An investigative hearing into a deadly mid-air collision of a US Army helicopter and a passenger plane that killed 67 people in Washington has revealed a discrepancy in the chopper's altitude displays.

Three days of investigative hearings on the deadly midair collision over Washington in January began Wednesday, aiming to reveal new insights into what caused the crash between a passenger plane and an Army helicopter that killed 67 people.

The National Transportation Safety Board has begun a three-day investigative hearing over the deadly midair collision between an American Airlines flight and an Army helicopter above Washington, D.C.,