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

Flight Management, Navigation -

Flight Management System

Operation

Boeing 737 Operations Manual

Actual Navigation Performance

[Option - Single FMC with GPS]

PO S SHIFT

3 / 3

G P S

P S ( L )

G P S - R

<210`/0.1 N M

220`

/0.0N M >

I R S

R S ( L )

I R S - R

<1 50` /3.9 N M

110`

/2.3N M >

R N P / A C T U A L

R A D I O

12 .0 / 5 . 1

035`

/1.4N M >

- - - - - - - -

- - - - - - - - - - - - - -

V STATUS>

<INDE X

Radio Position

035`/1.4NM

GPS(L) Position

Actual Navigation Performance (ANP)

210`/0.1NM

5.12NM

FMC Combined Position

IRS(L) Position

150`/3.9NM

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[Option - Dual FMC with GPS]

PO S SHIFT

3 / 3

F M

F M C - R

<2 74` /0.0 N M

094`

/0.3N M >

G P S

P S ( L )

G P S - R

<210`/0.1 N M

220`

/0.0N M >

I R S

R S ( 2 )

I R S - R

<1 50` /3.9 N M

110`

/2.3N M >

R N P / A C T U A L

R A D I O

12 .0 / 5 . 1

035`

/1.4N M >

- - - - - - - -

- - - - - - - - - - - - - -

V STATUS>

<INDE X

Radio Position

035`/1.4NM

GPS(L) Position

210`/0.1NM

Actual Navigation Performance (ANP)

5.12 NM

IRS(R) Position

110`/2.3NM

FMC Combined Position

IRS(L) Position

150`/3.9NM

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Boeing 737 Operations Manual

Lateral Navigation (LNAV)

LNAV provides steering commands to the next waypoint. If selected, LNAV

engages when laterally within 3 nautical miles of the active route leg. If outside of

3 nautical miles of the active route leg, LNAV engages if on an intercept heading

of 90 degrees or less and the intercept will occur before the active waypoint. FMC

LNAV guidance normally provides great circle courses between waypoints.

However, when an arrival or approach from the FMC database is entered into the

active route, the FMC can supply commands to fly a constant heading, track, or

follow an arc, as required by the procedure.

Waypoints

Waypoint (navigation fix) identifiers are displayed on the CDU and navigation

display.

The CDU message NOT IN DATA BASE is displayed if a manually entered

waypoint identifier is not stored in the database. The waypoint can still be entered

as a latitude/longitude, place-bearing/distance or place-bearing/place-bearing

waypoint.

FMC-generated waypoints contain a maximum of five characters assigned

according to the following rules.

Navaid Waypoint Names

VHF - waypoints located at VHF navaids (VOR/DME/LOC) are identified by the

official one, two, three or four character facility identifier. Examples:

• Los Angeles VORTAC - LAX

• Tyndall TACAN - PAM

• Riga, Latvia - RIX.

NDB - waypoints located at NDBs are identified by use of the station identifier.

Example:

• Fort Nelson, CAN - YE.

Fix Waypoint Names

Fixes with one-word names - waypoints located at fixes with names containing

five or fewer characters are identified by the name. Examples:

• DOT

• ACRA

• ALPHA.

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Long Waypoint Names

Names with more than five characters are abbreviated using the following rules

sequentially until five characters remain. Double letters are deleted. Examples:

• KIMMEL becomes KIMEL

• COTTON becomes COTON

• RABBITT becomes RABIT.

Keep the first letter, first vowel and last letter. Delete other vowels starting from

right to left. Examples:

• ADOLPH becomes ADLPH

• BAILEY becomes BAILY

• BURWELL becomes BURWL.

Keep the last letter, then delete consonants from right to left. Examples:

• ANDREWS becomes ANDRS

• BRIDGEPORT becomes BRIDT

• HORSBA becomes HORSA.

Fixes with multiword names use the first letter of the first word and abbreviate the

last word, using the above rules sequentially until a total of five characters remain.

Examples:

• CLEAR LAKE becomes CLAKE

• ROUGH ROAD becomes RROAD.

