Jeppesen-Radio-Navigation

Boeing 737 Flight Management Computer (FMC) Chapter 24

LIGHTS

Menu Page — This page displays if the FMC fails.

Message Light (Illuminated White) — The FMC contains an Alerting or Advisory Message for
display in the scratch pad. If non-message data is presently displayed in the Scratch Pad,
pressing the CLR Key displays the message. Pressing the CLR key clears the message and
extinguishes the light.

Fail Light (Amber) — When illuminated, the FMC test is in progress.

Call Light (White) — When illuminated, a user other than the FMCS is requesting control of the
CDU.

SYSTEM COMPONENTS

The single FMC contains stored databases of navigation and in-flight performance information.
This stored data is an electronic flight bag that contains information similar to (but more refined
than) the pages of Operations Manual Performance Data and the navigation publications that are
carried in a pilot’s flight bag.

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Chapter 24 Boeing 737 Flight Management Computer (FMC)

The two identical, independent CDUs provide the means for the FMC and flight crew to “talk” to
each other in familiar Air Traffic Control language. The crew may enter data into the FMC using
either CDU, although simultaneous entries should be avoided. The same FMC data and
computations are available for display on both CDUs. However, each pilot has independent
control over the information actually being displayed on an individual CDU.

DATABASES

Standard information stored in the permanent navigation database includes:

¾ The location of and other facility information for selected airports
¾ Runways
¾ VHF navaids

Other data that may be stored (at the airline’s option) includes:

¾ The company route structure
¾ Airways
¾ Waypoints
¾ SIDS, STARS, and approach procedures

This permanent navigation database contains two sets of time-sensitive data:

¾ Active data that is applicable to an initial 28 day time period
¾ Alternate data that is applicable to the time periods below

During pre-flight, the pilots select the data to be used via the CDU IDENT page. Updates to the
navigation database are performed by maintenance personnel using a magnetic tape cassette,
and are normally accomplished on a 28 day cycle corresponding to the revision date of navigation
publications.

If the permanent database does not contain all of the required flight plan data, the crew can
define additional airports, navaids, and waypoints and store them in either a supplemental or a
temporary navigation database. Use of these additional databases provides world-wide
navigational capability, with the crew manually entering desired data into the FMC via various
CDU pages. Information in the supplemental nav database is stored indefinitely, requiring specific
crew action for erasure. The temporary nav database is automatically erased at flight completion.

The performance database contains information on:

¾ Climb and cruise performance
¾ Thrust limits
¾ Maximum and minimum altitudes
¾ Maximum and minimum airspeeds for various configurations
¾ Drag characteristics

Maintenance personnel can refine the data by entry of correction factors for individual aeroplane
drag and fuel flow characteristics.

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Boeing 737 Flight Management Computer (FMC) Chapter 24

OPERATION

The CDUs are used during preflight to manually initialise the IRSs and FMC with dispatch
information such as:

¾ Present position
¾ Flight plan routing
¾ Zero fuel weight
¾ Planned cruise altitude

These CDU entries and the databases then form the starting point for FMC computations. The
FMC compares this planning information with actual data from a number of other sources
(including required IRS inputs). These other inputs provide dynamic real-time information on:

¾ Flight progress
¾ Aeroplane environment
¾ Systems status

The FMC then continuously computes:

¾ Aeroplane position
¾ ETAs
¾ Thrust (N1) targets and Limits and optimum/required speeds
¾ Recommended altitude for the selected vertical profile

Computations are displayed on the CDUs, while associated outputs provide roll, pitch and
airspeed commands to the AFDS (L NAV and V NAV modes) and thrust commands and limits to
the A/T.

FMC N1 Limits may be automatically displayed on the N1 Indicators. The N1 limit, or reduced
mode, used by the A/T is annunciated on the Thrust Mode Annunciator. During V NAV operation,
the FMC target airspeed may be automatically displayed on the Mach/Airspeed Indicators and
EADI speed tapes. Also, lateral and vertical navigation data may be displayed for reference on
the HSIs.

With LNAV and V NAV engaged, the CDU displays allow the crew to monitor proper FMS
operation and flight progress. With L NAV and V NAV disengaged, the displays provide a
reference, allowing the crew to fly the selected route/profile either manually or with conventional
autoflight modes. The CDUs also:

¾ Provide “what if” previews of flight plan options
¾ Make revisions to the flight plan
¾ Provide reference data

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Chapter 24 Boeing 737 Flight Management Computer (FMC)

LATERAL NAVIGATION

Lateral outputs from the FMC are normally referenced to a direct great circle course (either on or
off airways), but can be referenced to a fixed heading or course when flying a published
procedure.

