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.

Three days of investigative hearings on the deadly midair collision between a military helicopter and passenger plane over Washington begin Wednesday morning and may reveal new insights into the crash

A National Transportation Safety Board hearing revealed that the U.S. Army was aware of altitude discrepancies on Black Hawk helicopters.