Airfield traffic circuit

Airfield traffic pattern

An airfield traffic pattern is a standard path followed by aircraft when taking off or landing.

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.

Wind direction

Pilots prefer to take off and land facing into the wind. This has the effect of reducing aircraft speed over ground and hence reducing the distance required to perform either maneuver.

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.

Layout

Traffic patterns can be defined as left-hand or right-hand, according to which way the turns in the pattern lie. They are usually left-hand because most small airplanes are piloted from the left seat (or the senior pilot or pilot in command sits in the left seat), and so the pilot has better visibility out the left window. Right-hand patterns will be set up for parallel runways, for noise abatement or because of ground features (such as terrain, towers, etc.). Helicopters are encouraged, but not required, to use an opposite pattern from fixed wing traffic due to their slower speed and greater maneuverability. Because the active runway is chosen to meet the wind at the nearest angle (upwind), the pattern orientation also depends on wind direction. Patterns are typically rectangular in basic shape, and include the runway along one long side of the rectangle. Each leg of the pattern has a particular name:

  • The section extending from the runway ahead is called the departure leg or upwind leg.
  • The first short side is called the crosswind leg.
  • The long side parallel to the runway but flown in the opposite direction is called the downwind leg.
  • The short side ahead of the runway is called the base leg.
  • The section from the end of base leg to the start of the runway is called the final approach or final.

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.

Procedures in the pattern

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.

  • In the United States, aircraft usually join the pattern at a 45° angle to the downwind leg, abeam midfield. They may also join straight in along the final leg, if there is no conflict with other traffic.
  • In Canada, aircraft at uncontrolled airports usually cross the airport at midfield at pattern altitude from the upwind side, turning onto the downwind leg. At controlled airports, the tower typically directs aircraft to join the downwind leg, base leg, or straight in to the final leg.
  • In Europe, aircraft usually join the pattern at a 45° angle to the downwind leg, in the beginning of the downwind leg.
  • In South Africa, aircraft joining uncontrolled airfields will typically join 2,000ft AGL overhead the airfield to observe the windsock and signal square and then descend on the 'dead side' (the right side of the field where a left-hand circuit is in force and vice versa) to the circuit altitude, after which it will cross upwind threshold to join on the downwind leg.
  • Fast aircraft, for example military jets, may enter the pattern with a run-and-break. The aircraft flies at speed along the final leg, and makes a sharp, high-G turn above midfield to lose speed and arrive on the downwind leg at pattern altitude and in landing configuation.

Similarly, there are conventions for departing the pattern.

  • In the United States, aircraft usually depart the pattern either straight out along the runway heading, or with a 45° turn in the direction of the crosswind leg.
  • In Canada, aircraft usually depart straight out along the runway heading until at circuit altitude, at which point they may turn as desired. At controlled airports, the tower typically gives instructions for what turn to make on departure.

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".

Contra-rotating circuit patterns

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.

Altitudes

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.

Benefits

The use of a pattern at airfields is for air safety. Rather than have aircraft flying around the field in a haphazard fashion, by using a pattern pilots will know from where to expect other air traffic, and be able to see it and avoid it. GA pilots flying under Visual Flight Rules (VFR) will not be separated by air traffic control, and so the pattern is a vital way to keep things orderly. Although, at tower-controlled airports, ATC will provide traffic advisories for VFR flights on a work-load permitting basis.

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.

Helicopters

Helicopter pilots also prefer to land facing the wind and are often asked to fly a pattern on arrival or departure. Many airfields operate a special pattern for helicopters to take account of their low airspeed. This is usually a mirror image of the fixed-wing pattern, and often at a slightly lower standard height above surface level; as noted above this altitude is usually 500 feet above ground level. However due to helicopters' unique maneuverability, helicopter pilots often choose not to enter the pattern, and make a direct approach to the helipad or apron they wish to land on.

References

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