United Airlines has apologised for temporarily reviving the flight numbers of two hijacked planes used in the September 11 attacks.
The airline said the move was inadvertent and they would not be used again.
Call numbers 93 and 175 were mistakenly assigned to two flights by Continental Airlines, which has merged with United.
According to Reuters, United declined to explain how the numbers found their way back onto flights. Flight attendants urged the company to permanently retire them, while pilots said the move was insensitive.
Flight 175, a Boeing 767, took off from Boston and was one of two planes flown into New York’s World Trade Centre.
Flight 93, a Boeing 757, took off from New Jersey’s Newark International Airport and crashed into a Pennsylvania field.
Investigators believed the hijackers of that plane wanted to hit the US Capitol or the White House.
Two other planes used in the attack were flown by American Airlines. Flight 11, a 767 from Boston, hit the World Trade Centre, and Flight 77 from Washington Dulles, crashed into the Pentagon.
AMR Corp, parent of American Airlines, long ago retired those two flight numbers.
