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 National Transportation Safety Board questioned witnesses Friday on collision avoidance technology and organizational systems to manage risk. It is the third and final day of investigative hearings probing January’s midair collision between a US Army Black Hawk helicopter and an American Airlines regional jet,

In January, 67 people died when an American Airlines passenger flight crashed with a military helicopter near Washington, D.C.