WATCH: NTSB member presses witnesses on civilian risk during D.C. plane crash hearing

At the start of three planned days of hearings, federal investigations closely examined the discrepancies with altimeters — vital instruments that measure how high an aircraft is flying — and whether that was a factor in the Jan. 29 collision between an Army helicopter and a regional jet that killed 67 people at Ronald Reagan National Airport.

J. Todd Inman, a member of the National Transportation Safety Board, asked the panel testifying Wednesday if these known discrepancies were more of a design issue than a maintenance one.

The discrepancies that have been seen can be explained by a “stack of the difference errors” involving the devices’ limitations and factors such as the effects of temperatures on flights, said Steve Braddom, retired U.S. Army colonel.

“I think you can explain that deviation as being within the tolerance band for those different errors,” he said.

“How much tolerance should we have for aviation safety when our civilian lives are at risk?” Inman shot back.

When the board member didn’t receive what he found to be a satisfactory response, he said, “I think it should be zero.”

Scott Rosengren, an Army engineer, spoke up, saying the Federal Aviation Administration’s requirement of at least 500 feet of separation between aircraft in an airspace is a concern for him, given the known discrepancies with altimeters.

“The fact that we have less than 500-foot separation is of concern to me personally,” he said.

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