United States | News & Politics

Barometric pressure based altimeter can’t be used at low altitudes due to reading low. Helicopter should have been using radio based altimeter. Helicopter was at least 125 feet higher than the upper limit for that airspace

Investigators say the collision occurred at an altitude of 278 feet — even though the Army helicopter shouldn't have been flying above 200 feet on that portion of the route. NTSB Chairwoman Jennifer Homendy has previously said the Black Hawk pilots might not have been aware of how high they were because their barometric altimeter may have been displaying the wrong altitude. And Army representatives said this week that it wasn't unusual for barometric altimeters to be off by that much.

“In my experience, a difference of 80 feet or so between the radar altitude and the barometric altitude would not be cause for alarm," U.S. Army Chief Warrant Officer Kylene Lewis told investigators.

Radar altimeters measure the distance between an aircraft and the ground, while barometric altimeters use air pressure to estimate altitude.

NTSB member J. Todd Inman pressed others from the Army and from Sikorsky Aircraft, which manufactures Black Hawk helicopters, about why the barometric altimeters used weren't more accurate, especially since Army helicopters sometimes pass near civilian flights.

“You just told me, I believe, all of the equipment in this manufacturing process is within the tolerances for safety. How much is that tolerance?" Inman asked. "I think it should be zero."

The military representatives said at the hearing that the Army is considering changes to the flight manual to let pilots know about discrepancies with altitude readings.

Before the accident, FAA staff had raised concerns about the Army helicopter route's proximity to commercial flights landing at the airport. The 12th Aviation Battalion's Bravo Company, which operates out of Virginia's Fort Belvoir, ran training missions in the area related to priority air transport and continuity-of-government operations.

Allen, the former DCA tower operations manager, said a working group at the airport had considered the possibility of moving what was known as Route 4 or even eliminating part of it. "And both of those options we were told we were unable to do due to continuity of government operations or security," Allen said.

During the hearings, Homendy blasted the FAA for not acting on the safety concerns of its employees. "Are you kidding me? Sixty-seven people are dead! How do you explain that? Our bureaucratic process?" she said. "Fix it. Do better."