Accident at Des Moines, Iowa, 22 May 1953

Accident narrative

On May 22, 1953, a Resort Airlines Curtiss disintegrated in flight during a thunderstorm and crashed in a field about 11 miles northeast of Des Moines, Iowa. Both pilots, the sole occupants, were killed.

**The flight**

The aircraft was operating a military contract ferry flight from Cheyenne, Wyoming, to O'Hare Airport in Chicago, carrying neither passengers nor cargo. Captain Bowen F. Marshall and Copilot Samuel B. Aronson took over the aircraft in Cheyenne. Captain Marshall had 8,106 flying hours, including 3,622 in this equipment, and Copilot Aronson had 2,593 hours, with 393 in this aircraft. The incoming crew reported no mechanical or radio discrepancies.

Before departing, the pilots visited the Weather Bureau in Cheyenne. All current weather information, including three severe weather warnings, was available to them. The flight departed at 0132 on an instrument flight plan at 7,000 feet.

The crew made routine position reports over Nebraska. At 0337, the flight reported to the Omaha Interstate Airways Communication Station that it was over Omaha at 7,000 feet. Three minutes later, the pilot requested a descent to 3,000 feet. Air Route Traffic Control denied the request because a westbound flight was proceeding at 2,600 feet between Des Moines and Omaha. The flight was given the latest en route weather and the 0327 Des Moines weather. There were no further radio contacts. Des Moines tower personnel were notified of the crash by telephone at 0517.

**What the investigation found**

Wreckage was scattered over a considerable area. The main wreckage struck the ground in a near-vertical descent. Examination of the cockpit showed the throttles advanced halfway and the clocks stopped at 4:12 and 1:13. The engines and propellers showed no evidence of in-flight malfunction.

The right outer wing panel and various tail surfaces had separated prior to impact. The right wing's lower surface buckled in compression and its upper surface failed in tension and bending. Because the airplane was lightly loaded, the Board noted that down gusts alone at cruising speed could not have caused this failure; excessive airspeed and maneuvering loads from a loss of control were necessary to exceed the wing's design strength.

Damage to the left horizontal and vertical tail surfaces indicated they were struck by the detached right wing panel before the tail group disintegrated. This evidence included a hole punched through the left elevator by a gray-painted object moving rearward, and a sliver of wing deicer boot rubber wedged in a tear on the vertical fin.

Investigators discovered a pre-existing mechanical failure in the right aileron trim tab motor assembly. The outboard trunnion supporting the tab motor had backed out of its bracket and was missing. Rust-colored deposits on the threads indicated the motor had been insecure for a considerable period. The remaining inboard trunnion was bent 15 degrees and its safety wire was broken. Directly above the bracket, an 18-inch hole had been punched through the top skin of the wing from the interior by the tab motor before the right aileron separated.

Maintenance records showed the aileron support bracket had been signed off without discrepancy during its last two major inspections, though the Board noted it was unknown if the discrepancy existed at those times.

Weather analysis showed a squall line moving eastward, accompanied by severe thunderstorms and a pressure jump line, which are often accompanied by violent turbulence. The Board noted that the aircraft was likely in clouds at 7,000 feet, making it difficult for the pilots to visually ascertain that they were entering a violent storm area.

The Board analyzed that under normal conditions, the remaining inboard trunnion and bracket interference were evidently sufficient to retain the tab motor. However, it was probable that when the aircraft encountered the extreme turbulence of the thunderstorm, the loads forced the tab motor out of its housing, piercing the wing. This displacement would have actuated the trim tab erratically, producing a strong tendency for the aircraft to roll. The Board believed this rolling tendency, combined with the severe weather, caused the loss of control and subsequent wing failure.

**Probable cause**

The Board determined that the probable cause of this 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."