Washington DC - An investigative hearing into a deadly mid-air collision of a US Army helicopter and a passenger plane that killed 67 people in Washington has revealed a discrepancy in the chopper's altitude displays.

The National Transportation Safety Board (NTSB), the US agency tasked with examining major accidents, held hearings from Wednesday to Friday, with rigorous questioning of experts and various other parties, including regulators and air traffic controllers.

There were no survivors in the January 29 mid-air collision involving the Sikorsky Black Hawk military helicopter and a Bombardier CRJ700 operated by a subsidiary of American Airlines.

The passenger plane from Wichita, Kansas was coming in to land at Reagan National Airport – just a few miles from the White House – when the Army helicopter on a training flight collided with it.

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After examining flight recorder data, the NTSB first reported a discrepancy in the helicopter's altitude readings on February 14.

As part of the investigation, tests were conducted with three of the same helicopter models – Sikorsky Black Hawk Lima – belonging to the same battalion.

The findings revealed this week showed differences between the altitude indicated by the radar altimeter and the barometric altimeter on the aircraft.

Investigator Marie Moler noted that the altimeters "showed an 80- to 130-foot (24- to 40-meter) difference in flight," although the differences were within 20 to 55 feet in a controlled test environment.

"Once the helicopter rotors were turning and producing lift and thrust, the altimeter readings lowered significantly and stayed lower throughout the flights," Moler said.

NTSB chair Jennifer Homendy called the discrepancy significant, calling for more investigation.

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"I am concerned. There is a possibility that what the crew saw was very different than what the true altitude was," Homendy said.

"A 100-foot difference is significant" in this case, she added.

In the Potomac River area where the collision occurred, helicopters are required to stay below 200 feet, officials said during the hearings.

President Donald Trump was quick to blame diversity hiring policies for the accident, although no evidence has emerged that they were responsible.

Federal Aviation Agency air traffic control specialist Clark Allen told the hearing there was sufficient supervisory staff present in the control tower that night.

The collision was the first major plane crash in the US since 2009 when 49 people were killed near Buffalo, New York.