Traffic Global – Airport Design

One of the key design decisions in Traffic Global was to avoid using any additional files to help with navigation, and to avoid any special, Traffic Global-specific rules or encodings. All the data needed comes directly from files that are required by X-Plane itself and would be fully used by X-Plane itself. Although this does impose some restrictions, the benefits are that most existing airports work, at least to some extent, with no further changes or downloads required.

If an airport does need modifications, this work can be done with existing and well-understood tools (i.e. WED) and those changes uploaded the X-Plane Scenery Gateway, for the whole community to benefit, whether they use Traffic Global or not.

Runway Flows

Traffic Global uses exactly the same rules as X-Plane does when deciding on the runway-in-use. Larger airports will usually use predefined “flows”; sets of rules that determine which runways are in use, depending on various conditions. The flows are tested in order and the first matching flow is used.

One are that seems to cause some confusion is multiple runways in use. Many larger airports will have different runways for different purposes, or parallel runways, in use at the same time. The official documentation clearly says “The first flow to ‘pass’ will be used” so how is it possible to have multiple runways operating?

This is done by ensuring that the rules for different runways are different. If two runways belonging to the same flow have identical rules, then the first one and only the first one will be used. The simplest ways to make sure that both runways are used are to either set them for different aircraft types, or probably more usefully, set them to have different ranges for the flight’s on-course heading.

For example, consider two parallel runways as in this screenshot:

Example of parallel runway flows

Since the first acceptable runway will always be used, this example shows that 27R will always be used for “Jets”, “Turboprops” and “Props” because those are accepted on both runways, and 27R is listed first. However, note that both runways have no “min/max legal On-course Heading”. It would make sense to limit arrivals on 27R to those arriving from the north and arrivals on 27L to those arriving from the south. By doing this, you would move 50% of the arrival traffic onto the other runway instead of concentrating it all on the first in the list.

Traffic Global uses the “Traffic Type”, “Operations” and “min/max legal On-course Heading” values for runways, and all rules for selecting flows.

X-Plane offers no way of determining weather conditions at a specific location, only at the user’s location, so this is used for all airports. In practise this is unlikely to cause problems since you are most likely to be concerned with runway-in-use at an airport very nearby.

Any runway that is completely inaccessible, i.e. that has no runway entry or exit taxiways defined, will not be used.

Taxiways

When routing around the airport, Traffic Global uses all available data for taxiways except the “ILS Precision Area”. Runway entry and exit zones are treated with different rules to general taxiways and aircraft will not travel along a taxiway that is too small.

If absolutely no other route is available, the “One way” and “Size” rules will be ignored. There are a number of airports which have, for example, one-way taxiways leading to parking slots or terminal cul-de-sacs which would otherwise never allow aircraft to leave. These rules will only be ignored if no possible route exists; an extremely long and convoluted route will always be used before ignoring taxiway rules.

The exact route that is chosen depends on a complex set of heuristics which are likely to change, and so is deliberately not document. Rather than having designers deliberately create airports with stupid taxiway layouts to try to enforce a specific route, it is preferred that the airport taxiway routes are designed sensibly.

In general though, Traffic Global will select the route with the lowest cost at the time. The cost of a route will vary in time, since it is affected by how many other aircraft are on that route, and which directions they are travelling in. The length of each segment in the route, the sharpness of corners, the width of the taxiways, whether or not they are on or off runway and several other factors are considered when calculating a route’s cost, and this will be re-calculated potentially many times as an aircraft moves around the airport, depending on the level of congestion.

A problem that has been seen at many airports is that taxiways do not extend fully into parking areas such as terminals. When calculating how to enter or exit a parking slot, the nearest accessible taxiway is extremely important so if no taxiway is nearby, either the pushback may be inappropriate or, more likely, the parking spot will simply be marked as unusable.

In this screenshot (of the default KMIA), the highlighted terminal parking spots would be less than ideal because the taxiway does not extend behind the parking spots. Taxiways do not need to be added leading to each individual parking spot, only nearby either in front or behind.

