According to a Department of Defense Inspector General (DOD IG) report, Boeing agreed to provide $1 million in replacement parts for its F-15 fighters in a confidential settlement over a November 2007 incident in which an F-15C Eagle broke apart in midair. An investigation into the incident found that Boeing provided “defective or non-conforming” parts to the Air Force. According to the DOD IG’s summary of the investigation, the accident was caused by the failure of the upper-right longeron, a thin strip of material to which the aircraft’s skin is attached. The contract required the longeron to be .10 inches thick, but the investigation found that Boeing-supplied longerons varied in thickness from 0.039 to 0.073 inches.