(b)
In the intercept phase, the glide slope receiver signal is applied to a modulator amplifier (Fig. 15). The approach progress display GLIDE SLOPE annunciator is illuminated amber. This indicates that the vertical beam sensor is armed. The VBS, connected to the output of modulator amplifier, senses glide slope level. When this
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Nav/Mode Select Holding Coil Logic Circuit (Auto App) 5C8
22-11-0 Figure 19 Oct 20/86
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BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.
output signal level decreases to a predetermined value, the VBS changes state. This action terminates the intercept phase and is termed glide slope engaged. At this point the approach progress display GLIDE SLOPE annunciator illuminates green.
(c)
The capture phase begins when the vertical beam sensor changes state to provide an interlock voltage output and starts a 10-second timer. The vertical beam sensor changes state when the glide slope receiver signal decreases to a predetermined level. During the capture phase, an ac glide slope capture bias signal is applied to summing point 4 past de-energized switch PS-14 to command a predetermined rate of descent. An altitude rate signal from the air data computer is also applied to summing point 4 so that output from summing point 4 is proportional to deviation from the commanded rate of descent. With switch PS-4 open, the output from summing point 4 is routed through the electrical bus synchronizer to summing point 2. Output from summing point 2 causes movement in the pitch computer servo assembly in which the gear changer has selected the numerically higher gear ratio. The resultant output from the CT causes the valve amplifier to command elevator movement. Ten seconds after the vertical beam sensor changes state, switches PS-4 closes and PS-5 opens. This change terminates the capture phase.
(d)
The tracking phase of the automatic glide slope control mode begins when the 10-second timer, started by VBS state change, causes switches PS-4 and PS-5 to change state. This change removes altitude rate deviation signal (derived at summing point 4) from summing point 2, and applies this signal, after additional filtering to summing point 8. The glide slope deviation signal is applied to the gain programmer as well as to the vertical beam sensor. This deviation signal does not pass through the gain programmer until the 10-second timer turns on easy-on switch PES-1 in the programmer control circuit. The easy-on switch causes the gain programmer to gradually allow passage of the glide slope signal to summing point 8 and, past open switch PS-6 to summing point 2. The altitude rate signal is combined with the glide slope deviation signal at summing point 8 to provide damping for the glide slope mode. Output from summing point 8 is routed through the vertical path filter to summing point 6. The vertical path filter limits commanded pitch attitude changes while in the glide slope mode. Output from summing point 2 provides rotation of the pitch computer in which the gear changer has selected
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the numerically higher gear ratio to form an electromechanical integrator. Movement in this integrator results in output from the CT which is applied to summing point 6. Summing point 6 output causes the valve amplifier to command elevator movement necessary to maintain flight path in the glide slope beam center. Integrator operation results in high long-term gain used to eliminate beam standoff errors. Use of control wheel steering after termination of the intercept phase, disengages the glide slope mode. As the airplane descends along the glide slope beam, a radio altimeter (when installed) signal is routed to the altitude programmer. If the radio altimeter signal is valid, it is routed to the programmer control. A switch within the programmer control changes state when signal level corresponds to a 1500-foot altitude. This change of state grounds an interlock voltage and routes the radio altimeter signal through the programmer control to the gain programmer, through which the glide slope signal is passing. Amplification of the glide slope signal in the gain programmer is reduced as a function of radio altimeter signal. This gain reduction compensates for the increase in glide slope beam intensity as the transmitter is approached. If the radio altimeter fails or is not installed, amplification of glide slope deviation signal in the gain programmer is then reduced as a function of time, starting at the beginning of the capture phase.
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