Category: Air Traffic Control

ATC Taxi Layouts

One of the major components of the new ATC system in X-Plane v10 are the taxi layouts. We sometimes internally refer to this as the “taxi network” because what it’s REALLY comprised of is a network of taxi segments attached to other segments at taxi nodes. The concept is pretty simple, a node connects two or more segments together and it’s a place where a decision could be made by the ATC system in order to change directions. Every single land-based towered airport in the world has it’s own taxi network.

To give you an idea of what a network looks like, here’s one for Seattle (click to see it clearer)

White segments are runways, green-ish (my wife would say “seafoam”) segments are taxiways, red segments are taxiways inside of an active-zone* and the white dots are the nodes. For ground operations, this is what ATC knows about and it’s the ONLY thing ATC knows about. It doesn’t know about buildings or obstacles in the way, it only knows about the pavement. Anytime ATC wants you to get from one point to another, it has to give you a routing composed of a connected list of these segments. The only exceptions are the start and end segments. ATC is only allowed to go “off the grid” to get you onto or off of the network. So if you’re at a gate and you request a taxi to runway 34L, it first tries to figure out the fastest way to get you onto the network. It does this by calculating whether you’re closest to a node or a segment on the network, choosing the closest one and then giving you a straight line to that point. Now you’re on the network. The opposite happens for end points.

So that’s how you get in and out of the network, but what happens along the way? It’s not all that different than the GPS in your car. ATC analyzes essentially every possible routing from your start/end nodes and chooses the one with the lowest cost. Note that I said lowest cost, not shortest distance. Think about the best way to get to work in the morning. Do you take all back roads? Do you maximize highways? Do you avoid tolls? Your decisions are based on far more criteria than the mere distance. The ATC system is no different. Distance is just one of the contributing factors that goes into it’s cost calculations.

Some (but not all) of the other things in the costs are segment type, segment usage, segment angle and the aircraft’s mode of operation. These can be fairly complicated so I’ll summarize them quickly. Segment’s have different types. They can be one way or two way just like streets in a city. Obviously the cost of using a one way street depends entirely on the direction you want to go. If you want to go the legal direction, the cost is nothing out of the ordinary. If you want to go the wrong way, the cost is REALLY high. In other words…only do it if you have no other options! The final segment type is “runway”. ATC tends to avoid taxiing you on the runways because that’d be terrible for airport efficiency…especially if the runway is an active runway. Another contributor to cost is whether or not the segment is already marked to be used. In other words, if another aircraft is taxiing southbound on a particular segment and you want to taxi northbound, that’s a potential problem, so the cost is high. If you’re going in the same direction, it’s no big deal. Segment angle is important as well. ATC does it’s best not to issue routings that result in steep taxi angles. 747’s have trouble making 150 degree turns for instance. Lastly, there’s your mode of operation. Remember how I said ATC hates using runways for taxi purposes? There’s an exception. When you land, it’s often beneficial to continue taxiing on the runway since you’re already on it and you’re already going quite fast. All of these factors contribute to the cost weighting and the route chosen for you is the one with the lowest cost at the time.

So where do these networks come from? Where does the metadata come from? Well, there are thousands of airports in the world that need these taxi networks so that ATC can provide service. It was important to us that ATC would function at any airport out of the box so Ben wrote some insanely cool algorithm that runs “on-the-fly” when you load up an airport. The algorithm looks at the pavement, erodes it inward leaving the “average” center points of all of the edges. This works pretty well but it’s not without it’s problems. The first problem is that it has no clue what the pavement is to be used for. Is it an apron? Is it a taxiway? Is it an access road? Is it a shoulder on a runway? Pavement is generic in X-Plane so there’s no way to know. The result, as you can see in the screenshot above, is that sometimes you get routings on small access roads. Another problem is algorithmic “noise” that’s causes the taxi lines to be quite zig-zagged and wavy. This is why the AI planes often look drunk when they’re taxiing. Often, the segments are not “reduced” or “smoothed” as much as we’d like. You can see the entrances to each runway tend to have a “Y” shape to them because of the “fillet” on the taxiway. This is a byproduct of the erosion code. There is code in there to reduce it as much as possible and “snap” the edges to the runway but it has to be generic enough to work reasonably well at thousands of different airports. Most of the time it causes no problems but there are cases where the “Y” becomes extremely wide because the runway has a thick shoulder created with a strip of pavement. One final problem is the lack of metadata. The active-zones are stamped out around the runways proportional to their size so seldom do they line up with any painted hold-short lines. We have no clue what the taxiways are named. I know what you’re thinking “Why not use the painted taxi markings and signs to aid in the algorithm’s decision making”. That was actually one of the first things we tried and found the data to be too sporadic to be useful and it made the algorithm very slow and complicated.

