Boeing 737 - 600/700/800/900. Operations Manual (1997 year)

Flight Management, Navigation -

Navigation Systems Description

Boeing 737 Operations Manual

GPS System Schematic

GPS

TRANSMITTER

GPS

TRANSMITTER

GPS

TRANSMITTER

GPS

SENSOR UNITS

ANTENNA

GPS SENSOR UNIT

LEFT

RIGHT

FLIGHT MANAGEMENT COMPUTER

POS RE

2 / 3

F M C

G S

FMC Position

N47

W122

18.6

437

K T

I R

N47

W122

435

K T

I R

N47

32.4

W122

438

K T

Nav Sensor

G P S

Positions

N47

W122

G P S R

N47

32.4

W122

R A D I O

N47

W122

G P S U P D A T E

OFF

GPS UPDATE

ON/OFF

NAV OPTIONS

2/2

11.20.4

October 15, 2001

Flight Management, Navigation -

Navigation Systems Description

Boeing 737 Operations Manual

Inertial System

The inertial system computes airplane position, ground speed, and attitude data for

the DUs, flight management system, autoflight system, and other systems. The

major components of the inertial system are the air data inertial reference units

(ADIRU), an inertial system display unit (ISDU), IRS mode select unit (MSU),

and an IRS transfer switch. For information about the air data part of the system,

see chapter 10. The ADIRUs provide inertial position and track data to the FMC,

and attitude, altitude, and airspeed data to the CDS. Each ADIRU has an IRS

section and an air data section.

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 navigation 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 78 degrees 15 minutes North or South 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. Alignment time varies from five minutes

to seventeen minutes depending on airplane latitude.

Magnetic variation between 82 degrees north and 82 degrees 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 the latitude/longitude position is not within 4 NM of the origin airport, the CDU

scratchpad message VERIFY POSITION is displayed. If the entered

latitude/longitude position does not pass the IRS internal comparison tests, the

scratchpad message ENTER IRS POSITION is displayed.

October 15, 2001

11.20.5

Flight Management, Navigation -

Navigation Systems Description

Boeing 737 Operations Manual

The flashing ALIGN light alerts the crew that the position entered does not pass

one of the two internal comparison tests and should be checked for accuracy. If the

entered position does not agree with the last stored position, the first internal test

is failed, and the ALIGN light will flash. If the same position is reentered, the IRS

will accept the position and continue the alignment process. A second internal

position test compares the entered latitude with the system-computed latitude. If

this test is failed, the ALIGN light will again flash. If two consecutive entries of

the same position do not pass the second internal position test, the FAULT light

will illuminate. If the test is passed, the IRS will proceed to complete the

alignment process and enter NAV mode.

During transit or through-flight stops with brief ground times, a thirty second fast

realignment and zeroing of ground speed error may be performed by selecting

ALIGN while the airplane is parked. Present position should be simultaneously

updated by manually entering latitude and longitude prior to selecting NAV.

Note: If the airplane is moved during alignment or fast realignment, the IRS

automatically begins the full alignment process.

Loss of Alignment

If an IRS loses both AC and DC power, the alignment is lost. Alignment can be

lost if the MSU switch is moved out of the NAV position.

If alignment is lost in-flight, the navigation mode (including present position and

ground speed outputs) is inoperative for the remainder of the flight. However,

selecting ATT allows the attitude mode to be used to relevel the system and

provide an attitude reference. The attitude mode requires approximately thirty

seconds of straight and level unaccelerated flight to complete releveling. Some

attitude errors may occur during acceleration, but will be slowly removed after

acceleration stops.

The attitude mode can also provide heading information, but to establish compass

synchronization the crew must manually enter the initial magnetic heading. Drift

of up to 15 degrees per hour can occur in the IRS heading. Therefore, when in

attitude mode, an operating compass system must be periodically cross-checked

and an updated magnetic heading entered in the IRS, as required.

IRS Entries

Manual IRS entries of present position or magnetic heading are normally

accomplished on the POS INIT page of the FMC/CDU. The ISDU may also be

used.

11.20.6

October 15, 2001

Flight Management, Navigation -

Navigation Systems Description

Boeing 737 Operations Manual

IRS Power

The IRSs can operate on either AC or DC power. The left IRS is normally powered

from the AC standby bus, and the right IRS from the AC transfer bus 2. If AC

power is not normal, either or both systems automatically switch to backup DC

power from the switched hot battery bus. Backup DC power to the right IRS is

automatically terminated if AC power is not restored within five minutes.

