The principle is simple enough. Planes coming into land will be spaced out by time, rather than by distance. But it has taken a team at NATS (formerly National Air Traffic Control Services) four years of scientific study to make sure it is safe, and Heathrow is about to become the first airport on the planet to test it out.
It all comes down to wind.
Wind is the biggest single cause of landing delays at London's Heathrow airport, messing up flights on around 65 days each year.
If airliners are fighting against a headwind, even if they maintain the same speed through the air, they take longer to reach the runway. And that creates delays.