Aircraft in the pattern at a busy airport can create problems if you’re not careful. A pilot flying the pattern can generally make their legs as long or as short as they want. If you have other aircraft on approach, or maybe someone else in the pattern, you could potentially end up with a conflict. Therefore, the following is recommended:
Plan ahead for how you are going to work departures and arrivals into the pattern. The sooner you start planning, the more options you will have.
Give aircraft specific instructions about when to contact you such as, “Report midfield downwind,” or, “Report 3 mile final.” This can prevent you from forgetting about an aircraft.
All aircraft should ideally either be given a landing clearance or told to extend their downwind before they reach midfield downwind.
If you are falling behind, don’t be afraid to limit the amount of traffic in the pattern or to close the pattern altogether. Reserve this option for when you really can’t handle any more traffic. However, if you can, allow the pilots to have fun and fly.
With practice you should be aware of what is going on in your airspace, and be thinking ahead about what your plan is to manage the traffic. There are many tools you have available to manage traffic in your pattern.
Follow - Point out the aircraft you want them to follow. Once the pilot reports the other aircraft in sight, instruct them to follow that aircraft, along with any other information they need. They should now maintain their own spacing.
Extend - Tell the pilot to extend their pattern leg. You can extend the upwind, downwind, and crosswind legs. They should remain within tower airspace. You can either give them a point to turn or specify that you'll call their next leg of the pattern.
360s - Tell the aircraft to "make left/right 360." If no one is behind them, this will add an easy delay. This is usually done on the downwind with the turn made away from the runway (opposite the pattern direction). If the pilot makes a standard rate turn, it will take 2 minutes to complete.
270s - Similar to a 360, but in conjunction with a regular turn between legs in the pattern. The pilot will turn the opposite direction and make a 270-degree turn to the next pattern leg.
S-Turns - A pilot can make s-turns back and forth to create more spacing in front of them. Most common on final. This can help if you only need a bit more spacing. Take pilot proficiency into account.
Holding - Tell the pilot to hold around a visual point in the area. You can also just instruct them to remain outside the tower's airspace. If telling an aircraft to hold, you should tell them when they can expect further instructions. You may also hold aircraft on the ground until conditions allow them to depart.
Ideally the controller should always be planning ahead and never issue a takeoff clearance that could cause a traffic conflict. However, situations may arise when a takeoff clearance needs to be canceled. Note that you should give additional instructions to the pilot when able, but it's important to explicitly cancel the takeoff clearance and ensure the pilot reads this back.
"Cessna 123AB, cancel takeoff clearance, hold position."
The following are other bits of phraseology you will need for working VFR traffic.
VFR Traffic advisories are different from radar traffic advisories and are much more relaxed. In a traffic advisory from the tower, the main things you want to include are the aircraft's position and type:
"(Callsign), traffic ahead and to your right, Right base, Baron, report traffic in sight."
"(Callsign), traffic ahead and to your right, 2 mile final Citation, report traffic in sight."
If the second aircraft has the first in sight, you can say:
"(Callsign), follow that traffic, runway (XX), cleared for the option. number (2,3,4...)"
If you want to launch a departure before your traffic in the downwind turns base, point the departing aircraft out to the downwind traffic
"(Callsign), traffic departing runway (xx) Boeing 737, report traffic in sight."
"(Callsign), maintain visual separation from the B737, runway (xx), cleared for the option, CAUTION WAKE TURBULANCE."
If you end up having traffic on the runway or flying over the runway, that will cause a problem, you could say:
"(Callsign), go around, traffic on the runway."
"(Callsign), cancel takeoff clearance, traffic overhead the runway."
Aircraft approaching a towered airport but not intending to land there (just to fly through the airspace) must contact the control tower. They should be given a minimum altitude to maintain (usually at least 500 feet above the traffic pattern, "Maintain at or above 2000 feet") or a route restriction ("Remain north of highway 26") to keep them away from arriving and departing aircraft. When time permits, you should give transiting pilots traffic advisories to help them avoid other planes. That being said, VFR pilots in Class Delta airspace are responsible for their own safety and separation.