An investigation of the failure found that the propeller had been installed incorrectly following recent maintenance. Routine maintenance such as replacing the alternator belt or starter on a Grumman requires the removal of the propeller prior to removing the nose bowl, which increases the frequency of prop removal/reinstallation—a very critical task to perform properly.
The crankshaft flange has an indexing “pin” that ensures the propeller is properly oriented—or “clocked”—on the flange during installation. The spinner back plate is sandwiched between the propeller and the crank flange, and has a corresponding hole for the pin to pass through into the prop.
Unfortunately, it is very easy to align the spinner back plate incorrectly during the installation of the propeller, causing the pin to punch a new hole in the spinner back plate and making it appear that all is well as the propeller is torqued down. However, all is not well because the remains of the newly punched-out hole can be caught between the propeller and back plate. This situation prevents the surfaces from mating correctly and, as the trapped material wears down, the space is freed up and the prop bolts no longer hold the propeller tightly to the crankshaft flange.
The scenario described above is exactly what happened in Inhofe’s case. As soon as the trapped aluminum had worn away, the resulting stress on the loose bolts caused them to shear, and the prop landed well before the aircraft did. In a strange twist of fate, one of Inhofe’s high school classmates found the propeller, resulting in a reunion of sorts for the propeller, the classmate, and the Senator back at the airport.
