Boeing reported on Sunday that some of its 737 aircraft, including many MAX 8 planes, may still have faulty parts in their wings.

This report comes as one of several which have held Boeing from flying once again.

Boeing is currently working with international aviation bodies, national regulatory bodies, and airlines around the world to find a solution to the now long-standing technical concerns surrounding 737 MAX 8 aircraft. Boeing is currently working with the US Federal Aviation Administration (FAA) to reach out to airlines that fly 737 aircraft and advise them to check their slat track assemblies for all MAX and NG aircraft. At the moment, some of the tracks may not be meeting manufacturing standards and may be in need of replacement. Boeing and the FAA have stated that if these parts are found to be defective, they should be replaced before the planes take off again. At the same time, the FAA reports that these parts would not cause a plane to crash if they’re defective, but it could cause damage to the aircraft.

The FAA and Boeing will continue working together to monitor the malfunctions in many 737 aircraft. The FAA is set to issue a directive requiring airlines to inspect and if need be repair their slat track assemblies within 10 days. The issue affects 148 track slats produced by the same supplier. Boeing reported that they believe this malfunction is present in 20 737 MAX planes and 21 737 NG planes.