The visual circuit, leg by leg

Why aerodromes have a circuit

An aerodrome without a control tower still has several aircraft taking off and landing on the same strip of tarmac. The circuit gives them all one agreed path, so everyone knows where to look for everyone else, and each arrival lines up with the runway in the same predictable way. It also keeps you close to the field for most of the time, which matters if the engine stops at 1000 ft.

For a student it does a second job. A circuit packs a take-off, a climb, level flight, four turns, a descent, an approach and a landing into a few minutes, with a check, a call or a change of configuration on every leg. That is why the first solo flights are circuits.

The legs of the circuit, one by one

The heights and speeds below are the ones the circuit trainer uses for a typical single-engine trainer on a 1000 ft circuit. Your aircraft’s flight manual and your aerodrome’s published procedures come first.

Take-off and climb out (the upwind leg)

Full power, a quick look at the rev counter and the temperatures and pressures, then eyes outside. Climb straight ahead on the extended centreline at about 75 kt, allowing for any crosswind so you do not drift off the line. At a safe height do the after take-off checks: fuel pump off, landing light as required, flaps up if you used them. Do not turn until at least 500 ft above the aerodrome, and look into the turn before you start it.

Crosswind

A 90° turn, still climbing, takes you across the upwind end of the runway. Level at circuit height (attitude, then power back, then trim) and look down the downwind leg before you turn onto it, because that is where joining traffic appears.

Downwind

The long leg, flown parallel to the runway in the opposite direction, level at 1000 ft and about 90 kt. At a small aerodrome it sits somewhere around three-quarters of a nautical mile to a mile out; the trainer uses 0.8 NM. This is the quiet leg, so it is where the radio call and the pre-landing checks go.

Base

Turn base when the threshold is about 45° behind your wingtip. On base, the trainer’s sequence is carburettor heat to hot, power back, the first stage of flap once the speed is in the white arc (the flap operating range on the airspeed indicator), then trim for the approach speed and start down. Base is also where you judge the final turn.

Final

Roll out lined up with the runway, set the landing flap, and fly the approach speed: 70 kt in the trainer. Check that the runway and the approach behind you are clear. Hold the aiming point still in the windscreen, using power for the path and attitude for the speed, and in a crosswind put the wing down into wind so the wheels touch pointing along the runway.

Downwind checks and the radio calls

Most schools use a mnemonic for the checks on downwind; the letters vary from school to school and aircraft to aircraft. The trainer uses BUMPFICH:

• Brakes: off, with pressure.
• Undercarriage: down and fixed.
• Mixture: rich.
• Primer: locked.
• Fuel: on the fullest tank, pump on.
• Instruments: engine temperatures and pressures in the green.
• Carburettor heat: checked.
• Hatches and harnesses: secure.

Circuit calls in the UK follow CAP 413, the CAA’s radiotelephony manual. The downwind call is made once you are established on the downwind leg, normally abeam the upwind end of the runway; if you have already passed the downwind end, it becomes “late downwind”. A base call is made where the aerodrome asks for one. The final call is made once you have turned onto final; if that turn is more than 4 NM from touchdown, the report is “long final”. At a small field with an air–ground radio station, the calls sound like this, with the callsign abbreviated because the station has already used the short form (you may only abbreviate it after they have):

• Downwind: “Wizard Farm Radio, G-IZ, downwind, runway 24, to land.”
• Base: “G-IZ, base.”
• Final: “G-IZ, final, to land.”

At an aerodrome with air traffic control you then wait for a landing clearance. With an air–ground or information service you are given the runway and traffic information, and the decision to land is yours.

Try it: Open the circuit trainer and pick the “Still air” lesson: tick off each check, call and configuration change on the leg it belongs to, and the debrief lists anything missed or done on the wrong leg.

How wind bends the circuit

The circuit is a rectangle over the ground, not a set of headings. Fly each leg on the runway direction plus or minus 90° in a crosswind and the wind reshapes the whole thing. Take runway 24 with a left-hand circuit and 20 kt from 150°:

• On downwind, heading 060°, the wind is from your right and pushes you steadily in towards the runway. The spacing shrinks along the leg.
• The base turn therefore comes too close in, and the same wind is now behind you on base, carrying you at the centreline faster than the turn can cope with.
• You overshoot the centreline and finish with a scramble on short final.

With the wind from the other side, the downwind leg drifts away from the field instead: a longer base, a long flat final, and less chance of reaching the runway if the engine stops. The fix in both cases is the same as in navigation: on every leg the wind blows across, point the nose into wind by the drift angle so the track, not the heading, follows the rectangle. A leg flown straight into wind or straight downwind has no drift, only a slower or faster groundspeed; in this example that is the crosswind leg (into wind) and base (downwind). The drift and wind correction guide covers how to work that angle out.

1. Before take-off, look at the windsock and work out which legs have the crosswind and which have the headwind or tailwind.
2. On the climb out and on final, crab into wind to hold the extended centreline.
3. On downwind, fly a heading off the reciprocal by the drift and watch the runway: if the gap is closing or opening, change the heading.
4. With a tailwind on base, start the final turn early and expect to roll out with drift already on.
5. With a strong wind straight down the runway, your groundspeed is high downwind and low on final, so turn base a little earlier than usual to avoid a long, slow final.

Try it: In the circuit trainer, fly “Blown in, no allowance” and then “The same wind, allowed for” back to back: identical wind, and the only difference is whether the headings carry the drift.

Right-hand circuits and the overhead join

Left-hand is the default because the pilot in command sits on the left and can see the runway throughout. An aerodrome publishes a right-hand circuit where it needs one, often for noise or terrain, and everything is simply mirrored: the same checks and calls on the same legs, the runway out of the other window.

Arriving from elsewhere, the join the CAA recommends is the standard overhead join. You cross the aerodrome at 2000 ft above it (or the height it publishes), read the windsock and the traffic, descend on the dead side (the side away from the circuit) to circuit height, and cross the upwind end of the runway into the crosswind leg. Some aerodromes prefer a downwind, base or straight-in join instead; the local procedures say which.

The US traffic pattern: what changes

In the United States the circuit is called the traffic pattern, and the legs are the same, though the climb out is usually called the departure leg. Pattern altitude for light aircraft is commonly 1000 ft above the airport, flown on the local altimeter setting (QNH), not QFE. Turns are to the left unless the airport publishes otherwise.

The usual way to join is a 45° entry to the middle of the downwind leg at pattern altitude, rather than the overhead join. At an airport without a tower, pilots announce their own position on the CTAF (common traffic advisory frequency), addressed to the airport’s “traffic” and ending with the airport name, since several fields can share a frequency.