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.

The army helicopter that crashed into a passenger jet over Washington DC was flying too high, investigators have concluded.

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