Accident Summary
After takeoff, the airplane was observed at low altitude trailing smoke from the left engine. Witnesses reported the wings 'tipping' back and forth before a wing dropped and impacted the ground. All 12 occupants, including 11 parachutists and the pilot, were killed. The parachutists were found in the center part of the fuselage with no evidence of restraint usage.
Investigation Findings
Examination of the left engine revealed a supercharger bearing failure. The left engine had been recently installed by non-certificated personnel after being inactive for 18 years without preservation. The airplane had flown approximately 184 hours since its last annual inspection, with no record of a subsequent 100-hour inspection. The left propeller blades were found in an intermediate position between the operating range and the feathered position. The left propeller had been changed several weeks prior to the accident, but there was no evidence that it had ever been successfully cycled to the full feather position.
Maintenance and Propeller Issues
The left propeller feathering motor relays were not recovered. The operator and pilots were unaware of Hamilton Standard Service Bulletin 657, which recommends full-feather checks every 30 days. The left propeller's inability to fully feather likely contributed to the loss of control.
Probable Cause
The probable cause, as determined by the official investigation, was inadequate maintenance and inspection by the operator, which resulted in an engine power loss during the critical takeoff phase of flight. Additionally, the pilot did not, or was unable to, attain a full feather position on the left engine propeller, which would have most likely enabled the airplane to sustain minimum control airspeed.