Inspections of some older Boeing jets have turned up structural cracks in more than three dozen aircraft, raising a new safety issue for the company already dealing with two deadly crashes involving a newer version of the same plane.

Boeing said on Thursday that airlines worldwide have inspected 810 planes following an order from US safety regulators.

Of those, 38 – or 5 per cent – had “findings” requiring repairs.

Airlines are under orders to inspect certain Boeing 737 NG planes for cracking in a part that helps keep wings attached to the fuselage.

Boeing declined to identify which airlines found problems, but Brazilian carrier Gol said it has grounded 11 planes so far. In the US, Southwest Airlines has grounded two.

Southwest spokeswoman Brandy King told Reuters the company had inspected more than 200 high-use airplanes and found cracks on two planes, which had been grounded.

She declined to say if cracks had been found on any other jets but said the airline planned an update next week.

It is not known if the fault affects any of Boeing’s planes in Australia.

The NG is a version of the popular 737 that has been produced since the 1990s.

Boeing is replacing it with the 737 Max, but those planes have been grounded worldwide since March after two crashes killed 346 people.

The cracks in the NG planes occurred in a part called a pickle fork because of its shape.

Cracking in that component was found recently in some 737s that were being converted from passenger use to cargo jets.

The US Federal Aviation Administration gave airlines a week, ending on Thursday, to inspect 737 NGs that had made at least 30,000 flights.

A larger group, more than 1700 planes with at least 22,600 flights, must be checked before they fly another 1000 times.

An FAA spokesman said “a small number” of US-based planes had been removed from service while Boeing worked on instructions for repairing or replacing the parts.

Boeing is analysing airline inspection results, and its technical experts are deciding the best repair plan, a company spokesman said.

The FAA said it was working with international safety regulators to understand what caused the cracks to form.

-with AAP