Incident Overview
The incident involved a Piper PA-30 airplane that lost its propeller assembly during flight. A witness reported observing the aircraft spiraling downward in a manner consistent with a flat spin, accompanied by visible smoke.
Examination Findings
Post-incident examination of the fractured propeller mounting flange revealed that the propeller mounting flange had separated from the remainder of the crankshaft. The separation was caused by fatigue cracking that originated between the lightening holes in the flange.
Applicable Directives
An Airworthiness Directive (AD) and a Service Bulletin (SB) exist that specifically address the engine when installed on the Piper PA-30. These inspection requirements were prompted by cracking problems resulting from bending of the mounting flange during certain aerobatic-type maneuvers, primarily those that introduce stresses from gyroscopic action.
Weight and Balance
Following the loss of the propeller, the aircraft's weight and balance were calculated at 1,531 pounds with a center of gravity (CG) located 70.09 inches aft. For the specific aircraft, registration N320L, the approved CG envelope ranges from 60.75 to 66.75 inches aft. Thus, the post-separation CG was outside the permissible limits.