Qantas is facing calls to ground its entire fleet of Boeing 737s after claims two of the aircraft have developed cracks in a crucial part of the wing structure.

Key points: Qantas's engineering boss said calls to ground its fleet were "completely irresponsible"

Qantas's engineering boss said calls to ground its fleet were "completely irresponsible" The airline is inspecting 33 of its Boeing 737s

The airline is inspecting 33 of its Boeing 737s There have been similar problems with Boeing 737s around the world

The airline on Wednesday said it would immediately inspect 33 Boeing 737 planes after a crack was found on one of them during a scheduled maintenance check.

The airline did not confirm if cracks had been found in any other planes.

However, the Australian Licensed Aircraft Engineers Association (ALAEA) claimed a crack in a second jet was found last night.

It said both cracks were on a part of the plane called the "pickle fork", which holds up the wings.

"These aircraft should not be flying," ALAEA boss Steven Purvinas said.

"The area where the crack is takes the load off the wing and all the fuel it carries.

"As long as Qantas is unaware which aircraft do or don't have cracks, they should ground the entire fleet until they know which are safe to fly."

The airline said the cracks did not compromise safety and that calls to ground its fleet were unreasonable.

"These are completely irresponsible comments," Qantas engineering boss Chris Snook said.

"We would never operate an aircraft unless it was completely safe to do so."

Earlier this month, the US Federal Aviation Authority ordered inspections of all Boeing 737 NG model aircraft with more than 30,000 flights.

Qantas said none of its planes had completed that many flights, but aircraft with over 22,600 cycles would be checked by Friday.

The ALAEA said all 75 aircraft in Qantas's 737 fleet should be grounded and inspected regardless of their flight history.

"The checks only take about an hour so they should be able to get an aircraft, send some engineers out there, and if there are no cracks there, they can fly it," Mr Purvinas said.

"When aircraft have serious issues in the air it's usually something very small like this escalating very fast."

Qantas fleet safety captain Debbie Slade said calls for the entire fleet to be grounded were "completely unnecessary".

She said the remaining 42 planes, due to their age, did not need to inspected for another 1000 take-off and landing cycles, which covers about seven months.

"I completely understand the concern [of passengers]," she said.

"It's quite a scary word 'crack'. But it's in a structural component, it's a very small part of that component and the structural integrity of an aircraft is made up of many components."

She said Boeing had "assured" Qantas the aircraft were safe to be flown for the next 1000 cycles, even if there is a crack present in one of the components.

Boeing said earlier this month 38 planes worldwide were discovered with pickle fork cracks and grounded for repair.

Qantas's Boeing 737s are used on many domestic routes and selected overseas services to New Zealand, Indonesia and Fiji.