K77766
Compass System Component Location 505
34-21-02 Figure 1 Oct 20/84
Page 2
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.
E. The compass system interfaces with the flight instruments and other airplane systems through the flight instrument switching network (Fig. 3).
(1)
The magnetic heading signal is compensated for magnetic error and applied to the compass coupler, where it is synchronized with the platform heading signal from the DG. The magnetic heading output is applied to the automatic flight control system (AFCS), to the radio digital distance magnetic indicator (RDDMI) and horizontal situation indicator (HSI). The magnetic heading is supplied to the AFCS mode controller and flight control computers where it is resolved with a heading select signal to form a preset heading error signal.
(2)
The magnetic heading is also recorded by the flight recorder system.
F. The compass system requires 115 volts (400 Hz) for primary power, 5 volts ac for panel illumination, 26 volts ac for synchro transmitter excitation, and 28 volts dc for relay control. Primary power for the captain's system is supplied from the essential flight instrument bus and standby power. Primary power for the first officer's system is supplied from flight instrument bus No. 2.
2. Flux Valve
A. The flux valve contains a magnetic detector which senses the direction of the horizontal component of the earth's magnetic field and utilizes this reference to generate an ac signal representative of the airplane's magnetic heading.
B. The flux valves are mounted on prealigned brackets in the vertical stabilizer to avoid magnetic disturbances. Each flux valve is preindexed by the vendor to the plate upon which it is mounted thus permitting temporary flux valve replacement without system recalibration.
C. The flux valve consists of a pendulous magnetic detector, universally mounted in a damping fluid with up to 30 degrees freedom in roll and pitch. In straight and level flight he detecting element of the flux valve senses the horizontal component of the earth's magnetic field and thus determines the magnetic heading of the airplane. The sensitive element consists of a three-wire, wye connected, output winding on a three-legged, high permeability core. When the unit is energized by a 400-Hz single-phase voltage, it produces an 800-Hz single-phase voltage in each leg. The relative magnitudes and (in or out) phase relations of these voltages, define the angular orientation of the sensitive element relative to the horizontal component of the earth's magnetic field.
505
Oct 20/84 BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 34-21-02 Page 3
D. As the primary coil is excited by 400 Hz alternating current, the reluctance of the wye connected output winding is alternately raised and lowered. The change in reluctance, in effect alternately draws the earth's magnetic lines of force in and out of the secondary coils. As the earth's magnetic lines of force cut the secondary coils, voltages are induced in each coil. The magnitude of the voltages in each coil depend upon their orientation relative to the earth's magnetic field. The coil most nearly with the magnetic lines of force has the highest voltage induced.
3. Directional Gyros
A. The directional gyro provides the heading reference for the compass system. Its gimbals may be torqued by two torquemotors, the slaving torquemotor being used to precess the gyro to the magnetic reference generated in the flux valve and the leveling torquemotor to level the gyro with respect to gravity. The former motor is driven by a slaving amplifier and the latter is controlled by means of an electrolytic switch (Fig. 2).
B. A logic circuit monitoring the gyroscope wheel speed, gyroscope leveling, nutation brake operation, heading synchro excitiation and compass system signal null are used to operate two malfunction relays in the directional gyroscope. The contacts of these relays are used in the flag warning circuits.
H59149
Directional Gyro Schematic 505
34-21-02 Figure 2 Oct 20/84
Page 4
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.
505 Compass System Interface Block Diagram
Oct 20/84 Figure 3 34-21-02
Page 5
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.
4. Compass Controller
A. The compass controller provides mode switching, set heading control, and SYNC annunciation for the compass system. A toggle switch on the front panel selects either the DG or SLAVED mode of operation, a set heading knob allows the flight instrument compass cards to be manually set to a desired heading, and an annunciator is provided to indicate system synchronization.
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