't enough. So what kept them from deploying?
::::::::Thrust reversers typically deploy automatically _ six seconds or less after a plane's wheels touch the runway, said Paul Czysz, professor emeritus of aerospace engineering at Saint Louis University.

In this case, a computer used to calculate landing specifications "assumed they would go on immediately," he said.

Investigators haven't determined whether the pilots tried to deploy the thrust reversers manually, NTSB spokesman Keith Holloway said.

The flight's captain told investigators he did try immediately upon landing but could not. The first officer said he deployed the reverse thrusters after he noticed they weren't working, the report said.
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So the system designed to drop the reversers on landing failed to fire automatically. The question of "why?" is the one that needs to be answered and there's still nothing to say what the reason was one way or another. OK, next comes the question of what the crew did in response to that failure and was it appropriate? The captain says he tried to deploy them manually immediately upon landing. I assume he actually meant "immediately upon seeing they didn't pop automatically." So, that's about 6 seconds into landing. Figure it takes a second or two to 1) realize they aren't coming down on their own and 2) actuate the reverser control. We're up to 8 seconds or so, which leaves about 10 seconds between that attempt and the first officer's successful deployment. So, the question there is, first, did the captain actually attempt to deploy them? Did he hit the control and it didn't work? Did he mistakenly actuate a different control? Was there something going on in the system that failed to respond to the captain's attempt but corrected in time for the first officers? Are there 2 separate controls for the reversers (1 for the captain, 1 for the F/O) and the captain's fail? If so, why?

You see that an investigation is a complicated thing and you need to answer all these questions before you can conclude what caused the accident. Recall that your conclusions are going to guide the actions of pilots everywhere from this point forward, so you need to be right the first time. To say nothing of what a finding of pilot error will do to the careers of the pilots of that plane. And for what it will do for the souls of the family of that child who died.

Tread carefully, boys. As always I'll pass along more when I know more.