Unnamed Point Waypoint Names

Unnamed turn points, intersections and DME fixes - if an unnamed turn point,

intersection or fix is collocated with a named waypoint or navaid on a different

route structure (such as low altitude routes or an approach), the name or identifier

of the collocated waypoint is used. Example:

• Unnamed turn point on J2 between the Lake Charles (LCH) and New

Orleans (MSY) VORTACs is coincidental with the Lafayette (LFT) low

altitude VORTAC. LFT is used as the identifier for the turn point.

Identifier codes for unnamed turn points not coincidental with named waypoints

are constructed from the identifier of a navaid serving the point and the distance

from the navaid to the point. If the distance is 99 nautical miles or less, the navaid

identifier is placed first, followed by the distance. If the distance is 100 nautical

miles or more, the last two digits are used and placed ahead of the navaid

identifier. Examples (NAVAID - DISTANCE - IDENT):

• INW - 18 - INW18

• CSN - 106 - 06CSN

• TCS - 89 - TCS89.

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Unnamed flight information region (FIR), upper flight information region (UIR),

and controlled airspace reporting points - waypoints located at unnamed FIR,

UIR, and controlled airspace reporting points are identified by the three-letter

airspace type identification followed by a two-digit sequence number.

Unnamed oceanic control area reporting points - positions in the northern

hemisphere use the letters N and E, while positions in the southern hemisphere use

the letters S and W. Latitude always precedes longitude. For longitude, only the

last two digits of the three digit value are used.

Placement of the designator in the five character set indicates whether the first

longitude digit is 0 or 1. The letter is the last character if the longitude is less than

100° and is the third character if the longitude is 100° or greater.

N is used for north latitude, west longitude. E is used for north latitude, east

longitude. S is used for south latitude, east longitude. W is used for south latitude,

west longitude. Examples:

• N50° W040° becomes 5040N

• S52° W075° becomes 5275W

• N75° W170° becomes 75N70

• S07° W120° becomes 07W20

• N50° E020° becomes 5020E

• S50° E020° becomes 5020S

• N06° E110° becomes 06E10

• S06° E110° becomes 06S10.

Procedure Arc Fix Waypoint Names

Unnamed terminal area fixes along a DME arc procedure - unnamed fixes along

a DME arc procedure are identified with the first character D. Characters 2

through 4 indicate the radial on which the fix lies. The last character indicates the

arc radius. The radius is expressed by a letter of the alphabet where A = 1 mile, B

= 2 miles, C = 3 miles, and so forth. Examples:

• EPH252°/24 = D252X

• EPH145°/24 = D145X

• GEG006°/20 = D006T.

An unnamed waypoint along a DME arc with a radius greater than 26 miles is

identified as an unnamed turn point that is not coincidental with a named

waypoint. Examples:

• CPR338°/29 = CPR29

• GEG079°/30 = GEG30.

When there are multiple unnamed waypoints along a DME arc with a radius

greater than 26 miles, the station identifier is reduced to two characters, followed

by the radius, and then a sequence character. Examples:

• CPR134°/29 = CP29A

• CPR190° /29 = CP29B

• CPR201°/29 = CP29C.

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Procedure Fix Waypoint Names

Marker beacons - a marker beacon is identified by the marker type identifier

followed by the runway number. Examples:

• Outer Marker 13R = OM13R

• Middle Marker 21 = MM21.

Runway-related fixes - waypoints located at unnamed runway-related fixes are

identified by adding a two-letter prefix to the runway number. The following list

is used to determine the appropriate prefix:

• RX - runway extension fix

• BM - back course marker

• FA - VFR final approach fix

• MD - minimum descent

• CF - final approach course fix

altitude

• FF - final approach fix

• A - (+ an alpha) step down fix

• IF - initial approach fix

• RW - runway threshold

• OM - outer marker

• MA - missed approach point

other than RW

• MM - middle marker

• TD - touchdown point inboard

• IM - inner marker

of RW.

Examples: OM25L, MM09, IM23, RW04, RW18L.

