Flight paths for a cloudy future

WHEN an Avro Lancastrian—a modified bomber with no gun turrets and a small amount of room for passengers—became the first scheduled flight to take off from Heathrow in 1946, the airport’s passenger terminal was just a row of tents. But it had plenty of room to grow. Within a year, it saw 63,000 passengers. Within five years that had grown to 796,000.

Now, with five terminal buildings (one closed at present) and 193 destinations, Heathrow welcomes 70m passengers a year. It is the world’s third-busiest airport; only Atlanta and Beijing see more people come and go. And room to grow is hard to find. Limits on how, and at what time of day, its two runways can be used mean they can take no more flights. The London suburbs press up against the perimeter.

The company that owns Heathrow has long wanted to build a third runway to the north of the existing two. The most ambitious form of such a plan, the company says, would increase the number of flights the airport could handle by 46%. But the idea is fiercely opposed by many who live near the airport. Under European Union (EU) law, big airports...