FMC navigational computations are based upon an FMC position established using radio inputs
and/or IRS present position. The FMC position may be based upon IRS data only, but available
DME inputs are normally used to refine and update the FMC position. Just prior to take-off, the
crew may set the FMC position to a point on the departure runway via the CDU TAKEOFF REF
page. Activation of the TO/GA button updates the IRS to this position.

Note that radio updating does not occur on the ground. Consequently, navigation position error
can accumulate in the FMC during transit or through flight stops. Fast realignment of the IRSs
with a new present position removes the errors. The errors will also be removed after take-off
when radio updating again becomes available.

Also note that the FMC is not certified as a “sole source of navigation” system. It is certified to
navigate accurately in conjunction with an accurate radio navaid environment.

VERTICAL NAVIGATION

Vertical outputs from the FMC are normally referenced to the best economy profile for:

¾ Climb
¾ Cruise
¾ Descent and holding

Computation of optimum speeds for the economy profile is based upon a company specified Cost
Index, which is the ratio of other operating costs compared to the cost of fuel. Changing the Cost
Index changes the computed optimum speeds. If desired, the crew may manually select any
speed profile other than economy.

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Boeing 737 Flight Management Computer (FMC) Chapter 24

REQUIRED TIME OF ARRIVAL (RTA) NAVIGATION

The RTA navigation mode is designed to assist the pilot in complying with a required time of
arrival at a designated waypoint such as the final approach fix, holding fix, or airport.

After inputting the appropriate waypoint and RTA to the FMC via the CDU, the FMC computes:

¾ A recommended take-off time
¾ Speeds required to comply with the RTA
¾ Progress information as the flight takes place

Speeds are automatically adjusted for in-flight winds and route changes by the FMC adjusting the
Cost Index. If the RTA is unobtainable under present routing and/or environmental conditions, the
FMC so advises by displaying an appropriate message for the most economical operation. The
pilot should try to meet the recommended take-off time, as a later take-off results in a higher cost
index than originally planned.

RADIO TUNING

The dual frequency scanning DME radios are automatically tuned by the FMC. The stations to be
tuned are selected based upon the best available signals (in terms of geometry and strength) for
updating the FMC position, unless the flight plan requires a specific station. The intersection of
two DME arcs determines radio position.

If the DME radios fail, or if suitable DME stations are not available, FMC navigation is based upon
IRS position information only. The FMC uses the two VHF NAV Radios for LOCALISER updating
during an ILS approach and by the crew for navigation monitoring.

The FMC automatically rejects unreliable navaid data during FMC position updating. However, in
certain conditions, navaids that are in error may satisfy the reasonableness criteria and provide
the FMC with an inaccurate radio position. One of the most vulnerable times is when a radio
position update occurs just after take-off. This usually manifests in an abrupt heading correction
after engaging LNAV. The position shift is visible on the EHSI map which shifts, the desired track
and runway symbol to a position significantly different from that displayed during the ground roll. If
the flight crew observes either of these indications, and an extended period of IRS NAV ONLY
flight follows, the FMC position should be carefully monitored.

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Chapter 24 Boeing 737 Flight Management Computer (FMC)

CAPT FMCS SCHEMATIC
CLOCK
AFDS
TOTAL
FUEL A /T FCCs MACH
AIR DATA AIRSPEEDs
COMPUTERS
FMC EADIs
DME RADIO
NAVIGATION PERFORMANCE EHSIs
PNEUMATIC DATA DATA BASE
SWITCH BASES N1s

POSITIONS PERMANENT DME
AIR-GROUND AUTO TUNE
SUPPLEMENTAL THRUST MODE
SENSING TEMPORARY ANNUNCIATOR

PMC ON / OFF CAPT F/O
LOGIC CDU CDU
FLAP

POSITION

IRSs

EFIS
CONTROL
PANELS

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Boeing 737 Flight Management Computer (FMC) Chapter 24

ELECTRICAL POWER

The FMC and both CDUs are powered indirectly by transfer bus No. 1. If electrical power is lost
for less than ten seconds:

¾ LNAV and VNAV disengage
¾ The FMC retains all entered data
¾ When power is restored the FMC/CDU resumes normal operation

If power is lost for ten seconds or more:

On the Ground — All pre-flight procedures and entries must be repeated when power is
restored.

In-flight — LNAV and VNAV disengage and the FMC retains all entered data. When
power is restored, the MOD RTE LEGS page displays with the Advisory Message
SELECT ACTIVE WPT/LEG.

A Software Restart resulting from the FMC entering an impossible computational state, such as
division by zero, may appear to the aircrew to be a temporary loss of electrical power. The CDU
momentarily blanks and then displays “FMC”. The display of the MOD RTE LEGS page with box
prompts in the Active Waypoint line quickly follows. The message SELECT ACTIVE WPT/LEG
displays.

