Aviation Topic of the Week
By Michael Oxner, November 2, 2003


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This week's topic:
Airport Operations Part 3b: The Traffic Circuit

Well, we've taxied out and taken off. Now we're going to look at the traffic pattern around airports, also known as the Traffic Circuit, and we'll look briefly at landings as a side topic, since these are part of an approach.

Definition
Purpose of the Circuit
Parts of the Circuit
Rules
    Radio Calls in the Circuit
    Out of Wind Patterns
    ATC Instructions
        ATC Directions in the Circuit
        Entering the Circuit
        Sequencing
Continuous Circuits
    Possible Operations
    "Cleared for the Option"
    Notes
    Advance Notice of Intent
ADDENDUM

Definition

The AIP RAC 4.3 is our primary reference for this subject. While not offering a specific definition, the traffic circuit can be considered to be a pattern, or a series of paths to be flown while in the vicinity of an aerodrome while approaching for the purpose of landing.

Purpose of the Circuit

The primary purpose of the traffic circuit is simple: To standardize the methods of approaching a runway to land, so that each aircraft is aware of the likely position of other aircraft in the vicinity of an aerodrome.

Parts of the Circuit

Have a look at the following diagram of the Standard Traffic Circuit. This is scanned directly from the AIP RAC 4.3, Figure 4.1.

Traffic Circuit Diagram

Each "leg" of the traffic pattern has a name, and they all basically make sense. The Downwind Leg, for example, has the plane flying with the wind and opposite to the direction of the landing. This, of course, assumes the aircraft circles the airport to land into the wind. This isn't always the case, even though the name of the leg doesn't change. The names of the legs and placement is the same for each runway, whether aligned into the wind at any given time or not. You'll also notice this traffic pattern consists mostly of left hand turns. I'll talk a little more about this rule, as well as others in the next segment.

Rules for the Traffic Pattern

The notes that accompany the diagram above from the AIP state that the traffic pattern depicted is normally flown at 1,000 feet Above Aerodrome Elevation. There are circumstances at some aerodromes that don't permit this altitude, such as nearby terrain or obstacles like towers. Some of these airports have modifications to the standard pattern, such as a slight bend in the downwind leg, for these reasons. Any variances to the paths or to the altitudes would be published in the Canada Flight Supplement in the entry for the airport in the PROcedures section. Unless otherwise stated, the left hand pattern depicted above shall be flown. Also, in some circumstances, the left hand pattern isn't the best option, and the reverse of the diagram above applies. In this case, right hand circuits are specified, typically for only one end of the runway, while the other keeps the left hand pattern. This has the aircraft over the same area, and away from the same area, when using either end of a runway.

The AIP also tells us a few more things about flying in the pattern. A lot of it comes down to airmanship, too. For example, aircraft entering the circuit are to do so in such a manner as to not cut off other aircraft already in the circuit. Very similar to driving a traffic circle or "roundabout": traffic in the circuit has the right of way. Aircraft are to conform as closely as possible to the circuit altitude (1,000 feet AAE unless otherwise stated), and the speed and size of the circuit being flown by other aircraft in the circuit already. Bear in mind such speeds may not be possible, so other modifications may be necessary, such as flying wider so as to prevent overtaking a preceding aircraft. Normally, only left hand turns are to be made in the vicinity of an airport, with the exceptions of a right hand turn to enter the pattern, or otherwise as approved by ATC. If a right turn is desired, the aircraft should leave the vicinity of the airport, either by flying far enough away or by climbing well above the circuit altitude.

Additionally, the AIP tells us that aircraft joining the circuit from the upwind side of the runway are to do so at circuit altitude and, while considering other traffic, join the circuit on the downwind leg. This is normally best accomplished by overflying the runway at or about the midpoint, rather than over the departure end, taking into account aircraft performance, wind and/or runway length.

Radio Calls in the Circuit

Whenever flying in the circuit, whether at a controlled or an uncontrolled airport, pilots are required to make certain position calls while in the traffic pattern. The downwind call is important to establish both intentions of the pilot following the current approach, and for ATC to set you up with sequencing instructions if necessary, or for other aircraft to figure out where you are so they can see you and determine if they should follow you or the other way around. Also, it's helpful to give a call on final. See below for further information on ATC Instructions. ATC may ask for additional reports, and these must be made if requested.