Initial power-up requires battery bus power available and the IRS mode selector

to be in ALIGN, NAV, or ATT. If the IRS is turned off, it must complete a full

realignment cycle before the airplane can be moved.

If AC electrical power is subsequently removed from the airplane, the switched

hot battery bus continues to supply electrical power to the IRS. The ON DC light

illuminates, and the horn in the landing gear wheel well sounds to alert

maintenance personnel that the IRS is on battery power.

When the IRS mode selector is turned OFF, the IRS remains powered for

approximately 30 seconds. The ALIGN light illuminates until the system is

completely shut down.

Inertial System Display Unit (ISDU)

The ISDU is located on the aft overhead panel and displays data according to the

position of the display selector and system selector. The ISDU also contains a

keyboard for entry of present position and heading.

Mode Select Unit (MSU)

The MSU is located on the aft overhead panel and is used to select the operating

mode for each IRS. Indicator lights on the MSU show status of each IRS.

IRS Transfer Switch

Should either IRS fail, the IRS transfer switch is used to switch all associated

systems to the functioning IRS.

October 15, 2001

11.20.7

Flight Management, Navigation -

Navigation Systems Description

Boeing 737 Operations Manual

IRS Instrument Transfer Switch Schematic

AFDS

FLIGHT

OUTBOARD

INBOARD

CONTROL

INBOARD

OUTBOARD

DISPLAY

COMPUTER

DISPLAY

UNIT

WEATHER

RADAR

AUTOTHROTTLE

COMPUTER

NO. 1 DEU

NO. 2 DEU

IRS

BOTH

ON L

ON R

NORMAL

BOTH

ON L

NORMAL

ON R

AUTOTHROTTLE

COMPUTER HAS

EACH FLIGHT CONTROL

AUTOMATIC SWITCHING

COMPUTER HAS

CAPABILITY BETWEEN

INTERNAL SWITCHING

THE LEFT AND

LOGIC DEPENDENT UPON

RIGHT IRSs

ALIGN

OFF

NAV

OFF

NAV

IRS TRANSFER SWITCH

ATT

POSITION

L IRS

Radio Navigation Systems

Automatic Direction Finding (ADF)

An automatic direction finding (ADF) system enables automatic determination of

magnetic and relative bearings to selected facilities.

[Option - With 2 ADF receivers]

Two ADF receivers are installed. The ADF bearing signals are sent to the pointers

on the DUs and the standby radio magnetic indicator. The audio is heard by using

the ADF receiver control on the audio selector panel.

11.20.8

Flight Management, Navigation -

Navigation Systems Description

Boeing 737 Operations Manual

If heading or track information is lost or invalid, ADF bearing pointers on the DUs

will be removed, and ADF bearing pointers on the standby radio magnetic

indicator will not display correct magnetic bearing. Relative bearings indicated by

pointers may be correct if the receiver is operating.

Distance Measuring Equipment (DME)

Two frequency scanning DME systems are installed.

The FMC autotunes DME receivers as necessary for position updating. During

normal operations, two different DME signals or a signal from a collocated

VOR/DME pair provide an accurate radio geographical position to the FMC. The

identifiers of DMEs currently providing update data to the FMC are displayed on

the NAV STATUS page 1/2. The radio position is displayed on the POS REF page

2/3. Specific DME station tuning for FMC position updating can be inhibited on

the NAV OPTIONS page 2/2.

The flight crew must manually tune the DME on the VHF navigation control panel

and the respective EFIS control panel VOR/ADF switch must be in the VOR

position for DME to be displayed on the CDS. DME distance is also displayed on

the CDS when the ILS receivers are tuned to a collocated DME and localizer

facility.

Instrument Landing System (ILS)

Two ILS receivers are installed.

The ILS receivers are tuned manually on the VHF navigation control panel. The

flight crew must manually tune the ILS for display on CDS. The ILS localizer and

glideslope can also be displayed on the standby attitude indicator.

LOC updating of the FMC occurs only after the ILS is manually tuned.The tuned

ILS frequency is displayed on the navigation display in the APP modes.

Navaid Identifier Decoding

[Option - PFD/ND]

The Morse code identifier of a tuned VOR, ILS, or ADF can be converted to alpha

characters. The decoded identifier is then shown on the PFD and ND. The crew

should monitor this identifier for correct navigation radio reception. The identifier

name is not compared with the FMC database.

