topic: Scenery

ATC Taxi Route Authoring

Basics

The ATC Taxi routes are used by X-Plane ATC to provide navigation guidance around the airport.  ATC does not use any information other than the taxi routes; it does not matter where concrete and asphalt actually are, only where the Taxi Routes are.

Taxi Route Authoring in WED

A taxi route is a collection of line segments. Each taxiway segment can be defined as:

• One way
• On a runway
• Departure/arrival hot zones
• ILS precision areas
• Size restricted

In order for the taxi route to function correctly, a few things must always be true:

• Taxi route segments cannot cross each other unless they are connected by a common node. WED automatically prevents this from happening by placing a node where two crossing taxi route segments overlap.
• The ENTIRE grid of taxi routes must be connected – there should be a continuous path from any one route to any other route.  (This means you CANNOT build an airport with disjoint movement areas, even if it exists in the real world!)
• Every runway that is used in ANY flow needs to have a taxi route on it so aircraft can get from and to the runway.

Segments are color coded when viewed in the ATC Taxi + Flow tab, to provide an immediate sense of the properties assigned to each segment.

Basic Taxi Route Segments

The most basic taxi route segment that can be defined is for taxiway on the pavement surrounding an airport. Typically these are drawn over the line markings that guide aircraft around the airport. They are a collection of nodes and segments which are color coded yellow in the map pane.

These segments are typically named to match the real world route letters. You can leave the name blank for very small connectors and routes along the gate line. They can also be defined as one-way routes if planes always use it in only one direction.

To make a new taxi route, select the taxi routes tool and set the name and other properties in the tool bar, then click in the map pane to place the taxi route segments. The taxi route tool does not support bezier curves, so use short segments to approximate curves.

If you cross an existing taxi route segment while creating a new taxi route, WED will automatically insert a node at the intersection.

Runway Taxi Route Segments

A runway taxi route is defined by selecting the appropriate runway from the Runway drop down menu. The Name, Departures, Arrivals, and ILS Precision Area fields will be all be set automatically by WED. Runway segments are normally color coded blue in the WED map pane, but may also be purple – if portions of that runway are also hot zones for other runways (see below for information on hot zones).

For all runway taxi routes:

• Taxi route nodes must be inside of the runway’s bounds. Only a small overflow zone past the ends of the runways is allowed.
• Taxi routes must be parallel to and as close as possible to the runway’s centerline.
• The runway must be sufficiently covered by the taxi route. This includes displaced thresholds, but excludes blast pads.
• All runway taxi routes must be properly connected to the rest of the taxi route, as discussed in the Basics section

The property “Size” of runway segments has no function at all in X-plane.

Hot Zones

Hot zones are a location on an airport with a potential risk of collision or runway incursion, and where heightened attention by pilots and drivers is necessary. Usually a segment of the taxiway closest to the runway, or where runways overlap, hot zones are color coded red or purple (when crossing or on a runway) in the map pane.

All taxi route segments inside the hold short lines of the real airport should be marked as hot zones. The presence of the hot zone will cause the AI aircraft to correctly hold short at those hold short lines.

If the real world airport is missing a hold short line on one side of the runway (this is rare, but does sometimes happen) you still need to mark a hot zone approximately where the aircraft would have to stop in order to be out of the way of runway operations.

Runways need hot zones too. When two runways cross, each one needs to have a hot zone marked near the crossing point of the other runway. If there are real land-and-hold-short markings (LAHSO) lines on the runway, this is a good place to start the hot zone. The hot zones are used when a runway is inactive (and being used as a taxiway), and also to detect conflicts between a landed aircraft that is taxiing off the runway and crossing arrivals and departures.

ILS Precision Areas

An ILS critical zone is a designated area of an airport that all physical obstructions must remain clear of when one or more Instrument Landing Systems are in use, to protect against signal interference or attenuation that may lead to navigation errors, or accident. ILS critical zones are color coded orange in the WED map pane.

