How flat does a runway need to be?

Question

I always assumed that big runways are flat (or more specifically straight: they might have a slight slope in either direction), but while watching some crosswind landing footage shot at Birmingham Airport I noticed that the runway looks really uneven, and based on the videos it looks like this hinders safe landing especially in adverse weather conditions.

• How common are uneven runways like this?
• Does an uneven runway make landings more difficult?
• Is there any standard on how flat a runway needs to be?

Answer 1

As an anecdotal answer to your first two questions, paved runways used by air carriers that have obviously noticeable vertical curves are a minority but not uncommon.

Unless they've redone the runway at Stansted, north of London, there's a significant hump in the middle. I used to operate through it at night in the early 1990s. As a bit of fun, we'd arrange legs so that a new first officer would have the leg into Stansted. On final at night the hump was not noticeable. However, when you touched down, the runway lights along the last half of the runway would disappear, at which point the captain would observe, "Hmm, not much runway left."

If you define a runway simply as a strip of land regularly used by aircraft, there's a whole lot of them that have vertical curves, and many with horizontal bends as well. It's gone now, but there used to be a strip at a lumber mill in North Fork, California, that had a 10-15 degree bend in the middle. McKenzie Bridge airfield just upriver from my home in Oregon has a steep incline on the east end.

Landings and takeoff on uneven runways are not significantly more complex than on straight runways. If the runway qualifies as an uphill/downhill runway, you generally takeoff downhill and land uphill. Touching down on the downside of a hump can make your landing feel a little softer than otherwise. Touching down on the upside of a hump makes it a little harder than otherwise.

Accommodating a horizontal curve is more challenging. It's worth remembering, though, that seaplanes often use a curved takeoff run.

Answer 2

Given that the runway is over 3000m, those undulations don't look very big. I doubt they had much to do with the landings in the video.

It's hard to make something that long completely flat. The FAA has advisory circulars for all sorts of stuff in and around an airport. One does cover flatness.

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.

Unfortunately, I could not find measurements of the undulations at BHX online, and I'm not going to estimate it from that photo. Would be nice to know how much above or below these particular recommendations it is.

Answer 3

Based on ICAO ( doc9157 Aerodrome design manuel part1 runways), the standard of vertical and horizontal slope of runways is:

For Horizontal Slope

1-1.5% for code letter C,D,E,F

1-2% for code letter A and B

Except intersections of another runway/taxiway where slope can be falter

And for Vertical Slope

Slope between max. and min evaluation of the runway centre line should be :

Code number 4: overall <1%, 1st and 4th quarter <0.8%, the other half <1.25%

Code number 3: overall <1%, 1st and 4th quarter <0.8%, the other half <1.25%

Code number 1/2: overall <2%

For slope change between 2 consecutive slope should be:

<1.5% for code number 3/4

<2% for code number 1/2

And for transition from one slope to another should be:

<0.1% per 30m code number 4

<0.2% per 30m code number 3

<0.4% per 30m code number 1/2

This is the global standard, some aviation authority such as FAA my have stricter rules