Due to the large variation in ground station identifier quality, the decode feature

may incorrectly convert the intended identifier name. Examples: the Hong Kong

localizer “KL” may show as “KAI,” or the Boeing Field ILS may show as “QBFI”

or “TTTT” instead of “IBFI.”

Pilots should verify the identity of the tuned navigation station from the audio

Morse code when the tuned frequency remains shown or an incorrect identifier is

shown.

11.20.9

Flight Management, Navigation -

Navigation Systems Description

Boeing 737 Operations Manual

Marker Beacon

[Option - EFIS/MAP]

Marker beacon indications for outer, middle, and inner marker are displayed on

the upper outboard corner of Captain’s and First Officer’s outboard display units.

[Option - PFD/ND]

Marker beacon indications for outer, middle and inner marker are displayed on the

upper right hand corner of the attitude display located on the Captain’s and First

Officer’s Primary Flight Display (PFD) units.

Very High Frequency Omni Range (VOR)

Two VOR receivers are installed.

The flight crew must manually tune the VOR on the navigation control panel for

display on the DUs and the standby radio magnetic indicator. VOR-DME radio

updating is available if the crew manually tunes a valid in-range VOR station.

[Option - EFIS/MAP]

Left and right VOR bearings are displayed on the DUs when a valid in-range

VOR station is tuned, the respective EFIS control panel VOR/ADF switch is in

the VOR position and the respective EFIS control panel POS switch is pushed.

The DUs also show course deviation.

[Option - PFD/ND]

Left and right VOR bearings are displayed on the DUs when a valid in-range

VOR station is tuned and the respective EFIS control panel VOR/ADF switch is

in the VOR position. The DUs also show course deviation.

VHF NAV Transfer Switch

Should either VOR receiver fail, the VHF NAV transfer switch enables selection

of the opposite VHF NAV receiver for display.

ATC Transponder

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. Altitude reporting capability is provided.

Transmissions are automatically enabled when the air/ground system indicates air

mode.

TCAS is also controlled from the transponder panel. The TCAS system is

described in Chapter 15.

11.20.10

Flight Management, Navigation -

Navigation Systems Description

Boeing 737 Operations Manual

Weather Radar

The weather radar system detects and locates various types of precipitation

bearing clouds along the flight path of the airplane and gives the pilot a visual

indication in color of the clouds’ intensity. The radar antenna sweeps a forward arc

of 180 degrees.

The radar indicates a cloud’s rainfall intensity by displaying colors contrasted

against a black background. Areas of heaviest rainfall appear in red, the next level

of rainfall in yellow, and 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 coastlines, hilly or mountainous regions,

cities, or large structures. Ground mapping mode can be useful in areas where

ground-based navigation aids are limited.

The radar system performs only the functions of weather detection and ground

mapping. It should not be used or relied upon for proximity warning or

anticollision protection.

The turbulence mode displays normal precipitation and precipitation associated

with turbulence. When the radar detects a horizontal flow of precipitation with

velocities of 5 or more meters per second toward or away from the radar antenna,

that target display becomes magenta. This magenta area is associated with heavy

turbulence. The detection of turbulence is automatically limited to a 40 nautical

mile range, regardless of the selected range.

[Option - Weather radar with IDNT]

The IDNT position activates the ground clutter reduction feature. Signals that are

determined to have a high probability of originating from ground returns will be

automatically removed from the display. Some portions of weather targets may be

removed as well. The IDNT position is provided for analysis by the pilot and is

not for continuous use.

[Option - With predictive windshear]

The weather radar also provides predictive windshear alerting below 1,200 feet

RA. On the ground or in flight below 2,300 feet RA, radar antenna scan sweep is

limited to 120 degrees with PWS enabled. Above 2,300 feet RA the radar sweep

reverts to 180 degrees. (Refer to Chapter 15, Warnings.)

11.20.11

Boeing 737 Operations Manual

Flight Management, Navigation

Chapter 11

Flight Management System Description

Section 30

Introduction

The flight management system (FMS) aids the flight crew in managing automatic

navigation, in-flight performance optimization, fuel monitoring, and flight deck

displays. Automatic flight functions manage the airplane lateral flight path

(LNAV) and vertical flight path (VNAV). The displays include a map for airplane

orientation and command markers (bugs) on the airspeed and N1 indicators to

assist in flying efficient profiles.

The flight crew enters the desired route and flight data into the CDUs. The FMS

then uses its navigation database, airplane position and supporting system data to

calculate commands for manual or automatic flight path control.