For airports with more than one approach to the same runway, the two letter prefix

may change to allow different identifiers for the same waypoint. The first letter

identifies the type of fix and the second letter identifies the type approach as

follows:

• C( ) - final approach course fix

•

( )L - localizer only( )B

• F( ) - final approach fix

-backcourse ILS

• P( ) - missed approach point

•

( )D - VOR/DME

• I( ) - initial approach fix

•

( )V - VOR only

• D( ) - minimum descent

•

( )S - VOR with DME points

altitude

•

( )N - NDB

• T( ) - touch down point

•

( )Q - NDB with DME points

• R( ) - runway centerline

•

( )M - MLS

intercept.

•

( )T - Tacan

•

( )I - ILS

•

( )R - RNAV.

Examples: CI32R, PV15, FN24L.

Unnamed turn points - unnamed turn points that are part of a procedure are

identified as a latitude and longitude waypoint. These include waypoints (except

conditional waypoints) defined by flying a course or track from a waypoint

(except conditional waypoints) to a radial or DME distance. These waypoints are

automatically entered in a route by selection of a procedure using these waypoints,

from the departures or arrivals page.

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Airport reference points - airport reference points are identified by the ICAO

identifier.

Duplicate Waypoint Names

Duplicate identifiers - should application of these rules result in more than one

waypoint having the same identifier, then a CDU page change occurs when an

attempt is made to enter the duplicated identifier. The page title is SELECT

DESIRED WPT. The page lists the latitude and longitude of waypoints with the

same identifier and the type of facility or waypoint. Selecting the

latitude/longitude of the desired waypoint enters the correct waypoint on the

original page.

Conditional Waypoint Names

Conditional waypoints are automatically entered into a route as a result of

selecting a procedure on a DEPARTURES or ARRIVALS page. Normally,

conditional waypoints cannot be manually entered on a route or legs page. These

waypoints are events when a condition occurs and are not at a

geographically-fixed position. The types of conditions are:

• passing through an altitude

• intercepting a course

• flying a heading to a radial or

• heading vector to a course or

DME distance

fix.

Altitude and course intercept conditional waypoints are displayed on the CDU

inside (parentheses) marks. The following diagram depicts conditional waypoints.

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EXAMPLE:

MEANING:

RTE LEGS

180

HDG

CDE

(1000)

140

HDG

BCD

(ABC180)

180` HDG

TO 1000'

280` FOR

HDG

VECTORS

TO CDE

(ABC-20)

090`R

HDG

ABC

(INTC)

020` HDG

340

140` HDG

TO INTC

TO 180`R

340` TO BCD

BCD

ABC180

090` HDG

280

HDG

ABC20

TO 20 DME

(VECTOR)

270

CDE

Note: All waypoints except BCD and CDE are examples of conditional waypoints.

Note: When (VECTOR) is the active leg and LNAV is not engaged, the FMC

automatically sequence to the next waypoint when within 3 nm of the next

leg. If (VECTOR) is the active waypoint and LNAV is engaged, 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.

Manually Entered Latitude/ Longitude Waypoint Names

Pilot defined waypoints entered as a latitude and longitude are displayed in a

five-character format. The first three characters are WPT followed by a two digit

sequence number. Latitude and longitude waypoints are entered with no space or

slash between the latitude and longitude entries. Leading zeroes must be entered.

All digits and decimal points (to 1/10 minute) must be entered unless the latitude

or longitude are full degrees. Examples:

• N47° W008° is entered as N47W008 and displayed as WPT01

• N47° 15.4’ W008° 3.4’ is entered as N4715.4W00803.4 and displayed as

WPT02.

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Manually Entered Place-Bearing/Distance or

Place-Bearing/Place-Bearing Waypoint Names

Waypoints entered as a place-bearing/distance or place-bearing/place-bearing

are identified by the first three characters of the entry followed by a two-digit

sequence number. Examples:

• SEA330/10 becomes SEA01

• SEA330/OLM020 becomes SEA02.

Manually Entered Along-Track Waypoint Names

Along-track waypoints are a special case of place-bearing/distance waypoints

applied to the current route. When a waypoint is desired on the route where none

exists, the along-track waypoint feature creates the desired waypoint without

creating a route discontinuity.

Along-track waypoints are entered using the waypoint name (the place), followed

by a slash and minus sign, for points before the waypoint, or no sign for points

after the waypoint, followed by the mileage offset for the newly defined waypoint.

