Background
The aircraft arrived at Cheyenne, Wyoming, at 0038 on May 22, completing a Civil Air Movement flight (military contract) from New York International Airport. The incoming pilots reported no mechanical or radio discrepancies. After servicing, Captain Bowen F. Marshall and Copilot Samuel B. Aronson relieved the crew to ferry the aircraft to Chicago, where another crew would take over.
Pre-flight and Departure
Both pilots visited the U.S. Weather Bureau Airport Station at Cheyenne for a weather briefing. All current weather information through 2330, including three severe weather warnings, was on file and available to them. The aircraft departed Cheyenne at 0132 on an instrument flight plan to O'Hare Airport, Chicago, via Green Airway 3 and Red Airway 42 at 7,000 feet. No passengers or cargo were carried, and the center of gravity was within permissible limits.
Flight and Communication
Routine position reports were made over Sidney, North Platte, Grand Island, and Omaha, Nebraska. The last report, at 0337, indicated the flight was over Omaha at 7,000 feet, estimating arrival over Des Moines. Three minutes later, the pilot contacted Omaha INSAC requesting a descent to 3,000 feet. Air Route Traffic Control, through Omaha INSAC, disapproved the request because a westbound flight was operating at 2,600 feet between Des Moines and Omaha; descent was unavailable until after Des Moines. The flight was given the latest weather between Omaha and Chicago, including the Des Moines 0327 weather. No further radio contacts were received.
Accident
At 0517, Des Moines tower personnel received a telephone call notifying them of the crash. The aircraft was destroyed and both pilots were killed.
Investigation Findings
The investigation revealed that the flight encountered a severe thunderstorm in the vicinity of Des Moines, resulting in in-flight structural failure. Prior to impact, the outboard trunnion supporting the right aileron tab motor backed out of its support bracket due to inadequate safetying. The right aileron tab motor tore loose from its bracket, punched through the wing, and was lost when the aileron separated. The right wing panel then failed at the attach angles to the center section due to a strong downward force exceeding the wing's design strength. The tail group was struck by the right wing panel before disintegrating. No malfunction or failure of the engines or propellers was found. All radio contacts were routine, and no emergency declaration was received.
Probable Cause
The probable cause of the accident was separation of the right aileron tab motor from its support bracket due to loss of its outboard trunnion while the aircraft was in the severe turbulence of a thunderstorm. These conditions resulted in a tendency to roll erratically, and in conjunction with the extreme turbulence, caused loss of control and subsequent overloading of the wing to the point of failure.