Future of Qantas 737 fleet hangs in the balance

The future of the Qantas 737 fleet hangs in the balance after cracks were found on three 737s earlier this week.

The company is grounding the planes for repairs after hairline cracks were found in the "pickle fork" structure between the wing and the fuselage.

An investigation is underway, and Australian Unions have called for the grounding of all 75 737s until all are inspected.

"Qantas are flying these planes, some have cracks and some do not," Australian Licensed Aircraft Engineers Association (ALAEA) federal secretary Steve Purvinas told news.com.au.

"Qantas have said this is not a safety issue... but these cracks are in the pickle fork, which is a load-bearing component of the wing.

"[Failure] may occur in turbulence or heavy landing... and this could lead to loss of control of an aircraft."

Aviation commentator Irene King says the results of the investigation could ground the fleet.

"There will be intensive dialogue going on between the regulator and the airline, and at some point they may well agree the whole fleet should be grounded," she told Newshub.

However Qantas has reassured would-be passengers there is nothing to worry about.

"Qantas will never fly a plane if we do not believe it is safe to do so. Our entire reputation - our brand - is built on our safety record," Qantas domestic chief executive Andrew David told media on Friday.

King adds that the 737 is "probably the most robust aircraft ever built". 

"Cracking in aircraft is not unusual. It's not an everyday event either, don't get me wrong on that. They would have already a very robust inspection regime," she told Newshub.

"That airplane has been around 40, 45 years. They will be doing a very, very vigorous inspection."