In some cases, multiple Software Restarts inflight can result in FMC failure. The FMC may be
reinitialised by removing AC power for more than 10 seconds after landing.

TERMINOLOGY

The FMC databases contain information from which the crew makes selections or modifications
to their specific flight plan. The terminology below describes the status of this information and
methods of crew interaction with the FMC/CDU.

EXECUTING

The term Executing refers to pressing the EXEC key when it is armed (Light Bar illuminated) and
is often written as EXECuting.

INACTIVE

Inactive refers to route or performance information that the FMC is not actively tracking. Such
information cannot be engaged to the AFDS or A/T. Inactive data is made active by EXECuting.

ACTIVE

Active refers to route or performance information that the FMC is actively tracking and that may
be engaged to the AFDS (L NAV and/or V NAV) or Alt. This is often written as ACTive.

Within the above general context, the term “active” may also specifically identify the current leg or
route segment being flown, or the upcoming waypoint toward which the aircraft is flying, for
example, when flying from ABC to DEF (the active Leg), the active waypoint is DEF.

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Chapter 24 Boeing 737 Flight Management Computer (FMC)

PAGE STATUS

The Page Title line indicates whether certain route and performance pages are inactive, active, or
modified. For example:

Inactive Title Active Title

RTE ACT RTE
ECON CLB ACT ECON CLB

MODIFICATION

Modification refers to proposed changes to active route or performance pages, as indicated by
the Page Status. Modified pages again become ACTive after EXECuting, and are often written as
MODified. For example:

Modified Title New Active Title

MOD RTE ACT RTE
MOD ECON CLB ACT ECON CLB

INITIALISATION

Initialisation is the process of checking the effective dates of the navigation database and
entering preflight data into the FMC and IRSs via the CDU.

LINE SELECT

This is the act of pressing a Line Select Key.

ENTER

To enter data on a page, is to line select that data from the Scratch Pad up to the desired line.
The data is initially entered in the Scratch Pad by a Keyboard entry or by copying existing page
data into the Scratch Pad by Line selection.

ACCESS

To access a page is to cause that page to be displayed on the CDU.

PROPAGATE

Certain types of data (for example, cruise attitude) are used on more than one page. Generally,
such data need only be entered one time. The FMC then automatically propagates (duplicates)
the entry to other applicable pages.

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Boeing 737 Flight Management Computer (FMC) Chapter 24

PAGE CONCEPTS

GENERAL

The FMC stores its navigation and performance information on electronic “pages”. These pages
display on the CDU through use of the Mode Keys. Pressing a key provides access to all pages
of data functionally related to that mode. This may be a single page, or multiple pages. Access
Prompts on the displayed page serve as an index that provides access to other related pages (if
any), or perhaps to a separate Index Page. Consider the following Display:

____________________

<INDEX TAKEOFF>

In this example, the complete index to other related pages is of such length it requires accessing
a separate index listing by line selecting <INDEX. However, one of the related pages concerning
take-off data is directly accessible by line selecting TAKEOFF.

Occasionally, a page may be selected by pressing either the Mode Key or a Line Select Key.
Consider the following display:

____________________
ROUTE>

In this example, page 1 of route data can be accessed by either line selecting ROUTE, or by
pressing the RTE Mode Key.

Also, line selecting an Access Prompt may occasionally be used to perform a function rather than
to select another page. Consider the following display:

____________________
ACTIVATE>

In this example, line-selecting ACTIVATE> arms the execution function, causing the Light Bar in
the EXEC key to illuminate.

PAGE SEQUENCE LOGIC

Although the FMC contains many displayable pages, proper page selection and execution are not
difficult. Automatic display of some pages by phase of flight, as well as access prompts on many
displays, provide assistance with the proper sequence of steps to initialise, activate, and fly the
desired flight plan.

For example, the diagram on the next page shows how the FMC guides the crew through the
required pages of a normal preflight. Upon initial power application, the IDENT page normally
appears. If the IDENT page does not appear, it may be accessed via the INIT/REF INDEX page,
as shown at the bottom of the diagram. After checking the displayed data, line select key 6R is
pressed to display the next logical page, POS INIT. The crew continues with each page, checking
and entering data as required, then line selecting 6R, until preflight is complete.

If a Standard Instrument Departure (SID) must be entered into the route, press the DEP/ARR
mode key for access to the DEPARTURES page. Following selection of the SID, line selection of
6R returns the display to the RTE page.

