Runway calculations eyed in MDW overrun probe

 - September 25, 2006, 10:28 AM

While the NTSB is far from concluding its investigation into the fatal nighttime overrun accident involving a Southwest Airlines Boeing 737 at Chicago Midway Airport (MDW) on December 8, the Safety Board has released preliminary findings that shed light on what was going on in the cockpit and with the weather before the crash.

For example, upon touchdown, “The flying pilot (the captain) stated that he could not get the reverse thrust levers out of the stowed position. The first officer, after several seconds, noticed that the thrust reversers were not deployed and activated the reversers without a problem.” Flight data recorder information shows that the thrust reversers were activated “about 18 seconds after touchdown or about 14 seconds before [the aircraft made] contact with a blast fence.”

Despite a tailwind (varying from 100 degrees at 11 knots before the accident to 160 degrees at five knots after the accident), the Southwest crew used Runway 31C “because it contained lower landing minimums for aircraft using the ILS approach.” The Safety Board noted that if the crew had chosen Runway 13C–“the runway most aligned with the wind”–the pilots would have been unable to land because it would have been below minimums.

The NTSB said the pilots agreed with the airline dispatcher’s assessment that conditions were suitable for landing on Runway 31C. An onboard laptop computer tool into which the crew entered the wind speed and direction, aircraft weight, runway conditions and braking action data supported that position, according to the pilots.

On the way to Midway from Baltimore, “The flight was contacted twice [by the dispatcher] and the appropriateness of using Runway 31C for landing was reaffirmed during both contacts.”

Preliminary calculations show that the airplane touched down more than 2,000 feet beyond the approach end of Runway 31C, “leaving about 4,500 feet of remaining runway, and was on the runway for about 29 seconds.”

Calculations also show that, for the runway conditions–contaminated by snow–and use of brakes and thrust reverser that occurred, “the stopping distance without hitting obstructions would have been about 5,300 feet (the actual stopping distance was about 5,000 feet).” In addition, “had the airplane landed into the wind, rather than with a tailwind, the stopping distance for a landing would have been about 1,000 feet less,” said the NTSB.

The 737 went off the end of the runway, rolled through an airport perimeter fence and onto a roadway, stopping after hitting two cars. One automobile occupant was killed and another seriously injured.