So if the algorithm has so many problems, why are we using it? Two reasons. First, the alternative is that no airports would have ATC capabilities on the ground until a user made a custom layout by hand. Second, the algorithm was created to be a stop-gap solution, not a substitute for human data! The REAL taxi networks need to come from a human! Once they are made, they’ll be added to the apt.dat and distributed with the sim moving forward. This means that over time, human data will replace the algorithm’s data and taxiing will get better. The taxi network generation is one of the major features of WED v1.2 which will be released in the coming weeks. Authors can create a network for their airports relatively easily. In my first time using WED, it took me only 2-3 hours to create the network and I’m completely clumsy using any kind of CAD type software. Someone who knows what they’re doing could do an equally complex airport in about an hour I would guess.

The layout in the first screenshot above is an autogenerated one. That’s NOT what you’ll be using if you go to KSEA in the simulator and that’s because I made a custom layout. If the sim sees a custom layout, it always prefers to use that. Here’s what my custom layout looks like.

So you’ll notice the lines are straight! There are no taxi segments through buildings anymore, there are no taxi segments on the access roads, there are no “Y” branches at the runway entrances/exits. Also, not visible from this height, the segments are lined up perfectly with the taxi paint and the active-zones perfectly coincide with the hold-short markings as well. I’ve also named each segment properly so that ATC can tell you exactly how to get to your destination e.g. “Taxi via Alpha Charlie Two Foxtrot Delta”.

There really is no substitute for human data. It definitely results in a much more realistic and far less problematic experience.

*An active zone is a taxi segment that’s protected by ATC because it’s adjacent to or in some way affected by an active runway. This is how ATC knows what to protect with hold-shorts and it’s also how ATC knows when it’s time to talk to Ground vs. Tower.

Notes about the ATC system

Many of you have taken the time to tinker around with the new ATC system. Many of you have had a good experience, others have found yourselves frustrated for various reasons. My goal here is to try and explain what you can expect from the system moving forward and describe the causes of some of the troubles that you might be having.

First, as I’ve said before, what you see in the ATC system is just the tip of the iceberg. The features are going to expand moving forward. What you are looking at now is not the way the final system will work. If we waited months or even years before shipping X-Plane 10, I’d still be saying the same thing. The reason is that the ATC system has the potential to be very large and very comprehensive much like the scenery system. It will evolve over time. There will never be a lack of new things that would be cool to have, there will never be a lack of things that can be changed for added realism etc. We have to develop things in stages however and what you see is merely the first stage.

The first stage was designed with one goal in mind: to create a system that’s comprehensive enough to give AI aircraft a purpose. Having AI aircraft buzzing around creates a living airport environment. Coupled with Ben’s new roads and railways, the X-Plane world is no longer a static world, it’s one that’s alive! AI aircraft are very low maintenance. They don’t have many requests, they aren’t picky about their routings. They just want to depart from one airport and arrive safely at another airport and the current ATC does that just fine. We’ve also allowed YOU the user to interact with ATC at this point but humans are not as simple as the AI planes. Your goals are a lot more specific and detailed than the AI goals and the current ATC doesn’t handle that so well. That’s coming! That’s phase two however. Right now, your interaction with ATC allows you to “get into the system” so that you can have a turn to use the runway for takeoffs or landings.

For those of you unfamiliar with the real world ATC system, let me tell you what it’s NOT. It’s not a turn-by-turn Garmin navigation system. ATC’s job is never to be your navigator or co-pilot. Yes, they often do assist pilots but this is NOT their primary goal. Their primary goal is the separation of aircraft within their airspace. Often, when ATC is assigning you headings for vectors, it’s because it’s easier for them to put you exactly where they want you to be than it is to let you fly however you feel. They’re controlling your position for spacing, for sequencing and for efficiency. This typically only happens on approaches where numerous aircraft are all converging in tight spaces to use a single runway.

X-Plane is no different. If you file a flight plan with a blank route, you’re expected to go DIRECT to your destination. You will not be given instructions on how to do that. If you file a route, you’re expected to fly the route by yourself. You will not be told when to turn or how to get from fix to fix.

ATC is also not a weather service. If you want to know the weather, you have to tune to the various AWOS/ASOS/ATIS frequencies around the world. These were in v9 and they still exist today. Yes it’s true that real controllers will often give you weather information if you ask. We may add a little bit of that in the future but even in the real world, it’s not their job…they do it because they like to be helpful people.

The first stage of ATC right now will give you an IFR clearance after you file your plan. They will assign you a squawk code for radar tracking. They will issue you a routing to your assigned runway. They will ensure that you are only allowed to depart when it’s safe. They will stay with you until you’re near your arrival airport. They will provide vectors to your approach (visual/ILS) and then they will issue you a landing clearance and allow you to continue as long as the runway is safe to use. If the runway becomes unsafe, they will instruct you to go around for another attempt. When you land, they will issue an appropriate taxi route back to your gate. That is the extent of what is possible with the first stage. It’s not comprehensive but it’s a foundation to work off of.