From V1.0.8660, Traffic Global will try much harder to link parking to taxiways, based on extending the nearest accessible taxiway in a straight line. In this example, this would not be a good solution since in each case, the taxiway stops at a distinct change in heading and so the “invented” extension would pass through other parking or buildings. Simply adding a taxiway over the existing paint lines to the end of each terminal area would make all these parking spots behave much better.

Inaccessible parking spots

Parking Allocation

Traffic Global will use all available information when selecting a parking spot. An aircraft will only use an appropriate spot under most circumstances.

Airline reservation restrictions are used when deciding if a parking spot is usable. A parking spot can be reserved for one or more airlines by adding ICAO codes to the “Airlines” field, as defined in the apt.dat specifications:

zero or more space separated 3-letter airline codes

Airline restrictions can be optionally disabled by the user, with Traffic Global’s “Never Steal Parking” option. This is on by default but when switched off, an aircraft will use a parking spot reserved for another airline if, and only if, it is unable to find any other usable parking first. This option was added because a number of airports have all parking spots reserved exclusively for one airline, effectively blocking all other airlines from that airport.

Available parking spots will be chosen according to a preference order determined by the “Ramp Start Type”:

  1. Gate
  2. Hangar
  3. Tie-down
  4. Misc

Some parking spots are always automatically removed or marked inaccessible. Any parking that would block a taxiway or runway is automatically marked as unusable, because an aircraft parked there would totally block movement of all other aircraft. These parking spots are very often added as convenient starting spots, such as at the hold-short points for runway access.

Other Airport Data

The “Tower Viewpoint” location is used as the default position of the custom tower views (free look and following an aircraft), including the height. This location appears to be wrong in a great number of existing airports, often with latitude/longitude switched, the elevation incorrect, or simply located many miles away from the actual airport. In each of these cases it is ignored and Traffic Global will use the centerpoint of all the airport’s runways instead. Use WED’s “Zoom Selection” feature (Ctrl+/ ) to check the location of your viewpoint.

Since V1.0.8660, a very basic pseudo-ATC call has been added which will respond to a keypress with a designated runway and will reserve parking for the user. This works by using the aircraft’s current COM frequency and matching it against all nearby airports. If your airport has any radio facility at all, please make sure the ATC frequencies are set correctly.

If you are working with an older airport, it is worth noting that WED does not automatically upgrade the airport’s metadata when you re-export the apt.dat . This can lead to a situation where you have made changes but the airport remains largely unusable by Traffic Global, since some key records are not present before X-Plane 11.30. If this is happening, check WED’s “Export Target” setting, on the File menu, and ensure it is set to 11.30 or newer.

Checking In the Simulator

The acid test of an airport is to watch how it is being used by the simulator. The first step is simply to watch the area of the airport that you are concerned with to see what happens. If you want more specific information, typically regarding parking accessibility, the best way to do this is to open Traffic Global’s “Flight Plan” view (Ctrl+F1 by default on Windows) and zoom in close to your airport.

This view, from V1.0.8660, will have an optional legend showing the meaning of the various lines and symbols which should allow you to quickly see the state of your parking, runways and taxiways.

Traffic Global’s Flight Plan View

Summary

Ideally, Traffic Global will use existing airports with no modifications. If an airport is behaving poorly, hopefully this article will show how to improve that behaviour but it not intended as a guide on how to specialise an airport to work with Traffic Global; all changes should simply be to clarify existing designs in a way that would benefit any product using X-Plane’s airport definitions.

Traffic Global’s rules are highly likely to change over time as situations are reported where an airport’s design is already reasonable but is being interpreted badly so please do not try to second-guess or force a specific behaviour! Especially regarding real-world correct taxi routes, X-Plane offers no way to designate a particular taxiway as being the correct one to use in any given situation since these would need to be associated with the active runway flow. People have spent hours or days minutely tweaking taxiways to force aircraft down a specific route only to find that a small change in wind direction, or even time of day, completely breaks their airport.

Traffic Global should not require any special or unusual design considerations but it certainly benefits from having the airport being cleanly laid out. It is hoped that this article will help you to understand which areas of your airport’s layout have the largest effect on Traffic Global’s interpretation of it, and where some of the common problems lie.

About the Author: jimkeir

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