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on video B727 Flight Controls - Pitch Control

 



PITCH CONTROL
Pitch control of the 727 is accomplished through two independent elevators and a stabilizer. There are balance panels to assist in elevator movement. The elevators are powered jointly by systems "A" and "B" and will operate normally with either system alone. With loss of all hydraulic power, the elevator tabs act as control tabs, the same as the inboard aileron tabs. Full elevator movement by means of the control tabs is only about 50% of the movement available in powered operation. The actual position of the elevator with respect to the stabilizer can be observed by referring to the indices on the left and right sides of the rudder and elevator position indicator. Under normal flight conditions the elevator pointers should be centered.
To provide an artificial feeling, an elevator feeling computer is installed. With inputs of system "A" and "B" hydraulic pressure, pitot-static pressure and stabilizer position the feel computer furnishes proportional feel to the elevator control system. The elevators feel the computer does not supply boost to the elevator. When substantial differences in the computer pressure outputs occur, the elevator feels differential pressure light on the SID's panel will come on. It indicates a possible erroneous control feel. When the light is on the pilot should avoid abrupt elevator inputs. Spring tension and aerodynamic forces give feel when the elevators are operated with no hydraulic power.
Pitch trim is accomplished by repositioning the stabilizer. The stabilizer can be controlled by either of two electric motors or a manual system. Both of the electric motors and the manual system operate the same jackscrew, which drives the stabilizer. The high-speed electric trim motor is controlled by these main electric trim switches on the control wheels. Each main electric control consists of two thumb switches; one is for motor power, the other clutch power. Both have to be activated to move the stabilizer. This is a safety feature to prevent one faulty switch from causing a runaway stabilizer. The other trim motor is slow speed, and is controlled by the cruise trim switch on the control stand. The autopilot uses the slow speed motor for pitch trim. Manual trimming of the stabilizer is accomplished by using the cranks stored in the trim wheels located on either side of the control stand. The manual trim will override either electrical trim motor.
When the stabilizer is being trimmed electrically by either the main electric trim switch or cruise trim switches the stabilizer trim light on the control stand will come on. This indicates that one of the trim motor circuits is energized. When the autopilot is engaged, however, the cruise trim motor runs continuously. To avoid continuous illumination of the lights it is deactivated during autopilot operation. The two lever switches to the right of the light are cut-out switches to remove electrical power from the motors.
Trim indices, located on both sides of the control stand, show the position of the stabilizer units. The green band denotes limits of stabilizer trim in %MAC. If the stabilizer trim is not in the green band for takeoff, an intermittent horn will sound when the throttles are advanced toward takeoff setting. To prevent the stabilizer from running to the stops there are electrical trim limits, both nose up and nose down. When the main trim reaches one half-degree nose up during nose down travel, control automatically switches to the cruise trim motor. Once the electrical limits are reached, the stabilizer can be trimmed slightly further manually. As the stabilizer is trimmed into the range near the nose up limit, the neutral elevator will gradually move a few degrees up from fair. This provides more effective stabilizer trim and more nose down elevator capability when the airplane is trimmed nose up. Operation of this mechanical linkage can be noted in the cockpit by aft movement of the control column when the stabilizer is trimmed into the range where the system if effective. To stop a runaway stabilizer a brake stabilizer is installed. A control force opposite to the direction of the runaway will engage the brake. Once engaged, the stabilizer brake should automatically disengage when opposing pressure is relieved. If it does not, pulling the stabilizer brake release knob will mechanically release the brake.



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