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Chapter 24 Boeing 737 Flight Management Computer (FMC)

When the EXEC key illuminates, it must be pressed before continuing in order to activate the
entered data. The RTE page requires line selection of the ACTIVATE prompt before the EXEC
key will illuminate. This two-step procedure protects the crew from inadvertent activation of
unintended data.

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Boeing 737 Flight Management Computer (FMC) Chapter 24

The FMC/CDU is designed to automatically preserve the most capable modes of navigation and
guidance that can be maintained with the equipment and navigation aids available. If an error or
system failure results in reduced capability (downmoding), then the FMC may generate a crew
message for display in the CDU Scratch Pad. If other system inputs to the FMC should fail,
affected CDU Displays are blanked to prevent the display of misleading or erroneous data. For
example, loss of the total fuel input causes all performance-related data to be blank.

The messages and FMC internal responses provide for an orderly transition from full FMC-guided
flight to less automated capability, if required.

CDU MESSAGES

There are two categories of CDU messages. Alerting Messages have the highest priority and
identify a condition which must be acknowledged and corrected by the crew before further FMC-
guided flight is advisable or possible. Advisory Messages have lower priority and inform the crew
of CDU entry errors or system status.

The generation of any message causes the white CDU MSG light to illuminate. Alerting
Messages also illuminate the amber FMC Alert Light on each pilot’s instrument panel. Placing the
test switch to position 1 or 2 also illuminates the lights. Pressing the switch extinguishes the light.

If the Scratch Pad is empty, any message is displayed immediately when generated. Some
messages will displace an existing Scratch Pad entry and are also displayed immediately when
generated. Other messages will not be displayed until the Scratch Pad has been cleared;
however, the MSG Light will still be illuminated. A new entry in the Scratch Pad overrides any
displayed message. Messages caused by CDU entry errors are displayed only on the associated
CDU; other messages are displayed on both CDUs.

When multiple messages have been generated, they are “stacked” for display in priority
sequence, or in the order of their occurrence if of the same priority. As each message is cleared,
the next message in the stack displays. The CLR key on the CDU clears most messages as does
correcting the condition. Other messages are cleared by changing the displayed page, which will
delete the entry which caused the message.

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Chapter 24 Boeing 737 Flight Management Computer (FMC)

WAYPOINT

The term “Waypoint” may refer to either a specific waypoint name or, in a generic sense, to any of
the below definitions.

WAYPOINT IDENTIFIERS

Flight crew may enter stored waypoint identifiers manually on either the RTE or RTE LEGS
pages, or they may be entered automatically as part of a company route designation. The
following are valid CDU entries for published waypoint identifiers stored in the permanent
navigation database (five characters maximum):

¾ Waypoint identifier (waypoint name)
¾ Navaid identifier
¾ Runway number
¾ Airport ICAO identifier

CREATED WAYPOINTS

If the permanent nav database does not contain the desired stored waypoint(s), then the crew
can define new (previously unstored) “Created Waypoints” on the RTE or RTE LEGS pages.
Created Waypoints are keyed into the Scratch Pad as any of the following:

¾ Place Bearing/Distance (for example, COV150/50) where the “place” is any identifier
already stored in either the permanent, supplemental, or temporary nav databases.
This Scratch Pad entry could also be a transfer from the FIX INFO page.

¾ Place Bearing/Place Bearing (for example, COV080/TNT190) is the intersection of
bearings from two different “places”.

¾ Along-Track Displacement (for example, COV/-10) is the distance either side of an
existing flight-plan waypoint.

¾ Latitude and Longitude (for example, N5732.8W00410.3).

When the Scratch Pad entry is line-selected into the route, the FMC assigns a sequential
identifier number to Created Waypoints.

For the first three types of waypoints, the FMC uses the first three letters of the reference
waypoint, followed by the sequential number of times that reference has been used. For example,
COV150/50 would be assigned the identifier COV01. Then COV080/TNT190 would receive the
assignment COV02.

Latitude/Longitude entries use WPT as the first three letters.

The waypoints are automatically stored in the temporary nav database for one flight only. On the
NAV DATA pages, entry of the FMC-assigned identifier on the WPT IDENT line provides a
display of the parameters originally keyed-in to define that waypoint.

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Boeing 737 Flight Management Computer (FMC) Chapter 24

Alternatively, Created Waypoints can also be initially defined using crew-assigned identifiers on
either the SUPP NAV DATA or REF NAV DATA pages. This method allows waypoints to be
defined in any of three FMC categories:

¾ Waypoints
¾ Navaids
¾ Airports

Entries defined on the SUPP NAV DATA pages (accessible on the ground only) are automatically
stored in the supplemental nav database until deleted by the crew. Entries defined on the REF
NAV DATA pages are automatically stored in the temporary nav database for one flight only.

