The engineers' union wants the airline to ground the entire fleet until all 75 Qantas 737s are checked, but isn't demanding the same of rival company Virgin Australia.

The three Qantas planes have been grounded until hairline cracks in the "pickle fork" structure between the wing and fuselage are repaired.

Qantas found the first case of cracking in an aircraft with just under 27,000 cycles of landings and takeoffs last Friday and, this week, identified two more instances during an audit of all 33 of its 737s to have had at least 22,600 flights.

The US Federal Aviation Administration earlier this month ordered airlines to check within a week any 737s that had completed more than 30,000 cycles. Planes with more than 22,600 cycles have to be checked within seven months.

Qantas on Friday said the three affected planes would return to service by the end of the year. The airline will work to minimise passenger disruption.

"Qantas will never fly a plane if we do not believe it is safe to do so," Qantas Domestic chief executive Andrew David told reporters in Sydney on Friday.

"Our entire reputation - our brand - is built on our safety record."

The Australian Licenced Aircraft Engineers' Association wants all Qantas 737s grounded until they are checked.

Association federal secretary Steve Purvinas on Friday accused Qantas of putting profits ahead of passenger safety.

"They think the reputation of Qantas, that was built up over nearly 100 years, can be risked in order to make more money," Mr Purvinas told AAP.

He also accused the Civil Aviation Safety Authority of being under Qantas's thumb after the regulator said there was no evidence the full fleet should be grounded.

"It will only take an hour per aircraft for Qantas to check these planes," Mr Purvinas said.

"The reason they won't do that is because if they find more cracks they'll have to ground more aircraft and it will cost them more money."

Mr Purvinas denied he was holding Qantas to a different standard to Virgin which has cleared its 19 planes with more than 22,600 cycles. Virgin has 78 Boeing 737s in total.

The union boss said if Virgin had found any cracks on the 19 planes he would have called for the airline to ground its full fleet.

As it is, he says all planes on the "much younger" Virgin fleet should be checked while remaining in operation.

Qantas and CASA dispute Mr Purvinas's allegations.

Mr David said the engineers' union "completely misrepresented the facts" and questioned if Mr Purvinas's public comments were politically motivated.

"Why go to the media and call for us to ground (aircraft) and then (use) scaremongering tactics that quite honestly undermine the professionalism of everybody in this business," Mr David said.

CASA spokesman Peter Gibson said the organisation would send inspectors to both Qantas and Virgin hangars. He also defended the authority's independence.

"Our role is to check safety standards are being met, but we also have to work co-operatively with those involved, so it's a balancing act," Mr Gibson told AAP.

Mr David declined to reveal the cost of repairing the three planes or if Boeing would foot the bill.

Australian Associated Press