The FMS can automatically tune the navigation radios and determine LNAV

courses. The FMS navigation database provides the necessary data to fly routes,

SIDs, STARs, holding patterns, and procedure turns. Lateral offsets from the

programmed route can be calculated and commanded.

For vertical navigation, computations include items such as fuel burn data,

optimum speeds, and recommended altitudes. Cruise altitudes and crossing

altitude restrictions are used to compute VNAV commands. When operating in the

Required Time of Arrival (RTA) mode, the computations include required speeds,

takeoff times, and enroute progress information.

Flight Management Computer (FMC)

The basis of the flight management system is the flight management computer.

Since the term FMC is universally understood, it is used here for standardization

and simplification.

The FMC uses flight crew-entered flight plan information, airplane systems data,

and data from the FMC navigation 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 DUs. The EFIS control panels are used to select

the desired information for the navigation displays. The mode control panel is

used to select the autothrottle, autopilot, and flight director operating modes.

Refer to the following chapters for operation of these other systems:

• Chapter 4, Automatic Flight

• Chapter 10, Flight Instruments, Displays.

11.30.1

Flight Management, Navigation -

Flight Management System

Description

Boeing 737 Operations Manual

The FMC and CDU are used for enroute and terminal area navigation and to

supplement primary navigation means when conducting other types of

nonprecision approaches.

[Option - Dual FMC]

The dual FMC installation is certified as a “sole source” navigation system.

Airplanes equipped with two FMCs are certified to operate outside radio navaid

coverage. The second FMC serves as a backup, providing complete navigational

functions if the other FMC fails.

[Option - Dual FMC]

With a dual FMC installation, one FMC is always designated as primary. This is

controlled by the position of the FMC Source Select switch. Refer to Chapter 11,

FMC Source Select Switch.

[Option - Dual FMC]

The primary FMC:

• allocates navaid tuning and updating functions between FMCs

• insures synchronization between FMCs

• controls CDU displays

• provides input to the autopilot

• provides input to the autothrottle system

[Option - Dual FMC]

Positioning the FMC Source Select Switch to BOTH ON L or BOTH ON R

isolates FMC operation to use only the left or right FMC respectively. In the

NORMAL position, the left FMC is primary by default. Although the aircrew can

enter information into either CDU, the primary FMC is responsible for

synchronizing this information with the secondary FMC and updating both CDU

displays.

When external position updating is not available, the FMC uses the IRS position

as reference. When the IRS is the only position reference, the FMC applies an

automatic correction to the IRS position to determine the most probable FMC

position. This correction factor is developed by the FMC’s monitoring IRS

performance during periods of normal position updating to determine the typical

IRS error value. It is important to note that, when external position updating is not

available, navigation accuracy may be less than required. Flight crews should

closely monitor FMC navigation, especially when approaching the destination.

The accuracy of the FMC navigation should be determined during descent phase

by using radio navaids and radar information if available.

Note: Inaccurate position updating may cause the airplane to deviate from the

desired track.

11.30.2

October 15, 2001

Boeing 737 Operations Manual

Flight Management, Navigation

Chapter 11

Flight Management System Operation

Section 31

Introduction

When first powered, the FMS is in the preflight phase. As a phase is completed,

the FMS automatically transitions to the next phase in this order:

• preflight

• descent

• takeoff

• approach

• climb

• flight complete.

• cruise

Preflight

During preflight, flight plan and load sheet information are entered into the CDU.

The flight plan defines the route of flight from the origin to the destination and

initializes LNAV. Flight plan and load sheet information provide performance

information to initialize VNAV.

Required preflight information consists of:

• initial position

• performance data

• route of flight

• takeoff data.

Optional preflight data includes:

• navigation database

• RTA data

• SID

• cruise wind

• STAR

• reduced takeoff and climb

thrust limits.

Each required or optional data item is entered on specific preflight pages.

Preflight begins with the IDENT page. If the IDENT page is not displayed, it can

be selected from the IDENT prompt on the INIT/REF INDEX page. Visual

prompts provide assistance in selecting the appropriate CDU pages. Preflight

pages can be manually selected in any order.

After entering and checking the necessary data on each preflight page, the lower

right line select key is pushed to select the next page. When ACTIVATE is selected

on the RTE page, the execute light illuminates. The EXEC key is then pushed to

complete the task of making the route active before continuing with the preflight.

If a standard instrument departure (SID) is to be entered into the route, the

departure/arrival (DEP/ARR) page is selected. After selecting the desired SID, the

resulting modification must be appropriately linked to the existing route and

executed. This can be accomplished on the RTE or RTE LEGS page.

