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Twenty-five victims were never found, including Bill Fortenberry. For years, his son Ken believed the navigator was awaiting rescue on a desert island. (NASM)

The Mystery of the Lost Clipper

The Civil Aeronautics Board and the FBI abandoned the case 47 years ago, but two amateur detectives are still looking for the cause of the Pan Am 944 crash

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The governor inside a constant-speed propeller helps translate engine power to blade pitch. Gyrating flyweights (A) in the governor are driven by the engine. When the engine’s power is increased or decreased, the change in centrifugal force causes the L-shaped flyweights to open or close an oil flow valve (B), which controls the amount of oil flowing through the oil transfer tube (C) into the prop dome. Inside the dome, pressure from the oil shifts the piston-and-roller assembly (D), which twists the cam to adjust the pitch of the blades. If oil drains from the prop dome—perhaps due to a failure of the oil transfer ring (E), which connects the oil transfer tube to the spinning prop shaft—centrifugal force wrenches the spinning blades to their lowest angle of attack, perpendicular to the air flow. This condition makes it impossible to feather the blades. Without resistance from the blades—which aren’t doing any work—the engine drives the propeller too fast, causing it to over-speed.

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