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.

Washington, DC (CNN) — In more than 32 hours of testimony across three days the National Transportation Safety Board probed virtually every detail of what led up to the January 29 midair collision between a US Army Black Hawk helicopter and an American Airlines regional jet, operated by PSA airlines.