Runway length corrections

An aircraft needs more runway when the air is thinner and when the runway slopes uphill. Thin air reduces engine thrust and wing lift, so the aircraft must reach a higher ground speed before it can fly. Air density falls with altitude and with temperature, which is why high, hot airports need long runways.

Planners start from the basic runway length: the length the critical design aircraft needs at sea level, in standard atmosphere, on a level runway with no wind. This is then corrected. The ICAO Aerodrome Design Manual suggests, where aircraft performance data are not available, increasing the length by 7 percent for every 300 m of elevation, then by 1 percent for every degree Celsius by which the aerodrome reference temperature exceeds the standard-atmosphere temperature at that elevation. If the two corrections together exceed 35 percent, a specific study is needed rather than these rules. A further increase of 10 percent per 1 percent of effective runway gradient is applied for take-off.

The aerodrome reference temperature is the monthly mean of the daily maximum temperatures for the hottest month. The standard-atmosphere temperature falls by about 6.5 °C per 1 000 m from 15 °C at sea level.

These rule-of-thumb corrections are for early planning. Final runway length is set from the aircraft manufacturer's performance charts for the design aircraft and the routes to be flown, including take-off weight, which depends on stage length.