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For example shown here, the flight is heading to the centre of the airport, until it gets close when it then moves into it's landing path. This is an inefficient use of time, fuel and emmissions, why can't the airport tell the aeroplane exactly which runway and direction it will land earlier so it can more efficiently head towards the approach.

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    $\begingroup$ Sequencing... There is more than one plane trying to land or take off... $\endgroup$
    – Ron Beyer
    Commented Jan 26, 2021 at 1:18
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    $\begingroup$ Why can't they still sequence with a wider area? To maintain separation they could tell a plane to slow down so that when it reaches the straight runway approach it's correctly spaced between other flights (sorry errors in my terminology). $\endgroup$ Commented Jan 26, 2021 at 1:45
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    $\begingroup$ With a wider area when controllers have to make changes the corrections required by the planes may be much larger. Suppose for example in your diagram that ANZ1115 has to be diverted to a runway with an easterly approach when it's already far to the south lined up with the runway it was originally assigned. That's going to be a big circular path. $\endgroup$ Commented Jan 26, 2021 at 2:01
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    $\begingroup$ One of the posters here is a controller @expeditedescent I believe, who would be the best person to answer. $\endgroup$
    – John K
    Commented Jan 26, 2021 at 3:15
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    $\begingroup$ Related aviation.stackexchange.com/questions/43160/… $\endgroup$ Commented Jan 26, 2021 at 8:47

4 Answers 4


In general, IFR flights through controlled airspace use airways (highways in the sky) to fly between waypoints. The particular flight you show looks like it arrived via the N774 airway to a waypoint MARLN:

Airway to MARLN

The traffic flow around busy airports like Kingsford Smith (YSSY) in Sydney is usually structured using Standard Instrument Departures (SIDs) and Standard Terminal Arrival Routes (STARs). Your example was probably on the MARLN5 RNAV STAR:

(from the Jeppesen chart for MARLN5 at YSSY)

From somewhere after the JAKLN waypoint, the air traffic controller (ATC) would have instructed the flight to turn right to intercept their final approach path (probably to runway 34R).

All of this structure (airways and procedures around busy airports) exists to allow for an efficient flow of traffic that can be easily managed by ATC. There are many things to consider when these paths are designed:

  • avoiding conflicts between aircraft (e.g. between departing and arriving aircraft or between flights from different airports in the area)
  • restricted airspace (e.g. airspace reserved for the military or avoiding flights near sensitive areas, like nuclear power plants)
  • noise abatement (especially near large cities you want to avoid overflying the city center at low altitudes)
  • terrain (not really relevant for Sydney)

Nonetheless, one can deviate from these fixed routes. ATC might clear an aircraft on a more direct route if traffic and restricted airspace allows this (very common right now due to the pandemic). Some countries have even removed all high altitude airways and replaced it with Free Route Airspace.

The problem with setting an aircraft on a direct route for final approach interception is that this would have to be done far away from the airport. The aircraft needs time to descend and slow down (usually more than 100 NM). At this point they are typically not handled by the approach controller for the airport they are landing at, but still by the Area Control Center (ACC). This would require significantly more coordination between various controllers and is probably not very efficient in busy airspace. During the current pandemic with significantly less traffic, you might see it though.

  • $\begingroup$ Thanks for your answer. From your chart, if when at Marlin the flight would be told the final approach, it could then start turning to go direct towards Jakln and onto the landing approach. It sounds like Free Route Airspace aims to do something like this (but maybe not yet this close to the airport). So it sounds like it's just down to more coordination between the approach controller and the ACC, and potentially more computer automation to determine the most efficient route while still working within constraints (terrain, noise, distancing between aircraft, go rounds etc). $\endgroup$ Commented Jan 26, 2021 at 8:56
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    $\begingroup$ When ATC tells you "cleared direct to JAKLN" when you are at MARLN, then you suddenly have to descend faster. Airliners already fly an idle descent path (meaning engines are at idle), so descending faster is only possible with speedbrakes. That may save some time, but not really much fuel. That requires knowing about the shortcut much earlier and therefore starting the descent earlier. $\endgroup$
    – Bianfable
    Commented Jan 26, 2021 at 9:04
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    $\begingroup$ I see, though to me that just says ATC need to communicate this earlier so the aircraft can descend earlier. $\endgroup$ Commented Jan 26, 2021 at 9:11
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    $\begingroup$ Exactly, they need more communication and coordination between different controllers. We definitely see a move towards this (like Free Route Airspace for high altitudes), which is probably supported by newer technology and clever computer algorithms. Since you make a valid point (saving fuel is always a good thing), I guess we will see more direct routing in the future, but sometimes aviation is slow to move on... $\endgroup$
    – Bianfable
    Commented Jan 26, 2021 at 9:19
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    $\begingroup$ Another factor might be wind direction (and hence landing direction). Don't know about Sidney, but around here it's not uncommon for wind direction to change during the day. If your flight is aiming for an approach from the south, and the wind shifts to favor an approach from the north, you've wasted even more time. $\endgroup$
    – jamesqf
    Commented Jan 26, 2021 at 16:50

Most airports have approaches designed to be as straight-in as possible for the reasons you describe. Cases like this one in Sydney are in the minority (for major airports at least).

