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CWSU Aircraft Compression

     Compression may occur in moderate to strong wind conditions when the Air Route Traffic Control Center (ARTCC) controllers hand-off the aircraft to the Terminal Radar Approach Control (TRACON). The TRACON takes a flow of aircraft that are lining up and preparing for approach to the arrival airfield. The compression is not due to miscommunication but is directly related to the wind flow relative to the stream of air traffic. One way to think about it is driving along the interstate at 65 miles per hour and as you crest the next hill the traffic has slowed down to about 45 miles per hour. All of a sudden the spacing between you and the rest of the vehicles is greatly reduced.

When aircraft begin their descent they the ARTCC will decrease their altitude to flight level (FL) 250 and work them down to an altitude of roughly 10,000 feet before handing them off to the TRACON. The following example shows that when the difference of wind speeds between FL250 and 10,000 ft is 40 knots or greater traffic it creates the potential for congestion. Much like the car on the interstate example, the aircraft in the stronger tailwind can easily overtake the aircraft now with a head wind or a lighter tailwind.

A graphic illustrating the potential for air traffic compression along the western ARLIN approach to Phoenix International Airport.  The quote from the area supervisor is as follows: Compression on approaches creates problematic turns and possibly results in overshooting turns.  Additionally if PHX is on an east flow with a strong tail wind miles in trail becomes an issue for handing off aircraft.  Yet, some advantages can be using the winds when possible to slow aircraft by turning them into the wind. -Steve Wright, Supervisor Southwest Specialty, Albuquerque, NM; 
Air Route Traffic Control Center

Compression can also be a problem in the TRACON, especially if there are significant head and/or tail wind differences between 10,000 ft and the airfield approach. This is why Center Weather Service Meteorologists (CWSU) briefs the ARTCC controllers and provides TRACON compression graphics. This is an example of the meteorologists providing "in-house" verbal and weather impact graphics as well as internet graphical support to the National Airspace System. A TRACON compression example graphic is shown below for the Philadelpia TRACON:

A screen capture for the Philadelphia TRACON compression forecast for altitudes 6000 ft and below