What kinds of things might you expect in the next stage? (NOTE: these are not promises. The details of future features have not been finalized but i’m trying to offer an idea of roughly where we’re headed). VFR operations. Requesting higher/lower altitudes in flight. In-air conflict prevention and resolution. Changing destinations in flight. Requesting specific runways. Requesting specific approaches…etc. etc. Please do not make feature requests at this time. I’m just rattling off some ideas from memory.

I have one or two remaining stability issues to clear up before I can fix some of the current usability issues that seem to have come up. For now, here’s an explanation of some of the issues so that you may at least understand what’s going on under the hood. I will say, the current system is a lot like a fire alarm in that it only takes one simple problem like a dying battery to be REALLY annoying. But like a dying battery, a relatively simple fix can make the problem disappear.

1. “I filed a FP but ATC isn’t talking to me” – That’s because you haven’t asked for anything. Tune your COM1 radio to the appropriate controller for an IFR clearance. At big airports, this is the Clearance Delivery controller. At smaller airports, this can be Ground or Tower.
2. “How do i know what frequency to be on?” – You really only need to know the frequency for two people. The guy who’s going to give you your IFR clearance, and the ground controller. After that, you will be instructed by ATC which frequency you should be on for the remained of you flight. You can get the DEL/GND frequencies from a variety of places. First, in the sim you can go to the local map view, enable Airports and then click on the airport that you want the frequency of. Then look at the popup. You can also use real world charts or resources.
3. “ATC is giving me a routing that’s through gates and buildings” – This is because the taxi layouts are auto-generated and have no idea where the buildings are. In the future, airport authors will create detailed taxi layouts that will completely alleviate this problem at their airports.
4. “I taxi to the runway and nothing happens” – If you were talking to ground, he should have handed you off to tower. If he didn’t, then perhaps you didn’t pull up enough. Make sure to taxi to the very end of the yellow arrows…not necessarily the hold short lines pained on the ground. Again, when human authors make taxi layouts, these should align perfectly (as they do in KSEA) but for autogenerated layouts, they may not. Trust the arrows, not the paint.
5. “I contact tower holding short of the runway but i don’t get a takeoff clearance” – This is because it’s not safe for you to takeoff. You have to be patient and wait. There could be someone taxiing on the other end of the runway, there may be an arrival that’s too close, or the last departure may still be in the way.
6. “I’m constantly being told I’m off course” – This is because you are… The problem however is probably not your fault. If you don’t file a route and you leave the routing blank, ATC assumes you’re going “direct”…the current bug is that direct is from the center of the departure airport to the center of your arrival airport. So what happens is that you takeoff, you fly runway heading, you get yourself adjusted, clean up the aircraft….by this time, you can be MILES from the center of the airport where the route line began. Now you decide to “proceed direct” your destination but you’re now paralleling the course that ATC expected you to fly. The real solution to this in the future is me fixing this in various ways but for now, try to fly a course track that emanates from the center of the airport or file a real routing. Real routings may sometimes have the same issues for the same reasons but will be probably be less severe.
7. “I’m constantly hearing AI planes getting yelled at for their altitudes” – yeah this is because some of the AI planes are being asked to climb to FL360 or so and they seem to be climbing in an inefficient manner. They end up running out of airspeed at FL330 or so and then are not smart enough to step climb up the rest of the way so they just stay there on a border-line stall. Austin will need to tweak the AI planes in the future.
8. “ATC’s not talking to me after I depart” – Why should he talk to you? You’re the pilot, your job is to fly the plane the way you filed. ATC is NOT a turn-by-turn GPS. You need to know how to get to where you’re going on your own and you are expected to do so. The ONLY time you can expect vectors currently is when you get within 30nm of your arrival airport. At that time, ATC will issue you vectors for your visual or ILS approach.
9. “ATC forgot about me after issuing me some vectors” / “ATC is vectoring me away from the airport” – Perhaps ATC is not issuing the tightest vectors in the world if you’re in a small GA plane, but it’s not THAT far off. ATC (much like in the real world) typically issues vectors to final using a vectoring Tee (See below). Yes, you may be told to fly away from the airport. Yes he may give you headings that seem out of the way. He will not forget about you. Hang in there. Depending on your arrival angle, you may be given some shortcuts. In the future, I can add some tighter shortcuts as well but it’s actually not that far off. I think in it’s worst case, you can expect an 8nm intercept to final.

So here’s a vectoring tee like the one used by X-Plane’s ATC. The numbers represent the headings you could expect for this sample runway (09/27). Depending on whether you’re approaching from the north or the south, you’ll be given only half of this Tee. Let’s pretend that you were south of the airport…you’d be given a right turn heading 090, then a left turn heading 360, then a left turn heading 330, then finally a turn to 270. Depending on your aircraft type, you may be required to fly away from the airport by several miles. This is normal. ATC has not forgotten about you. You will be turned back toward the airport when the time is right.

So that’s a quick dump of my brain regarding ATC at the moment. There will be more posts in the future about future features and what you can expect. The purpose of this post was to just give you a glimpse of the status of things and alleviate some confusion all around since it’s a completely new paradigm for many people.