The supplemental and temporary databases share storage capacity for:

¾ 40 Navaids
¾ 6 Airports

The entries being stored in either nav base on a “first come, first served” basis. For the Waypoint
category, exclusive storage is reserved in the temporary database for 20 entries (including those
created on the RTE or RTE LEGS pages). An additional 20 Waypoints (up to a maximum of 40)
can be stored in either the temporary or supplemental data base on a “first come, first served”
basis.

CONDITIONAL WAYPOINTS

The preceding waypoints all refer to geographically fixed positions. Waypoints that are not
geographically fixed are called Conditional Waypoints, and are imbedded within stored
procedures. They are displayed on the CDU in parenthesis. They cannot be entered manually.
Conditional Waypoints are displayed as any of the following:

¾ (1500) altitude condition
¾ (COV.100) VOR radial crossing condition
¾ (COV-l0) DME crossing condition
¾ (INTC) intercept course to next waypoint
¾ (VECTOR) maintain heading indefinitely

When (VECTOR) is the active leg, the FMC does not automatically sequence to the next
waypoint. The next waypoint becomes active only upon EXECution of the procedures for
proceeding direct to a waypoint or intercepting a leg to a Waypoint.

The crew should delete entries that are no longer required when any storage category is full to
make space for additional new entries. Created Waypoints cannot be stored in the database
Runway Category.

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Chapter 24 Boeing 737 Flight Management Computer (FMC)

24-24 Radio Navigation

INERTIAL REFERENCE SYSTEM

IRS DISPLAY

DSPL SEL

PPOS WIND 1 N 3

TK / GS HDG / STS 2 E

TEST BR WH 6

45

7 S 9

8

SYS DSPL EMT 0 CLR

LR

ALIGN ON DC ALIGN ON DC
FAULT DC FAIL FAULT DC FAIL

ALIGN NAV ALIGN NAV

OFF ATT OFF ATT

L IRS R

IRS MODE SELECTOR

The IRS Mode controls the operating mode of the respective inertial reference system (left or
right).

OFF: Alignment is lost. All electrical power is removed from the system after a
30-second shutdown cycle.

ALIGN: Used for initial alignment. The aircraft must be parked.

Moving the selector from OFF to ALIGN initiates the alignment cycle.
The selector may be moved to NAV during the cycle.

Moving the selector from NAV to ALIGN automatically updates alignment
and zeroes groundspeed error (fast realignment).

The aircraft’s present position should be manually updated, but is not
required.

Return the selector to NAV.

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Chapter 25 Boeing 737 Navigation Equipment and Flight Management

NAV: Navigation is a detented position. The system enters the NAV mode after
completion of the alignment cycle and entry of present position.

It provides full IRS data to aeroplane systems for normal operations.

ATT: Attitude is a back up mode. It provides only attitude and heading
information.

Attitude Information is Invalid means the Attitude flag is in view until ALIGN
light is extinguished.

Heading Information is Invalid means the heading flag is in view until the
actual magnetic heading is manually entered and the ALIGN light extinguished.

Position and groundspeed information is lost until after the IRS is aligned
on the ground. The selector must be cycled through OFF before
reselecting ALIGN or NAV.

ALIGN LIGHT (WHITE)

Illuminated (Steady) The respective IRS is operating normally in either the:

¾ ALIGN mode
¾ Initial ATT mode
¾ The shutdown cycle

Illuminated (Flashing) Completion of Alignment cannot be completed due to IRS
detection of one of the following errors:

¾ Significant difference between previous and entered
positions or an unreasonable present position entry

¾ No present position entry

Extinguished IRS is not in the ALIGN mode:

¾ Mode selector is in NAV and Alignment is completed. Full
IRS data is available.

¾ Mode selector is in ATT and Attitude information is available.
Heading information is also available following entry of the
initial magnetic heading.

FAULT LIGHT (AMBER)

Illuminated A system fault that affects the respective IRS ATT and/or NAV
modes has been detected.

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Boeing 737 Navigation Equipment and Flight Management Chapter 25

ON DC LIGHT (AMBER)

Illuminated The respective IRS is operating on DC power from the switched
hot battery bus (AC power not normal).

If on the ground, the ground call horn in the nose wheel well
sounds providing an alert that a battery drain condition may
exist.

Momentary illumination is normal during alignment self test (DC
power normal).

DC FAIL LIGHT (AMBER)

Illuminated DC power for the respective IRS is not normal.

If the other lights are extinguished the IRS is operating normally
on AC power.

