55-18-02 DOUGLAS: Applies to All Model DC-6 Series Aircraft That Are Not Equipped With the New Douglas Type Propeller Reverse Thrust Mechanical Throttle Lockgate. \n\n\tCompliance required as soon as practicable but not later than January 1, 1957. \n\n\t1.\tBecause of instances of inadvertent propeller reversing due to malfunctioning of the solenoid operated throttle locks and improper operation of throttles at time of touchdown on landing, a more positive means of preventing inadvertent movement of the throttles into the reverse segment of the throttle quadrant must be installed. \n\n\t2.\tThis modification consists of the installation of a protective device which will require a separate and distinct motion by the flight crew member accomplishing the reversing, in order to place the throttles in reverse pitch range. It must also provide safeguards against disarming or unlocking of this protective device when the throttle levers are further forward than idle setting or, if it is possible to unlock at a forward power setting, movement of the throttles toward idle from slow cruise or landing gear warning positions shall reengage the lock prior to reaching the idle position. \n\n\t3.\tDouglas Service Bulletin DC-6 No. 557 revised October 15, 1954, covering this modification is an acceptable method of compliance. This bulletin also provides for the continuous operation of the feathering motors during reverse thrust, installs timers to control feathering pump operation during feathering, and deletes the propeller governor pressure cutout switch from the holding circuit. These latter changes, although desirable, are not mandatory. \n\n\t4.\tIt is possible that some aircraft have already been modified by installation of a protective device which differs in arrangement and detail from the provisions of the Douglas Bulletin. Such alternate designs may be acceptable if the objectives of this directive, as expressed in paragraph 2, have been met. However, because of the many technical considerations involved, all alternate methods of accomplishing this modification should be referred to FAA for engineering evaluation and approval.