Airport Planning: Transverse Gradient

Transverse Gradient

A typical cross section of a runway is shown in Fig. 6-23. The FAA and ICAO specifications for transverse slope on the runways are given in Tables 6-10 and 6-11, respectively. It is recommended that a 5 percent transverse slope be provided for the first 10 ft of shoulder adjacent to a pavement edge to ensure proper drainage.

aLighting of obstacle penetrations to this surface or the use of a VGSI, as defined by the TERPS order, may avoid displacing the threshold.

b10,000 ft is a nominal value for planning purposes. The actual length of these areas is dependent upon the visual descent point position for 20:1 and 34:1 and decision altitude point for the 30:1.

cAny penetration to this surface will limit the runway end to nonprecision approaches. No vertical approaches will be authorized until the penetration(s)

is/are removed except obstacles fixed by function and/or allowable grading.

dDimension A is measured relative to departure end of runway (DER) or TODA (to include clearway).

eData collected regarding penetrations to this surface are provided for information and use by the air carriers operating from the airport. These requirements do not take effect until January 1, 2009.

fSurface dimensions/obstacle clearance surface (OCS) slope represent a nominal approach with 3 o GPA, 50 TCH, 500 HAT. For specific cases refer to

TERPS. The obstacle clearance surface slope (30:1) represents a nominal approach of 3 o (also known as the glide path angle). This assumes a threshold crossing height of 50 ft. Three degrees is commonly used for ILS systems and VGSI aiming angles. This approximates a 30:1 approach angle that is between the 34:1 and the 20:1 notice surfaces of Part 77. Surfaces cleared to 34:1 should accommodate a 30:1 approach without any obstacle clearance problems.

gFor runways with vertically guided approaches the criteria in Row 7 is in addition to the basic criteria established within the table, to ensure the protection of the glide path qualification surface.

hFor planning purposes, sponsors and consultants determine a tentative decision altitude based on a 3 o glide path angle and a 50-ft threshold crossing height.

These specifications are used to site the lo threshold so that approach and departure procedures associated with that runway are not adversely affected by existing obstacles or terrain. The siting specifications vary depending on a number of runway

use conditions, including

•        The   approach   speed   of   arriving   aircraft

•        heTapproach category of arriving aircraft

•        Day   versus   night   operations

•        Types   of   instrument   approaches

•        The   presence   of   published   instrument   depar

•        The   use   of   the   runway   by   air   carriers

Runway end siting requirements are often the most confusing as well as overlooked element of runway planning. Care should be given to fully understand the purpose of the planned runway, the type of aircraft that will be using the runway, the current and future instrument approach procedures associated with the runway, and of course any terrain or obstacles in the vicinity.

Should an object penetrate any of the surfaces at the site of a runway, Displacing the threshold allows the airport planner to design runways with sufficient lengths to accommodate aircraft departures, while also allowing arrivals to safely approach the runway by maintaining sufficient clearance from upstream obstacles. Displacing the threshold does carry the penalty of reducing available runway lengths for landing. The FAA recommends avoiding the need for displaced thresholds when possible, but recognizes their benefits in the wake of no

other alternatives.