INERTIAL REFERENCE SYSTEM

GENERAL

Two independent Inertial Reference Systems (IRS) are installed consisting of:

¾ Inertial Reference Units (IRU)
¾ Mode selectors
¾ IRS Display Unit

Each IRS has three sets of laser gyros and accelerometers that replace the conventional
mechanical gyros and compass system. The IRSs are the aeroplane’s sole source of attitude and
heading information, except for the standby attitude indicator and the standby magnetic compass.

In their normal navigation mode, the IRSs provide:

¾ Attitude
¾ True and magnetic heading
¾ Acceleration
¾ Vertical speed
¾ Groundspeed
¾ Track
¾ Present position
¾ Wind data

IRS outputs are independent of external navigation aids.

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Chapter 25 Boeing 737 Navigation Equipment and Flight Management

ALIGNMENT

ALIGNMENT ON THE GROUND
An IRS must be aligned and initialised with the aeroplane’s present position before it can enter
the NAV mode. The position is normally entered through the FMC CDU during alignment. If the
position cannot be entered through the FMC CDU, it may be entered through the IRS Display
Unit. The aeroplane must remain stationary during alignment.

Normal alignment between 70°12’N and 70°12’S is initiated by rotating the IRS Mode Selector
from OFF directly to NAV. The IRS performs a short DC power test, during which the ON DC
Light illuminates. When the ON DC light extinguishes and the ALIGN light illuminates, the IRS
has begun the alignment process. Aeroplane present position should be entered at this time. The
IRS automatically enters the NAV mode after approximately 10 minutes, and the ALIGN light
extinguishes.

High latitude alignment, at latitudes between 70°12’ and 78°15’, require an extended alignment
time. The mode selector must be left in the ALIGN position for 17 minutes, then rotated to the
NAV position. The IRS then immediately enters the NAV mode.

Magnetic variation between 73°N and 60°S is stored in each IRS memory. The data
corresponding to the present position are combined with true heading to determine magnetic
heading. If magnetic information is unavailable, special navigation equipment is required to
provide true heading to the EHSIs.

FAST REALIGNMENT ON THE GROUND
During transit or through-flight stops with brief ground times, a 30-second realignment and
zeroing of groundspeed error may be performed by selecting ALIGN from NAV while the
aeroplane is parked. Present position should be simultaneously updated by manually entering
latitude and longitude prior to reselecting NAV.

Moving the aeroplane during alignment or fast realignment (ALIGN light illuminated) causes the
IRSs to automatically begin the full 10 minute alignment process over again.

LOSS OF ALIGNMENT
If an IRS loses both AC and DC power, the alignment is lost. Moving the mode selector out of the
NAV position causes the alignment to be lost as well.

If alignment is lost during flight, the navigation mode (including present position and groundspeed
outputs) is inoperative for the remainder of the flight. Selecting ATT allows the attitude mode to
be used to re-level the system and provide ADI attitude. The attitude mode requires
approximately 30 seconds of straight and level unaccelerated flight to complete the re-level
process. Some attitude errors may occur during acceleration, but will be slowly removed after
acceleration.

The attitude mode can also provide heading information, but to establish compass
synchronisation the crew must manually enter the initial magnetic heading. Thereafter, drift of the
IRS heading will occur (up to 15° per hour). When in ATT mode, the crew must periodically cross
check an operating compass system and enter an updated heading in the IRS as required.

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Boeing 737 Navigation Equipment and Flight Management Chapter 25

IRS DISPLAY UNIT

System Display Selector Displays the left (L) or right (R) IRS for the data displays.

DISPLAY SELECTOR

The Display Selector selects the desired function or data for the data displays. Displays are for
the IRS selected with the System Display Selector.

TEST: The knob is spring loaded to return to TK/GS, and TEST is used only
during alignment. All lights in the data displays and on the mode selector
unit momentarily illuminate, followed by a 10-second internal self test.

Positioning the Master Lights Switch on the centre instrument panel
to TEST illuminates all lights in the data displays and on the mode
selector unit.

TK/GS: When set to track and groundspeed, the left window displays present
true track (course), and the right window displays present in-flight wind
speed (knots).

PPOS: The Present Position detent displays present latitude and longitude.

WIND: In this position, the left window displays present in-flight true wind
direction, and the right window displays present in-flight wind speed
(knots).

HDG/STS: In the Heading Status mode, the left window displays true heading. The
right window displays any applicable maintenance status codes (last two
digits).

During IRS alignment, the right window also displays the minutes remaining until
alignment is complete. The window displays 7 (at the third digit) until the time remaining
reaches 6 minutes. The display then counts down in one minute intervals.

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Chapter 25 Boeing 737 Navigation Equipment and Flight Management

BRIGHTNESS CONTROL

Brightness control adjusts the brightness of the data displays and is located in the centre of the
display selector.

