New York (CNN Business) Boeing, already struggling with safety problems that grounded its 737 Max airplanes, now faces the limited grounding of an older version of that key jet due to separate safety concerns.

Structural cracks have been discovered in 38 737 NG's, or Next Generation jets. The cracks were found in inspections ordered by the Federal Aviation Administration a week ago. According to Boeing, as of Wednesday there had been 810 planes inspected, and 38 jets require repairs. That is roughly 5% of the planes that were inspected.

The cracks are on what is known as a "pickle fork," a part of the fuselage that helps to attach the wings to the aircraft. The problem was discovered when some used passenger jets were being stripped down for conversion to freighter jets, according to a person familiar with the problem. Boeing notified the FAA, which then ordered the inspections.

There are about 6,800 of the 737 NG jets already in service around the world. It is not clear how many need immediate inspections.

Read More