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MAINTENANCE MANUAL
(b)
The control law for FL CH is developed in the speed section of the FCC pitch channel. This control law maintains the airplanes speed to the airspeed or mach number selected in the IAS/MACH window of the MCP.
(c)
During FL CH mode, the following interfaces provide input to the speed function to maintain the requested IAS or MACH (these inputs are used primarily to determine a reference angle of attack). 1) Flap control unit (FCU) - two angle of attack references
are developed; one for flap up (zero units of trim) and one
for flap 1 to 30 units
2) Inertial Reference Unit (IRU) - provides pitch angle
3) Air Data Computer (ADC) - provides calibrated airspeed
(CAS) and indicated airspeed (IAS)
(5) Vertical Navigation (VNAV) Mode
(a)
The VNAV mode is requested by pressing the VNAV switch/light on the MCP, provided the appropriate logic is satisfied. In VNAV mode, the Flight Management Computer (FMC) provides navigation information and the Autothrottle provides the appropriate thrust to provide for an optimal flight path (Ref 34-61-00/001).
(b)
The VNAV portion of the FCC pitch channel provides the control law to fly the FMC calculated profile. The VNAV control law primarily provides for airplane stability and vertical acceleration limit control.
(c)
The VNAV has three sub-modes which are annunciated on the PFDs when active. These sub-modes are: VNAV SPD, VNAV ALT and VNAV PTH. 1) VNAV SPD is active anytime the FMC guidance commands are
controlling the speed of the airplane via the elevator. This sub-mode provides the same type of control as the Speed through the elevator portion of the FCC.
2) VNAV ALT is active anytime the FMC guidance commands are controlling the airplane to capture or maintain an MCP selected altitude.
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3) VNAV PTH is active anytime the FMC guidance commands are controlling the airplane to a particular path via the elevator.
(6) Approach (APP) Mode
(a)
The approach mode is selected by pressing the APP switch/light on the MCP. During approach mode, the pitch channel controls glideslope tracking, flare, and nose-let-down.
(b)
Glideslope is armed when APP is selected. When the glideslope mode is armed, it has no effect on the current pitch axis mode (mode that was active when APP was selected). Once the capture criteria is met, glideslope is engaged and the previous pitch mode becomes inactive. The capture criteria for glideslope is based on vertical position and vertical speed relative to the glideslope beam. The Vertical Poisition section of the FCC pitch channel provide the control law for capturing and tracking the glideslope beam. When glideslope is initially engaged, a command to exponentially capture the beam is provided by the FCC. Once the beam is within the specified capture range, the control law provides for tracking. The glideslope capture and tacking functions are similar to those for aquiring and holding an altitude in cruise mode.
(c)
Once the airplane reaches a radio altitude of less than 50 feet, flare is automatically engaged (flare was previously armed when APP was selected). The flare command is also controlled by the Vertical Position section and is computed at the same time as glideslope tracking commands. If the airplane is not at the desired target altitude and sink rate (1.5 ft/sec) when flare is engaged, a command will be given to exponentially capture them.
(d)
When the air/ground relays sense that the airplane has touchdown and the pitch angle reduces to 2 degrees, the flare command disengages and the nose-let-down function assumes control. Nose-let-down is developed in the Pitch Attitude Integrator portion of the pitch channel. The control law commands the airplane to slowly let the nose of the airplane down on the runway and hold a 2 degree pitch-down command. The pitch-down command keeps the nosewheel firmly on the runway during rollout.
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