When you drive on a road, there are of course signs that tell you where to drive, when to stop, and what precautions to take. Similarly, pilots need people, tools, and signs to guide them in take-offs and landings. As for people, you already know the importance of air traffic controllers. Approach controllers and flight coordinators, for example, indicate to aircraft when and where to transit in order to maintain smooth traffic flow around the airport. Inside the plane there are also instruments and rules for orientation in the air. And again at airports, apart from markings on the runway there are also other elements which aren’t visible but still very necessary for proper traffic flow. These are part of what is known as the airport traffic circuit.
Now, when flying the vicinity of or into/out of an airport, decisions on how to proceed are normally left to the air traffic control tower, with a procedure divided into two parts. Approach manoeuvres are transitional between the flight itself and take-off and landing manoeuvres. Whereas the term “traffic circuit manoeuvres” refers to those done in the vicinity of the airport after take-off or before landing, a set of predefined trajectories which manage traffic around an airport, following a specific sequence which rarely varies.
Sections of the Traffic Circuit
Imagine a rectangle, but with rounded corners (because an airplane, when making a 90-degree turn, makes a quarter of a circle). The runway is in the central part of one of the long sides of the rectangle. Taking this into account and eliminating this runway from the set of segments, these are the five segments which make up the traffic circuit:
The Headwind or Upwind Leg
The first section after take-off until the first 90º turn, this is the distance from the end of the runway to the turn (if to the left, all turns in the traffic circuit shall be to the left to form the rectangle; the same applies if the circuit is made to the right). It’s referred to as headwind/upwind because in theory an aircraft always flies against the wind, so it references the direction the wind is blowing with respect to the runway.
The Crosswind Leg
This is the one that goes from the first 90° turn to the second 90° turn, and forms the short side of the rectangle. During this leg, the wind hits the aircraft from the side on the outside of the circuit and the aircraft rises to approximately 1,000 feet, but the altitude can depend on many factors.
The Downwind leg
This third section forms the side parallel to the runway. After turning a total of 180°, you get to the tailwind leg. This is the section of the traffic circuit that is used for both take-off and landing. An altitude of 1,000 feet is maintained.
The Base Leg
The fourth leg corresponds to the other short side of the rectangle, i.e., it runs parallel to the crosswind leg and is part of the landing circuit and therefore the descent.
The Final Approach
This is the remaining distance before the runway.
Other Points of Interest
Looking at the section names of the traffic circuit, you may wonder if the wind always blows in the same direction. But clearly not. These are names that have been assigned to each leg and serve as a guide for communications around the airport.
Speaking of communication, the traffic circuit is identified by the runway number where planes land or take off from (pilot-control tower interactions will use the runway number as a reference).
The traffic circuit, as expected, is not exited or entered arbitrarily. There are a series of rules, such as entering and exiting the circuit at a specific altitude of each leg (unless the controller says otherwise); joining the downwind leg at a 45º angle; and exiting the circuit at the same angle on the downwind or crosswind leg. These are some of the most basic rules (which of course can be modified by the air traffic controllers) and are aimed at avoiding accidents and improving airspace visibility.
It’s possible that due to traffic, weather, etc. the traffic circuit may not be entered or exited directly, so the control tower indicates that a 360º turn must be performed, always subject to a standard (turning 3º per second). The pilot may also be asked to lengthen, widen or shorten the traffic circuit on sections if conditions require it.