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The 1997 Supplemental Environmental Impact Statement (EIS) for Sea-Tac Airport 's Master Plan estimated the community noise impacts associated with operations on all the Airport's runways, including the third runway. The noise impact estimates were based on Federal Aviation Administration (FAA) assumptions on how the runways, including the third, would be used in 2010.
Airport runways are designated by numbers based on compass headings. Each runway has two designations based on which direction an aircraft is arriving or departing. At Sea-Tac, when an aircraft is pointing to the north, the runways are designated with the number 34 (north flow). When they are pointed to the south, the designation is 16 (south flow).
Sea-Tac's three runways are further designated by their relation to each other -- left, center and right (noted next to 16 or 34 as L, C, or R) Thus, runway designations for Sea-Tac are 34L, 34C, and 34R in north flow. In south flow, they are 16L, 16C, and 16R. Therefore, the third runway is known officially as 34L/16R. The runway closest to the terminal is 34R/16L and the center runway is 34C/16C.
The Overall Runway Use table's first column -- “EIS assumptions for 2010” -- shows the percentage of total arrivals or departures that was predicted for each runway in 2010 based on the entire year of operations.
The next column -- “Actuals to date” -- shows the percentage of actual arrivals and departures since the third runway opened on November 20, 2008 for each runway. At the end of November 2009, when we have gathered a years worth of data, this column will tell us how the actual runway usage on all runways compares to the EIS 2010 predictions. Partial year totals will reflect the predominant seasonal traffic flow, i.e. during the winter months, south flow is more common, while in the summer, north flow is more common. The tables are updated every two weeks. Please note that no real conclusions about runway usage can truthfully be drawn until at least a full year of data for the three operating runways is collected.
If you are interested in the daily runway numbers, they are in the Daily Runway Use Summary table. The daily numbers are the basis for the Overall Runway Use table.