This allows controllers to guide aircraft into and out of crowded airspace with smaller separation standards than it was previously possible to do safely.

Yes, pilots had to navigate through more crowded airspace.

But with airplanes moving at hundreds of miles an hour in often crowded airspace, American safety experts worry about such practices.

Even in crowded airspace, it was able to keep its distance so no other aircraft could see it, changing altitudes or maneuvering far enough away to keep out of sight.

Now and again throughout the morning, I studied them from the roof of Headquarters as they rumbled overhead, clumsily-to the eye of killer-ship Helmsman-maneuvering in the tight constraints of a very crowded airspace.

But Tom Bock, an F.A.A. official, said that his agency was planning an unrelated redesign of the region's crowded airspace.

The F.A.A. spokesman said near-collisions were fairly common, particularly in the crowded airspace over southern California.

Aviation specialists point to the difficulty of fitting another airport in this crowded airspace.

The most crowded airspace in Europe is over South-East England where, this year alone, air traffic controllers are filing a record number of overload reports.

That has put growing pressure on the Swiss controllers, who oversee some of the most crowded airspace in Europe, including a slice of southern Germany, where the two planes flew into each other at 36,000 feet.