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.

The new problems come as Boeing continues efforts to win approval of a fix for the newest version of the 737, the 737 Max. All 737 Max jets have been grounded since March following two fatal crashes - one in Ethiopia and one in Indonesia. The two crashes, which killed 346 crew and passengers, are believed to have been caused by an automatic safety feature on that jet.

Boeing had hoped to have the final version of the fix approved by the FAA early this month, but it appears it will not make that latest target date.

Boeing (BA) has not disclosed how many airlines have the 38 NG jets on which the new problems have been discovered. More than 100 airlines own some version of the planes covered by the inspection order. The jets are a version of the workhorse single-aisle airplane that Boeing sold just before the introduction of the 737 Max. Their model names are the 737-700, 737-800 and 737-900.

Boeing's statement suggests the fix for the NG planes is more straightforward than the solution to get the 737 Max back in the air. But it could not give a time frame for how long those planes will be grounded.

"Safety and quality are Boeing's top priorities. Boeing regrets the impact this issue is having on our customers worldwide," said the aircraft maker. "Boeing is actively working with customers to develop a repair plan, and to provide parts and technical support as necessary. We are working around the clock to provide the support needed to return all airplanes to service as soon as possible."

The FAA order said airlines had a week to inspect planes that have made more than 30,000 flights. They have a year to inspect jets with at least 22,600 flights. Even planes with fewer flights than that must be inspected once they have 1,000 additional flights going forward from the Oct 3 order date. A plane used by a major airline can have 1,200 to 1,500 flights during the course of a year.

So far only two airlines have disclosed that they have jets that required grounding. Southwest (LUV) said late Wednesday that it had grounded two of the more than 700 NG jets in its fleet due to the problem. It has conducted inspections on more than 200 of its jets. Brazilian carrier Gol said it grounded 11 of its jets.

American Airlines (AAL), Delta (DAL) and United (UAL), three of the larger US airlines with the jets, said that they have not discovered any problems in the planes that have been inspected so far. They all said none of their planes have more than the 30,000 flights that required immediate inspections.