Incident Description
A cargo flight departing Memphis, Tennessee, with no passengers on board, experienced an uneventful flight until the captain perceived a vibration on the left side of the aircraft. Believing the propeller to be the origin, the captain asked the first officer if he had ever seen the propeller in the feathered position; the first officer replied that he had not. The captain then instructed the first officer to go into the passenger cabin and visually inspect the left propeller in the feathered state.
Seconds after the first officer moved to the cabin, the aircraft began a series of violent gyrations. These movements threw the captain and a flight attendant against the interior of the cabin. The aircraft lost several thousand feet of altitude, descending from 11,000 feet, before the flight crew managed to regain control. The crew then landed the aircraft without further incident.
Investigation Findings
A review of the event and the reported vibration, which was felt only by the captain, failed to reveal any mechanical problem that would have produced a vibration. The procedures employed by the captain were not in accordance with FAA or company-approved directives.
Aftermath
The aircraft landed safely, and no further details on injuries or damage were provided in the investigation source.