Flight Details
Flight 329 departed Des Moines on a VFR night plan at 1608 for Omaha, Nebraska. The aircraft's gross weight was 45,215 pounds, which was 1,685 pounds less than the allowable maximum of 46,900 pounds. According to company records, the load was properly distributed with respect to the center of gravity.
In-Flight Events
The climb to 5,000 feet was uneventful, but at that altitude the crew noticed vibration and a slight fore-and-aft movement of the control column. The climb was continued to 6,000 feet, where the aircraft was leveled off and power was reduced. As the vibration persisted, the captain attempted to dampen it by engaging the autopilot, but this was unsuccessful and it was immediately disengaged. The first officer then lowered the flaps, first to 5 degrees and then to 15 degrees, without any noticeable effect. The fasten seat belt sign was turned on, and the captain instructed the first officer to advise the company of their difficulty via radio.
Emergency Procedures
Shortly after, a sudden failure in the control system was felt, and it became extremely difficult to maintain any semblance of elevator control. The first officer again tried lowering the flaps, this time to the 24-degree position, but as this did not help, he returned them to the 15-degree position, where the most favorable results were attained. The first officer transmitted a Mayday distress call and stated that they were attempting to return to Des Moines but were experiencing control trouble. The buffeting became so severe that the copilot was required to help the captain hold the control column. The buffeting then lessened, and the captain directed the first officer to depressurize the aircraft and instruct the flight attendant to prepare passengers for an emergency landing. This was accomplished. By that time, the aircraft had descended below 3,000 feet. Both throttles were retarded in turn to determine if the trouble was caused by one of the engines, but this proved ineffective.
Accident and Evacuation
The vibration built up to a high level, and another failure in the control system was felt, causing the aircraft to enter a steep climb. As a stall seemed imminent, the captain quickly moved the propellers to a high RPM and advanced the throttles forward until about 50 inches of manifold pressure was indicated. The aircraft then nosed over and began a very steep dive. During the rapid descent, the captain reduced power and headed toward open country to his right. When the aircraft reached 500 feet above the ground, the captain succeeded in flaring, and the aircraft struck the ground in a flat attitude. All occupants quickly deplaned as soon as the aircraft stopped.
Probable Cause
The official finding determined that the probable cause of this accident was a series of omissions made by maintenance personnel during a scheduled inspection, which resulted in the release of the aircraft in an unairworthy condition and an almost complete loss of elevator control during flight. The following findings were reported: during a routine maintenance inspection, an explanation was not written on the non-routine job card that a bolt had been removed and replaced finger-tight pending arrival of a new bolt; final inspection of the servo tab system failed to disclose its unairworthy condition and the aircraft was released for service; vibration backed off an unsafetied nut in the servo tab system, resulting in a sequence of structural failures that ended in almost complete loss of control of the aircraft elevators.