Taxi Route Sizes

Each taxi route segment has a size letter, which restricts the use of that segment by aircraft larger than the specified size. The sizes depend on only the aircraft’s wingspan in X-Plane and are identical to ICAO aircraft size classifications:

Size        Wingspan        Examples

A        < 15m        C172, B58

B        < 24m        King Air C90

C        < 36m        B737, A320, MD-80

D        < 52m        B767, A310, MD-10

E        < 65m        B777, B747, A340

F        < 80m        A380

Although this is useful to prevent oversized aircraft visibly hitting their wings on nearby scenery objects, care must be taken to not specify small sizes for no valid reason. The maximum size of aircraft that may taxi or be present at a given airport is limited by the largest size taxi route size specified in any of the ramp starts at that airport, plus runway size restrictions. If there is no path from any ramp start to the active runway wide enough to accommodate that aircraft, the ATC system may come to a full halt.

Taxi Route Obstructions

A similar “stuck forever” situation can occur if ramp start positions that can be used by AI (of type Tiedown or Gate) are on or close to taxi routes. ATC route planning will not check for such obstructions ahead of time, and if the shortest route to the runway happens to go along a route obstructed by a parked aircraft, AI traffic will stop indefinitely as it cannot re-route.

Validation

WED 1.5 (and greater) now has more validations to ensure airport ATC taxi routes are accurate and sensible.

If there are no ATC runway use rules specified, all runways are examined. Otherwise, only the runways mentioned in a runway use rule that also have at least one runway taxi route of the same name are checked during validation.

WED will display the boundaries where ATC taxi routes need hotzone after any failed validation in purple.

Final Tips

A good rule of thumb is to avoid “micromanaging” taxi routes. ATC and Ground Vehicle routes are intended to get traffic close enough to their destination that the built-in logic can taxi the vehicles to their final destination as needed. Specify only the preferred routes, as in general X-Plane can not tell non-plausible, obsolete or rarely used routes from the ones where you want aircraft to go.

Avoid connecting taxi routes directly to ramp starts so that the aircraft has room to maneuver between the taxi network and the ramp start for the best, most realistic results.

Use the fewest segments possible to specify only the desired & safe taxi routes. The taxi route is determined without checking for obstructions along the entire route–only the very next segment of the route is checked ahead of time, and the aircraft will not reroute if it becomes stuck.

ATC Flow Authoring in WED

Basics

Real world ATC is all about efficiency – runway space at major airports is limited, so ATC aims to use the existing runways, taxiways, and airspace in the most efficient manner to arrive and depart as many planes per hour as possible. The “flow” system in WED/X-Plane aim to model real world procedures that were designed for real world efficiency.

An airport ATC “flow” defines how the runways in an airport are used. Each flow tells ATC:

• Which runways are used, and in which directions
• Whether the runway is used for takeoffs, landings, or both
• What kinds of planes use which runways.

Real world flows are often named after the direction of traffic, e.g. “east flow” and “west flow” but these names are never exposed to pilots. In the same manner, WED’s flows are named for reference and log output only and are never displayed to X-Plane users.

Flows are picked based on wind and weather conditions so aircraft can land and take off into the wind. They are also sometimes based on noise abatement – the route the aircraft flies may be restricted to not fly over residential areas at low altitude and high power.

Only one “flow” is used at an airport at a single time – each flow is designed so that all of the runways used in the flow can be used at the same time safely to have maximum efficiency at the airport.

While X-Plane doesn’t move as many airplanes as KORD, we support the same kinds of rules for realistic routings and flow.

A detailed example: KBOS

Boston Logan has five runways that it uses for major operations: two parallel runways (4L/4R), two near-parallel runways (33L and 32) and one additional runway (9). The airport also has a noise restrictions: no aircraft ever land on runway 14 or depart on runway 32.

