Flow Control & Sequencing

The Sequence

Aircraft can only land one at a time, and once they are established in final approach they, for all intents and purposes, have priority over other traffic and vehicles which might want to use the runway.

Aircraft arriving at Perth are sequenced at intervals of +2+2+3 minutes at the threshold. (The extra minute of the third arrival builds some leeway into the sequence.) During instrument approaches, used during poor weather, another minute, at least, is added because aircraft tend to fly slower and require longer finals.

These intervals not only reflect the time needed for aircraft to clear the runway before the following one lands, but also the limited airspace available to the approach controller, due to the proximity of Pearce, to vector aircraft should the sequence close up. There are no fast exits off the runways at Perth. Heavies on RWY 24 may use the full length, need to turn and backtrack a short distance to exit the runway. This must be catered for in the sequence.

The responsibility for determining this sequence rests with the Flow controller, operating from a separate console in the Perth TCU. Sequencing is a joint effort involving the Flow, KNG, LCI, JAR sectors, Approach and pilots.

Due to the complexity of developing a sequence, requests for track shortening, for reasons of a more expeditious route, are refused by the sector controllers and not passed on to the Flow. This is due to the disruption such requests cause as the Flow works on the sequence, and the conflictions it causes with departing traffic. The Flow will track shorten aircraft when the traffic situation permits.

The Role of Katanning and Lancelin Sectors

As aircraft come into radar coverage, KNG begins building the initial sequence. KNG is responsible for placing aircraft on the appropriate STARs and like-type aircraft in trail. That is, KNG must achieve in-trail spacing of:

· 12nm for jets with no closing speed (±10knots)

· 8nm for non-jets with no closing speed (±10knots)

These distances will ensure the 2 minute interval at the threshold.

KNG determines the order of the sequence by the "untouched" estimate of the aircraft at the LCI/KNG boundary after their STAR and transition have been issued. By vectoring and speed reduction, jets and non-jets may be placed in parallel sequences to allow the faster aircraft through. KNG may not increase an aircraft's speed without Flow approval.

These traffic streams are handed off to LCI who blends the streams as they near Perth. Additionally, LCI brings into the sequence lower level aircraft. Both KNG and LCI can see the order required in a window on their screens.

The problem for the Flow in determining this order is that low-level aircraft come into radar contact later, and closer to Perth, than the higher traffic. The problem for the LCI and KNG controllers is that they have to slip these aircraft into the sequence. The Flow and the LCI/KNG controllers may adjust the sequence by speed control - increasing or decreasing speeds - or track shortening individual aircraft. Some aircraft may be track stretched by vectoring or given an orbit. If the sequence is becoming too congested, the Flow will issue holding.

Holding

In fine weather, Perth can land 25 aircraft in one hour. That is just a simple mathematical fact. It follows that if 30 aircraft estimate Perth in, say, 60 minutes, the +2+2+3 sequence won't work. Delaying action is necessary and this is achieved by holding aircraft at holding points at the LCI/KNG boundary (80nm from Perth). Aircraft are generally held in the holding stack in the order that they will leave it, with the first at the bottom and later entries coming in at the top and being stepped down.

Holding may occur closer to Perth in ad hoc holding patterns when, for instance, an aircraft makes a missed approach. LCI will issue holding instructions which include a description of the pattern to be flown.

And don't forget, while this is happening Perth Tower is attempting to depart traffic. If a busy departure burst is building, the tower controllers must liaise with the Flow to build more gaps in the arrivals sequence so that aircraft can depart, further slowing the sequence. To delay departures too much simply means that gates, parking bays and taxiways are full or congested, further delaying arrivals. It's a balancing act.