When all required preflight entries are complete, the preflight status prompts on

the TAKEOFF REF page are no longer displayed.

11.31.1

Flight Management, Navigation -

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Operation

Boeing 737 Operations Manual

Takeoff

The takeoff phase begins with selection of TO/GA and extends to the thrust

reduction altitude where climb thrust is normally selected.

Climb

The climb phase begins at the thrust reduction altitude and extends to the top of

climb (T/C) point. The T/C point is where the airplane reaches the cruise altitude

entered on the PERF INIT page.

Cruise

The cruise phase begins at the T/C point and extends to the top of descent (T/D)

point. Cruise can include step climbs and en route descents.

Descent

The descent phase begins at the T/D point or when either a level change or vertical

speed descent is initiated. The descent phase extends to the beginning of the

approach phase.

Approach

The approach phase begins two miles from the first waypoint of a published

approach or approach transition selected from the ARRIVALS page.

Flight Complete

After landing, the flight complete phase clears the active flight plan and load data.

Some preflight data fields initialize to default values in preparation for the next

flight.

FMC and CDU Terminology

The following paragraphs describe FMC and CDU terminology.

Active - flight plan information currently being used to calculate LNAV or VNAV

guidance commands.

Activate - designating an entered route as the active route for navigation. It is a

two step process:

• push the ACTIVATE prompt

• push the execute (EXEC) key.

Altitude restriction - a crossing restriction at a waypoint.

Delete - remove FMC data and revert to default values, dash or box prompts, or a

blank entry using the DELETE key.

11.31.2

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Boeing 737 Operations Manual

Econ - a speed schedule calculated to minimize operating cost. The economy

speed is based on the flight crew CDU-entered cost index. A low cost index

reflects high fuel costs and results in a lower cruise speed.

Enter - placing an entry into the CDU scratchpad and then line selecting the

information to the desired location. New characters can be typed, or existing data

can be line selected into the scratchpad.

Erase - removing flight crew-entered information, which has resulted in a

modification, by pushing the ERASE prompt.

Execute - making modified information part of the active flight plan by pushing

the EXEC key.

Inactive - route, climb, cruise, or descent information not currently being used to

calculate LNAV or VNAV commands.

Initialize - entering information required to make the system operational.

Message - information the FMC automatically writes in the scratchpad to inform

the flight crew of a system condition.

Modify - active data that is changed but not yet executed. When a modification is

made to the active route or performance mode, MOD is displayed in the page title,

ERASE appears next to line select key 6 left, and the execute key illuminates.

Prompt - CDU displays that aid the flight crew in accomplishing a task. Prompts

can be boxes, dashes, or a careted (< or >) line to remind the flight crew to enter

or validate information.

Select - pushing a key to obtain the desired information or action, or to copy

selected data to the scratchpad.

Speed restriction - an airspeed limit associated with a specified altitude or

waypoint.

Waypoint - a point on the route. It can be a fixed point such as a latitude and

longitude, VOR or ADF station, airway intersection, or a non-fixed point such as

a conditional waypoint. A conditional waypoint is not necessarily associated with

a land reference; it reflects a time position, or altitude requirement. An example

of a conditional waypoint is “when reaching 1000 feet.”

11.31.3

Flight Management, Navigation -

Flight Management System

Operation

Boeing 737 Operations Manual

Maintenance Index Page

The MAINT BITE INDEX page is available only on the ground and provides

access to data for use by maintenance personnel.

INIT/R EF INDEX

1 / 1

<OFFSE T

MAINT>

MAINT BITE INDEX

1 / 1

<FMC S

ENGINE>

<DFC S

APU>

<A/ T

FQIS>

<ADIR S

<CD S

<INDE X

F MC DOWNLOAD>

INDEX

Displays the INIT/REF INDEX page.

Navigation Position

[Option - With GPS]

The FMC determines present position from the IRS, GPS, and navigation radios.

The FMC uses its calculated present position to generate lateral steering

commands along the active leg to the active waypoint.

[Option - Dual FMC]

When the FMC Source Select Switch is positioned to NORMAL, the left FMC

becomes primary, however, data from both FMCs is combined to determine a

composite position and velocity for guidance and map displays.

11.31.4

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Boeing 737 Operations Manual

FMC Position Update

[Option - With GPS]

On the ground, the FMC calculates present position based on GPS data. If GPS

data is not available, the FMC calculates present position based on IRS data.

