NTSB investigators found Friday that the aircraft involved in the mid-air collision over DCA lacked certain collision avoidance systems

A three-day National Transportation Safety Board hearing on the deadliest U.S. aviation accident in decades dug into problems with altimeters, chopper routes and the busy Washington, D.C., airspace.

The NTSB heard testimony from air traffic controllers, the Federal Aviation Administration and the Army about the mid-air collision of an Army helicopter and passenger plane.