Based on this, KBOS has four possible main flows:

“Northeast” VFR flow:
Jets land on 4R
Jets depart 4L and 9
Heavy jets depart 4R
Props land and depart 4L

“Southwest” flow:
Jets land on 27
Jets depart 22R
Heavy jets depart 22L
Props land 22L and hold short of 27

“Northwest” flow
Jets depart 27
Heavy jets depart 33L
Props depart 27
Jets land 33L and 27
Props land 34

“Southeast” flow
Jets land 15R
Props land 15R and 15L (15L is tiny)
Jets depart 15R
Props depart 14

These flows are ordered from most efficient to least efficient for the airport. The southeast flow is a huge bottleneck because KBOS can’t land on runways 9 or 14; all planes have to land on one runway. It is also hard for ATC to land a prop behind a jet because of wake turbulence rules and the difference in speeds. By comparison the northeast flow provides the highest operations rate, with jets landing parallel to props (with each stream of aircraft packed tightly) and room for completely independent departures on runway 9.

Flows in WED & X-Plane

Selection Rules and Priority

The main rules for flow selection in X-Plane are:

• Only one flow may be running at a time.
• X-Plane will evaluate each of your flows in the order they appear in WED – the top flow in the hierarchy is evaluated first.
• The first flow in which all of its rules “pass” is selected.

This means flows should be arranged in WED so that the ideal, most-used flow is first. It will be picked the majority of the time and will only be bypassed if it absolutely cannot be used. In our KBOS example, you would want to arrange the flows in WED in the order they’re listed above (ordered by high to low efficiency).

It is important that for any conditions at least one flow be selected; given bad enough weather conditions in real life the airport might be shut down, but in X-Plane the airport must remain open. So it may make sense to have your last flow have no rules, so it is a “catch-all”.

Flow basics & rules

Each airport in WED can have one or more flows – if no flow is provided, X-Plane just makes one up.

Flows contain two kinds of data:

• Runway use rules tell which runways will be used.
• Restrictions that control whether the flow can be used at any given time in the sim (time & wind rules).

Runway Use Rules

Each flow has one or more “runway use” that describes a particular runway with use restrictions.

Runway use rules describe:

• The end of the runway to start from
• Departure frequency for aircraft departing from this runway using this rule
• Whether it is used for arrivals, departures, or both
• The type of aircraft operations
• Departure direction range
• Initial assigned heading range

Flows also have a “pattern” runway for VFR traffic patterns, but X-Plane ignores the Pattern Runway and Pattern Direction fields on flows because we don’t support VFR traffic patterns yet. You must still include a runway use – just having the traffic pattern runway will not make that runway used. (The VFR pattern runway on the flow doesn’t tell us what kinds of planes use the runway, etc.)

The runway use rule for the pattern runway should include both arrivals and departures. But do not set a flow to allow arrivals (or departures) on both ends of a runway, or you risk head on collisions! For example, do NOT arrive on 4R AND arrive on 22L. In the majority of cases, if a runway allows arrivals and departures at the same time they will go the same direction and use the same runway.

It is okay to have a runway in a flow more than once – you might need this for complex rules.

Example:

KBOS north flow:
4R – arrivals, jets + heavies
4R – departures, heavies

In this case we need to use the 4R runway use twice. For arrivals jets and heavies land 4R, but for departure, ONLY heavies use 4R. The logic behind this case is the heavies need the long runway 4R for departure, but the jets can depart runway 9 for more operations.

You can set this example up in WED two different ways with the same result.

Option 1:
Create a use rule and set traffic type to Jets & Heavy Jets
Set Operations to Arrivals only
Create another use rule
Set traffic type to Heavy Jets
Set operations to departures

Option 2:
Create a use rule
Set traffic type to Heavy Jets
Set operations to departures & arrivals
Create a new rule
Set traffic to Jets
Set operations to arrivals only

The departure frequency field is required but is not fully implemented in X-Plane. Unless you have created a custom ATC frequency file, this field will be rewritten by X-Plane to match its 23 ATC frequency sectors. (ATC departure frequencies can be specified in WED but will only appear on the local map as pilot reference. They are not actually in use by X-Plane at this time.)

