An airport consists of several components out of which runways form the most important part of any airport design. The design of an airport runway involves finding the length of the runway. The length of an airport runway depends on a number of factors. Also, different corrections need to be applied before arriving at the final runway length for a proposed site. As we have covered the basic runway length calculation in our previous blog, in this blog we have covered the corrections required to arrive at the final length of the runway.
Assumptions in Basic Runway Length Calculation
Three important assumptions that require correction are given below.
Airport at mean sea level - Altitude correction
Standard temperature of 15°C at MSL - Temperature correction
Level gradient - Gradient correction
1. Altitude Correction
As we move up from the mean sea level, the air density decreases. Due to this decreased air density, it becomes difficult for aircraft to generate enough lift within basic runway length. Therefore, a longer runway length is required with an increase in altitude i.e., runway length ∝ altitude.
7% increase in runway length for every 300m rise in elevation from the mean sea level is the correction recommended by ICAO.
For instance, if an airport is at 600m above mean sea level and has a basic runway length of 2000m, then correction for runway length will be, = (7/100) * 2000 * (600/300) = 280 m, and the corrected runway length will be, = basic runway length (i.e., 2000m) + correction (i.e., 280m) = 2280m.
2. Temperature Correction
An increase in temperature again decreases the air density and as air density decreases, a greater length of the runway is required for reasons as discussed in altitude correction. Therefore, runway length ∝ temperature.
To calculate temperature correction, the temperature at a given elevation must be found as follows,
Standard temperature at elevation (STE) = Temperature at MSL (i.e. 15°C) - (0.0065 * Elevation),
Note that this temperature is not the reference temperature at the airport. The airport reference temperature is calculated as follows,
Airport reference temperature (Tr) = Ta + (1/3) * (Tm - Ta),
where,
Ta - the average temperature of the hottest month
Tm - monthly mean of the max daily temperature of the same month
For instance, if May is the hottest temperature at the proposed airport site, then the average temperature of the hottest month (Ta) is the average of the average daily temperature (the average daily temperature is the average of max temperature at day time and minimum temperature at night time), and monthly mean of the max daily temperature (Tm) is the average of max temperature at day during the same month.
1% increase in runway length for every 1°C rise in airport reference temperature (Tr) above the standard temperature at elevation (STE) is the correction recommended by ICAO.
Temperature correction = Runway length * (1/100) * (Tr - STE)
Note that gradient correction should be applied before temperature correction. The Combined value of both altitude and temperature correction must not be greater than 35% of the basic runway length as per ICAO. If it exceeds 35%, ICAO suggests going for in-depth analysis.
3. Gradient Correction
When a runway has ups and downs along its path, lift generation is affected. Therefore, runways need to be corrected for gradients as well.
20% increase in runway length for every 1% increase in effective gradient is the correction recommended by ICAO.
Gradient correction (in m) = (20/100) * Runway length * Effective gradient(in %),
The effective gradient is the max difference in elevation between the highest point and lowest point of the runway divided by total runway length.
Example Problem
Question: The elevation and the airport reference temperature of an airport are 535m above the mean sea level and 22.65°C respectively. The length of the runway required for a design-aircraft under the standard condition is 2000m. Apply the corrections required in sequence as per ICAOand find the corrected runway length after applying temperature correction.
Solution:
Basic runway length = 2000m
Elevation = 535 m
Airport reference temperature (Tr) = 22.65°C
Effective gradient = 1%
Altitude correction
7% increase in runway length for every 300m rise in elevation from the mean sea level,
Altitude correction = (7/100) * basic runway length * (535/300)
Altitude correction = (7/100) * 2000 * (535/300) = 249.67 m
Corrected runway length = 2000 + 249.67 = 2249.67 m
Temperature correction
Standard temperature at elevation (STE) = Temperature at MSL (i.e. 15°C) - (0.0065 * Elevation)
Standard temperature at elevation = 15°C - (0.0065 * 535) = 11.52°C
Temperature correction = Runway length * (1/100) * (Tr - STE)
Temperature correction = 2249.67 * (1/100) * (22.65 - 11.52) = 250.39 m
Corrected runway length = 2249.67 + 250.39 = 2500.06 m
Corrected runway length = 2500 m (approx)
Check
Combined correction (i.e., 2500 - 2000 = 500 m) should be less than 0.35 * 2000 (i.e., 700 m). Therefore, corrected length is ok.
Practise problem
Refer to the below-mentioned video lecture for more insights.