What happened
The Mitsubishi MU-2B-35 landed with visible damage on the underside of the left wing, specifically midway between the engine and the tip tank, extending outboard to the tank. The pilot reported encountering turbulence while cruising at 250 knots. In response, he reduced speed to 170 knots before resuming cruise speed at 250 knots.
The investigation
A detailed examination of the left wing revealed a broken lower front wing spar and a bent rear lower wing spar. Mitsubishi conducted a separate analysis of the damage and determined that the structural failure was likely caused by either a hard landing or landing with excess fuel in the tip tank. Investigators noted it was probable the aircraft had been flown several times with this wing damage already present.