The 737 NG is the third-generation 737 and version before the now grounded 737 MAX.

The U.S. Federal Aviation Administration said Wednesday that aircraft operators must inspect 165 Boeing 737 NG airplanes for structural cracks within seven days after the issue was found on a small number of planes.

Boeing Co notified the FAA of the issue after it discovered structural cracks on an aircraft undergoing modifications in China. The FAA said subsequent inspections found similar cracks in a small number of additional planes. The FAA said planes with fewer flights will eventually get inspected. The order covers 1,911 U.S. registered planes.

The 737 NG is the third-generation 737 and version before the now grounded 737 MAX, which is not impacted by the issue. Boeing said Friday it has been in contact with 737 NG operators about the discovery of cracks, but added that "no in-service issues have been reported."