A three-day National Transportation Safety Board hearing about the collision of a flight from Wichita, Kan., and a U.S. Army helicopter dug into problems with altimeters, chopper routes and busy airspace.

Three days of hearings held by federal investigators this week shed new light on the deadly midair collision last January between an American Airlines regional jet and an Army Black Hawk helicopter just outside Washington, D.C.

Everyone on board both the jet and the helicopter — 67 people — were killed in the crash. It was the deadliest U.S. aviation accident in decades.

Here are some of the biggest revelations from the National Transportation Safety Board hearing.

The Army helicopter's altimeter may have been wrong — and that wasn't unusual

On the night of Jan. 29, the three-person

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