When approaching a runway that slopes downhill, it's all too easy to arrive well above the desired glideslope. This article explains why.

 

 

Tutorial: Approach Judgment in the Great Basin



Dealing with Runway Slope

Many visiting pilots experience difficulty judging approaches when flying in the Great Basin.  As with so many other difficulties in aviation, the problems are in ‘the basics’ and stem from the way so many pilots are taught to fly.

A great number of pilots who learned to fly in locations with little or no terrain relief were taught to judge approach slopes by reference to the apparent shape of the runway—that is, by in effect measuring the vertical angle between the runway surface and the approach path. The glider (or any other heavier-than-air aircraft), however, “sees” the approach slope as the vertical angle between the horizontal and the approach path. What’s the difference? After all, didn’t we all learn in high school geometry that when two parallel lines are intersected by a third line, opposite interior angles are always equal? Of course we did.

The problem arises in the careless assumption that the two “parallel lines” in the above theorem are indeed parallel. In other words, this amounts to saying that “flat” and “level” are synonymous. In flat country, this assumption is true often enough that many pilots and their instructors behave as if it is always true. But in the Great Basin, where the valley floors are typically covered with adjoining alluvial fans, a piece of ground that looks “flat” is rarely, if ever, level—and so the runway often has an appreciable slope.

Flying an approach to a sloping runway, under the assumption that the angle between the approach slope and the runway is the controlling factor, has resulted in numerous accidents and close calls.

Flying an approach by reference to the vertical angle between the horizon and the approach path, however, will always work, no matter what the terrain might be. It will work in flat country, and—unlike the common technique of judging approach slope by reference to the runway--it will work in the Great Basin, too. It is the safest and most professional way to judge a visual approach, bar none.