Definition
An aerodrome circuit is defined by the CAA/FAA as:
"A pattern around which aircraft fly when arriving at an airfield, usually rectangular in UK but not necessarily elsewhere. The circuit (known as the pattern in USA) is aligned with the active runway and may be either left- or right-handed. Dead side is the opposite side of the circuit pattern in operation from which arriving aircraft join for landing."
Explanation
As the definition suggests, the aerodrome circuit (henceforth referred to as the circuit) is a pattern around which aircraft approach an airfield. The circuit is an elongated rectangle, with the long sides being over and parallell to the runway.
Why do pilots need a circuit?
The primary use for a circuit is the practising of landing and taking off. It would be extremely inefficient if student pilots only landed after each hour long flight as landing is a true art1. With circuits a pilot can take-off as with a normal flight, then fly the circuit which brings him back to the approach for landing. The student can then attempt to land but once on the ground does not brake, instead applying full power and taking off again to flying the circuit once more. In this way a student can make up to 10 take-offs and (hopefully) landings in the hour of flight time instead of one.
The circuit is also used when aircraft approach an airfield to make only one landing. In this case only the downwind, base and final legs are used2. This allows the air traffic controller to organise aircraft on the approach and maximise efficiency aswell as giving pilots a good idea of where other planes are in relation to them.
Flying the circuit
A circuit is started when the pilot powers up the engine on the departure runway. When accelerating down the runway the aircraft is entering the circuit. Once airborn the aircraft flies straight ahead on the climb leg. At 500 feet the aircraft will turn left (or right in a right hand circuit) and continue to climb to 1000 feet, the standard circuit height.
At 1000 feet the aircraft will again turn left so that it is flying parallell with the runway of departure. This is callled the
downwind leg. On the downwind leg the pilot informs
ATC of his intentions in the following manner:
Pilot XX: Pilot XX, Downwind, Touch and go
ATC: Pilot XX, Touch and go, Call Final
Pilot XX: Call Final, Pilot XX.
The pilot then carries out his
downwind checks. These vary depending on the aircraft being flown, but generally include checking the brakes are off, the gear are down and that you have fuel etc.
Approximatly 1 minute after flying past the threshold of the runway, the pilot begins a 90o to the left(or right) and is on base. At this point the pilot begins to lose height and try to get the aircraft ready to hit the floor.
When able, the pilot turns the aircraft to be on the runway centre line. The plane is now on finals. The pilot informs tower in this manner:
Pilot XX: Pilot XX, on finals, touch and go
ATC: Pilot XX, clear to land, runway XX, surface wind XX
Pilot XX: Cleared for touch and go, runway XX, Pilot XX
The pilot then flies perfectly down to the runway for the perfect landing, he hopes.
Sources and references
- www.caa.co.uk
- www.pakair.com
- AFE Air Law, Navigation and Communication textbook
- AFE Flying Training Manual
- Personal Flying expereince
Footnotes
1. I can say this with experience as a student pilot who, "arrives" more than lands gracefully.
2. See "Flying the circuit" later in node.