Monday, June 11, 2007

A Skewed Runway

This is a composite picture obtained from the United States Geological Survey. The top picture is a view of the runway called 1L at Washington-Dulles International Airport. Look at the tire skid pattern from the landing airplanes.

The real runway is many times longer than it is wide. The images shown here have been stretched across the width of the runway and shrunk along the length to better see the pattern of use. Aside from this mild distortion, the tire skid pattern has not been altered.

The aiming point for landing pilots is indicated by two broad white rectangular marking stripes about 1,000 feet from the end of the runway. Touchdown zone markers are groups of one, two, or three rectangular bars every 500 feet arranged on either side of the runway center line.

This runway at Washington-Dulles airport is so long (11,500 feet) that pilots need not land exactly on the aiming point for safe operation. This along with the fact that they definitely don’t want to land short of the runway accounts for the skewed tire skid pattern. Note that although the skid pattern is skewed along the length of the runway it is symmetric across the width of the runway.

The skewness along the length of the runway shows that most of the airplanes land within 1,000 feet of the aiming point, but some land, as indicated by the skid pattern, much further down the runway. The symmetry across the width of the runway is, of course, due to the two sets of wheels of the landing gear and how accurately the pilots hit the centerline of the runway.

The second runway picture shows the entire 1L/19R runway (albeit distorted to fit the page). Notice now the U-shaped distribution of tire skids, as we see the accumulated skid marks from airplanes landing from both directions.

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