[Option - FMC U10.2 and later]

If GPS UPDATE is OFF, the FMC updates position to the takeoff runway

threshold when a TO/GA switch is pushed. When making an intersection takeoff,

the intersection data must be entered on the TAKEOFF REF page. If GPS

UPDATE is ON, the TO/GA update is inhibited. GPS UPDATE is on the NAV

OPTIONS page.

[Option - Runway position update via the CDU only]

On the ground prior to takeoff, FMC position update to the takeoff runway

threshold position can be done on the TAKEOFF REF page.

[Option - With GPS]

In flight, the FMC position is continually updated from the GPS, navigation

radios, and IRS. Updating priority is based on the availability of valid data from

the supporting systems.

FMC position updates from navigation sensor positions are used in the following

priority order:

• GPS

• two or more DME stations

• one VOR with a collocated DME

• one localizer and collocated DME

• one localizer.

The station identifiers and frequencies of the selected radio navigation aids are

displayed on the NAV STATUS page 1/2.

FMC logic selects the GPS position as the primary update to the FMC position. If

all GPS data becomes unavailable, the FMC reverts to radio or IRS updating.

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 a specific

station is required by the flight plan. Radio position is determined by the

intersection of two DME arcs.

11.31.5

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Boeing 737 Operations Manual

If the DME radios fail, or if suitable DME stations are not available, FMC

navigation is based on IRS position information only. The two VHF Nav radios

are used by the FMC for localizer updating during an ILS approach and by the

crew for navigation monitoring.

Note: The FMC is designed to automatically reject unreliable navaid data during

FMC position updating. However, in certain conditions, navaids which 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 takeoff. This is usually manifested

in an abrupt heading correction after engaging LNAV. The position shift

can be seen on the map which will shift the desired track and runway

symbol to a position significantly different from that displayed during

ground roll.

[Option - FMC U10.3 and later]

Note: If the flight crew observes either of these indications, the FMC should be

carefully monitored.

When adequate radio updating is not available, navigation display map mode may

display a shift error. This error results in the displayed position of the airplane,

route, waypoints, and navigation aids shifted from their actual positions.

An across track, undetected map shift may result in the airplane flying a ground

track that is offset from the desired track. An along track, undetected map shift

may result in the flight crew initiating altitude changes earlier or later than desired.

In either case, an undetected map shift may compromise terrain or traffic

separation.

Map shift errors can be detected by comparing the position of the airplane on the

navigation display map mode with data from the ILS, VOR, DME, and ADF

systems.

Navigation Performance

The FMC uses data from the navigation systems to accurately calculate the

position of the airplane. The current FMC position is shown on line 1 of the POS

REF page 2/3.

[Option - With GPS]

The FMC position is derived from a mathematical combination of the positions

determined by the IRS, radio, and GPS systems. It represents the FMC’s estimate

of the actual position of the airplane. Its accuracy varies according to the accuracy

of the other position determining systems.

[Option - FMC U10.4 and later]

Note: If the GPS position update is excessive, GPS updating is suspended until

the GPS position can be determined to be reasonable.

11.31.6

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Boeing 737 Operations Manual

Actual Navigation Performance (ANP)

Actual navigation performance (ANP) is the FMC’s estimate of the quality of its

position determination. It is shown on POS SHIFT page 3/3 and on RTE LEGS

pages. ANP represents the estimated maximum position error with

95%

probability. That is, the FMC is 95% certain that the airplane’s actual position lies

within a circle with a radius of the ANP value around the FMC position. The lower

the ANP value, the more confident the FMC is of its position estimate.

Required Navigation Performance (RNP)

The FMC supplies a default required navigation performance (RNP) value for

takeoff, en route, oceanic, terminal, and approach phases of flight. RNP can also

be supplied by the Navigation Database or may be entered by the crew. Actual

navigation performance should not exceed RNP.

[Option - FMC U10.3, U10.4 or U10.4A]

If ANP exceeds the RNP value, the UNABLE REQD NAV PERF-RNP message

appears. RNP is shown on POS SHIFT page 3/3 and on RTE LEGS pages.

[Option - FMC U10.5 and later]

If ANP exceeds the RNP value, an amber UNABLE REQD NAV PERF-RNP

message appears on the map display. RNP is shown on RNP PROGRESS page 4/4

and on RTE LEGS pages.

11.31.7

Boeing 737 - 600/700/800/900. Operations Manual (1997 year) - page 57