A Virgin Atlantic flight that was scheduled to travel from London to New York had to turn around less than an hour into the trip on Monday.
According to CNN, it was learned during the flight that the first officer had not completed the airline's "final assessment" test.
A spokesperson for Virgin Atlantic called the incident a "rostering error."
The spokesperson added that both pilots were licensed and qualified to undertake the fight, but the first officer hadn't completed the company's internal training protocols which "exceed" industry standards.
The first officer was replaced after landing back at Heathrow Airport. The officer was replaced with another pilot and the plane reportedly made it to New York nearly three hours late.