U.S. Transportation Secretary Sean Duffy says it is “unacceptable” that two commercial aircraft had to abort landings at a Washington airport this week because of an Army Black Hawk helicopter that was flying to the Pentagon.

The episode followed a fatal collision between a military helicopter and a commercial jet in January, and prompted concern and outrage among officials.

The Black Hawk was trying to land at the Pentagon’s heliport, bringing it close to a pair of airliners, according to the Federal Aviation Administration.