737 Max
© Reuters / Lindsey WassonFILE PHOTO: A 737 Max aircraft is pictured at the Boeing factory in Renton, Washington, US, on March 27, 2019.
Boeing's troubled 737 MAX model, alongside its NG aircraft, has been hit by another safety issue as the plane manufacturer revealed that some aircraft parts were improperly manufactured and would need replacement.

Up to 148 leading edge slat tracks - aerodynamic surfaces on a plane's wings, which allow it to operate at a higher angle of attack - had defects, the company told the US Federal Aviation Administration. All the faulty parts were produced by a Boeing sub-tier supplier and could affect up to 133 NG and 179 MAX planes worldwide.

Although complete failure of a slat track is unlikely to cause a plane crash directly, it could still lead to aircraft damage in flight, the FAA said in a statement. The faulty parts could be potentially susceptible to premature wear-out or cracks.

The FAA also said it would soon issue an airworthiness directive to mandate Boeing service actions worldwide.

The news comes as the US aircraft manufacturer is still reeling from another scandal involving its 737 MAX models. All planes of this model were earlier grounded worldwide following two deadly crashes that were blamed on the aircraft's malfunctioning flight control system.

In late May, the company admitted that it "fell short" and failed to install a safety feature that could have prevented both catastrophes, which killed 346 people in total. So far, the Boeing 737 MAX aircraft remain grounded as the company has yet to present a software upgrade to the FAA.