The collision between a passenger flight and an Army helicopter near Washington, D.C., on Jan. 29 marks the first fatal disaster involving a U.S. commercial airliner in 16 years.

The Federal Aviation Administration’s website boasts that the U.S. has the world's toughest qualifications for airline pilots.

The airspace around Washington, D.C., is congested and complex — a combination aviation experts have long worried could lead ...