Page:CAB Accident Report, Eastern Air Lines Flight 304.pdf/4

 {| The aircraft can be controlled longitudinally by use of the elevators or variable incidence horizontal stabilizer. The elevators are operated by movement of either control column through two independent cable systems to elevator control tabs. The elevators are connected together by a torque tube at the rear spar of the horizontal stabilizer. The friction tolerance in this system is ± 5 to 6 pounds. Tabs on the trailing edge provide aerodynamic boost to control inputs. Most of the pilot's stick force is provided by a load feel mechanism with two opposing preloaded springs which establish a neutral point of the elevator control system. Resistance of these springs to motion of the control column is greatest near the neutral point (See Figure 1, Attachment C).
 * 1
 * 610631
 * 2,590 hours
 * 6,766 hours
 * 2
 * 611786
 * 4,151 hours
 * 7,876 hours
 * 3
 * 610636
 * 3,927 hours
 * 6,857 hours
 * 4
 * 611573
 * 734 hours
 * 6,320 hours
 * }
 * 3,927 hours
 * 6,857 hours
 * 4
 * 611573
 * 734 hours
 * 6,320 hours
 * }
 * }

The nosedown pitching moment encountered in high speed flight is offset in the DC-8 by the Pitch Trim Compensator (PTC) system which applies noseup control through the elevator system. Operation of the PTC is also required in the low altitude, high speed regime below mach effect to improve stick force characteristics as speed increases. This system consists of an electrical computer, an electrical actuator, spring loaded linkages, and a mechanical indicator. The computer senses mach effect at high altitude and dynamic pressure below 20,000 feet, and provides the electrical signals to the actuator which actually moves the copilot's control column. The actuation begins at either Mach 70 or 310 knots and increases in displacement and rate up to Mach 88 or 410 knots. The maximum input is 36 pounds of stick force. Actuation of the PTC is indicated by the extension of a plunger from a flexible cable housing attached to the left side of the copilot's control column. There is no measurable correlation between the amount of indicator showing and the degree of actuator extension. A three-position switch located on the left side of the control pedestal permits normal operation, testing of the system in the spring-loaded test position, and an override position which may be used to retract the actuator in the event of a malfunction.

Longitudinal trimming of the aircraft is accomplished by hydraulic or electric actuation of the horizontal stabilizer. The hydraulic motor trims at a rate of 1/2 degree/second through a range of 10 degrees aircraft noseup (ANU) to two degrees aircraft noseodown (AND), and is actuated by manipulation of dual toggle switches on either control column, or by split "suitcase" handles mounted side by side on the center console. The electric motor trims at a rate of 1/17 degree/second, and is actuated by dual toggle switches on the center console, or by the autopilot. Both motors provide power through differential gearing to a drive shaft