The Army helicopter that collided with a plane over D.C. in January had incorrect altitude readings, contributing to the aircraft getting too close.

A test flight in May over the Potomac River found that the barometric altitude was consistently lower by about 100 feet than the radio and geometric altitudes over the water. The helicopter's barometric altimeter was also obscured during the flight, investigators said.

Flight instruments probably led the Black Hawk crew to believe the helicopter was lower than it actually was before the collision with a commercial airplane on Jan. 29.