Runway uses also include the departure direction range. This is the rough direction from the airport that the aircraft will fly (e.g. if we depart KBOS for San Francisco, our direction is west). This is specified in the Legal on-course hdg min/max in WED. Given two runways that can both depart an aircraft, X-Plane will pick a runway based on departure direction when possible. This lets you specify things like “north departures use 27R, south departures use 27L”, to avoid aircraft crossing in-air.

Runway uses also contain an initial heading. These are the ATC Assigned hdg min/max fields in WED. This is the heading that ATC will assign the aircraft off of the runway – it can be a range so that ATC can “fan out” aircraft for faster departures. This is another case where two runway uses can be used, e.g. from our example KBOS northeast flow:

9 – departures – jets, initial heading 090, departure heading 000 – 360
9 – departure – props, initial heading 150, departure heading 090 – 180
4L – departure – props, initial heading 360, departure heading 180 – 090

In other words, most props will depart runway 4L and head directly north. But props departing to the south or east will depart runway 9 and immediately turn south-east, getting out of the way of jets. Jets will take off runway 9 and go straight.

(The southeast runway 9 heading lets small airplanes going to cape cod go straight out toward their destination – the jets fly straight to get out over water and to not make noise over the city of Boston.)

Note that departure direction and initial direction do not have to match. E.g. at KBOS a jet whose departure direction is West might still get an initial direction of East – this means the plane initially flies the wrong way (usually for noise abatement) and is then turned around.

Time, wind and visibility rules

Flow rules come in three flavors:

• Time rules define during what times of day the flow may be used.
• Wind rules define under what wind conditions the flow may be used.
• Visibility rules define what visibility is required to use the flow.

You can have more than one wind and time rule for a flow; if you do, the flow will pass if any of the rules of that type pass. In other words, you can do this:

Flow “northeast”
Wind < 4 knots, 0-360
Wind < 15 knots, 270 – 090
Flow “southwest”
Wind < 15 knots, 090 – 270

In other words, if the wind is less than four knots or the wind is coming from the north, we use the northeast flow. This “or” behavior is useful because often airports will use the most efficient flow when the wind is very low.

Similarly, the time rules can have more than one time, e.g. for two peak periods.

For all weather-related rules (wind, visibility), you specify the ICAO code of a METAR reporting station. For major airports, this is the airport’s ICAO, but for small satellite airports, you can use the ICAO code of a larger nearby airport. This is for cases where a small GA airport’s runway use must be changed to affect the runway use of the neighboring large airport, or where several airports must adjust their runway uses all at once due to proximity.

Typically an airport with multiple runways will have higher priority “VFR” flows with higher visibility requirements and multiple runways in use, and then lower priority “IFR” flows with no visibility requirements. This lets X-Plane’s ATC use more runways in good conditions and just one runway in bad conditions.

Debugging Flows and Flow Changes

When the weather changes, X-Plane will wait one minute before beginning to change flows. When it starts to change flows, the sim goes through three phase:

• Departure drain. All new departures are held, but existing departures are allowed to depart using the old flow. All arrivals continue to arrive using the old flow.
• Arrival drain. Once the number of non-held departures falls to zero, all arrivals not on final are re-routed to the new flow. Existing arrivals are allowed to land on the old flow.
• Complete change-over. Once the existing legacy arrivals have landed, the new flow is fully in effect and the ground hold is lifted.

Note that if the weather changes back to the old flow in phase 1, the old flow is resumed; if the weather changes back in phase 2, we continue to the new flow.

The logic behind step 1 is: we have to let all taxiing departures depart on the old runway because there might not be enough room for aircraft to turn around on the pavement. While this is happening we don’t re-route arrivals to avoid a head-on collision. The logic behind step 2 is: as long as pending arrivals are arriving in the old runway, we can’t release the gate hold because the departing aircraft might block the arrivals’ route to the gate.

By default the ATC system logs some information about flow changes to Log.txt, but you can turn on advanced and detailed logging by setting the art control atc/debug/rwy_flow to 1. Detailed logging starts after the art control is set. Log.txt can be viewed in real-time using the “developer console” menu item in the Special menu.

