A crack was discovered in one of its 13 wing spars of the F-35C durability testing ground article last month. The Pentagon has determined that modifications are needed on existing jets to ensure full life operation.
By U.S. Navy video by Mass Communication Specialist Eli K. Buguey [Public domain], via Wikimedia Commons
No aircraft has been grounded because of the crack and the U.S. Navy is still on course to declare Initial Operating Capability (IOC) of the C-model in August 2018.