DATA DISPLAYS

Two windows display data for the IRS selected with the System Display Selector. The display
selector normally determines the type of data displayed. Keyboard entry of present position or
magnetic heading overrides the selected display.

The last digit of each window is for a decimal place.

ENTER KEY

Illuminated: The integral cue lights zz illuminate upon keying N, S, E, W, or H
Press: entries. When keying is complete the key is pressed.

The cue lights extinguish and the keyed data is simultaneously entered
into each IRS following completion of a valid self-test for data sensibility.
The data displays are again controlled by the display selector.

KEYBOARD

The keyboard provides for manual IRS entry of present position or magnetic heading.
The keyboard functions independently from the display selector position and the L or R
position of the System Display Selector.

Alpha Keys The data displays are controlled by the keyboard when the N, S, E, or W
Press: (latitude/longitude) or H (heading) key is pressed. Alpha keys arm the
keyboard for numeric entries.
Numeric Keys
Press: Permits manual entry of present position (latitude/longitude) when
either ALIGN light is illuminated.

Clear Key Permits manual entry of present magnetic heading when either mode
Illuminated: selector is in ATT.

The integral cue lights illuminate following an ENT operation if the
self test determines the data to be of an unreasonable value (entry
not accepted by the IRSs).

Press: Extinguishes the cue lights. If the cue lights are already extinguished,
pressing CLR clears the associated data display of data keyed in but not
yet entered (or not accepted). The data displays are again controlled by
the display selector.

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INSTRUMENT TRANSFER SWITCH/INSTRUMENT TRANSFER SWITCH
LIGHT

IRS TRANSFER SWITCH
Should either IRS fail, the crew uses the IRS Transfer Switch to switch the flight instruments
attitude and heading source to the functioning IRS.

INSTRUMENT TRANSFER SWITCH LIGHT (AMBER)
When illuminated, one or both of the Instrument Transfer Switches has been moved from the
normal position.

DME SYSTEM

A dual scanning DME is installed. This consists of two DME interrogators, each of which rapidly
alternates between the manually tuned frequency (VOR/ILS) and an automatically tuned
frequency for FMC use.

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RADIO DISTANCE MAGNETIC INDICATOR

1 DME Indicators 300 nm maximum search for all DME stations
Warning Flags: Warning Flag – Electrical power lost

DME receiver powered, but not
receiving a DME station, or during
Agility-Tuning

2 Bearing Pointer Number 1 Warning Flag in View
VOR Mode: Power failure
VHF NAV Signal unreliable
ADF Mode: Power Failure

3 Heading Warning Flag In View Selected compass signal is invalid.
Power failure

4 Bearing Pointer Number 2 Warning Flag in View
VOR Mode: Power failure
VHF NAV Signal unreliable

ADF Mode: Power Failure

5 Bearing Pointers Signals to the VOR Bearing Pointers are not affected

by the VHF NAV Transfer Switch.

Narrow Pointer: Uses signals from the VHF NAV receiver
Number 1, or ADF receiver number 1

Wide Pointer: Uses signals from the VHF NAV receiver
Number 2, or ADF receiver number 2

6 ADF/VOR Bearing Pointer Switches When pressed, selects the ADF or
VOR for the bearing pointer.

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VOR/ILS NAVIGATION

Two navigation receivers and control panels are installed. The panels are used to tune related
VOR and ILS frequencies. VOR/ILS information is displayed on the RDMI and the EHSI VOR/ILS
mode.

The VHF NAV Transfer Switch is used to switch the EHSI to a functioning receiver in the event of
a failure of the number one or the number two navigation radio, or the loss of navigation
information to an EFIS display.

VOR Test Switch: With a VOR frequency tuned and a course of 000 selected:
Press: The course deviation bar centres
VOR Bearing Pointer indicates 180°
TO-FROM Ambiguity Indicator shows a FROM indication

ILS Test Switch: With an ILS frequency tuned, pressing switches provides test indications.

Frequency Selector: Manually selects the desired frequency

Frequency Indicator: Indicates the frequency selected by the frequency selector or by the
automatic tuning system

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Mode Selector Switch: Selects between automatic and manual tuning of the frequencies

Marker Beacons: Each pilot has a set of marker beacon lights that show outer, middle, and

airways beacon passage. One marker beacon receiver operates both
sets of lights.

The high-low switch adjusts the sensitivity of the receiver.

Airways (White): Illuminates over an inner or airways marker beacon
Middle (Amber): Illuminates over a middle marker beacon
Outer (Blue): Illuminates over an outer marker beacon

MARKER BEACON SWITCH

High: High sensitivity of the receiver
Low: Low sensitivity of the receiver

ADF NAVIGATION

Two ADF receivers are installed. The ADF bearing signals are sent to the pointers on the EHSI
and RDMI. The audio is heard by using the ADF Receiver Control on an audio selector panel.

