This is a presentation of Taxiway Design Section 4: Basic Taxiway Geometry

by the Federal Aviation Administration.

This section discusses the variables of basic taxiway curve geometry.

Standardization.

Standardization enhances safety. Pilots make fewer errors when they know

what to expect, and this is why the FAA recommends standard taxiway

geometry in the form of taxiway widths, turn angles, and turn radii.

As covered in our previous videos and Advisory Circular 150/5300-13,

“Airport Design”, the first step in designing a taxiway system is to determine

the proper straight section taxiway width. These materials also provide

the standard Taxiway Edge Safety Margins (or TESMs) and maximum

Main Gear Widths (or MGWs) associated with each Taxiway Design Group (or TDG).

Standard Turns.

In addition to standard straight section widths, the “Airport Design” AC explains why a

good taxiway intersection comprises no more than three paths for an airplane to follow.

Ideally, these three paths are: straight ahead, a 90-degree turn to the left, and a

90-degree turn to the right, as represented on this diagram of a 90-degree

4-way taxiway intersection.

When 90-degree turns are not possible, we recommend basic changes

in direction, or deltas, of: 30 degrees

45 degrees,

and 60 degrees.

An intersection may include supplements to these angles, namely: 120 degrees,

135 degrees,

and 150 degrees.

However, to navigate these tight turns, a pilot must maneuver at minimum speed.

Taxiway Turn Pavement Geometry.

As discussed in Section 2: Airplane Maneuvering, we based the geometry of the

pavement in a taxiway turn on the path taken by the landing gear of theoretical airplanes

up to the TDG to be accommodated, using Cockpit-over-Centerline maneuvering.

To review, the theoretical "design airplane" has the longest Cockpit to Main Gear

distance (or CMG) and widest MGW in its TDG. Although they are used to define

the limits of the TDG, there are no actual airplanes with these dimensions.

The "Taxiway Turn- Less Than 90 Degree" figure from the “Airport Design” AC

shows he dimensions we used to define each turn. Landing gear paths are

dependent upon the delta of the turn and the centerline radius, which is the

first dimension we determined when establishing FAA taxiway turn geometry guidance.

For turns of 90 degrees or more, we based the centerline radius on a maximum

nose gear steering angle of 50 degrees. We chose this angle to reduce tire

scrubbing, after consulting with aircraft manufacturers. For turns of less than 90 degrees

we chose the centerline radius, to allow the pilot to maintain taxiing speed.

We determined landing gear paths by modeling aircraft movements for the basic

turn deltas of: 30, 45, 60, 90, 135, 150, and 180 degrees

using all critical landing gear configurations for each delta and TDG combination.

We used this model to design the standard geometry of these turns for each TDG.

These basic layouts will accommodate the theoretical "design airplane" in the TDG

and all airplanes in lower TDGs.

We define the rest of the curve using three widths, three lengths, and three radii.

W-0: the shortest distance between the taxiway edge and the centerline

or one half of the straight section taxiway width;

W-1: the distance from the end of the first taper to the centerline;

and W-2: the distance from the end of the second taper to the curve tangent.

L-1: the centerline length of the first taper;

L-2: the centerline length of the second taper;

and L-3: the length along the centerline tangent between the end of the

second taper and the point of intersection of the curve.

R-Fillet: the radius of the inner curve, which is often zero as shown here;

R-Centerline: the radius of the centerline curve;

and R-Outer: the radius of the curve of the outer taxiway edge.

Note that these three radii are not concentric.

For each TDG, tables in the AC like the one shown here show the values

for these widths, lengths, and radii for each standard delta.

Taxiway Turn - Special Cases.

One special case in an intersection allowing an airplane to reverse direction

from one taxiway to a parallel taxiway.

When designing parallel taxiways with crossover taxiways, engineers and

planners must consider both the TDG and ADG. The necessary separation

between parallel taxiways is the greater of that determined by the ADG wingtip

clearance and that determined by the TDG minimum turn radius.

Let's look at the specifications for TDG-5.

Completing a 180-degree turn with a maximum nose gear steering angle of no

more than 50 degrees requires a turn radius of 120 feet, or a separation

of 240 feet, between parallel taxiway centerlines. This would appear to be

two consecutive 90-degree turns. However, it is only possible to use two standard

two consecutive 90-degree turns. However, it is only possible to use two standard

90-degree turns when airplanes will not reverse direction between parallel taxiways.

90-degree turns when airplanes will not reverse direction between parallel taxiways.

