Understanding the Circuit

Understanding the Circuit

By Tom Parker

Purpose
Often times, the airspace immediately above an airport can get quite busy. To enable orderly flow of traffic around the airport and to prevent conflicts between aircraft, a standardized traffic pattern was established. Understanding this pattern will better help you determine the flow of traffic around an airport and enter that flow when the time comes.

The Elements of the Circuit
The circuit is a rectangular path around the active runway. It is composed of five elements: upwind, crosswind, downwind, base, and final as shown in the illustration below. The direction of the traffic flow is usually determined by the wind such that takeoff and landing are into the wind. Because the winds my not be straight down the runway, it may be necessary to adjust your aircraft heading such that you achieve a rectangular ground track. (ie: "Crabbing") 

The size of the pattern is determined by the speed of the aircraft. The faster the aircraft, the larger the pattern should be. For smaller GA type aircraft, the downwind leg should be ½ to 1 mile from the runway. For larger aircraft, this pattern can be up to 5 miles. It should be sized so that you can make a safe, gradual descent through downwind, base, and final.

Upwind
The upwind leg is that leg immediately following takeoff and is aligned with the runway heading. It starts from the point of takeoff and ends at the point where you make the turn to crosswind, and follows that line created by extending the runway centerline. 

When flying in the pattern, you must determine the point at which to make the turn to crosswind. The guidelines are to make sure that it?s beyond the departure end of the runway and that the aircraft is within 300 feet of the traffic pattern altitude (TPA).

Crosswind
The crosswind leg is that leg following the upwind leg. It is perpendicular to the runway heading. It starts at the point you finish your turn from upwind, and ends at the point you begin your turn to downwind. Upon reaching the traffic pattern altitude and when in the correct position, the pilot should begin a level turn to downwind.

Downwind
The downwind leg is that leg immediately following the crosswind leg. It starts at the point you finish your turn from crosswind and ends at the point you being your turn to the base leg. It is parallel to the runway and flows in the opposite direction of your intended landing. It is during this leg that you complete your landing checklist. In many cases, it?s also the time that you begin your descent. As a general rule of thumb, you begin your descent when abeam the arrival end of the runway. (ie: ?abeam the numbers?)

When flying downwind, you must determine when to begin your turn to base. General guidelines are to begin your turn when the line between your aircraft?s position and the arrival end of the runway form a 45? angle. If another aircraft is ahead of you in the pattern on final, you should extend the downwind leg until the line between your aircraft and the other aircraft form a 45? angle. You must also consider the wind conditions. You should ensure that you turn at a point such that you can achieve a safe, gradual descent to the touchdown point.

Base
The base leg is that leg following the downwind leg. It begins at the point you complete your turn from downwind and ends at the point you begin your turn to final. It is perpendicular to the runway heading. Having started the descent on downwind, this is a descending leg.

Final Approach
Final Approach is the last leg of the pattern. It?s that leg immediately following the base leg. It begins at the point you complete your turn from base and ends at the point your aircraft touches down on the runway. It follows that line created by extending the runway centerline. This is the most critical leg as it?s where you much continually judge your height and airspeed to touchdown at the desired point on the runway.

Determining the Pattern Altitude (TPA)
The standard Traffic Pattern Altitude (TPA) for small GA aircraft is 1000? AGL. For turbojets, the standard is 1500? AGL. However, the TPA is set by the airport and can vary on occasion. To find out the actual altitude, you should reference an Airport / Facilities Directory (A/FD) which contains information pertinent to that airport or search online to find this information. It?s readily available in several locations.

Determining the Pattern Direction


Entering the Pattern