At an airport, the pattern (or circuit in the Commonwealth) is a standard path for coordinating air traffic. It differs from "straight in approaches" and "direct climb outs" in that aircraft using a traffic pattern remain close to the airport. Patterns are usually employed at small general aviation (GA) airfields and military airbases. Most large airports avoid the system, unless there is GA activity as well as commercial flights. However, a pattern of sorts is used at airports in some cases, such as when an aircraft is required to go around.
The exception to this rule is at alpine airports, 'Altiports' where the runway is on a severe slope. In these instances, takeoffs are made downhill and landings uphill, with the slope aiding in acceleration and deceleration.
Many airfields have runways facing a variety of directions. The purpose of this is to provide arriving aircraft with the best runway to land on, according to the wind direction. Runway orientation is determined from historical data of the prevailing winds in the area. This is especially important for single-runway airports that don't have the option of a second runway pointed in an alternate direction. A common scenario is to have two runways arranged at or close to 90 degrees to one another, so that aircraft can always find a suitable runway. Almost all runways are reversible, and aircraft use whichever runway in whichever direction is best suited to the wind. In light and variable wind conditions, the direction of the runway in use might change several times during the day.
The Pilots Operating Handbook (POH) displays the maximum demonstrated crosswind component for the aircraft, this figure is based on a pilot with average experience and, in most cases, could easily be exceeded by an experienced pilot. Many pilots set their own crosswind limitations based on their skill. High-wing aircraft are more difficult to control in crosswinds compared to low-wing aircraft.
While many airfields operate a completely standard pattern, in other cases it will be modified according to need. For example, military airfields often dispense with the crosswind and base legs, but rather fly these as circular arcs directly joining the upwind and downwind sections.
Aircraft are expected to join and leave the pattern, following the pattern already in use. Sometimes this will be at the discretion of the pilot, while at other times the pilot will be directed by air traffic control.
There are conventions for joining the pattern, used in different jurisdictions.
Similarly, there are conventions for departing the pattern.
There is also a procedure known as an "orbit", where an aircraft flies a 360° loop either clockwise or anticlockwise. This is usually to allow greater separation with other traffic ahead in the pattern. This can be the result of a controller's instruction. If at the pilot's initiative, the pilot will report e.g. "(tail ID number or flight number) making one left-hand orbit, will advise complete".
In cases where two or more parallel runways are in operation concurrently, the aircraft operating on the outermost runways are required to perform their patterns in a direction which will not conflict with the other runways. Thus, one runway may be operating with a left-hand pattern direction, and the other one will be operating with a right-hand pattern direction. This allows aircraft to maintain maximum separation during their patterns, however it is important that the aircraft do not stray past the centerline of the runway when joining the final leg, so as to avoid potential collisions. If three or more parallel runways exist, as is the case at Bankstown Airport in Australia, then the middle runway(s) can, for obvious reasons, only be used when either a straight in approach is used or when the aircraft joins the pattern from a very wide base leg.
An airfield will define a circuit height or pattern altitude, that is, a nominal level above the field at which pilots are required to fly while in the circuit. Unless otherwise specified, the standard pattern height is 1000 ft AGL (above ground level), although a pattern height of 800 feet above ground level is relatively common. Helicopters usually fly their pattern at 500 feet above ground level. Extreme caution is exercised by pilots flying the published traffic pattern altitude as this may contribute to mid air collisions.
A pilot undergoing training will often fly many patterns, one after another. Usually, each landing is followed immediately by a take off and further pattern; this is called a touch and go, or roller.