(d)
Altitude Select Engage
1) The altitude select engage logic is generated and is momentarily zero at F/D disengage. The input requires altitude select engage valid and altitude select arm to be logic one. The altitude select arm logic requires that the ALT HOLD switch not be pressed and that the FMC does not command a target altitude in a direction away from the MCP altitude when in the VNAV mode. the ALT SELECT/HOLD mode must be active for altitude select arm to be logic one.
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2) The altitude select engage logic is required to establish the altitude select mode in the ALT SEL/HOLD signal development.
(e) Altitude Hold Engage
1) The altitude hold engage logic is generated and is momentarily zero at F/D disengage. The altitude input can be generated three ways (provided altitude hold/engage valid is logic one). a) If the ALT HOLD switch/light dot bar matrix on the MCP
is lighted and ALT SEL/HOLD mode is active.
b) When the FMC (in VNAV mode and not in ALT SEL ENG) commands the airplane to fly to a target altitude in a direction away from the MCP set altitude.
c) In the V/S mode, the V/S display set to zero will also generate altitude hold engage.
(12) Altitude Select/Hold Control Signal Flow and Monitoring (Fig. 10)
(a) Vertical Position Time Constant (Natural Frequency)
1) The vertical position time constant is generated for the three regions in which altitude capture can occur, to provide a safe and smooth transition path to the selected altitude. The three regions of concern are overshoot (region I), acceleration limit (region II) and no acceleration limit (region III). Additional requirements for the time constant calculation are vertical acceleration command limit, delta altitude command and delta altitude rate command. The delta altitude command provides the initial path command and the delta altitude rate command provides the initial path rate command at ALT CAP or when the airplane achieves the proper delta altitude and rate to smoothly achieve the selected altitude.
2) The three regions in which ALT CAP occurs defines the logic which activates time constant calculation for one iteration period. The time constant value is limited between 0.01 and 0.50 radian/second for a smooth transition path generation.
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(b)
Vertical Position Limit
1) The vertical position limit is generated by a logic 1 input from ALT CAP in either overshoot limit region or acceleration limit region. The vertical position limit is required for switching logic in the vertical command processor to input vertical acceleration command limit when the altitude is captured in the overshoot or acceleration limit regions.
(c)
Vertical Acceleration Command Limit
1) The vertical acceleration command limit is a fixed value of vertical acceleration from the IRS at the time of ALT CAP, or it is 3.2 ft/sec (0.1g) prior to ALT CAP. Either of these fixed values will control the airplane until the altitude rate is zero or the absolute value of the rate of approach to selected altitude is reduced to the normal capture limit.
2) The sign of HDDCC is opposite that of delta altitude rate command. This provides an acceleration that will bring the airplane closer to the desired altitude. The altitude capture (ALT CAP) logic is required for vertical acceleration command limit control.
(13) Pitch Channel - Flight Level Change (Fig. 13)
(a)
The flight level change mode is entered when the FLCH switch/light is pressed with the autopilot system in any mode except control inactive. Flight level change mode can exit into any autopilot mode except takeoff mode by pressing the appropriate mode button.
(b)
If a different autothrottle mode is entered, the AFDS exits FLCH and enters the V/S mode. Two submodes, VNAV ARM and G/S ARM, are possible in FLCH mode. Automatic transition to G/S occurs if G/S is ARMED when G/S is captured. The FMA on the EADI will display G/S in the pitch mode armed annunciation section.
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