Geometrical standards for Runways (Transportation Engineering)

Geometric standards for runways

Geometric standards for various runways elements are as follow;

1.  length

2.  width

3.  longitudinal gradient & profile

4.  transverse gradient

5.  runway intersections

1) length

Selection of length of a runway is the most important decision in the planning of landing area. Runway lengths for various types of airports recommended by ICAO are for sea level elevation, standard atmospheric conditions;

·        59⁰F or 15⁰C

·        29.92”barometer pressure

·        zero effective gradient

·        No wind

The basic lengths recommended by FAA are based on the same conditions except that temperature of 100⁰F (41⁰C above standard) has been included in the length. Necessary corrections have to be applied for changes in elevation, temperature and grade, at actual site of construction of the airport.

Correction for runway length

The following three corrections are required be applied for calculating the lengths of runways for all types of airports.

Correction for elevation

The ICAO and FAA recommends that the length of a runway for standard conditions at sea level be increased at the rate of 7% per 1000ft (300m) elevation above mean sea level.

Correction for temperature

ICAO recommends that length corrected for elevation, shall be further increased at the rate of 1% for every 1⁰C that the aerodrome reference temperature exceeds the temperature of the standards atmosphere for that elevation. However FAA corrects the runway length for the elevation of the airport but not for temperature, since the basic length at sea level is for a temperature of 100⁰F.

Correction for gradient

ICAO does not contain specific recommendations concerning the effect of the runway gradient on the runway length. Instead ICAO recommends that the effect of the runway gradient be based on the analysis of aircraft performance.

FAA recommends that the runway length corrected for the effect of altitude be further corrected for the runway gradient at the rate of 20% for each 1% of effective gradient. Effective gradient is defined as the maximum difference in runway profile elevation divided by the entire runway length.

2) Width

A runway is made up of a paved load bearing area and adjacent shoulders. The runway (paved load bearing area) combined with shoulders is called landing strip. The shoulders are normally not paved but contains of mechanically stabilized soil with or without grass cover and are only used during an emergency landing.

Experience show that the distribution of air traffic loading on the runway is such that the central portion of the paved area is subjected to maximum loading which goes decreasing towards the edges. Minimum width of landing strip has been recommended as 500 ft (150m) in non-instrument runways and 1000 ft (300m) in the case of instrument runway.

3) longitudinal Gradient & Profile

Frequent grade changes not only restrict the sight distances and increase the runway length needed for landing and taking off but also jeopardize the safety of the aircrafts flying at high speeds during the take off operation. It is for this reason that the ICAO and FAA have limited the maximum longitudinal gradient on any portion of the runway to 1.5% and maximum effective gradient to 1%.

Longitudinal intersecting grades should be connected with proper vertical curves. A rate of change of grade of 0.3% for every 100ft length of the vertical curve has been recommended by ICAO.

4) Transverse Gradient

Runway transverse grades are meant to be provided for purposes of quick drainage. According to ICAO and FAA, transverse gradient should not exceed by 1.5%. Minimum value has however not been specified. If the cross-grade is less than 0.5% drainage may not be satisfactory. Steeper transverse grades are provided for the shoulders. According to ICAO, for all classes of airports, shoulders should not exceed 2.5% for 500’ wide runway & 5% for 1000’ wide runways.

5) Runways Intersections

When two runways, both at zero grade, intersecting each other, there is no significant problem or laying out the intersection. But if the runways are at grade, the design of intersection assumes considerable importance. For appropriate design of intersection, it is necessary to have smooth profile of each runway at the intersection, so as to avoid any abrupt change in grade. But we can’t provide flat gradients at intersection as it might results in the ponding of water at the intersection. So a compromise has to be made by reducing grade of both runways considerably & by providing adequate drainage facilities at the intersection. 

Geometric Standards for Taxiways

A taxiway is a strip of paved areas connecting the runway to the apron. The speed of the aircraft when it runs on the taxiway is less than that on the runway at the time of taking off or landing. Some standards for taxiway design and construction are not as stringent as for runways.

1.      The length of a taxiway depends upon the distance of the apron from the entry end or the exit end of the runway.

2.      Considerable width suitably maintained shoulders are provided in case of taxiway. Width of taxiway is much less as compared to the runways. However when taxiways are to be used by turbine engine aircraft, the ground surface on either side of the taxiway over which the engines may overhang should be so prepared and maintained to prevent the ingestions of any loose stones other objects by engines.

3.      The ICAO recommends a longitudinal grade of 3% (max) and the rate of change of longitudinal grade at 1% per 100ft and transverse grade of 1.5% (max) for all classes of airports.

4.      The design of intersection of taxiway with runways and other taxiways is simple as compared to intersection of two runways sine the grades along the taxiway can be safely made flatter for a considerable distance on either side of the intersection.

5.      The sight distance for taxiways should be so provided that the surface is visible from 10’ (3m) height upto a distance of 1000 ft (3