The Aeromexico DC-9 and the Piper Cherokee Archer that collided in midair over Cerritos, Calif., last August should have been visible to each other for at least a minute before the crash, experts believe. One if not both of the pilots probably saw the other plane coming. That chilling fact confirms what experienced flyers already know: simply spotting an oncoming plane is not enough to avoid it. The pilot must then gauge whether the other craft's speed and bearing pose a threat. In crowded airspace, the risk of error is high.
For this reason, the FAA and the industry have been...