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.

The National Transportation Safety Board is launching a three-day investigation hearing on the D.C. plane crash that left 67 people dead.

Three days of investigative hearings on the deadly midair collision over Washington in January begin Wednesday.