(4)
LOC Mode
(a)
The LOC mode provides roll commands to capture and track the selected ILS localizer beam path. Localizer signal information for CAT IIIa equipped airplanes is obtained from a dual port LOC antenna mounted on the nose radome lower bulkhead (Ref Chapter 34). The antenna provides LOC signals to both VHF navigation receivers through two isolated coax relay switches. Contacts on each of the coax relays provide an interlock of the LOC (ILS 28 vdc) logic signal to the autopilot, if a switch failure occurs the autopilot will not engage in LOC or APPROACH modes.
(b)
The AP/FD is armed in the LOC mode by tuning the VHF navigation receiver to the desired localizer frequency, selecting the appropriate runway heading on the appropriate PSC knob and pressing the VOR/LOC button on the MCP. The controls and displays function identically to those of the VOR mode. The LOC capture and LOC O/C will perform as described in single channel autopilot approach.
(c)
The LOC mode can be disengaged by pressing the VOR/LOC button, tuning to a VOR frequency, applying sufficient force to trigger the CWS high detent override, or by pressing the HDG SEL button. VOR LOC is annunciated of the FMA white or black when this mode is armed and black on green when this mode is engaged.
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May 20/82 BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 22-11-01 Page 33
G. Pitch Axis
(1)
The pitch axis block diagram (Fig. 10) represents the complete AFCS pitch control system from input signals, control annunciation F/D output, AP output to control surface commands, stabilizer trim and feedback signals.
(2)
The pitch axis schematic (Fig. 11, Sheets 1 and 2) shows internal computation within the FCC for digital and analog computers. The digital computer provides cruise and F/D mode computations and the analog computer provides AP approach mode computation. Analog signal equivalent circuits are used within the digital computer area to show various signal relationships.
(3)
The analog (approach) computer can be divided into AP approach computer and command and output computer.
(a)
The command and output computer input is at switch S9. This switch selects between approach or cruise computers. Summing point (13) receives position is provided for the valve amplifier output signal prior to system engagement by S7, S6 summing point (7) in the digital computer then back to summing points (2) and (13) in the analog computer. When the AP system is engaged the output of summing point
(13) is amplified and positions the transfer valve in the AP actuator. The output computer also provides positional information for the stabilizer trim system.
(b)
The AP approach computer provides computation for the approach (glideslope) mode. Control of mode switching is provided by the vertical beam sensor and associated mode logic. The glideslope signal is gain programmed by radio altitude signal then applied thru the vertical path filter to summing point (8). Vertical acceleration which is summed with altitude rate, filtered and together with attitude is applied to summing point (8). This composite signal is provided to S9 in the command and output section. Summing point (9) provides a pitch down signal at glideslope capture to improve beam capture performance. Prior to glideslope mode engagement, elevator position signals and neutral shift sensor (16) together with attitude, provide the input signal for the pitch integrator. This provides a smooth transition when glideslope is engaged. After glideslope engagement programmed glideslope deviation controls the integrator to ensure beam centering.
(4)
The digital computer can be divided into AP cruise computer and F/D computer.
(a)
The AP cruise computer receives data via the 429 data bus from the mode control panel, Performance Data Computer and Digital Air Data Computer. This data is shown in analog format. Vertical speed, altitude acquire and altitude hold signals and speed mode signals are applied to summing point (5) when the respective mode is engaged. The signal is then gain controlled by CAS and applied to summing point (7) via two paths when in CMD mode. One path is thru the vertical path filter to summing point (7) and the other thru summing point (6) and an integrator to summing point (7). Versine is also added at summing point (7) to prevent altitude loss during turns at high bank angles. The CWS signal is also applied to summing point 6. CWS is applied only when control column force exceeds the detent level. This composite signal is attitude and rate limited then applied to summing point 2, summed with attitude and altitude rate and then routed to S9 in the command computer.
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