Traffic Global – Choosing the Runway

There are a lot of people asking a completely fair question: Why does Traffic Global not use the same runway as [FlightRadar24|X-Plane|Pilot2ATC], delete as appropriate. It feels like it should be straightforward. Weather is this, airport’s rules are this, therefore land on 08R. Dead easy.


The problem is that there are a load of different, uncommunicative systems involved plus, at least in some cases, maybe just a little misunderstanding of what’s going on. Today I’ll talk about what’s involved and why things can’t just play nice with each other.

Weather is Hard!

(Especially hailstones.)

First up is the weather, which is quite well known as being a tad awkward to pin down in the real world, and the simulated world is just as awkward even if it’s for different reasons.

Start with the simple case: X-Plane is set to manual weather input, you choose the weather when you set the flight up, and no other plugins that change the weather are being used. Life is good: The entire planet has the same weather and X-Plane is happy to tell you so. Runway flows can be chosen at startup – before startup, even – and they will never change. This one, at least, is easy.

Okay, so on to the next: X-Plane is set to use real-world weather, but there are still no weather plugins installed. This is where things begin to be maybe a little different to how you’d expect them. Switching on “real-world weather”, you’d expect to get… well… real-world weather. Like, if you look at a weather forecast for a given place, that’s what the weather is inside X-Plane too. You’re only partly right.

X-Plane – at least according to the documentation – refreshes its view of the world’s weather once an hour. During that hour, the real-life weather might change. Maybe not by much, or maybe a lot, but it will change. Possibly with the exception of Scotland, where it’s still gonna be raining if it’s not that one week in August when the tourist board photographers are out in force, except for the west coast where it’s gonna be whatever hasn’t had a turn in the last half hour. So, if you check the weather after you’ve been flying for a while, it might not match the actual, honest-to-goodness real weather at your destination airport as closely as you might expect.

The next bad news is that the weather report for a given airport can potentially be many hours old before X-Plane downloads them. They’ll probably be within the last hour for busy airports but for others they could be quite a long way out of date. Still, that’s the best we’ve got, so probably better use it. That’s fine until you check against a realtime forecast and find that the wind’s backed by 80 degrees since the weather report was taken. We’re beginning to get some real differences here.

Finally, at least for weather, let’s add in a weather plugin – xEnviro, ActiveSkyXP, RWC, and others. These might use current weather reports just like X-Plane does, and of course they’re subject to the same problems for the same reasons. They will almost certainly do a lot more frequent updates. That keeps you up to date – at least within the last hour – for busy places but if the most recent data is hours old, downloading it again doesn’t change anything.

Each of these will also be doing its own processing on the weather – that is, guessing what’s going on in the gaps – and then feeding the results into X-Plane’s weather system for display. That means that they will set X-Plane’s weather mode to “manual”.

Huh? No, you mean real-world weather, surely?“. Nope. If X-Plane is set to real-world weather then it will be doing its own downloading, filtering and merging and if you’re a weather plugin, you don’t want that. You want X-Plane to take whatever weather it’s given and roll with it. That means… yup, manual weather mode, except that the plugin adjusts the manual weather in realtime.

Remember, those realtime adjustments are going to be based on weather that could be several hours out date if you compare it with the “look out the window” test, and totally made up – sorry, interpolated – if there’s no actual weather reading nearby.

As a quick aside, at least one weather plugin is really helpful and smooths the new weather reports in over a minute or so after they’re received. This is good in that the new weather appears gradually instead of suddenly. It’s less good (i.e. bad) when the “old” weather state is, as it is at the start of a flight, whatever X-Plane has stored in the flight definition – typically 0 knots from 0 degrees. This means that when Traffic Global starts allocating flights, after you’ve clicked the “Start Flight” button but before the actual simulation starts, they’re based on calm conditions which then change by up to 180 degrees over the first 60 seconds of the simulation.

Another little wrinkle in the process is that in manual weather mode, X-Plane can’t be told what the weather is at different places. Whatever weather you set is the weather across the globe. So, most if not all of the weather plugins are going to be continually changing the global weather to be whatever they think is the local weather at your plane’s current location. If you’re in your simulated plane you want to see the local weather out the window, right? If you want accurate information from an airport 50 miles away, on the other hand…

So, let’s summarise. Traffic Global gets its weather from X-Plane, because X-Plane is guaranteed to be present if you are using Traffic Global. X-Plane’s weather is likely to reflect the real-world conditions anywhere between a few minutes and several hours ago, not the actual weather reported this instant by somebody on the ground in a given location. And while X-Plane obviously handles different weather in different places, plugins can only read or write the weather at precisely your location and nowhere else.

Even with real-world weather switched on and a plugin updating it frequently, what you see in the simulator is unlikely to match what a true realtime report tells you except if the weather’s been stable for some time. Traffic Global uses the simulator’s weather report because whatever weather the simulator has is the weather that the simulated airports have.

In even shorter form: “Real-world” simulated weather is highly likely to be at best similar to actual right-now real-world weather.

Reduced to one sentence: Take a bow, FlightRadar24, the butler’s got your coat.

No, harder than that.

