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 National Transportation Safety Board will hold three days of hearings on the fatal midair collision between a passenger jet and an Army helicopter

Incorrect altitude readings on the Army helicopter that collided with a passenger plane over Washington, D.C., in January contributed to the aircraft getting too close, but air traffic controllers warned about the hazards helicopter traffic presented years before the crash.