Airport Runways: Flatness And Precision In Aviation

It's easy to assume that airport runways are completely flat, but this isn't always the case. While some runways are very flat, others have noticeable differences in elevation. For example, one user describes a runway that starts at the same level as the terminal area, then dips down to 5m below, rises to 5m above, and stays at that height. Another user describes a runway that has a hump in the middle to allow water to drain off to each side.

The flatness of airport runways varies

It is hard to make something that is over 3000m long completely flat. The FAA has advisory circulars for all sorts of stuff in and around an airport, including one that covers flatness. This document has recommendations on gradients, including that vertical curves for longitudinal grade changes are parabolic, and that the length of the vertical curve is a minimum of 300 feet (91m) for each 1.0% of change. A vertical curve is not necessary when the grade change is less than 0.40%.

Almost no runways are truly flat. All runways have a slope grade, and some are humped in the middle so that water drains off to each side.

Some runways have a slope

It is difficult to make something that is over 3000m long completely flat. Therefore, some runways have a slope. For example, the runway at Exeter Airport has been described as being 'like a roller coaster'. The runways at Schiphol Airport are very flat, with less than one foot of elevation difference from the beginning to the end of the runway.

All runways have a slope grade, which can be found in airport facility directories. The runways at Galilee Airport are humped in the middle, so water drains off to each side. The centre line of the runway might be about a foot higher than either edge.

The FAA has advisory circulars for all sorts of stuff in and around an airport, including one on flatness. Airport Design Advisory Circular Page 77 has recommendations on gradients, including that vertical curves for longitudinal grade changes are parabolic. The length of the vertical curve is a minimum of 300 feet (91 m) for each 1.0% of change. A vertical curve is not necessary when the grade change is less than 0.40%.

Some runways have a hump in the middle to drain water

While it's commonly assumed that airport runways are completely flat, this is not always the case. Some runways have a hump in the middle to drain water, with the centre line of the runway perhaps a foot higher than either edge. This is the case at Galilee airport, for example.

The runway at Exeter Airport has been described as 'like a rollercoaster', while the RWY 11/29 at VIE is 'far away from being flat'. The RW 22 in Prague has been described as having 'huge waves' in it.

The FAA's Airport Design Advisory Circular Page 77 has recommendations on gradients, including that vertical curves for longitudinal grade changes are parabolic. The length of the vertical curve is a minimum of 300 feet (91 m) for each 1.0 percent of change. A vertical curve is not necessary when the grade change is less than 0.40 percent.

The FAA has recommendations on gradients

It is difficult to make a runway completely flat, and there are almost no truly flat runways. However, the FAA does have recommendations on gradients. Airport Design Advisory Circular Page 77 has recommendations on gradients including: Vertical curves for longitudinal grade changes are parabolic. The length of the vertical curve is a minimum of 300 feet (91 m) for each 1.0 percent of change. A vertical curve is not necessary when the grade change is less than 0.40 percent.

Some runways are humped in the middle so that water drains off to each side. This means that the centre line of the runway might be a foot higher than either edge. The runways at Schiphol (AMS) are very flat, with less than one foot of elevation difference from the beginning to the end of the runway. However, some runways have a huge difference in elevation. For example, the rwy27 threshold at one airport is level with the terminal area, but then it dips down to about 5m below, and then rises to about 5m above, and then stays at that height until the rwy9 threshold.

Some runways have huge waves in them

It's hard to make something that long completely flat. Some runways have huge waves in them. For example, the runway at Exeter Airport has been described as being like a roller coaster. The RWY 11/29 at VIE is also far from flat. The runway at Prague airport looks like it has huge waves in it.

The runways at Schiphol (AMS) are very flat. All five runways have less than a one-foot elevation difference from the beginning to the end of the runway.

The FAA has advisory circulars for all sorts of stuff in and around an airport, including one that covers flatness. Airport Design Advisory Circular Page 77 has recommendations on gradients, including that vertical curves for longitudinal grade changes are parabolic. The length of the vertical curve is a minimum of 300 feet (91 m) for each 1.0% of change. A vertical curve is not necessary when the grade change is less than 0.40%.

The runways at Galilee are humped in the middle, so the water drains off to each side. The centre line of the runway might be about a foot higher than either edge.

Frequently asked questions