Exit Taxiway Geometry
The function of exit taxiways, or runway turnoffs as they are sometimes called, is to minimize runway occupancy by landing aircraft.
Exit taxiways can be placed at right angles to the runway or some other angle to the runway. When the angle is on the order of 30 o , the term high-speed exit is often used to denote that it is designed for higher speeds than other exit taxiway configurations. In this chapter, specific dimensions for high-speed exit, right-angle exit (low-speed) taxiways are presented. The dimensions presented here are the results obtained from research conducted many years ago and subsequent research conducted by the FAA.
The earlier tests were conducted on wet and dry concrete and asphalt pavement with various types of civil and military aircraft in order to determine the proper relationship between exit speed and radii of curvature and the general configuration of the taxiway. A significant finding of the tests was that at high speeds a compound curve was necessary to minimize tire wear on the nose gear and, therefore, the central or main curve radius R2 should be preceded by a much larger radius curve R1.
Aircraft paths in the test approximated a spiral. A compound curve is relatively easy to establish in the field and begins to approach the shape of a spiral, thus the reason for suggesting a compound curve. The following pertinent conclusions were reached as a result of the tests :
1. Transport category and military aircraft can safely and comfortably turn off runways at speeds on the order of 60 to 65 mi/h on wet and dry pavements.
2. The most significant factor affecting the turning radius is speed, not the total angle of turn or passenger comfort.
3. Passenger comfort was not critical in any of the turning movements.
4. The computed lateral forces developed in the tests were substantially below the maximum lateral forces for which the landing gear was designed.
5. Insofar as the shape of the taxiway is concerned, a slightly widened entrance gradually tapering to the normal width of taxiway is preferred. The widened entrance gives the pilot more latitude in using the exit taxiway.
6. Total angles of turn of 30 o to 45 o can be negotiated satisfactorily. The smaller angle seems to be preferable because the length of the curved path is reduced, sight distance is improved, and less concentration is required on the part of the pilots.
7. The relation of turning radius versus speed expressed by the formula below will yield a smooth, comfortable turn on a wet or dry pavement when f is made equal to 0.13.
8. The curve expressed by the equation for R2 should be preceded by a larger radius curve R1 at exit speeds of 50 to 60 mi/h. The larger radius curve is necessary to provide a gradual transition from a straight tangent direction section to a curved path section. If the transition curve is not provided tire wear on large jet transports can be excessive.
9. Sufficient distance must be provided to comfortably decelerate an aircraft after it leaves the runway. It is suggested that for the present this distance be based on an average rate of deceleration of 3.3 ft/s2. This applies only to transport category aircraft. Until more experience is gained with this type of operation the stopping distance should be measured from the edge of the runway.
A chart showing the relationship of exit speed to radii R1 and R2, and length of transition curve L1
ICAO has indicated the relationship between aircraft speed and the radius of curvature of taxiway curves
For high-speed exit taxiways ICAO recommends a minimum radius of curvature for the taxiway centerline of 275 m (900 ft) for aerodrome code number 1 and 2 runways and 550 m (1800 ft) for aerodrome code number 3 and 4 runways. This will allow exit speeds under wet conditions of 65 km/h (40 mi/h) for aerodrome code number 1 and 2
runways and 93 km/h (60 mi/h) for aerodrome code number 3 and 4 runways. It also recommends a straight tangent section after the turnoff curve to allow exiting aircraft to come to a full stop clear of the intersecting taxiway when the intersection is 30 o . This tangent distance should be 35 m (115 ft) for aerodrome code number 1 and 2 runways and 75 m (250 ft) for aerodrome code number 3 and 4 runways .
A configuration for an exit speed of 60 mi/h and a turnoff angle
of 30 o is shown in Fig. 6-34. The FAA recommends that the taxiway centerline circular curve be preceded by a 1400-ft spiral to smooth the transition from the runway centerline to the taxiway exit circular curve. ICAO recommends the same geometry for both of these highspeed exits. Right-angle or 90 o exit taxiways, although not desirable from the standpoint of minimizing runway occupancy, are often constructed for other reasons. The configurations for a 90 o exit and other common taxiway intersection configurations are illustrated in Fig. 6-35. The dimensions labeled in Fig. 6-35 are determined by the aircraft design group of the design aircraft. These dimensional standards are provided in Table 6-23.
Location of Exit Taxiways
The location of exit taxiways depends on the mix of aircraft, the approach and touchdown speeds, the point of touchdown, the exit speed, the rate of deceleration, which in turn depends on the condition of the pavement surface, that is, dry or wet, and the number of exits.
