Background
Apache Airlines Flight 33 was a regularly scheduled air taxi service from Tucson to Phoenix, Arizona, on May 6, 1971. The aircraft involved was a De Havilland Dove Model 104-7AXC, modified under a Carstedt CJ600A conversion, registered as N4922V. The flight departed Tucson International Airport at 1253 Mountain Standard Time under visual meteorological conditions, carrying 10 passengers and a crew of two. The fuel load was 140 gallons.
Accident Sequence
At approximately 1315, ground witnesses near Coolidge, Arizona, observed the aircraft flying northwest. Several witnesses reported hearing loud engine noises that then ceased. Two witnesses described the aircraft initially descending at a slight angle, which steepened into a 45-to-50 degree dive. No smoke, fire, or in-flight breakup was observed by witnesses. High scattered cumulus clouds were present in the area. A pilot who flew through the region after the accident reported encountering severe turbulence at about 4,200 feet near Phoenix. The 1258 surface weather observation at Phoenix indicated a measured ceiling of 4,800 feet broken clouds, 25,000 feet overcast, visibility 40 miles, wind from 250° at 8 knots, and an altimeter setting of 29.78 inches. The crew was current and qualified for the operation.
Wreckage and Investigation
The wreckage was found in a flat plowed field, forming two distinct craters with no impact marks between them. One crater, oriented north-south, contained most of the fuselage and left wing debris within 200 feet. The other crater, about 35 feet southeast, held the right wing and right engine remains. Scattered right wing parts lay northeast of that crater. Only one piece, a fairing from the lower aft wing root, was found 1,160 feet northwest of the fuselage crater. The aircraft was heavily fragmented, with greater damage on the right side. Both engines were recovered attached to wing structure, with no pre-existing damage to mounts. Examination revealed no evidence of malfunction or failure prior to impact; neither engine was producing power at impact. Both propellers were found in the feathered range. Disassembly of fuel controls, pumps, and propeller governors showed only impact damage. All 12 occupants died.
Probable Cause
The official probable cause was the inflight failure and subsequent separation of the right wing. This failure resulted from a fatigue fracture in the lower main root joint fitting, which propagated from an area of corrosion and fretting damage caused by design deficiencies. These deficiencies went undetected because surveillance of the supplemental type certification process and modification programs was inadequate to assure compliance with design and inspection requirements.