22-12-00
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A
757
MAINTENANCE MANUAL
Pitch Axis Control
Figure 1 (Sheet 2)
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757
MAINTENANCE MANUAL
(b)
The basic mode of the pitch channel is vertical speed (V/S). Altitude hold is V/S equal to zero. Both are active whenever a flight director is on or an autopilot is engaged in CMD. Altitude hold is manually selected by pressing the HOLD button on the MCP. Vertical speed is manually selected by pressing the V/S button on the MCP or is automatically selected when CMD is engaged without selecting a mode. Altitude hold is automatic whenever the vertical speed is less than 300 feet per minute.
(c)
Altitude select control is available whenever a flight director is on or an autopilot is engaged in CMD. A vertical speed may be selected and V/S pushbutton pressed. The airplane then climbs or descends to the altitude selected on the MCP. If in a climb or descent at the time the F/D is on or the autopilot is engaged,the airplane continues at the existing vertical speed until the selected altitude is reached. The mode then automatically transfers to altitude hold. The V/S or HOLD pushbuttons do not have to be pressed if the vertical speed and altitude differences are greater than altitude hold capture limits.
(4) Programmed Pitch Attitude Control
(a)
Any of the programmed pitch attitude control functions can be used with a flight director engaged or an autopilot engaged in CMD. During sustained elevator deflections, the stabilizer may be retrimmed. The FCC generates auto stabilizer trim commands for the SAM. The SAM drives the stabilizer trim system until FCC pitch attitude circuits return elevators to neutral and remove auto stab trim commands (Ref 22-22-00).
(b)
The flight management computer system provides pitch steering commands when VNAV is selected. The airplane is then guided to the vertical flight profile programmed in the FMCS.
(c)
Programmed pitch attitude control functions provide autopilot or flight director guidance to capture and track the glideslope centerline during an ILS approach. Additional programmed functions provide automatic compensation for windshear and perform the flare maneuver prior to touchdown. Vertical position correction provides a consistant touchdown point on the runway. A controlled pitchdown attitude maneuver lowers and holds the nose wheel down after the main gear touchdown. If a go-around is initiated, flight director or autopilot guidance establishes a pitch-up attitude and positive rate of climb. Go-around mode disengages at 500 feet, or when another mode is selected. A selected vertical mode is then used for airplane guidance.
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757
MAINTENANCE MANUAL
C. Flight Control Computer Pitch Axis Control Inputs and Outputs (Fig. 2)
(1)
Inputs include mode requests from the MCP, servo and surface position from the APCS, flight reference data from IRUs and ADCs, and navigation data from ILS and RA systems. Vertical steering, rate target altitude, and airspeed/mach commands are supplied by the FMC. Auto stabilizer trim data is received from the stabilizer position module, stabilizer trim and elevator asymmetry limit module, and elevator neutral shift transducer.
(2)
The FCC outputs include mode status discretes, air speed/mach number, and vertical speed data to the MCP. Autopilot pitch commands are sent to the autopilot pitch control servos and stabilizer trim and elevator asymmetry limit module. Flight director pitch bar command, pitch armed and engaged mode annunciation data is sent to the EFIS symbol generators (Ref 34-22-00).
2. Component Details
_________________
A. AIRPLANES WITH CONTROL WHEEL STEERING;
Elevator Control Force Transducer (Fig. 2A)
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