What happened
During a positioning flight, the pilot of the aircraft noticed that the airplane required aileron trim to maintain level flight. The aircraft subsequently landed without incident.
The investigation
A subsequent examination of the aircraft revealed two missing rivets on the underside of the left wing, located between the heat shield and a factory-installed doubler. Investigators found that the lower forward spar cap was cracked at wing station (WS) 114, specifically at a rivet hole situated just outboard of the outboard engine beam. Additionally, both the forward and aft web doublers were split near the end of the inboard wing spar web at WS 112.5. The skin also exhibited a crack extending from the front spar lower cap up to the first stringer location behind the front spar.
The aircraft had been operated for approximately 20,510 total hours. Under FAA AD 2000-23-01, the forward, aft, and auxiliary wing spars were required to undergo inspections for cracks at intervals not exceeding 110 hours time-in-service.
Upon removing the cracked portion of the forward spar, skin, and the aft web doubler for further examination, investigators identified fatigue damage. This damage originated at the forward portion of the rivet hole on the spar cap at WS 114. Blackening and corrosion were observed on the fracture surface, which was consistent with the fracture being present during the aircraft's last inspection, which had occurred approximately one week and 12 operational hours prior to the incident.