New Report Only Deepens Mystery of Midway Runway Accident
Dec. 16, 2005 — -- The National Transportation Safety Board has just released some preliminary information on last week's runway overrun accident at Chicago's Midway Airport, but those hoping for a quick explanation will be disappointed. The report raises new issues about what went wrong aboard Southwest Airlines Flight 1248.
This, however, is the very nature of accident investigation. Let me explain.
First, the most important consideration is that there is never, ever just one cause behind a major airline accident.
An airliner's landing sequence is a series of complex steps and decisions, all made against the background of thousands of established procedures, the company and professional culture, and legal and regulatory structures.
A single pilot decision may be based on company and Federal Aviation Administration decisions and practices, information provided (or omitted) by airfield authorities, decisions (such as the dispatcher's contribution) made in distant places, or even a sudden change in the weather. In fact, virtually all of such considerations may have been involved at Midway when the Boeing 737-700 overran Runway 31C and rolled onto a highway.
The plane crashed into a car, killing 6-year-old Joshua Woods.
The NTSB's interim report raises several issues, each of which may (or may not) be a crucial contributing factor:
1. Air-traffic control authorities chose to use Runway 31C even though there was a tail wind, probably because the instrument approach procedures to guide pilots to Runway 13C, which runs in the opposite direction, were unusable due to low visibility.
This means that if Midway's tower switched to 13C, the airport would effectively be unusable. The FAA does not shut down an airport, by the way, but it can render the airfield unusable when the weather drops below legal minimums for whatever runway is in use.
2. The pilots, in conjunction with Southwest's dispatcher (who shares legal authority with the flight captain), apparently communicated several times regarding the ability to land on Runway 31C, given the difficult conditions reported by the airport. The question is whether those conditions were reported correctly and, if not, were any significant deviations the result of human error, equipment failure, or some other systemic problem (such as delayed reports).
3. The pilots listened several times to the ATIS -- the automated radio signal broadcasting current conditions at Midway -- and used the reported figures in an onboard laptop system authorized by Southwest to determine when a landing would be both safe and legal. In aviation terms, legal means in compliance with regulations and procedures.
If the crew's conclusions were wrong, did it result from operator error (i.e., someone entering the wrong data), untimely or bad information from the ATIS, a computer program glitch, or some very human failure such as misreading the result? The NTSB will need to pay close attention to why this process designed to enhance safety apparently indicated the landing would be safe and legal.
4. The NTSB reports the crew elected to use maximum autobrakes, which would seem to be a very good and conservative decision. The report found the autobrakes began delivering maximum braking as soon as the main wheels were on the ground and, in addition, the report indicates that the aircraft touched down inside or very close to the appropriate landing "zone" and had about 4,500 feet in which to decelerate and stop. Yet, post-flight NTSB analysis shows that with the tail wind, snow conditions and slipperiness -- as well as an apparent delay in deploying the thrust reverse system on the two engines -- the aircraft would have needed more than 5,300 feet to stop. Here we have a difficult problem.
For many decades, thrust reversal was never taken into account when computing how much runway a pilot needed to legally and safely stop. Yet the NTSB says its post-crash conclusion that 5,300 feet of runway was needed is based on partial usage of thrust reversers. That would mean the only safe landing of Flight 1248 required full and rapid use of thrust reversers. This is a troublesome inconsistency.
5. Perhaps the most worrisome and puzzling indication from this report is that the aircraft was on the runway for 18 seconds before the thrust reversers deployed, and while the captain could not deploy them, the first officer was able to do so. Even if the landing was only legal with the use of thrust reversers, they are normally out and functioning within four to seven seconds of touchdown -- at least 10 seconds sooner than what happened on Flight 1248.
What the report does not mention, however, is that the aircraft itself has to mechanically "know" that it is on the ground before an interlock will permit the reversers to open. That air-ground switch system is located on one of the main gear struts, and yet another NTSB focus will be whether anything was wrong, even momentarily, with that switch. A significant delay in sending an "on the ground" message to the thrust reversers could explain why the first officer succeeded. Then again, it might not.
So the information presently available about Flight 1248 raises many more questions, but it also helps focus the investigation. It is important to remember the NTSB's job is not to assign blame -- which is counterproductive to improving air safety -- but to discover every single contributing factor that could produce a future accident.
Every such contributing element has to be found and fixed, and it is clear in the case of Flight 1248 the complexity of accident means that just pointing a finger -- at the pilots or anyone else involved -- will accomplish nothing.
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