Out of Wind Patterns

Many times, the traffic circuit doesn't meet the concept of landing into the wind. The names of the legs don't change, however. For example, in the above diagram, we'll say the wind is actually coming from the left side and blowing toward the right. If no other traffic is using the "active runway" which would normally be the one pointing into the wind, an aircraft approaching from the left hand side of the diagram may ask for and get approval from ATC to land on the opposite end, landing with the wind. In this case, the Final Leg would still be the last leg that approaches the runway. Similarly, an aircraft who wants to park at an apron on the left hand side of the airport as seen above might want to land out of wind to reduce taxi distance upon landing. If he's approaching from the top of the diagram, he'll join the Downwind Leg, despite the fact that he now flying into the wind, by turning right. After the right turn into the circuit, the next turns to get to the runway would all be left in the pattern, and therefore the pilot would be flying a left hand circuit to the out of wind runway.

ATC Instructions

At a controlled airport -- remember that this is one where ATC is currently providing an airport control service -- ATC may approve an approach to a runway opposite to the established pattern. In fact, this is often desirable for reasons of efficiency and for separation. For example, faster aircraft might be better placed on the right hand side while the slower aircraft fly the left hand circuit. This reduces the likelihood of an overtake, and allows the controller the flexibility of telling aircraft when to turn base leg to help sequence the traffic flying different speeds.

That brings us to another issue. ATC has the authority at a controlled airport to direct traffic in the circuit. This could include approval, or direction, for an aircraft to fly left or right hand patterns. It can also include directions that help sequence aircraft, such as telling an aircraft to "do a 360 to the right for spacing". A 360, for those who don't know, would simply be a 360° turn used to delay you just a little bit. This kind of tactic is used primarily when the pilot of an aircraft hasn't seen the aircraft he is being sequenced to follow, or if ATC needs to "build a hole", to get a departure out for example. Another tactic is to have an aircraft in a left hand pattern make a 270° turn to the right and rejoin base leg, rather than a 90° turn to the left from the downwind leg. Yet another tactic is to have an aircraft "extend downwind". I have worked with tower controllers who have employed this in a few ways: telling an aircraft to extend until he sees his traffic on a long final, extend until further advised (this means ATC will tell the pilot when to turn base leg), and one controller even told pilots to "extend downwind for two minutes" making a self-canceling restriction.

When entering a traffic circuit, ATC has a few different ways of clearing aircraft in. A standard phraseology is, "Cleared to the circuit". This means that ATC is telling you to join the circuit on the Downwind Leg in the standard pattern for the runway (remember, normally left unless otherwise stated). Note this may require an aircraft to overfly the airport on crosswind to join a left hand circuit as discussed above. A tower controller can also allow an aircraft to join a circuit at any other point, too. If, for example, it's more convenient for you to join right base, rather than left downwind, ATC will often even state that right away by saying, "cleared right base runway 33", or something similar.

As mentioned earlier, the downwind call is important. If you were cleared directly to base leg, or to final if cleared "straight in", make this call as you enter what would be a normal distance from the airport for a traffic circuit. ATC will normally give you a sequence and maybe even instructions at this point. Some examples follow:
If you're not number one, ATC should have provided you with the aircraft type your are to follow, and where they are. You should have a good look to find this aircraft, and advise ATC if you have it in sight or not. If you do, ATC will likely direct you to "follow" the aircraft. Make sure you keep him in sight so you know how big and how fast to fly. They expect that you will follow at a safe distance to allow for possible wake turbulence (if the preceding aircraft is of a heavier weight category) and keep enough spacing between you so the preceding aircraft can land and vacate the runway before you get too close. If you don't have your traffic in sight, they'll issue alternate instructions like some kind of delaying tactic to either build enough space in their eyes or to help you see your traffic.