An automatic direction finding (ADF) system enables automatic determination of magnetic and
relative bearings to selected facilities.

If heading or track information is lost or invalid, EHSI ADF bearing pointers do not display, and
RDMI ADF bearing pointers do not display correct magnetic bearings. Relative bearings indicated
by pointers may be correct, if the receiver is operating.

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Frequency Indicators: Two windows showing the frequency selected for ADF 1 and ADF 2

ADF Mode Selector: Two selectors for each ADF

OFF: No electrical power to the receiver
ANT: Audio reception optimised. No ADF bearing sent to the RDMI.
ADF: Audio reception is possible. ADF bearing sent to RDMI
TEST: Bearing pointer indicates 45° left of lubber line for a valid test

ADF Gain Control: Adjusts the receiver gain of the respective ADF

Tone Switch: Position 1 or 2 adds tone to the respective ADF receiver
Central position disables tone

SSR TRANSPONDER SELECTIONS

Two ATC Transponders are installed and controlled by a single control panel.

The ATC Transponder System transmits a coded radio signal when interrogated by ATC ground
radar. The system also provides altitude reporting capability, allowing altitude information from a
selected ADC to be transmitted to the ATC radar facility.

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ATC Identification Code Selectors: Select the ATC code

ATC Code Indicator: Displays the code selected by the code selectors
“F” appears if a fault in the transponder system is detected

Altitude Reporting Switch: Enables altitude reporting

Identification Pushbutton: Transmits the identification signal

Transponder Select Switch:

STBY: Transponder mode disabled
L or R: System connected to Number 1 or Number 2 ADC
AUTO: Some systems have an AUTO system that activates the transponder

when an air/ground switch in the main landing gear shows the aircraft
to be airborne. The transponder is deactivated when the aircraft is
shown to be on the ground.

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WEATHER RADAR

The weather radar is an X band colour radar system used for:

¾ Weather detection and analysis
¾ Ground mapping

The system detects and locates various types of precipitation bearing clouds along the flight path
of the aircraft, and gives the pilot a visual indication of their intensity by using colour contrast.

The radar indicates the clouds rainfall intensity against a black background.

¾ Areas of heaviest rainfall appear in red
¾ The next level in yellow
¾ The least rainfall in green

In map mode, the radar displays surfaces in red, yellow and green (most reflective to least
reflective).

These displays enable identification of:

¾ Coastlines
¾ Hilly or mountainous regions
¾ Cities or large structures

Ground mapping mode can be useful in areas where ground-based navigation aids are limited.

The WX/T mode displays normal precipitation and precipitation associated with turbulence. When
the radar detects a horizontal flow of precipitation, with velocities of 5 or more metres per second,
toward or away from the radar antenna, that target display also becomes magenta. This magenta
area is associated with heavy turbulence.

The detection of turbulence is automatically limited to 50 nm range regardless of the selected
range.

The IDNT mode activates the ground clutter reduction circuits. Signals determined to have a high
probability of originating from ground returns are automatically removed from the display. Some
portions of weather targets may be removed as well. The IDNT mode is provided for analysis by
the pilot and is not for continuous use.

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EFIS Control Panel – Top Right Section

Range Selector: Selects the desired range in nautical miles for:

¾ HSI MAP
¾ Plan, and
¾ Weather radar displays

Weather Radar Switch: Press to activate the weather radar display

TFR: Activates the radar display on the left or right HSI
WX/T: Activates display of detected precipitation and turbulence within 50 nm
WX: Activates display of detected precipitation
MAP: Activates display of detected ground returns
IDNT: Activates ground clutter suppression in WX mode, normal operation position
would be off
TEST: Displays maintenance test pattern

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GAIN: Rotating gain control. Manually sets receiver sensitivity to enhance

¾ Ground
¾ WX
¾ WX/T
¾ MAP

TILT: Manually controls antenna tilt position from 15° up to 15° down.

GAIN UCAL: Illuminates when gain is improperly set.

RADIO ALTIMETERS

The low range radio altimeters provide indication of aeroplane height above the ground up to
2500 ft absolute altitude. Each pilot’s EADI indicates the radio altitude.

When the Captain’s RA is inoperative:

¾ All modes of the GPWS are inoperative
¾ Autopilot A channel should not be selected for approach
¾ Autothrottle automatic retard during landing flare is inoperative

When the First Officer’s RA is inoperative:

¾ Autopilot B channel should not be selected for approach

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