Correction For Elevation, Temperaure & Gradient

The basic runway length (discussed above) is for mean sea level elevation, having standard atmosphere conditions.
Necessary corrections are applied for elevation, temperature and gradient for actual site conditions.

A. Correction for Elevation

As the elevation increases, the air density reduces. Thus, lift on the wings of the aircraft reduces and the aircraft requires greater ground speed. To achieve greater speed, longer runway length is required.
ICAO recommends that the basic runway length (BRL) should be increased at the rate of 7 percent per 300 m (1000 ft) rise in elevation above the mean sea level.

Table of Contents

Correction for Temperatur emperature

where,

T_a = monthly mean of average daily temperature for the hottest month of the year.

T_m = monthly mean of maximum daily temperature for same month of the year.

Rise in airport reference temperature has the same effect as the rise in elevation.
According to ICAO, BRL (after have been corrected for elevation) should be further increased by 1% for 1°C rise in airport reference temperature above the standard atmosphere temperature at that elevation.
Temperature gradient for standard atmosphere temperature at elevation from Mean Sea Level (MSL) to the altitude where temperature is –15.6°C is (–) 0.0065°C per meter.
Temperature gradient become zero above the altitude where temperature is –15.6°C.
Standard atmospheric temperature at mean sea level is taken as 15°C.

ICAO recommends that if combine correction exceeds 35% of BRL, then these correction should be further checked by specific studies at the site by model testing.

In steeper gradient longer length of runway is required so that it can attain desired ground speed.
ICAO recommends no correction for gradient.
FAA recommends that after temperature and elevation corrections, BRL should be increased by rate of 20% for every 1% of effective gradient.
Effective gradient is defined as maximum difference in elevation between highest and lowest points on runway divided by total runway length.

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