Continuous Circuits

The traffic circuit is often seen in two lights: A method of approaching an airport, and as a training path. Everyone who trains to get a pilot license invariably ends up doing circuits for one or more entire flight, or does one or more circuits after a lesson before landing for the last time. The idea is to practice flying at the most dangerous times: when the aircraft is closest to the ground, on take-off and landing. You take-off, turn Crosswind, then turn Downwind (and make your downwind call), turn Base and then Final. Then you execute a landing. The difference here is, you take-off again for another run in the circuit rather than exiting the runway. There are a few different options open for runway use at an airport as the result of any approach, whether IFR or VFR.
  1. Full Stop: Simply what it means. You're done flying and you land and clear the active runway as per normal.
  2. Stop and Go: This operation completes a landing with braking to a complete stop, typically with a short period of time on the runway (we're not talking an hour or two, now), and then a take-off commenced from wherever the aircraft comes to a complete stop after the landing.
  3. Touch and Go: An operation where the aircraft is flown to a landing roll, the aircraft is "cleaned up" for take-off while still rolling, then power is applied and a take-off commenced without stopping the aircraft. This is the most common operation for aircraft training in the circuit.
  4. Low Approach: This is simply an approach to landing without the landing. Before making contact with the runway, the pilot applies power and begins to climb out instead of landing, cleaning up flaps and gear as appropriate. Also known as a "go around".
Another phraseology allowed for is "the option". The AIP, RAC 4.4.3, tells us when you are "cleared for the option runway 34", you can do any of the above operations without further approval from ATC. This gives the pilot free will to change his mind without having to talk to ATC. It also allows for some training opportunities, too. For example, the instructor tells his student to do a low approach only and go around, then pulls the power on the student as he initiates the go around to see if he can handle himself in an engine failure at that moment. A clearance for the option is normally only used in light traffic times.

There are several notes to keep in mind for the above operations, too. For a full stop, it doesn't necessarily mean a full stop on the runway, rather an end to the flight. For the stop and go, don't sit on the runway too long. If you need more than a few seconds on the stop, give tower a guess at how long you need and see if they can approve that operation. Also, a heavier weight category aircraft departing before you land means a strong possibility of wake turbulence for the take-off part of your circuit, so ATC may tell you, "cleared for a stop and go, tower will call the go" or something like that. They may say something else like, "For wake turbulence, make this one a full stop. Cleared to land runway 29, I'll issue further when you're on the runway". Lastly for the stop and go, if you need to backtrack to the threshold for the take-off, you need approval from TWR, and you should secure this ahead of time. Not asking for it ahead of time may require ATC to get you to vacate the runway for other aircraft, and then have you taxi on taxiways to reposition the aircraft later. Low approaches don't have a restriction on the altitude as far as how low you can go. Quite commonly heard is an expression something like, "Not below 200 feet". If equipment and personnel, vehicles or other aircraft are using the runway for whatever reason, ATC may even deny a touch and go or other operation rather than get the others off the runway for you and say, "cleared low approach only, not below 200 feet". In either case, this means not below 200 feet above the runway, not above sea level.

Pilots should advise ATC of their intended operation, preferably in the downwind leg with the normal downwind call. "Tower, Alpha Bravo Charlie downwind runway 26, touch and go". This allows ATC time to plan other aircraft movements while they still have time to modify your flight path if necessary. If the desired movement is not communicated until the aircraft is on final, ATC may not be able to provide you with what you want.

ADDENDUM

It was pointed out to me that I missed one note here: ATC will not normally initiate a clearance for "the option". A pilot is expected to ask for such a clearance. MANOPS 336.13 says that a pilot must request a clearance for the option in order to receive it, and it still is dependent on current traffic. Thanks again to Greg Phelan for keeping an eye on me.




That turned out to be a bigger topic than I had planned. Like most of the others I've done, I guess. I intended to cover off circuits from both a piloting and a controlling point of view, to allow some information for both sides to work from. If you feel I've left something out, don't hesitate to write me. My e-mail address is moxner@nbnet.nb.ca. Next week, I have a special guest writer, Ken Greenwood, who has provided us with some good information on military manoeuvers at airports. I know this one will be appreciated. Thanks again for taking the time to read!