Each day, approximately 50,000 aircraft move through the US National Airspace System. Air traffic controllers (ATCs) prevent collisions between aircraft operating in the system and provide the "safe, orderly and expeditious movement" (1) of aircraft between one point and another.

In order to prevent these collisions and maintain safety, ATCs keep aircraft separated from each other by adhering to different rules. This article will explain basic radar separation procedures. The most common three types of separation used in a radar environment are vertical, lateral (side to side), and longitudinal (one in front of another).

VERTICAL

ATCs assign altitudes for aircraft to maintain using two different intervals of distance. The first interval is 1,000 feet between all aircraft altitudes 28,000 feet or below. For example, an aircraft flying west to east would be assigned 20,000 feet. A second aircraft flying east to west could be assigned 21,000 feet. With that separation, these two aircraft can pass directly above and below each other safely. The second interval is 2,000 feet between all aircraft assigned altitudes 29,000 feet and above. Therefore, an aircraft flying west to east could be assigned 29,000 feet. A second aircraft flying east to west could be assigned 31,000 feet. With that separation, these two aircraft can pass directly above and below each other safely.

There is one more factor ATCs must comply with when they use altitude separation; the direction the aircraft are flying. All aircraft heading north or east are assigned odd numbered altitudes. All aircraft heading west or south are assigned even numbered altitudes. A common mnemonic for this rule is "knee-odd, swee-ven."

Since ATCs are required to maintain a minimum of 2,000 feet between aircraft at and above 29,000 feet, the "knee-odd, swee-ven" rule is modified slightly. North and eastbound aircraft are assigned 29,000 feet, 33,000 feet, and so on. West and southbound aircraft are assigned 31,000 feet, 35,000 feet, and so on.

LATERAL

ATCs are required to keep all aircraft at least three or five miles apart if the aircraft are going have less than the required altitude minima. ATCs that control airspace immediately surrounding airports are allowed to use three miles while ATCs that control the airspace between airports must use five miles. This is where radar begins to be valuable.

One way ATCs ensure lateral separation is by assigning vectors to aircraft to make them pass side by side with the correct amount of miles between them.

LONGITUDINAL

In the radar environment, longitudinal separation is closely related to lateral separation. The difference is that ATCs maintain the required distance between aircraft traveling in the same direction, one after the other. ATCs can assign each aircraft a certain speed to maintain so that a faster aircraft does not creep up on a slower aircraft and lose the required separation minima.

These are the three basic tools that ATCs use to ensure the safety of all aircraft under their control, but there are many more. It takes years to train as an air traffic controller. By complying with the above safety requirements, ATCs successfully direct the movement of thousands of aircraft safely each day.

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(1) From FAA Order 7110.65 also known as the ATCs bible.

http://www.fly.faa.gov/Products/Information/infor mation.html Link to a screen shot showing over 5,000 aircraft operating in the NAS at one time.

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