I mentioned that X-Plane will only allow plugins to read and write weather for your precise location. This has a second unwelcome effect in that if Traffic Global wants to select the correct runways at, say, all airports within 100 miles of you, they will all get whatever weather you would currently see out your plane’s windows if you were on the ground immediately beneath you, no matter where you are. If you actually are on the ground at an airport that’s fine, but if you’re in the air then the weather that will be reported near the airport might be quite different. As you get closer to the airport, the weather at the airport will get closer too, so that’s all fine, right?

Well, it would be if it wasn’t for a second consideration: Everybody else.

What do you mean, I need to queue?

Problem the second: Other people. In this case, other aircraft. As you get closer to the destination airport, the weather gets closer to whatever the sim thinks it should be in that area. All good so far. Clouds thin, the windspeed drops, the wind direction changes by ten degrees or so, and the airport’s flow rules choose the right runway for that set of conditions. Or, more likely, they don’t.

A flow rule is a set of conditions that tell an airport, and anyone using that airport, what runways are in use. They’re based on a number of things: time, visibility, ceiling, but most importantly, wind speed and direction. If the wind direction changes, a different flow becomes active and that determines what runways are used, and in which direction.

Wind’s from the east, land into wind, it’s a bit of a no-brainer. But what about all the other planes that were assigned runways ten minutes ago when the wind was different, whether that difference is down to X-Plane’s weather observer being sat in your co-pilot seat or an actual change in conditions? They’re already on final, turning onto approach, lined up at the end of the runway waiting for access… You need to allow a bit of time for all of those to clear. That means you can end up with an inappropriate flow active for a short while after the conditions change. I hope that in the real world this is handled a little more elegantly but within the simulator we struggle to get decent weather reports for right here, right now let alone the future, so lagging behind the weather by a few minutes is the best option available.

Each airport in the area will have different traffic active at any given moment, so while one airport might be able to change the flow immediately, another might take quite a while longer to activate the new flow.

And what if the wind changes back again soon after? Again, in the real world I hope that the controllers are taking forecasts into account and not changing flows back and forth, but inside the simulator… yeah. So, Traffic Global deals with this by just refusing to change the flow too often otherwise you can end up with an airport being permanently backed up because everything’s waiting for a flow to clear, all the time. All it takes is a wind variation of a few degrees around a certain heading or a knot or two either side of the speed boundary, to do this if that heading or speed happens to be very close to the changeover between two different runway flows.

So, Traffic Global’s flow selection might lag behind the weather for ten minutes or so if the airport’s busy. It might switch back and forth between different flows too often because it doesn’t have access to forecasts, only the weather that’s already happened. That weather is probably wrong for this airport if you’ve flown in, because X-Plane only reports the weather at your location, and it may even then be based on weather that’s hours out of date compared with the real world.

On top of that, the real-world airports will also have their own rules about when to change flows. It almost certainly won’t be based on the weather right this instant, but either lag a little behind or lead the weather changes, and will still have to wait for aircraft to clear the old flow before the new one can start if they want to avoid an almighty pileup which is generally seen as A Bad Thing.

Combine those together and we… hey, FlightRadar24, are you still in the building?

But you’re not even using the same runway that X-Plane’s ATC gives!


X-Plane’s ATC system is as shy as its weather system. You, dear pilot, can interact with it by pushing buttons. Plugins can’t. There’s no way to ask X-Plane what it thinks the correct flow is for a given airport, just like there’s no way to ask X-Plane what the weather is for a given airport. There’s also no way to tell X-Plane where other aircraft are; the ATC system is aware of its own AI planes and nothing more.

Same airport, same weather – you said that already – so same runway. Gotcha.

Kind of. Yes, if you’re at or very close to the airport then Traffic Global will see the same weather that X-Plane does for that airport. One thing, though: Traffic. X-Plane has absolutely no idea that there’s any traffic in the area, except you. So, it does what Traffic Global does, and what real airports do: it works out what the local conditions are, it looks around for aircraft that are busy using a runway they’ve already been allocated – that’ll be none – and it changes the flow immediately.

Traffic Global, on the other hand, has two on final to parallel runways, a bunch more in the circuit to land, a queue at the end of a long runway with only one parallel taxiway, and decides on a whim to be nice and not to set the tanks on them and sweep the debris off the tarmac just because this one really picky sod about thirty miles out point-blank insists that some website says we should be using a different one.

So just let me choose the active flow so that it matches the real world and that solves the problem.

So as you approach an airport you bring up a dialog box, tell the airport what flow to use by picking it from a list – just like real pilots do – and then watch your radar and TCAS systems go blank followed by a bunch of airliners popping into existence around you, all in the name of… uh… realism…?

So, uh, what runway’s in use, then?

Traffic Global says 08L. X-Plane says 08R. FlightRadar24 – really, somebody call security – says it’s 35.

Question: When you approach a real-world airport to land in a real-world aircraft which will real-world kill you if you insert it into another one, who do you contact for approach instructions:

  1. A well-known realtime aircraft tracking website
  2. The regional controller, such as Farnborough or London Center, which doesn’t have a scoobie about what’s going on once you drop beneath radar altitude
  3. The airport itself, which actually knows about local conditions and traffic and is issuing instructions to all other nearby pilots

Option 1 is… OK, just release the hounds.
Option 2 is X-Plane.
Option 3 is Traffic Global.

About the Author: jimkeir