SYDNEY (AP) — Australian airline Qantas has rejected calls to ground its Boeing 737s after claims a second aircraft in its fleet was found with a crack in its wing structure.

Qantas on Wednesday said it had found cracking in one 737, following calls from the U.S. Federal Aviation Administration for all airlines to check their aircraft.

On Thursday, the Australian Licensed Aircraft Engineers Association, which represents ground crews, said another 737 was found with "a cracked primary wing structure," and urged Qantas to ground its fleet of the planes.

But Qantas' head of engineering, Chris Snook, said the call was "completely irresponsible, saying the airline would never operate a plan unless it was safe.

Snook added: "Even when a crack is present, it does not immediately compromise the safety of the aircraft."