Mostly returned to service in February this year - but 137 is rather close to the number which were found to have non-standard longerons but cleared to fly. Perhaps not a coincidence. I think that without remedial work (which would cost money, & take them out of service for a while), they probably have a reduced fatigue life, & may need frequent inspection.I thought the USAF will only have 129 F-15Cs unless the 140 or so F-15s that where grounded last year are back in service and not retired.
Report on returning to service