22-12-00
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A
747-400
MAINTENANCE MANUAL
(h)
If NOT in multi-channel engage, manual trim switch is pressed.
(i)
If in multi-channel engage, two identical autoland sensors from different channels fail or invalid
(j)
If in multi-channel engage, local program pin status invalid
C. Pitch Axis Modes of Operation
(1)
Takeoff (TO) Mode - flight director only
(a)
The takeoff mode is automatically engaged when either or both flight director (F/D) switches on the MCP are placed in the ON position while the airplane is on the ground.
(b)
Takeoff mode sets the pitch command on the Primary Flight Display (PFD) to 8 degrees while the airplane is still on the ground. This F/D pitch command is a fixed reference position for takeoff mode since AFDS does not provide F/D guidance commands for takeoff rotation. Once the airplane is rotated, pitch commands are developed in the Speed portion of the pitch channel. The reference angle of attack used for the speed control law is a function of the stick-shaker pitch angle, for radio altitude less than 1,000 feet. For radio altitude greater than 1,000 feet, the reference angle of attack is a function of the stall velocity and flap position.
(2)
Vertical Speed (V/S) Mode
(a)
Vertical speed mode entry is possible provided transition logic and acceptance criteria are satisfied (Ref 22-10-00/001). Vertical speed may be entered by default when the autopilot is engaged or flight director turned ON and no other pitch mode has been selected. Vertical speed is also entered while the autopilot is engaged by pressing the V/S switch/light on the MCP.
(b)
Prior to engagement of the V/S mode, with the AP/FD system active, the vertical speed pitch computations are synchronized to the airplanes present vertical speed to provide for a smooth transition. During this synchronization period, the pitch attitude integrator provides a constant output for airplane pitch axis stability.
(c)
The FCCs are continuously providing present vertical speed (which it receives from IRS) to the MCP to be displayed in the VERT SPD window when the V/S switch/light is pressed. This allows the VERT SPD window to be synchronized to the present vertical speed.
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22-12-00
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
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747-400
MAINTENANCE MANUAL
(d)
Once synchronized and all the engage logic is satisfied, V/S is engaged and AFDS provides control of the airplane as selected on the VERT SPD window. Vertical speed computations rely on: 1) Inertial Reference Unit (IRS) - inertial vertical speed,
pitch rate, and roll attitude 2) Air Data Computer (ADC) - total airspeed, angle of attack, and indicated airspeed
(e)
Various damping terms, integrator, limiters and time delays in the Vertical Speed and pitch attitude integrator provide for smooth pitch axis control and dampen rapid changes in the VERT SPD window.
(f)
With V/S engaged, pitch attitude is limited to |30 degrees and pitch rate changes are limited to 0.1g during V/S capture.
(3)
Altitude Hold (ALT HOLD) Mode
(a)
The altitude hold mode is manually selected by pressing the HOLD switch/light below the ALT select knob on the MCP. Altitude hold is also selected automatically as a sub-mode as follows: 1) In V/S mode, vertical speed equals zero. 2) In FL CH and the airplane deviates from the path to aquire
the MCP selected altitude. AFDS will hold the present altitude (when the error was detected).
(b)
When ALT HOLD is selected manually or is defaulted to as a sub-mode of another mode as described above, the Vertical Position control block provides pitch-axis control of the airplane. While in ALT HOLD mode, the Vertical Position control law is relying on the following input for the indicated information: 1) IRU - vertical acceleration and vertical acceleration limit 2) ADC - Barometric altitude
(4)
Flight Level Change (FL CH) Mode
(a) Flight level change mode is entered when the FL CH switch/light on the MCP is pressed, provided the appropriate logic is satisfied. Normal operation of the FL CH mode is with the autothrottle (A/T) system armed. However, FL CH mode may be used with the A/T system off. With the A/T system armed and FL CH selected, the FMC provides a constant thrust to aquire a particular vertical speed. The vertical speed that the FMC calculates is primarily a funcition of the magnitude of the change in altitude that is requested (difference between present altitude and MCP selected altitude).
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