Airport Planning: Sight Distance and Longitudinal Profile

Sight Distance and Longitudinal Profile

The FAA requirement for sight distance on individual runways requires that the runway profile permit any two points 5 ft above the runway centerline to be mutually visible for the entire runway length. If, however, the runway has a full length parallel taxiway, the runway profile may be such that an unobstructed line of sight will exist from any point 5 ft above the runway centerline to any other point 5 ft above the runway centerline for one-half the runway length.

The FAA recommends a clear line of sight between the ends of intersecting runways. The terrain must be graded and permanent objects designed and sited so that there will be an unobstructed line of sight from any point 5 ft above one runway centerline to any point 5 ft above an intersecting runway centerline within the runway visibility zone. The runway visibility zone is the area formed by imaginary lines connecting the visibility points of the two intersecting runways. The runway visibility zone for intersecting runways is shown in Fig. 6-21. The visibility points are defined as follows:

1. If the distance from the intersection of the two runway centerlines is 750 ft or less, the visibility point is on the centerline at the runway end designated by point .

2. If the distance from the intersection of the two runway centerlines is greater than 750 ft but less than 1500 ft, the visibility point is on the centerline 750 ft from the intersection of the centerlines designated by point b in Fig. 6-21.

3. If the distance from the intersection of the two runway centerlines is equal to or greater than 1500 ft, the visibility point is on the centerline equidistant from the runway end and the intersection of the centerlines designated by points c and d.

The ICAO requirement for sight distance on individual runways requires that the runway profile permit an unobstructed view between any two points at a specified height above the runway centerline to be mutually visible for a distance equal to at least one-half the runway length. ICAO specifies that the height of these two points be 1.5 m (5 ft) above the runway for aerodrome code letter A runways, 2 m (7 ft) above the runway for aerodrome code letter B runways, and 3 m (10 ft) above the runway for aerodrome code letter C, D, or E runways.

It is desirable to minimize longitudinal grade changes as much as possible. However, it is recognized that this may not be possible for reasons of economy. Therefore both the ICAO and FAA allow changes

aApplies also to runway safety area adjacent to sides of the runway. bMay not exceed 0.8 percent in the first and last quarter of runway. cA minimum of 3 percent for turf.

dA slope of 5 percent is recommended for a 10 ft width adjacent to the pavement areas to promote drainage.

eFor the first 200 ft from the end of the runway and if it slopes it must be downward. For the remainder of the runway safety area the slope must be such that any upward slope does not penetrate the approach surface or clearway plane and any downward slope does not exceed 5 percent.

fFor each 1 percent change in grade.

gNo vertical curve is required if the grade change is less than 0.4 percent. hDistance is multiplied by the sum of the absolute grade grade changes in percent. Source: Federal Aviation Administration [6].

longitudinal gradient and longitudinal grade changes to 2 percent for runways serving approach category A and B aircraft and 1.5 percent for runways serving approach category C, D, and E aircraft. ICAO limits both longitudinal gradient and longitudinal grade changes to

2 percent for aerodrome code number 1 and 2 runways and 1.5 percent for aerodrome code number 3 runways. For aerodrome code number 4 runways the maximum longitudinal gradient is 1.25 percent and the maximum change in longitudinal gradient is 1.5 percent. In addition, for runways that are equipped to be used in bad weather, the gradient of the first and last quarter of the length of the runway must be very flat for reasons of safety. Both the ICAO and the FAA require that this gradient not exceed 0.8 percent. In all cases it is desirable to keep both longitudinal grades and grade changes to a minimum.

Longitudinal slope changes are accomplished by means of vertical curves. The length of a vertical curve is determined by the magnitude of the changes in slope and the maximum allowable change in the slope of the runway.

*May not exceed 0.8 percent in the first and last quarter of runway for aerodrome code number 4 or for a category II or III precision instrument runway for aerodrome code number 3.

†Difference in elevation between high and lo ‡For each 1 percent change in grade.

§Distance is multiplied by sum of absolute grade changes in percent minimum length is 45 m.

The number of slope changes along the runway is also limited.

The FAA requires that the distance between the points of intersection of two successive curves should not be less than the sum of the absolute percentage values of change in slope multiplied by the 250 ft for airports serving aircraft approach category A and B aircraft and 1000 ft for airports serving aircraft approach category C, D, and E aircraft.

The ICAO requires that the distance between the points of intersection of two successive curves should not be less than the sum of the absolute percentage values of change in slope multiplied by 50 m (165 ft) for aerodrome code number 1 and 2 runways, 150 m (500 ft) for aerodrome code number 3 runways, and 300 m (1000 ft) for aerodrome code number 4 runways. ICAO also specifies that the minimum distance in all cases is 45 m (150 ft).

For example, for an FAA runway serving transport aircraft, that is, approach category C, D, or E aircraft, if the change in slope was 1.5 percent, the required length of vertical curve would be 1500 ft. Vertical curves are normally not necessary if the change in slope is not more than 0.4 percent. The FAA specifies a minimum length of vertical transition curve of 300 for each 1 percent change in grade for runways

*For facilities for small aircraft only.

†Satisfies the requirement that no part of a separations may be needed to achieve this result.

‡Forealevelsup to elevation 6000 ft. Increase by 1 ft for each 100 ft of airport elevation above 6000 ft.

serving approach category A and B aircraft and 1000 ft for each 1 percent change in grade for airport serving approach category C, D, and E aircraft. ICAO specifies a minimum length of vertical transition curve

of 75 m for each 1 percent change in grade for aerodrome code number 1 runways, 150 m for each 1 percent change in grade aerodrome code number 2 runways, and 300 m for each 1 percent change in grade for aerodrome code number 4 runways.