 The Douglas Aircraft Company designed a system that would prevent the accidental reversal of propeller blades in-flight. During development of the DC-6, the company installed a system that cut electrical power to the mechanisms which rotated the blades while the airplane was in the air. When there was enough weight on the landing gear (which would only be the case when the aircraft was on the ground), a switch which supplied electrical power to the mechanisms was closed — meaning that when the aircraft touched down the blades could be reversed and thus the airplane could be slowed. When the switch was closed, a red flag would swing into view in the cockpit of the aircraft, warning the crew that the blades could be reversed. Should the switch fail to close upon landing, the flag could be raised manually and electrical power to the mechanisms would be restored. When the aircraft took off, electrical power would be cut to the mechanisms so that the propeller blades could not be inadvertently reversed, and the red flag swung out of sight. Reverse thrust warning lamps, which would have warned the crew if the propellers were reversed, were not fitted on Mainliner Idaho.
