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

Investigators probing the January midair collision of a passenger plane and an Army helicopter over Washington that killed 67 people found the chopper was flying higher than it should have been and its altitude readings were inaccurate.