I remember back in June and July of 2002, I investigated two firefighting aircraft accidents, one in Colorado and the other in California. A Lockheed C130-A crashed in Walker, CA; both its wings separated from the fuselage during a fire retardant drop. The second was a Consolidated-Vultee PB4Y-2 in Estes Park, CO; it lost its left wing during a retardant drop.
The C130-A was manufactured in the early 1950s and the PB4Y-2 was manufactured to fight in the Pacific theater of WWII in 1944. The unusual thing about both of these aircraft was that they were considered Public Use aircraft; they didn’t subscribe to the same federal rules that regular aircraft of that size follow. As a result, the maintenance requirements of their inspection programs were way out of date; they did not have aggressive structural inspection programs and, unfortunately, did not have to. For aircraft that old and with the stresses those airframes were subjected to, the lack of a comprehensive inspection program was ludicrous. As a result, the causes of both accidents were almost identical.
Public Use applies to many types of aircraft, from fire fighters to police helicopters to State aircraft. It would be interesting to see if the rules governing Public Use aircraft have been revised in the last thirteen years.