A preliminary analysis of the flight data and voice recorder on board a Black Hawk helicopter leading up to the collision with a commercial flight over Washington, DC, on January 29, indicated the helicopter’s altimeter may have been inaccurate and the pilots may not have heard some calls from the Reagan National Airport control tower,

The crash was the deadliest U.S. air crash in nearly 25 years, claiming the lives of all 67 people on board both aircraft.

A Black Hawk helicopter crew may not have heard key instructions from air traffic control before it struck an American Airlines jet near Washington DC a fortnight ago, according to black box data.