Metering, in the ATC sense of the word, is controlling the release of aircraft from an airport, often for purposes of managing en-route or arrival congestion issues. Metering requirements are generally established by a Traffic Management Unit (TMU) controller or an en-route or TRACON controller. When metering requirements are in effect, it is the responsibility of the affected ground and tower controllers to ensure traffic departs in a timely manner.
The simplest metering requirement is a TRACON controller asking for a set time between aircraft departing for the same airport. This is often the case during events or a group flight with a number of aircraft all going to the same place. In this instance, you may hear something to the effect of, "we need three minutes between departures bound for Las Vegas."
As a Ground controller, how you sequence departing aircraft can make or break the airport's efficiency. Following our above example, if you have five aircraft going to Las Vegas, you should look to insert other aircraft not bound for Las Vegas into the departure stream between the Vegas departures to maximize runway throughput.
For this to make sense, it may be important to understand why metering restrictions get imposed in the first place. Throughout the National Airspace System, there are various requirements for the amount of space that ATC must keep between participating aircraft. In the en-route environment, for example, we like to see aircraft on the same route no closer than 20 miles in trail (MIT).
If our five aircraft bound for KLAS were to all leave KSAN as quickly as they could be safely departed by the Tower, they would not have sufficient spacing in the en route environment. On the ground, we can stop an aircraft where they are to create the necessary spacing. But once those aircraft are airborne, they're much harder to "park" somewhere.
However, that same radar controller can easily handle a departing aircraft going to a different airport. They won't be using the same route and, in turn, won't create spacing conflicts in the en-route environment. By departing other aircraft (not headed to KLAS) between the Vegas departures, we can maximize our use of the runway while not compromising en-route safety.
As you continue your training at ZLA, you will be introduced to more advanced metering techniques and procedures. But the foundation remains the same -- managing spacing on the ground is much easier, more efficient, and more realistic than trying to fix that problem in the air.
If the airport ground environment becomes congested with departures it will become necessary to hold aircraft at the gate to ensure the smooth flow of ground operations. You should make this determination when you see the ground environment becoming saturated. It is easier to hold aircraft at the gate then to allow them to taxi into a saturated environment. Continuing to allow aircraft to taxi can cause a clogged ground environment, stopping the movement of aircraft, and cause an ineffective metering program.
If you determine the airport ground environment is becoming saturated (such as running out of room to move aircraft), you should notify the delivery controller to advise aircraft to request pushback from ground.
When the aircraft calls for taxi, inform them to hold at the gate and an approximate time they can expect to hold at the gate. This is not the same as Gate Holds where the arriving airport has become saturated.
“Hold at the gate. Expect a 20 minute delay for pushback.”
You must note the sequence the aircraft has called in so that you know which order to give aircraft pushback instructions. You should also apply the taxi order with the above metering principles for spacing aircraft for departure.