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 first of the National Transportation Safety Board’s three days of investigative hearings is underway to help determine what caused the deadly midair collision on January 29 between an Army helicopter on a training mission and American Airlines flight 5342 landing at Ronald Reagan Washington National Airport.

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