This is the approach route in question:

Sydney STAR

As to why YSSY specifically has such an inefficient approach? I'm not 100% certain but I can think of a few contributing factors. Sydney airspace overall is a mess, with two defence force zones, large GA airports, and restricted airspace over the harbour. Also, due to the proximity of the airport to the city millions of people are affected by aircraft noise. There are powerful lobby groups, and enough electoral seats under the flightpath that the federal government could easily fall if they took a pro-airport stance such as lifting the curfew or the hourly movement cap. Changes to any of the flight paths, even just over the ocean, require community consultation and could stir up a lot of angst. Basically, it's a huge job.

All of this leads to: the flight paths haven't had a major update in a long long time. I suspect that this route merely replicates what the route was before we received the flexibility of GPS/RNAV approaches, back when you basically flew VOR to VOR. Nowadays, you would only design the approach in this way if there was some other airspace restriction or terrain (but being over the ocean, neither of those apply).

Fortunately, with the new Western Sydney airport finally due to open in 2026, a whole re-think of the airspace is about to commence and the approaches will hopefully be made a tad more efficient.

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    $\begingroup$ I don't agree with your first paragraph. Many large airport, at least in Europe, have RNAV STARs similar to Sydney where all traffic is directed toward a downwind leg before joining final $\endgroup$ Commented Jan 26, 2021 at 9:03
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    $\begingroup$ Thanks, that makes sense, basically too many stakeholder issues to change flight paths, so instead they leave them in a few set paths. It's funny that it affects paths over the ocean too though. End of the day the current inefficiency gives me more time looking out the aircraft window observing the city so not complaining, but had been wondering why for a while. $\endgroup$ Commented Jan 26, 2021 at 9:08

When the landing approaches, pilot has to reduce the speed of the aircraft and maintain the lower speed and distance to avoid accident. Pilot has to slowdown or else he will not be able to land at the destination airport. Moreover due to the congestion in the aircorridor, pilot can be asked by ATC to take more rounds so that the landing is safe and enough time is provided for other aircrafts that are either landing or taking off or taxiing. Once I saw One International airline, suddenly abort landing and raise the altitude, may be due to ATC's instructions. I can only see the flight above as the engine made a roaring sound all of a sudden.


ATC isn't incentivized to reduce CO2 emissions. To do so would require re-architectiing their whole SIDs and STARs (to make them dynamic based on AC type, weather, traffic, and other factors). I vaguely recall reading about a (US) FAA initiative to modernize the vector airways to optimize for descent rates, etc but that will take years. Ultimately, it's the airline who bears responsibility to minimize fuel consumption and government who is responsible for separation.

I'm not myself an aviator, but I imagine that the downwind, crosswind leg, and final also give the crew a good feel of the surface meteorological conditions.

My final $0.02: ultimately the best way to minimize CO2 is to not fly for leisure. Look at youtube for all the frivolous flights of people globe trotting just because they can. It's simply un-sustainable.

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    $\begingroup$ Strongly disagree ... the purpose of ATC is "preventing collisions and expediting and maintaining an orderly flow of air traffic." Of course safety is #1, but it is very closely followed by efficiency. $\endgroup$ Commented Jan 27, 2021 at 8:03
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    $\begingroup$ @expeditedescent, he emphasized that ATC is NOT incentivized to reduce CO2. So what are you disagreeing with? $\endgroup$ Commented Jan 27, 2021 at 18:22
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    $\begingroup$ ATC is not incentivized to keep an airplane in the air longer that it needs to, its role is also to manage traffic and thus a "useless" flying aircraft is a burden for them. On top of that this is not true that there are no optimization of routes. STAR approaches are one and there are larger scale ones eurocontrol.int/concept/free-route-airspace $\endgroup$
    – Brice
    Commented Jan 27, 2021 at 19:12
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    $\begingroup$ From a purely theoretical standpoint I'm sure you could, as you say, just solve it as a "multivariate optimization math problem". However, this is the real world. You have to add a bunch of variables and random noise to your data, and also realise that the sensor input you receive is not necessarily 100% accurate. The day these issues dissapear, sure, you could optimize air traffic "perfectly". Until then, there is a reason ATC has not yet been replaced by computers. $\endgroup$ Commented Jan 28, 2021 at 6:54
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    $\begingroup$ For one thing, despite hugely complex mathematical models running 24/7 on massive supercomputers, we are unable to accurately predict the weather. An unexpected change in wind speed or direction at high altitude could result in an aircraft arriving several minutes earlier or later than expected. You would have no way of knowing in advance. Inevitably, even if your amazing mathematical model has spaced all the aircraft perfectly in advance, you are going to end up with two aircraft arriving on top of each other and having to deal with that on short notice. $\endgroup$ Commented Jan 28, 2021 at 7:00

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