Flow Mysteries

There are 3 strategic methods used for Flow control, on ground delays, airborne time in trail sequences and airborne miles in trail sequences; the latter two are effectively the same thing, but it’s hard to put a PA31 miles in trail with a B747 at 100NM from touch down.

The primary function of a tactical FLOW controller is to ensure that all available landing slots are utilised. The amount of landing slots varies on many things including aerodrome layout, current weather conditions, forecast weather conditions, noise abatement, legislative requirements.

Where I work, the FLOW Controller uses a tool, to help calculate landing slots, called MAESTRO. The FLOW chooses the arrival mode, such as MODE 7 (Jets via RIVET get 34L everything else gets 34R unless operationally require 34L) or 16PRMs, or 07ILS etc. Then they landing rate is chosen, normally this is done via a number of arrivals per hour; which then translates back to a ‘time between arrivals’. For example on 07ILS the arrivals rate may be 28 per hour, which is about 128 seconds apart.

MAESTRO uses the original arrival time (provided to it by TAAATS or manually manipulated by controllers or the FLOW) of each aircraft and applies a delay to an aircraft to ensure it lands in the calculated arrival slot.

For example, Bigplane1 is estimating landing at 00 and is number 1, so no delay is given assuming fast and shortening isn’t an option, Bigplane2 is estimating landing at 00+30 seconds and is number 2, so a delay is given to ensure that Bigplane2 lands at time 02 and so on.

So lets assume single runway operations with that 128 second arrival rate. It is pretty easy to follow in chart form, where ETA_FF is the actual estimate and STA_FF is the adjusted estimate for the sequence to work; for ease of management, slots are allocated 2,2,2,2,3 minutes between arrivals; so the 1st 4 are tighter than the rate and the 5th on time.

Callsign... ETA_FF..... STA_FF
QFA006... 1945.......... 1945
BAW015... 1945.......... 1947
SIA221... 1946............ 1949
UAL863... 1946........... 1951
ETI450... 1947........... 1954
UAE412... 1950......... 1956
RBA193... 1953......... 1958
VOZ623... 1955......... 2000
QFA400... 2000......... 2003
JST601... 2000......... 2005
RXA453... 2001......... 2007
EA2221... 2001.......... 2009
VOZ601... 2012......... 2012*
RXA774... 2013......... 2015*

You can see from the table that ETI450, despite being only 2 minutes behind number 1 in the sequence has been given a 7 minute delay; and VOZ601 has been allocated the landing slot 12 based on it’s estimate, but there may be opportunity to track shorten to achieve a landing time of 11; this may also flow on to the rest of the sequence.

MAESTRO shares the delays required between approach (TMA), and enroute. The first 1 to 2 minutes of the delay will be absorbed within the TMA (210K from Welsh or 250K from RIVET), so to enroute it will appear that no delay is required for BAW015 in the above table; yet the airplanes are effectively on top of the other so some spacing is required by enroute; otherwise it’s a step descent and lots of work etc. Then by slowing/vectoring BAW15 it impacts on the next aircraft etc. This also assumes same track into the TMA.

Where I work we have one jet track and one non-jet track into the Sydney TMA. Jets via RIVET and non-jets via BIK and ODALE.



All the Jet routes converge at TARAL, about 4 minutes flying time prior to RIVET with 4 major routes joining at CULIN about 8 minutes prior to RIVET.

Often for the purposes of separation, it is not prudent to let converging jets manage their own sequence times; despite the advantages it may give re VNAV etc. With 4 routes converging at CULIN almost never works to have a two minute gap managed by the pilots, as a two minute gap at RIVET may be a zero minute GAP at CULIN.

For separation we may want you doing a different speed than what we actually need for the sequence; this is when we may change your speed for descent. Then there are the occasions when we expect the aircraft in front of you, turning at MAKKA and TARAL to gain speed or loose speed depending on the winds and they just don’t.

Lets also not forget the fly-over and fly-past way-points that boeing and airbus use; Airbuses via CB-CULIN, and MAKKA-TARAL cut the corner (sorry smooth the corner) and save a good three track miles or so.

When we adjust your speed, preferably upwards rather than down, we will do so because the sequence permits it; but sometimes it’s needed to slow you down too. Sometimes the flow just puts another aircraft in “your” slot, for whatever reason (WX, Operational Requirements, MED priority) if this happens and you are inside 200NM it will cost you another 2 minutes minimum; and usually the whole sequence gets vectored and slowed.

On ground delays (CTMS) are strategically used in Sydney too; these are for flights within 2 hours only. They effectively use flight-planned arrival times to allocate an ETA based upon ETD at departure. With the exception of ‘departure aerodromes’, CTMS times are not considered by the flow or MAESTRO. Departure aerodromes include Canberra, Bathurst, Orange, Williamtown etc. If you have an on ground delay and are early to depart you may be penalized and held for your CTMS arrival slot; if you get away late you forfeit your slot and join the regular sequence. Another big impact is arrangements with airlines re departures from West Coast USA and Africa, where the maximum delay applied shall not exceed 10 minutes.

In my next blog for EMC, I’ll be explaining some of the methods we use to achieve the delays.