The purpose of the accident flight was as a normally scheduled air taxi flight between Tucson (KTUS) and Phoenix (KPHX), USA. The flight was operated by Apache Airlines, Inc. The aircraft crashed about 5 miles southwest of Coolidge. The accident aircraft, a De Havilland Dove, was originally produced with two reciprocating engines and seating for ten passengers and two pilots. The accident aircraft had been altered under a Supplemental Type Certificate (STC) that replaced the engines with Garret TPE-331 turboprop engines and lengthened the fuselage, allowing seating for 18 passengers. As part of this STC conversion, the wing lower main root joint fittings, which was done to accommodate the new engines as well as changes to the fuel system; the fittings were included with the alteration, and were made of heat treated alloy steel. This STC was developed by the Von Carstedt Corporation of Long Beach, California. Witnesses to the accident reported initially seeing the aircraft flying on a northwesterly heading. Engine noises were heard to suddenly cease, and the aircraft began a shallow descent. The descent steepened into a 40-50 degree dive. The aircraft soon struck a freshly plowed agricultural field. The main wreckage and right wing came to rest 35 feet apart. A wing root fairing was located 1,160 feet downwind of the main wreckage. Investigation revealed the right wing lower main root joint fitting had failed in fatigue, causing the wing to begin to separate in flight, eventually departing the aircraft completely after the aircraft had entered an emergency descent. The fatigue failure of the fitting occurred because the size and geometry of the fitting did not allow the complete and consistent hardening of the material throughout its thickness, causing soft areas within the part. Corrosion and fretting damage was also noted on the fitting. Probable Cause: “The inflight failure and subsequent separation of the right wing. This failure was the result of a fatigue fracture in the lower main root joint fitting which propagated from an area of corrosion and fretting damage which, in turn, was caused by design deficiencies. These deficiencies remained undetected because surveillance of the supplemental type certification process and the modification programs was not adequate to assure compliance with design and inspection requirements.”