http://www.casa.gov.au/airworth/pape...oCommander.pdf
The Story Behind 24 Aero Commander airborne wing-sheddings (by 1995) with a total of a further 35 a/c found, on the ground, to have cracked main-spars.
"Despite their robust appearance, at last count 24 Aero Commanders had lost wings in flight, 35 spars had been found cracked on the ground, and hundreds of other spars had defects caused by:
fatigue
corrosion
stress corrosion
static overload