You can also view more details about runway selection by setting the art control atc/debug/rwy_selection to 1.

Guide to Ramp Starts

Starting with X-Plane 10.50 and WED 1.5, ramp starts can be customized with a lot of data to help specify their use. Ramps starts should be designed based on their use in the real world and should never have aircraft scenery objects placed on them as this would interfere with the “Draw Parked Aircraft” and “AI Aircraft” functions in X-plane 10.50 and newer.

Ramp start type

The majority of ramp starts should be either Gate or Tie-down spots. Static aircraft will show up only at these ramp types.

Misc and hangar type ramp starts should only be used for human player starts.  X-Plane will ignore Misc type and will not spawn static aircraft there. When Hangar is set, X-Plane assumes the aircraft is in a building (and therefore not visible) and won’t spend resources placing aircraft there. AI aircraft will use the Misc and Hangar types as a last resort, if no tie down or gate spots are open.

Set only one type from the drop down here.

Equipment Type

This field determines what type of aircraft should use this parking spot. Set at least one type from the drop down here, but multiple types are supported on the same spot. The differentiation between “Heavy Jets” and “Jets” is historic – all aircraft size E or larger are usually classified as “Heavy” in X-Plane, while those of size D and below as “Jets”. It is recommended to always include both “Heavy Jets” and “Jets” as equipment types for all ramp starts of size D and larger to avoid creating unintended restrictions.

Size

This field determines what size (wingspan) aircraft can be drawn as parked aircraft at that spot. X-Plane will place aircraft that are marked in X-Plane’s scenery library as the specified size & one lower (i.e., size C will be used by size C & B aircraft).

The classification of these sizes follows ICAO Aircraft Size Standards and is as follows:

Ramp Operation Type

To further limit what type of aircraft can be shown as parked aircraft at that ramp, set an operation type. When set to “none,” no parked aircraft will be placed, but AI aircraft of a suitable size can still use this ramp start. Pick only one type from the drop down here.

Airlines

Case insensitive in WED, three letter ICAO codes as per the  Airline Codes Wiki Page separated by spaces.

If you have included more than one airline code in this spot, X-Plane will randomly pick one before the available liveries in the library are checked. If the airline code X-Plane selected is not found in the user’s library, a random result will be placed instead. So limit your definitions to airlines code most people actually have static aircraft installed for, otherwise most people will only see completely random liveries at those starts. X-Plane by default includes liveries for only a small number of major airlines, but these will always work. See the category lib/airport/aircraft/airliners/ for available liveries on ramp starts marked as Operation Type=’Airline’. Or the equivalent categories corresponding to the other operation types, if selected.

Designing sceneries with Ramp Starts

All ramp starts of types “Tiedown” or “Gate” must be placed to allow aircraft taxing along ATC taxi routes to safely pass any aircraft parked there. To check for interference, switch to the “Taxi+Flow” view mode in WED and verify the yellow aircraft shapes are completely clear of the ATC taxi routes depicted in any color, with preferably 10-20 feet of clearance to spare.

Debugging Ramp Starts & AI Aircraft

If AI don’t seem to be using your airport or ramp starts, there are a couple art controls that you can turn on to help debug the situation. (Note you will need to set the art control then make sure the airport reloads by leaving entirely and coming back.):

• atc/debug/log_spawn=1  (Prints log info on AI aircraft arrival & departure logic. Use developer console to see this in-sim)
• airp/debug_ramp_starts=1 (Leaves on screen a bunch of visual debug markers telling you about the ramp starts, so if a static airplane is missing or wrong you can SEE the data.)

Correctly Forming Taxiways and Junctions

Revision History:
         01/26/18   Updated with Joining Taxiways to Runways
         06/25/08   Updated With Taxiline Guidelines
         10/25/07   Initial Draft

Correct Formation of T Junctions

A “T” junction is any connection between lines in an airport layout where one line ends in the middle of another line. T junctions would include a T-shaped meeting of taxiway lines (shown in these examples), but the junctions formed by the edges of taxiway pavement are also T junctions and need to be formed using the same technique.

