Cracks have been found on some Boeing 737 NG planes.
The FAA has ordered the planes to be inspected.
Boeing discovered the cracks "while conducting modifications on a heavily used aircraft".
There were also several other planes with the same issue.
Boeing has not said where the cracks are located.
To date, there have been no problems reported due to the cracks. The Boeing 737 Max 8 planes that are presently grounded are not impacted.