Accident Overview
The airplane was maneuvering to deliver fire retardant when its left wing separated. Control was lost, and the aircraft crashed into mountainous terrain. A witness on the ground captured a series of photographs showing the left wing separating at the wing root, after which the remaining airplane entered a 45-degree dive in a counterclockwise roll.
Wreckage Examination
Examination of the wreckage revealed extensive areas of preexisting fatigue in the left wing's forward spar lower spar cap, the adjacent spar web, and the adjacent area of the lower wing skin. The portion of the wing containing the fatigue crack was obscured by the retardant tanks and would not have been detectable by an exterior visual inspection.
Similar Aircraft Inspection
An examination of two other air tankers of the same make and model revealed that the area where the failure occurred on the accident airplane was in a location masked by the airplane's fuselage construction.
Aircraft History
The airplane was manufactured in 1945 and served in the military until 1956. It was not designed with the intention of operating as a firefighting airplane. In 1958, it was converted to civilian use as an airtanker and served in that capacity until the accident.
Maintenance Procedures
The investigation revealed that the owner developed service and inspection procedures for the airtanker; however, the information contained in the procedures did not adequately describe where and how to inspect for critical fatigue cracks. The procedures were based on U.S. Navy PB4Y-2 airplane structural repair manuals that had not been revised since 1948.