Aviation In Canada

Friday, February 18, 2005

Runway Slope

Have you ever seen the markings on an airport diagram that say something like, "0.77% up," alongside one of the runways? Ever wonder what it means?

It's an indication of slope that can be reduced to a calculation, but it really means nothing without some experience to go with it. Think of a percentage and you realize it's based on 100. So this value above, being 0.77%, means 0.77 over 100. This is the rise over the run. An easier way to look at it is to multiply both by 10, to retain the same ratio, and work with 7.7 over 1,000. The reason I say this is easier is because we measure runways in 1,000's of feet and it makes it simple. This makes for a 7.7 foot rise over a 1,000 foot run. For example, a runway with a 0.77% slope that is 4,000 feet long will have a difference in elevation between thresholds of (7.7 * 4) = 31 feet (30.8 rounded up). If it were a 10,000 foot runway, it would be 77 feet in difference between one end and the other. Note that present this way, it doesn't include the fact that a long runway often rolls with a hill or dale in the middle of it, perhaps making a more significant slope in one area, rather than a steady grade all the way along its length.

Slopes on roads are marked the same way. If you see a sign warning you of a 16% slope, it means that over a 1,000 foot run, you'll drop 160 feet. Or over a 1,000m run, you'll drop 160m. In one kilometer, a drop of about 550 feet is pretty significant. Especially if you're facing the hill from the bottom on a bicycle -- or from the middle on the way down and your brakes have fail. Think about that for a split second and ask yourself: Would be ready for it if it were to happen to you?