Using two standard 90-degree turns to complete a 180-degree turn would result

in the nose gear steering angle exceeding the maximum allowable of 50 degrees.

This animation shows an airplane negotiating a crossover taxiway designed

specifically for a 180-degree turn. The centerline radius must be determined

based on the full 180-degree turn. This, in turn, determines the required

taxiway centerline to parallel taxiway-centerline distance, based on TDG.

Our example design is adequate if our Airplane Design Group is ADG-IV,

with its maximum wingspan of 171 feet, as shown here. These ADG-IV airplanes

(placed adjacent to each other on the parallel taxiways) have a wingtip clearance

of 69 feet, above the minimum requirement of 44 feet.

However, it is not possible to reduce the centerline-to-centerline separation because

this would increase the maximum nose gear steering angle to more than 50 degrees.

If the airplane design group is ADG-V (and the airplanes with a maximum wingspan

If the airplane design group is ADG-V (and the airplanes with a maximum wingspan

of 214 feet are again placed adjacent to each other on the parallel taxiways)

the wingtip clearance would be only 26 feet, well below the required 53 feet.

the wingtip clearance would be only 26 feet, well below the required 53 feet.

In this case, the required wingtip clearance for the ADG governs the design

of the parallel taxiways, so the centerline-to-centerline separation is 267 feet,

as shown in this updated design. Another design goal is to minimize the width

of the crossover taxiway. To achieve the minimum width for the crossover taxiway,

use the requirements of this maneuver to design the centerline radius and fillets.

In this particular example, the centerline radius remains 120 feet because the

addition of a 27-foot straight section does not significantly affect the maximum

nose gear steering angle.

We have also designed parallel taxiways with crossover taxiways for typical

combinations of ADG and TDG. We use the dimensions shown on this diagram

to define configurations controlled by TDG. Lengths L-1 and L-2 are the same

as in a typical curve. In this special case, L-3 is the distance along the centerline

from the end of the second taper to the center of the crossover taxiway.

The two radii, R-Centerline and R-Fillet, are once again similar to a typical curve.

We also use four widths. The first three ( W-0, W-1, and W-2) are typical.

The last width, W-3, is the width of the crossover taxiway at its narrowest point.

As shown here in this portion of a TDG table, the AC summarizes the

centerline-to-centerline distance and related widths, lengths, and radii for each TDG.

We use similar variables including a new length, L-4, to define configurations

controlled by ADG, as shown on this parallel taxiway design. Length L-4 is

the length of the straight section between two 90-degree turns.

The AC includes a table for all known combinations of TDG and ADG. This table

identifies the taxiway centerline-to-centerline distance, widths, lengths, radii,

and steering angle for each group. In many cases L-4 equals zero. Note that these are still

specific designs. The two 90-degree turns are not the same as normal 90-degree turns.

Finally, note that for existing construction there are several combinations of TDG and

ADG for which the maximum steering angle will exceed the standard 50 degrees.

As we noted earlier, W-3 is the width of the crossing taxiway. For higher TDGs,

W-3 can be wider than the runway, placing signs far from the pilot's eye. For this

reason, it is important to coordinate the design of the taxiway system with the local

Air Traffic Control tower. If the airport does not require the capability of reverse turns,

design the crossing taxiway accordingly.

As shown in this animation, when the capability for reverse turns is not necessary,

the crossover taxiway can be much more narrow. For such crossover taxiways,

the centerline layout is always two normal 90-degree turns.

Runway centerline to parallel taxiway centerline distances may be based on airspace

requirements. As shown here, a right-angle runway entrance taxiway is similar to a

crossover taxiway that includes a 180-degree turn. Note, however, that for runway

entrance taxiways, W-3 represents a different value: the distance between the

crossover taxiway centerline and the innermost taxiway edge at the line of symmetry

between the runway and the parallel taxiway centerlines.

Another table in the AC lists the widths, lengths, and radii for right-angle entrance

taxiways for all known combinations of TDG and typical runway centerline to

parallel taxiway centerline distances. TDG-3 does not appear in this table because

two standard 90-degree turns work for all typical TDG-3 runway-to-taxiway separations.

Design a right-angle runway exit taxiway, like the one shown in this animation,

by creating a mirror image of a right-angle entrance taxiway. Overlay normal taxiway

fillets on the runway/taxiway intersection. This method, as opposed to building fillets tangent

to the runway edge, allows the placement of lights and signs closer to the exit taxiway.

This has been a presentation of Taxiway Design Section 4: Basic Taxiway Geometry

by the Federal Aviation Administration.

Produced by Joint Venture Solutions.