Indonesian carrier Lion Air has found structural cracks in two Boeing 737 NG planes that have taken fewer flights than a US Federal Aviation Administration (FAA) threshold for checks, Indonesia's aviation safety regulator says.

Key points: Indonesian carrier Lion Air found cracks in two Boeing 737 NG planes

Indonesian carrier Lion Air found cracks in two Boeing 737 NG planes The airline has an tragic recent history after everyone on board a Boeing 737 MAX was killed when it crashed in 2018

The airline has an tragic recent history after everyone on board a Boeing 737 MAX was killed when it crashed in 2018 Several other airlines, including Qantas, have experienced cracking issues near the 'pickle fork' section of the plane

The discovery could make it more likely the FAA will require airline operators to inspect 737 NGs with fewer than 22,600 cycles, which had not been mandated previously.

Each cycle typically represents one flight, with a take-off and a landing.

The cracks are on what is known as the "pickle fork", a part that attaches the plane's fuselage, or body, to the wing structure.

An FAA spokesman said the agency had asked operators to report any cracks so it could assess whether it needed to change its inspection orders.

The Lion Air jets with cracks had fewer than 22,000 cycles and were now grounded for repairs, a spokesman for the airline said.

Lion Air performed the checks even though they were not yet required, to "ensure security and safety of the flights," he said.

Indonesia's Director-General of Civil Aviation, Polana Pramesti, said there were no plans for the country's aviation regulator to expand the inspections beyond the FAA directive.

The FAA requires immediate checks of aircraft with more than 33,000 cycles and inspections within the next 1,000 cycles for those with more than 22,600 cycles.

Boeing did not respond to an immediate request for comment. The manufacturer last week said just over 1,000 planes globally had met the threshold for inspections to date, and of those, fewer than 5 per cent had issues.

The cracks in the Lion Air planes were just the latest to be found after several airlines, including Qantas, found them on their aircraft.

Despite finding cracks in three 737 NG aircraft at the pickle fork structure, Qantas rubbished "irresponsible" calls for their fleet to be grounded.

"We would never fly an aircraft that wasn't safe," Qantas Domestic chief executive Andrew David said.

"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."

Southwest Airlines, Brazil's Gol Linhas Aereas Inteligentes and Korean Air Lines are among the carriers to ground planes after discovering cracks.

Lion's recent Boeing history

Lion Air has a shocking recent history with Seattle-based Boeing, which began in October of 2018 when one of the airline's newer 737 MAX jets crashed shortly after take-off from Jakarta, killing all 189 people on board and leading to heightened scrutiny on the carrier's maintenance operations by Indonesia's safety regulator.

That crash was the first time a Boeing 737 MAX jet crashed shortly after take-off but not the last and it was followed in March 2019 by an Ethiopian airlines jet crashing, also killing everyone on board.

The result of the second incident was the Boeing 737 MAX being grounded worldwide by US presidential decree.

Indonesian aviation investigators eventually found mechanical and design issues contributed to the crash of the Lion Air jet and that incorrect assumptions on how an anti-stall device — called the Manoeuvring Characteristics Augmentation System (MCAS) — functioned and how pilots would react played a part.

In October, explosive text messages from a former test pilot claimed Boeing knew about the dangerous behaviour of the MCAS system and that the MCAS was "running rampant" and more powerful than the company had told the FAA.