After completing checks on 33 of its 737 NG aircraft, Qantas has discovered structural cracks on three “pickle fork” structures.

The national carrier revealed this morning that it has discovered hairline cracks on three “pickle fork” structures on affected 737 Next-Generation aircraft, which have been temporarily taken out of service.

Qantas has finished undertaking inspections on 33 of its 737 NG aeroplanes that have operated on more than 22,600 flights, ahead of schedule, following a directive by the Federal Aviation Administration (FAA) amid an ongoing investigation by the aircraft manufacturer, Boeing.

The FAA has issued a mandate for all 737 NG models – which includes the 737-800 – to be inspected prior to the accumulation of 30,000 total flight cycles and within seven months of the mandate.

“As people would expect with Qantas, we’ve gone above what was required to check our aircraft well ahead of schedule,” Qantas Domestic CEO Andrew David said in a statement.

“We would never fly an aircraft that wasn’t safe. 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 also slammed Thursday’s calls from the Australian Licensed Aircraft Engineers Association (ALAEA) for Qantas’ entire fleet of 737-800s to be grounded, labelling them “irresponsible comments” that “completely misrepresented the facts”.

“Those comments were especially disappointing given the fantastic job our engineers have done to inspect these aircraft well ahead of schedule, and the priority they give to safety every day of the week,” he said.

Boeing, the manufacturer of the aircraft, has notified all of its worldwide customers of an issue affecting “pickle fork” structures on certain 737 NG model jets, which attach the aeroplane models’ wings to the body. The manufacturer is currently investigating the root cause of the issue.

Qantas is working with Civil Aviation Safety Authority (CASA) and Boeing to resolve this issue, which the airline said involves some complex repair work.

All three aircraft are expected to return to service before the end of the year.

Virgin Australia has also conducted checks on its 17 Boeing 737 NG aeroplanes and did not find any issues, CASA spokesman Peter Gibson told ABC News.

Gibson also pointed out how the situation has been handled by airlines and aviation authorities, as a sign of the aviation safety system working exactly as it is designed around the world and in Australia.

“Well, look, things develop during the lifespan of all aircraft types and that’s why you have got a system in place so that they are reported, the manufacturer can analyse the problem, and if there needs to be a new process, a rectification, a change of parts, whatever, then that’s done in a timely fashion and that’s exactly what has happened here,” Gibson said.

“So it is the aviation safety system working in a robust way, exactly as it is designed to right around the world and here in Australia.”

Following the Qantas announcement, a Boeing spokesperson said fewer than five per cent of 1,000 737 NG planes had cracks detected and were grounded for repair.