Weather May Be a Factor in Crash

ByABC News
December 9, 2005, 7:10 AM

Dec. 9, 2005 — -- Without question, bad weather strains the safety buffers that make flying safe. And while it will take some time for the National Transportation Safety Board to piece together all the contributing causes that led a Southwest Airlines jetliner to slide off the end of the runway Thursday evening in Chicago, the physical conditions at Midway Airport at that time already tell a tale of reduced margins that have obviously contributed.

This does not imply that the low visibility and the presence of a major snowstorm meant that making an approach and landing was improper, given what facts were known at the time. But there's no question that when a major winter storm blows into airports such as Midway, New York's LaGuardia or Washington's Reagan National, safe landings become more demanding.

Clearly, the Southwest crew faced deteriorated runway conditions. The 6,500-foot runway had been slickened by the snowstorm in progress, and whenever a runway is anything but dry, stopping a large jet touching down at over 120 mph becomes a more demanding series of steps. In addition, 6,500 feet is roughly half the length of major runways at airports such as Chicago's O'Hare or Atlanta's Jackson-Hartfield, runways long enough to enable pilots to finesse a soft touchdown and decelerate in a much more leisurely fashion.

The physiology of such a landing is important. Airline pilots expect to fly over the threshold of a runway at approximately 50 feet above the ground and set the main wheels on the ground no further than 1,500 feet down the runway. In Midway's case, that leaves approximately 5,000 feet in which to dissipate the speed and turn off. To achieve a safe "rollout" and stop, the pilots of late-model Boeing 737s have several automated systems to rely on, including automatic deployment of large panels on the wings called spoilers.

The pilots arm the automatic spoiler deployment system on final approach and, if working properly, the system will raise the spoiler panels as soon as the tires have touched the ground and "spun up" to a sufficient speed. This is important because the spoilers help decelerate the aircraft and help "spoil" some of the lift being generated by the wings, causing the big aircraft to settle more firmly onto the runway, an action which gives the tires more of a chance for braking effectiveness.

If the automated system does not work, however, the pilot is trained to instantly pull the spoiler lever back into the deployed position and then move his or her hand to the reverse thrust levers (part of the throttle controls). With the spoilers deployed, the thrust reversers take a few extra seconds to move into position and direct about half of the engine thrust forward to help slow the aircraft. Until the reversers lock into position, though, the pilot can't rev the engines up to apply maximum reverse thrust, and even though the sequence only takes a few more seconds, the aircraft is still moving very fast and eating up runway.

Outside the cockpit -- if the snow is too heavy -- the "runway distance remaining markers" that tell the pilots how much space they have left in which to stop may not be visible. On top of that, once the spoilers deploy, taking off again (a balked landing) is usually a prohibited maneuver that over the years has proven far more dangerous than continuing the landing.

Once the reverse thrust is applied (with the nose gear lowered to the runway), the pilot meters the brakes (operated by pressing on the top of the rudder pedals) as necessary to slow and almost stop before turning off onto a taxiway at the far end. If the runway is two miles long (more than 10,500 feet), very little wheel braking is needed. But when the length is 6,500 feet -- as with all of Midway's runways -- the braking required is more pronounced. When the runway is also slick with rain, snow or (the worst case) ice, heavy braking and maximum reverse thrust is required, and a very sophisticated system known as "anti-skid" becomes invaluable in keeping the tires from simply slipping along the surface rather than providing the friction to stop.

Before the plane gets close to the runway, however, there are regular radioed reports from the air traffic control tower about runway conditions, usually including a reading called an RCR value (for Runway Condition Reading) that has to be rapidly and correctly interpreted by the inbound pilots. The slipperiness of snow itself is not a constant, and some forms and temperatures make it more slick than colder, more powdery versions. The very amount (or depth) of the snow on the runway surface is also an important factor. The job of communicating the true runway condition to the inbound pilots requires much coordination and testing in a partnership between airport authorities (who take the RCR readings), the tower operators who report it by radio, the accuracy of such reports and the degree to which a heavy snowstorm may have already changed conditions for the worse. Therefore, even if every aircraft system works correctly and the pilots land in the proper position and apply the spoilers, reverse thrust and brakes in timely fashion, it will often be difficult to accurately predict whether the rollout will take 3,000 or the full 5,000 feet.

Obviously, on Thursday there wasn't enough room to stop.

When the conditions are clearly reported as too slick for such a relatively short runway, the pilots may elect to do a "missed approach" and fly to another airport with a longer runway or just wait for the runway to be plowed.

The safety system in almost all cases leaves nothing of substance to chance, but there is no question that some of the wide safety margins present on a dry runway become compressed, and when that happens, the margins for error (in reporting the true conditions, landing at the appropriate spot with the appropriate speed, or not applying spoilers, reverse thrust, or wheel brakes rapidly enough) become much more narrow.

Again, it will take some time and the sifting of hundreds of factors and facts before the chain of circumstances that permitted Thursday's accident to occur can be fully known. But even if the pilots landed too far down the runway or touched down with too much airspeed, it is never just a simple matter of pilot error. Instead, we can look to the culmination of a complex chain of actions and decisions.

At speeds of 120 to 140 mph, coming out of a quarter-mile visibility with the whitened runway appearing suddenly just ahead gives little time for adjusting flight path, yet sometimes a rapid change in wind speed or direction can vastly complicate the equation.

The mere fact that snowstorm landings are done on shorter runways literally thousands of times a year in the U.S. (and specifically at Midway) shows that the system's failure rate is all but infinitesimal. Once the true sequence of events has been thoroughly pieced together by the NTSB, the lessons learned from this accident will be rapidly applied to prevent a repeat. But the one thing even at this early stage we can be assured of is that no single problem, error, failure or communications glitch caused the Southwest flight to slide through the fence at the northwest end of the airport.