ERA12LA500 · Beech V35B

The pilot reported that he was conducting a cross-country instrument flight rules flight, and, during the cruise portion of the flight, he intermittently encountered areas of instrument meteorological conditions (IMC). About 1 hour 50 minutes into the flight, an air traffic controller advised the pilot of an area of moderate-to-extreme precipitation 20 miles ahead, extending along the intended route of flight for 100 miles. Eight minutes later, the pilot contacted an air traffic controller and requested a descent from 12,000 to 10,000 ft mean sea level (msl) "for weather," but he did not receive a reply to the request, and the airplane continued on-course. After an additional 8 minutes, the pilot reattempted to contact a controller but was interrupted by another pilot, and he again received no response. The pilot attempted to contact a controller a third time and requested a turn to get out of the weather. This time a controller responded and advised the pilot to turn left, but, just as the pilot initiated the left turn, the airplane encountered an area of severe turbulence. The pilot reported that, while in the turbulence, the airplane encountered an updraft that put the airplane in a 4,000 ft per minute climb and that the airspeed reached 253 knots, which exceeded the airplane's never-exceed airspeed. The airplane then encountered a downdraft, which caused the airplane to lose 3,000 ft of altitude, and the primary flight display simultaneously "went black." When the display returned, it showed a message advising the pilot to "level the wings" while the attitude and heading reference system realigned. The pilot subsequently used the standby instrumentation to control the airplane while he initiated an emergency descent. The airplane exited the turbulence and IMC about 4,000 ft msl, and, shortly thereafter, the propeller separated from the engine. The pilot subsequently performed a forced landing to a cornfield, and the airplane sustained substantial damage to the fuselage and both wings.

The pilot reported that he received a weather briefing before departing on the accident flight. According to audio recordings of the briefing, the weather briefer advised the pilot of the adverse conditions along his route of flight; the pilot replied, "Ok, I guess we'll deal with that when we get there, if we have to go around it or stop, that's fine." An air traffic controller again advised the pilot of the severe weather conditions at least 20 miles ahead of the encounter. It is unlikely that the pilot's initial unanswered request to descend to 10,000 ft msl would have prevented the weather encounter, and the pilot's second and third unsuccessful attempts to contact the controller occurred more than 15 minutes after he was first advised of the weather conditions. The pilot was clearly made aware that severe weather conditions existed along his route of flight, but he waited until too far into the flight to try and avoid them, which ultimately led to the flight's encountering the conditions that resulted in the in-flight loss of control.

The air traffic controller complied with the Federal Aviation Administration's minimum requirement for "additional services" by providing the hazardous weather information to the pilot when he first checked in, but additional information by the controller would have been valuable. For example, as the flight continued tracking directly into known heavy-to-extreme precipitation that other aircraft were deviating around, the controller should have realized that the pilot was not taking action to avoid the weather and either suggested a deviation or at least updated him about the proximity of the hazardous weather that the airplane was rapidly approaching. Regardless, it was ultimately the pilot's responsibility to avoid the severe weather.

The propeller hub was found detached from the airplane due to a failure caused by reverse bending fatigue of the mounting bolts connecting the hub assembly to the engine crankshaft mounting flange. All of the bolts exhibited features consistent with fatigue cracking in a circular direction along the same direction as the wear marks on the hub case aft face. The reverse bending failure of the hub mounting bolts was indicative of a loose connection between the hub and the crankshaft. None of the airplane's documented maintenance indicated that the propeller hub was removed during the year before the accident.

The engine's fractured connecting rod exhibited a small thumbnail fatigue crack on one side. However, this small amount of fatigue likely occurred after the fatigue cracking had begun on the propeller bolts. Cracking in the propeller bolts would likely have created unbalanced loading in all of the connected components, including the crankshaft and connecting rod. Once the propeller separated from the crankshaft, the crankshaft absorbed the entire load exerted by the engine, and this increased loading likely in turn increased the friction at the contact surfaces beyond the capacity of the lubrication. Without sufficient lubricating capacity at the journals, the material would begin to heat excessively, creating local material deformation. The underlying reasons for the loose connection between the propeller hub and the crankshaft could not be determined, but it is likely that the extreme forces encountered during the flight's weather-induced upset and loss of control resulted in the ultimate failure of the connecting bolts.

Verbatim NTSB analysis and probable cause from the NTSB dataset