The cracking issue was discovered by Boeing in September, leading the FAA to call for airlines to inspect planes that had operated more than 22,600 flights.
Indonesian airline Lion Air reported finding cracks on the "pickle fork" of two Boeing 737 jets with less time in service than meets the threshold for mandatory inspections, according to the Sydney Morning Herald.
Boeing first discovered in September that a component on certain planes was showing signs of stress damage sooner than expected, leading the FAA to order airlines to inspect 737 Next Generation, or "NG" jets, that had operated a certain number of flights. The order called for 737 NGs that had operated more than 30,000 flights to be inspected within a week, and jets that had flown more than 22,600 cycles to be inspected within the next 1,000 flights.
Both Lion Air planes had fewer than 22,000 flights, the Morning Herald reported.
The hairline cracks were found on a component of the plane called the' "pickle fork," a section that reinforces where the planes' wings join with its body.
The cracking issue has led the Australian airline Qantas to ground three of its jets for repairs after finding the cracks. South Korean airlines have grounded nine of their jets after discovering the cracks, Reuters reported. As many as 50 of the popular jets are estimated to have been grounded worldwide.
Boeing, regulators, and airlines maintain that there is no safety risk involved. The planes are designed to fly even with damage to the pickle fork thanks to redundant safety features, according to Stephen Fankhauser, a Swinburne University of Technology aviation expert in Australia who was cited by AFP.
The pickle fork issue on the 737 NGs is unrelated to the ongoing crisis surrounding the 737 Max, the newer generation of the workhorse jet.
The 737 Max has been grounded worldwide since March, when the second of two fatal crashes involving the type occurred. The first crash, in October 2018, involved a 737 Max flown by Lion Air. A combined 346 people were killed in the two crashes.
The Boeing 737 NG — which includes the 737-600, -700, -800, and -900 variants — is a backbone of commercial fleets around the world, with more than 7,000 sold.
In a statement, Boeing said it "regrets the impact this issue is having on our 737 NG customers worldwide," and affirmed that it is working to support affected customers.
Neither Lion Air nor the FAA immediately returned a request for comment.