This article review common assumptions made by members of fire service and airline industry that can prove fatal.
"Southwest 1248, runway three one center cleared to land, wind zero nine zero at nine, braking action fair to poor. Southwest Airlines (SWA) 1248, a Boeing 737-700, prepared for final approach and landing at Chicago's Midway airport. SWA 1248 was nearly two hours late as a result of weather delays in Chicago that kept the airplane waiting on the ground in Baltimore.
At 1912 hours, SWA1248 was lined up on the Instrument Landing Approach signal that would place them over the threshold of runway 31C. Landing conditions were not the best. Snow had been falling for six hours and visibility was a half-mile in moderate snow and freezing fog with a broken ceiling at 400 feet above ground level. Despite the fact that snow removal crews at Midway had been working steadily for hours, the runway braking conditions were reported as fair for the first half and poor for the second half of the runway.
Fair Versus Poor
Just as firefighters routinely do, the flight crew used information available to them, often based on assumptions that would assist them in making decisions. If the assumptions were incorrect, the resulting decisions could be faulty and potentially deadly. They were using alaptop personal computer (PC) to make a decision concerning their ability to safely land on 31C. The laptop program was not designed to take a mixed runway report (fair on the first half and poor on the second) to develop a calculation regarding how many feet it would take for the aircraft to come to a full stop under the prevailing conditions.
Though not happy with the results derived from the runway calculation, the flight crew agreed that they would pick up several hundred feet of runway margin when reverse thrust was applied. This maneuver would apply engine exhaust to slow the aircraft in combination with conventional braking. Reverse thrust was applied by moving two levers on the throttle console first to a neutral position and then to an engaged one.
"Brakes, Brakes, Brakes"
Traffic into Midway was "stacked" and in the 15 minutes before 1248 landed they listened to four other company flights touch down and roll-out on 31C. Hearing other SWA flights make a successful landing no doubt influenced the crew to attempt a landing in conditions far less than ideal. They also heard reports of deteriorating runway conditions.
SWA 1248 descended through 1,000 feet and picked up the runway dead ahead. They crossed the numbers and glided to a firm touch down about 1,250 feet past the threshold. The pilot was flying the aircraft and the first officer was monitoring. Both would later report being focused on the operation of the automatic braking system since they were using it for the first time. The pilot attempted to move the reverse thrust levers into position and indicated that he had difficulty in doing so. He also said that he felt the aircraft's anti-skid system engage and that the aircraft seemed to pick up speed. The first officer shouted "brakes, brakes, brakes", pushed the pilot's hand away from the reverse thrust levers and successfully engaged them. Manual braking was also applied at this point. Twenty seven seconds had passed since touchdown.
Still moving at 53 knots the aircraft departed the overrun, rolled through a blast fence, an airport perimeter fence, and onto a roadway where it struck a vehicle, killing one passenger and seriously injuring another.
Assumptions and Decisions
As is the case with all significant transportation incidents, the National Transportation Safety Board (NTSB) conducted an investigation. The NTSB found that the flight crew was certified, qualified and unimpaired. Similarly, the aircraft was properly certified, equipped, and maintained. Finally, the airfield was operating appropriately for the weather conditions.
And yet, a fatal accident occurred. Why?