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.

The NTSB is holding three days of hearings to uncover insights into what caused the fatal crash between the passenger jet and the army helicopter over Ronald Reagan National Airport in January.

It showed how the helicopter flew above the 200 feet (61 metres) altitude limit on the helicopter route along the Potomac River before colliding with the plane. Investigators said the flight data recorder showed the helicopter was actually 80 to 100 feet higher than the barometric altimeter the pilots relied upon showed they were flying.