COMPASS SYSTEMS -DESCRIPTION AND OPERATION
EFFECTIVITY
GJ ALL EXCEPT B-2509 and B-2510
1. General
A. The compass system is designed primarily to furnish the captain and first officer with information concerning the airplane's magnetic heading at all times during flight. Heading information is displayed on the airplane's radio magnetic indicators (RMI's) and the horizontal situation indicators (HSI's). Heading information is also fed to the autopilot, flight recorder, navigation systems, and steering computers.
B. Two compass systems are installed. Each consists of the following components: remote compass transmitter (flux valve), directional gyro, horizontal situation indicator, radio magnetic indicator, servo-amplifier, slaving amplifier, and instrument amplifiers. The servo and slaving amplifiers are contained in the compass racks. A portion of the circuitry in the flight instrument accessory box is also utilized for the flag warning circuits. Location of the components is shown in Fig. 1.
2. Remote Compass Transmitter (Flux Valve)
A. The flux valve contains a pendulous 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.
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 the slaving amplifier and the latter is controlled by means of an electrolytic switch (Fig. 2).
5C8
Jun 20/85 34-21-0 Page 1
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K79413 H24228
Compass System Component Location 5C8
34-21-0 Figure 1 Jun 20/83
Page 2
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.
513 Directional Gyro Schematic
Aug 15/68 Figure 2 34-21-0
Page 3
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.
B. A spin-down brake in the directional gyro prevents nutation of the gyro by holding the outer gimbal steady for 10 seconds after power is applied. At shutdown, the brake is reapplied to the gimbal 30 seconds after power is removed. This prevents the gyro from spinning around its vertical axis. The 30-second time delay of the spin-down brake at shutdown also prevents momentary power interruptions (such as changing inverters) from introducing errors into the heading information.
C. A logic circuit monitoring the gyroscope wheel speed, gyroscope leveling, nutation brake operation, heading synchro excitation and compass system signal null are used to operate two malfunction relays. The contacts of these relays are used in the flag warning circuits.
D. Heading information for the autopilot is obtained direct from directional gyro No. 1 or, if the compass transfer switch is in BOTH ON NO. 2 position, from directional gyro No. 2.
4. Radio Magnetic Indicator (RMI)
A. Heading information from the directional gyro is displayed on a rotating compass dial which is read against a fixed index at the top of the indicator. Synchro transmitters in the captain's RMI repeat this information to the captain's horizontal situation indicator (HSI) and VHF navigation system No. 1. The first officer's RMI provides a similar service for the first officer's HSI, flight recorder and VHF navigation system No. 2. In addition, on airplanes not incorporating SB 34-1057, the first officer's RMI feeds the captain's RMI when the compass transfer switch is set to BOTH ON NO. 2 or the captain's RMI feeds the first officer's RMI when compass transfer switch is set to BOTH ON NO. 1, on airplanes incorporating SB 34-1057, the RMI's are hand wired to their respective gyro with no switching capability, however switching is provided for the HSI's to display information from the opposite system.
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