According to the Federal Aviation Administration (FAA), a runway should have a maximum slope of 1.25% in any direction. Runways should be as flat as possible, however in reality hardly any runway will be really flat. A modern full length runway is 4 kilometers long (more than 13,000 feet).

While it might seem intuitive that a runway should be perfectly level, they are rarely "flat" in the mathematical sense. In fact, most runways are designed with a transverse crown, meaning they are slightly higher in the middle and slope downward toward the edges to facilitate water drainage and prevent hydroplaning during heavy rain. Furthermore, many airports have significant longitudinal slopes dictated by the natural terrain. The Federal Aviation Administration (FAA) and international bodies like ICAO allow for a gradient of up to 1.5% to 2% for transport-category runways. For example, Courchevel Altiport in France has an incredibly steep 18.6% grade to help decelerate planes on landing and accelerate them on takeoff. While a slight slope is manageable for modern aircraft, engineers must carefully calculate "effective gradient" because an uphill takeoff requires more runway length and engine thrust, whereas a downhill landing increases the stopping distance. Therefore, while a runway must be "smooth" to prevent landing gear damage, it is intentionally engineered with specific slopes and curves to balance aerodynamic performance with safety and environmental factors.