Boeing has hit yet another snag in its bid to get the troubled 737 MAX aircraft back into the skies, with fresh findings the MAX is among hundreds of Boeing craft suspected to be fitted with faulty parts.

America’s peak aviation watchdog announced on Sunday (US time) that more than 300 MAXs, as well as a number of older-model 737s, may be equipped with faulty wing slats. It will give the carrier 10 days to replace the parts.

The replacement call is another milestone in a horror past 12 months for the aircraft manufacturer, which had the MAX – its best-selling jet – grounded worldwide in March, after a fatal Ethiopian Airlines crash.

That crash was preceded by a similar Lion Air crash in Indonesia in October. The death toll for the combined crashes was 346.

There’s no definite date when the MAXs will be allowed to fly again – the latest reports suggest a July or August take-off date.

However, Emirates president Tim Clark said this week he would be surprised if the craft was back before Christmas.

What’s gone wrong now?

Wings slats are used during take-off and landing, and if they’re not working correctly, could cause damage to an aircraft during flight. They were made and supplied to Boeing by a sub-tier company.

Boeing responded to the Federal Aviation Administration, saying it identified 20 737 MAXs that are most likely carrying the faulty slats, but that it would further inspect 159 planes for the parts.

The world’s largest aircraft maker said another 21 737 NGs (a third-generation 737 that has been in production for 20-plus years) have suspect parts, with another 112 to be checked.

October’s Lion Air Boeing 737 MAX crash claimed the lives of 189 people. Photo: Getty

The planes have been sold to airlines across the world.

AAP reported Boeing has said it is “now staging replacement parts at customer bases to help minimise aircraft downtime while the work is completed”.

It should only take one to two days to fit the replacement pieces, once they’re available.

Safety overhaul

Investigations into the definite cause of the second 737 MAX crash are continuing, but Indonesian officials have indicated the pilots of the Lion Air craft tussled with the plane’s automatic safety system, which repeatedly pushed the plane’s nose down during take-off.

Boeing said it would overhaul its software system and pilot training procedure, before it would undertake a certification flight test. The company reported the software update was completed mid last month.

In Australia, two airlines ran the troubled aircraft: Fiji Airlines and SilkAir.

Boeing and the Civil Aviation Safety Authority were contacted for comment.

-with AAP