While the rules for flying transport aircraft are relatively precise, a certain amount of variability among pilots is bound to occur especially in respect to braking force applied on the runway and the distance from runway threshold to touchdown. The rapidity and the manner in which air traffic control can process arrivals is an extremely important factor in establishing the location of exit taxiways. The location of exit taxiways is also influenced by the location of the runways relative to the terminal area.
Several mathematical analyses or models have been developed for optimizing exit locations. While these analyses have been useful in providing an understanding of the significant parameters affecting location, their usefulness to planners has been limited because of the complexity of the analyses and a lack of knowledge of the inputs required for the application of the models. As a result greater use is made of much more simplified methods.
The landing process can be described as follows. The aircraft crosses the runway threshold and decelerates in the air until the main landing gear touches the surface of the pavement. At this point the nose gear has not made contact with the runway. It may take as long as 3 s to do so. When it does, reverse thrust or wheel brakes or a combination of both are used to reduce the forward speed of the aircraft to exit velocity. Empirical analysis has revealed that the average deceleration of air-carrier aircraft on the runway is about 5 ft/s2.
In the simplified procedure, an aircraft is assumed to touch down at 1.3 times the stall speed for a landing weight corresponding to 85 percent of the maximum structural landing weight. In lieu of computing the distance from threshold to touchdown, touchdown distances are assumed as fixed values for certain classes of aircraft. Typically these values range from 500 to 1500 ft from the runway threshold. To these distances are added the distances to decelerate to exit speed.
These locations are derived using standard sea-level conditions. Altitude and temperature can affect the location of exit taxiways. Altitude increases distance on the order of 3 percent for each 1000 ft above sea level and temperature increases the distance 1.5 percent for each 10 o F above 59 o F.
During runway capacity studies conducted for the FAA, data were collected on exit utilization at various large airports in the United States. T indicate the cumulative percentage of each class of aircraft which have exited the runway at exits located at various distances from the arrival threshold. It is recommended that the point of intersection of the centerlines of taxiway exits and runways, which are up to 7000 ft in length and accommodate aircraft approach category C, D, and E aircraft, should be located about 3000 ft from the arrival threshold and 2000 ft from the stop end of the runway. To accommodate the average mix of aircraft on runways longer than 7000 ft, intermediate exits should be located at intervals of about 1500 ft. At airports where there are extensive operations with aircraft approach category A and B aircraft, an exit located between 1500 and 2000 ft from the landing threshold is recommended. Planners often find that the runway configuration and the location of the terminal at the airport often preclude placing the exits at locations based on the foregoing analysis. This is nothing to be alarmed about since it is far better to achieve good utilization of the exits than to be too concerned about a few seconds lost in occupancy time.
When locating exits it is important to recognize local conditions such as frequency of wet pavement or gusty winds. It is far better to place the exits several hundred feet farther from the threshold than to have aircraft overshoot the exits a large amount of time. The standard deviation in time required to reach exit speed is on the order of 2 or 3 s. Therefore, if the exits were placed down the runway as much as two standard deviations from the mean, the loss in occupancy time would only be 4 to 6 s. In planning exit locations at specific airports, one needs to consult with the airlines relative to the specific performance characteristics of the aircraft intended for use at the airport.
The total occupancy time of an aircraft can be roughly estimated using the following procedure. The runway is divided into four components, namely, flight from threshold to touchdown of main gear, time required for nose gear to make contact with the pavement after the main gear has made contact, time required to reach exit velocity from the time the nose gear has made contact with the pavement and brakes have been applied, and time required for the aircraft to turn Mix Index* Exit Range from Arrival Threshold
?Mix Index is equal to the percentage of Class C aircraft plus three an aircraft with a maximum certified takeoff weight in excess of class D aircraft, where a class C aircraft is an aircraft with a maximum certified takeoff weight greater than 12,500 lb and up to 300,000 lb and a class D aircraft is an aircraft with a maximum certified takeoff weight in excess of 300,000 lb.
off on to the taxiway and clear the runway. For the first component it can be assumed that the touchdown speed is 5 to 8 kn less than the speed over the threshold. The rate of deceleration in the air is about 2.5 ft/s2. The second component is about 3 s and the third component depends upon exit speed. Time to turnoff from the runway will be on the order of 10 s. As may be observed in this
table, typical runway occupancy times for 60 mi/h high-speed exits are 35 to 45 s. The corresponding time for a 15 mi/h regular exit is 45 to 60 s for air carrier aircraft.