Investigators probing the January midair collision of a passenger plane and an Army helicopter over Washington that killed 67 people found the chopper was flying higher than it should have been and its altitude readings were inaccurate.

The details came out of the first day of National Transportation Safety Board hearings in Washington, where investigators aim to uncover insights into what caused the crash between the American Airlines plane from Wichita, Kansas, and the Black Hawk helicopter over Ronald Reagan National Airport.

The board opened the three days of hearings by showing an animation and playing audio and video from the night of the collision, as well as questioning witnesses and investigators about how the Federal Aviation Administration and the Army may have contributed to nation’s deadliest plane crash since November 2001.

The board’s final report won’t be released until sometime next year, but it became clear Wednesday how small a margin of error there was for helicopters flying the route the Black Hawk took the night of the crash.

The January nighttime incident was the first in a string of crashes and near misses this year that have alarmed officials and the traveling public, despite statistics that still show flying remains the safest form of transportation.

Animation, altimeter discrepancy

The hearing opened Wednesday with a video animation showing where the helicopter and airliner were leading up to the collision. It showed how the helicopter flew above the 200 feet (61 meters) altitude limit on the helicopter route along the Potomac River before colliding with the plane.

Investigators said Wednesday the flight data recorder showed the helicopter was actually 80 feet to 100 feet (24 to 30 meters) higher than the barometric altimeter the pilots relied upon showed they were flying. So the NTSB conducted tests on three other helicopters from the same unit in a flight over the same area and found similar discrepancies in their altimeters.

Dan Cooper with Sikorsky helicopters said that when the Black Hawk helicopter involved in the crash was designed in the 1970s, it used a style of altimeter that was common at the time. Newer helicopters have air data computers that didn’t exist back then that help provide more accurate altitude readings.

The NTSB tested Black Hawk helicopters from the same unit and found that their barometric altimeters were 80 feet to 130 feet lower than the altitude readings from a radio altimeter, according to the board’s Altimeter Testing Report.

A barometric altimeter measures atmospheric pressure to determine altitude, with pressure decreasing as altitudes increase. Radio altimeters bounce radio waves off of the ground or the surface of water to determine altitude.

Chief Warrant Officer Kylene Lewis told the board that she wouldn’t find an 80 to 100 foot discrepancy between the different altimeters on a helicopter alarming because at lower altitudes she would be relying more on the radar altimeter than the barometric altimeter. Below 500 feet (152 meters), Lewis said she would be checking both instruments and cross referencing them.

She said as long as an altimeter registers an altitude within 70 feet of the published altitude before takeoff the altimeter is considered accurate under the checklists.

Army officials said a discrepancy of 70 to 100 feet (21 to 30 meters) between the Black Hawk’s altimeters is within the acceptable range because pilots are expected to maintain their altitude plus or minus 100 feet.

The greater concern is that the FAA approved routes around Reagan airport that included such small separation distances between helicopters and planes when planes are landing.

“The fact that we have less than 500 foot separation is a concern for me,” said Scott Rosengren, chief engineer in the office that manages the Army’s utility helicopters.

But Rosengren said that “if he was king for a day” he would immediately retire all the older Black Hawk models like the one involved in this crash and replace them with newer versions of the helicopters.

Questions over the route

Army officials and the head of a local medevac helicopter company that flies around Washington told the board they believed air traffic controllers would never let them fly the helicopter route involved in the crash anytime a plane was approaching the runway.

Chief Warrant Officer David Van Vetchen said after the crash he talked to many of his fellow pilots and everyone had the same assumption that controllers would never allow them to fly across the path of the runway the American plane was approaching before the crash.

Citing the numbers for runways, Van Vetchen said that “100% of the time when I was on route four and 33/15 was active” he would be instructed to hold until after the plane landed or took off from that runway.

‘Stepped on transmission’

During the two minutes before the crash, one air traffic controller was directing airport traffic and helicopters in the area, a task that involved speaking to or receiving communications from several different aircraft, according to the NTSB’s History of Flight Performance Study.

The air traffic controller had spoken to or received communications from the Black Hawk helicopter, an airplane that was taking off, an Air Force helicopter, an airplane on the ground, a medical helicopter and an inbound flight that was not the American Airlines plane that would crash.

“All aircraft could hear the controller, but helicopters could only hear other helicopters on their frequency and airplanes only other airplanes,” the report stated. “This resulted in a number of stepped on transmissions as helicopters and airplanes were not aware when the other was communicating.”

Stepped on transmissions are those that are unheard or blocked because of other transmissions. The NTSB report provides a list of 29 separate communications between the airport tower and other aircraft during approximately the 1 minute and 57 seconds before the collision.

Previously disclosed air traffic control audio had the helicopter pilot telling the controller twice that they saw the airplane and would avoid it. Officials on Wednesday also raised the use of night vision goggles, which limit the wearer’s field of view, on the helicopter as a factor.

The animation ended with surveillance video showing the helicopter colliding with the plane in a fiery crash.

Investigations have already shown the FAA failed to recognize a troubling history of 85 near misses around Ronald Reagan National Airport in the years before the collision, and that the Army’s helicopters routinely flew around the nation’s capital with a key piece of locating equipment, known as ADS-B Out, turned off.

Proposed changes

U.S. Sen. Ted Cruz, a Republican, introduced legislation Tuesday to require all aircraft operators to use both forms of ADS-B, or Automatic Dependent Surveillance Broadcast, the technology to broadcast aircraft location data to other planes and air traffic controllers. Most aircraft today are equipped with ADS-B Out equipment but the airlines would have to add the more comprehensive ADS-B In technology to their planes.

The legislation would revoke an exemption on ADS-B transmission requests for Department of Defense aircrafts.

Homendy said her agency has been recommending that move for decades after several other crashes.

Transportation Secretary Sean Duffy said that while he’d like to discuss “a few tweaks,” the legislation is “the right approach.” He also suggested that the previous administration “was asleep at the wheel” amid dozens of near-misses in the airspace around Washington’s airspace.

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Associated Press writers Leah Askarinam, Ben Finley and Rio Yamat contributed to this story.