.
autopilot/flight director system (AFDS)

.
autothrottle (A/T)

. inertial reference systems (IRS). Each of these components is an independent system, and each can be used independently or in various combinations. The term FMS refers to the concept of joining these independent components together into one integrated system which provides continuous automatic navigation, guidance, and performance management.
The integrated FMS provides centralized flight deck control of the airplane’s flight path and performance parameters. The flight management computer, or FMC, is the heart of the system, performing navigational and performance computations and providing control and guidance commands.
The primary flight deck controls are the AFDS MCP, two control display units (CDUs), and two electronic flight instrument system (EFIS) control panels. The primary displays are the CDUs, electronic attitude director indicator (EADI), electronic horizontal situation indicator (EHSI), and thrust mode display.
The FMC uses crew entered flight plan information, airplane systems data, and data from the FMC navigation database and performance database to calculate airplane present position, and pitch, roll, and thrust commands required to fly an optimum flight profile. The FMC sends these commands to the autothrottle, autopilot, and flight director. Map and route information are sent to the respective pilot’s EHSI. The EFIS control panels are used to select the desired information for navigation display. The mode control panel is used to select the autothrottle, autopilot, and flight director operating modes.
Flight Management, Navigation -Navigation Systems Description 737 Operations Manual

Inertial System
The inertial system computes airplane position, ground speed, and attitude data for the flight instruments, flight management system, autoflight system, and other systems. The major components of the inertial system are the inertial reference units (IRUs), an inertial system display unit (ISDU), IRS mode selector unit (MSU), and an IRS transfer switch.
Inertial Reference System
Two independent IRSs are installed. Each IRS has three sets of laser gyros and accelerometers. The IRSs are the airplane’s sole source of attitude and heading information, except for the standby attitude indicator and standby magnetic compass.
In their normal navigation mode, the IRSs provide attitude, true and magnetic heading, acceleration, vertical speed, ground speed, track, present position, and wind data to appropriate airplane systems. IRS outputs are independent of external navigation aids.
IRS Alignment
An IRS must be aligned and initialized with airplane present position before it can enter the NAV mode. The present position is normally entered through the FMC CDU. If the present position cannot be entered through the FMC CDU, it may be entered through the ISDU keyboard. The airplane must remain stationary during alignment.
Normal alignment, between 70° 12’ north and 70° 12’ south latitudes is initiated by rotating the MSU switch from OFF to NAV. The IRS performs a short power test, during which the ON DC light illuminates. When the ON DC light extinguishes and the ALIGN light illuminates, the alignment process begins. Airplane present position should be entered at this time. The IRS will automatically enter the NAV mode after approximately 10 minutes, and the ALIGN light will extinguish.
High latitude alignment, at latitudes between 70° 12’ and 78° 15’, requires an extended alignment time. Alignment is initiated by rotating the MSU switch from OFF to ALIGN. After 17 minutes the MSU switch must be rotated to the NAV position. The IRS will then immediately enter the NAV mode.
Magnetic variation between 73° north and 60° south is stored in each IRS memory. The data corresponding to the present position are combined with the true heading to determine magnetic heading. If magnetic variation information is not available, special navigation equipment is required to provide true heading to the EHSIs.
Copyright . The Boeing Company. See title page for details.
11.20.2  December 1, 2000

Flight Management, Navigation -Navigation Systems Description

737 Operations Manual
On airplanes SN 26318, 27635, and 28085, magnetic variation between 82° north and 82° south latitudes is stored in each IRS memory. Operation between 82° north and 82° south latitudes is acceptable with the following restrictions:
.  operation is prohibited in the area bounded by 70° north and 82° north and
80° west and 130° west.

Beyond 73° north and 60° south latitudes:
.  
the EFIS navigation display may not be used
　
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