Airport Engineering
According to I.C.A.O. the recommended length of air ports is decided on

Standard sea level temperature (15°C)
All listed here
Effective gradient percentage
Sea level elevation

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Airport Engineering
Effective length of a runway is the distance between

Ends of the clear way on either side
Point of intersection of the glide path and the extended plane of the runway surface and the other end of the runway
Point of intersection of the obstruction clearance line and the extended plane of the runway surface, and the other end of the runway
Ends of the runway

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