The key points are:

• Two lines in a WED layout (of any type) should always meet at their endpoints.
• One line should not end in the middle of another line.
• This means that you must split a line at the point another line intersects it.

You should not simply attempt to visually line up your layout. Consider this T junction:

It looks correct but upon zooming it becomes clear that the taxiway line ends “near” the next line but is not actually on it. With no vertex to join to, this join can never be clean.

Junctions like this can suffer from all sorts of problems in X-Plane:

• The alignment error may look significantly worse in the sim–the only way to guarantee a good connection is to have two vertices with the exact same location.
• If one of the lines is a bezier curve, the approximation of the bezier in X-Plane may be different from WED, causing a disconnect.
• If these lines were taxiway edges, the “sliver” of a gap could cause bumps when a plane rolls over it or ugly rendering errors in the pavement.

The correct way to build a T junction is a two-step process.

Split one of the lines so there is a vertex at the location where the two nodes will meet. :

Then drag the connecting line to that vertex. In WED you can use the “snap to vertex” option to get precise positioning of the two vertices on top of each other.  :

This T junction will look correct at any zoom, and will render correctly in X-Plane.

Remember: close visual alignment in WED is not good enough! You must create an extra vertex and snap the vertices together to create precisely aligned layouts.

Correct Bezier T Junctions

When a T junction is made up of a bezier curve, another issue comes into play: besides needing the end vertices to be snapped together, the bezier curve handles must not be “over-curved”. Consider this layout:

It looks okay in WED, but note that the lower control handle of the bezier curve is on the left side of the straight taxiway line (and yet the bezier curve itself is on the right side. This is a mistake in the layout! Here’s what it looks like close up:

The problem is that the bezier curve control handle (by being on the left side of the taxiway line) pulls the bezier to left a little bit too much. The result is that the bezier crosses over the centerline before joining with it. This error looks small in WED, but in X-Plane, where the taxiway lines aren’t as smooth, the error is significantly more noticeable.

In order to fix this problem, the lower bezier control handle must be on the same side of the joining line as the curve itself–in this case on the right side.

This creates a bezier curve that looks good no matter how closely it is zoomed.

The key here is to err on the side of caution: position the bezier control point on the right side of the connecting line; do not trust the preview of the curve to show if there is a problem.

Do Not Add Extra Vertices to Make Smooth Curves

You must add vertices to make T junctions correct. However, you should use the smallest number of vertices possible to correctly describe your layout. For example: this is a bad taxiway line.

The taxiway line is bad because it uses a huge number of vertices – way more than necessary. The line should be built like this:

Use a small number of bezier curves to shape your lines. Do not add extra vertices in an attempt to make the line look smoother in X-Plane.

X-Plane uses an “error” heuristic to draw taxiway lines–basically it will add vertices to minimize the “squareness” of lines. The problem with adding vertices is that you have to add a very large number of vertices to force X-Plane to render at a higher quality. If you add a few vertices, you increase file size, download time, load time, and RAM use, and X-Plane simply adds fewer of its own points (since fewer are needed to reach the same amount of roundness). Only by adding an insane number of vertices can you markedly improve visual quality; this comes at the expense of RAM use for your layout.

Leave smoothness of lines up to X-Plane; this setting will be wired to the user interface. Some users will need lower rendering quality to use your airport on slower computers. If you add too many vertices, you simply assure that these users cannot fly to your airport at all.

Joining Taxiways to Runways

Taxiways should be drawn to the runway edge, not just to the runway shoulder edge:

Connecting the taxiway to the runway in this manner will result in correctly recessed runway edge lights and a better looking connection between the pavements. The taxiways are automatically drawn above the shoulders and below the runways. If you stop the taxiway at the shoulder edge, it often leaves a gap between the runway pavement and the taxiway.