Boeing Co. (NYSE: BA) on Sunday informed the Federal Aviation Administration (FAA) that some of its 737 Max and 737 NG jets may have defective wing parts. Boeing has reached out to all airlines that operate 737 planes and advised them to inspect their leading edge slat tracks.

Slats are moveable pieces on the front edge of a wing used to improve air flow at low air speeds. The track is the railing on which these pieces move forward and back. The good news is a faulty slat can’t take down an airplane. The bad news is a failed slat could lead to damage during flight.

Affected parts may be susceptible to premature failure or cracks, according to the FAA.

“We are committed to supporting our customers in every way possible as they identify and replace these potentially non-conforming tracks,” said Kevin McAllister, President & CEO of Boeing Commercial Airplanes. The Company is planning to provide replacement parts for its airline customers affected by the issue “to help minimize aircraft downtime while the work is completed”.

As directed by the FAA, any tracks which don’t meet regulatory strength and durability standards are required to be repaired within 10 days. This replacement work is generally a one or two-day fix.

The issue, discovered on Friday, affects 148 slat tracks. Boeing said it believes 21 737 NGs and 20 737 Max jets most likely have faulty tracks. Worldwide, 133 NG and 179 Max planes will be inspected for defects.

Shares of Boeing are down 3% over the past 12 months.