The little airport in the valley had one problem, and every pilot who flew there knew it by name. Between the runway and the open sky there was a ridge, and the way down to the runway went straight over it.
An aircraft coming in descends along a straight line. The ridge is not straight: it rises, rounds off, and falls away again. So the question the inspector came to answer was a simple one. Where, if anywhere, do the two meet?
There are only three possible answers, and she wrote all three on the board at the briefing room.
The line may cross the ridge at two places. That is the dangerous one: between those two places the aircraft would be below the rock.
The line may touch the ridge at exactly one place. That is the borderline case — the aircraft grazes the highest stone and clears everything else. No airport in the world accepts it.
Or the line may miss the ridge entirely. Then the approach is safe, and the only remaining question is by how much.
"You do not need to know where they meet to know whether they meet," the inspector said. "One calculation tells you which of my three lines you are on, and that is the one that decides whether we open the runway."
They were on the third line, with sixty metres to spare. The airport opened that spring.