Qantas has pulled three of its Boeing 737 planes from service after finding hairline cracks but said it would not ground all of its 737s.

Three planes have now been affected by cracks in the “pickle fork” structure between the wing and the fuselage. On Thursday, the aircraft engineers union said the airline should ground the entire 737 fleet.

But in a press conference on Friday morning, the airline said it had checked all the relevant planes, and would not ground any more so as to “minimise disruption to customers”.

Under global aviation regulations, 737s have to be urgently checked for cracks, within seven days, if they have completed more than 30,000 cycles. Planes with between 22,600 and 30,000 cycles, also have to be checked, but within 7 months.

One cycle is effectively one takeoff and landing.

For Qantas, all three of the planes with cracks were between 22,600 and 30,000 cycles. The airline has no 737s with more than 30,000 cycles.

On Friday, the airline said it had now checked all planes with more than 22,600 cycles, and found no further cracks.

Andrew David, the head of Qantas domestic said: “We are focused, right now, on those in scope, which is the 33 that have done between 22,600 cycles and 30,000 cycles”.

“Any between 22,600 and 30,000 cycles needed to be checked within seven months … and we’ve done them within seven days, as soon as we found the first one,” he said.

But, the airline will not be checking those planes below 22,600 cycles.

Qantas has 33 Boeing 737s that are between 22,600 cycles and 30,000. It has 42 737s that are below 22,600.

“Aircraft with low cycles – which 42 of our aircraft comprise of – [regulations are] at this stage, they do not need to be inspected,” David said.

David, and the head of engineering, Chris Snook, said that all three planes with cracks had more than 27,000 cycles.

They said the difference between 22,600 cycles and 27,000 cycles equals two and a half years operational flying time.

The next aircraft they need to check is two years away from 22,600 cycles, they said.

“Qantas will never fly a plane if we do not believe it is safe to do so”, David said.

Snook told media there were no concerns of cracks in planes with less than 26,000 cycles.

“Less than 26,000 ... is a threshold that has been very deliberately set [by regulators]”, he said.

The carrier said all three planes would return to service by the end of the year.

“Even where these hairline cracks are present they’re not an immediate risk, which is clear from the fact the checks were not required for at least seven months,” David said.

The Australian Licensed Aircraft Engineers’ Association on Thursday called for the immediate grounding of all 75 Boeing 737s until inspections of the entire fleet were undertaken.

“We say these things for the safety of the people who fly on these aircraft,” ALAEA federal secretary Steve Purvinas told ABC radio on Friday.

The union leader also criticised Boeing for outsourcing work in the United States.

“I don’t have any confidence in Boeing at all,” Purvinas said.

“They’re being very optimistic if they think these aircraft will be back in the sky by the end of the year.”

ButSnook, said the union was being “completely irresponsible”.

David added that they had “completely misrepresented the facts”.

“The action we’re taking is fully supported by the Civil Aviation Safety Authority,” he said.

Virgin Australia said it had already inspected 19 of its Boeing 737s and no cracks had been found.

Snook said the repairs on the affected planes would take a month.

“It is a complex repair,” he said. “Effectively, we replace the pickle forks with new pickle forks.”