The route course takes the place of the bearing which is not entered. The created

waypoint is then inserted over the original waypoint. The distance offset must be

less than the distance between the originating waypoint and next (positive value)

or preceding (negative value) waypoint. Latitude and longitude waypoints cannot

be used to create along-track waypoints. Examples:

• VAMPS/25 is 25 miles after VAMPS on the present route, and is

displayed as VAM01

• ELN/-30 is 30 miles before ELN on the present route, and is displayed as

ELN01.

Navigation Displays

The route is displayed on the navigation display in the map, map center, and plan

modes. The display color and format represent the following status:

• an inactive route is displayed as a cyan dashed line

• an activated but not yet executed route is displayed as a cyan dashed line

• the active route is displayed in magenta

• modifications to an active route are displayed as dashed white lines

• modified waypoints are displayed in white

• executed route offsets are displayed as a dashed magenta line.

Vertical Navigation (VNAV)

VNAV provides vertical profile guidance through the climb, cruise, and descent

phases of flight.

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Speed/Altitude Restrictions

VNAV controls the path and speed to comply with waypoint crossing restrictions.

Waypoint crossing restrictions are entered on the LEGS page waypoint line by

pushing the applicable key on the right side of the CDU. Barometric altitude

restrictions must be below the cruise altitude to be valid. Values entered as part of

a procedure and manually entered restrictions are shown in large font. FMC

predicted values do not act as restrictions, and are shown in small font.

[Option - With color]

A waypoint restriction is magenta when it is active. The restriction does not have

to be in line 1 to be active.

[Option - With color]

Modified waypoint restrictions are shaded white until they are executed. Speed

restriction entries require an altitude restriction at the same waypoint.

Waypoints can have altitude, airspeed or both airspeed/altitude restrictions.

All speed restrictions are considered by the FMC as at or below restrictions.

At or above altitude restrictions are entered with a suffix letter A (example: 220A).

At or below altitude restrictions are entered with a suffix letter B (example: 240B).

Mandatory altitude restrictions are entered without any suffix letter (example:

270).

Altitude restrictions that are between two altitudes are displayed with the lower

limit first, followed by a suffix letter A, then the upper limit, followed by a suffix

letter B (example: 220A240B).

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Thrust Reduction

Climb thrust is selected by pushing the N1 switch.

[Option - With automatic thrust reduction after takeoff]

Climb thrust is selected by pushing the N1 switch or automatically upon reaching

the thrust reduction altitude.

[Option - With quiet climb]

When cutback mode is selected ON, the FMC calculates and commands a thrust

cutback at the required cutback altitude. A new N1 is calculated during climb and

normal climb thrust is restored at the RESTORE altitude.

VNAV Engagement

VNAV commands an airspeed increase to the planned climb speed profile, limited

by configuration.

VNAV Climb

The VNAV climb profile uses VNAV SPD at the default climb speed or pilot

selected climb speed to remain within all airspeed and altitude restrictions that are

part of the SID entered into the active route. Autothrottle uses selected climb

thrust limit.

Note: Selection of ENG OUT on the CLB page provides the crew with advisory

engine out performance information.

If the climb speed profile cannot achieve an altitude restriction, the UNABLE

NEXT ALTITUDE scratchpad message is shown.

Climb Restrictions

VNAV enters the VNAV PTH mode to remain within departure or waypoint

restrictions. Speed maintained during this time can be:

• procedure based speed restriction

• waypoint speed restriction

• default VNAV climb speed

• manually entered climb speed.

Top Of Climb (T/C)

The point where the climb phase meets the cruise altitude is called the top of

climb. Approaching this point, the FMC changes from the climb phase to the

cruise phase. The T/C is shown any time the FMC calculates a change from a

climb phase to a cruise phase, such as a step climb.

The T/C point is shown on the map as a green open circle with the label T/C.

October 15, 2001

11.31.19

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MCP Altitude Intervention

[Option]

[Option - With VNAV ALT]

The altitude intervention switch may be used to resume climb after a temporary

level off.

[Option - With VNAV ALT]

Temporary Level Off

T/C

FL250

FL190

VNAV ALT

10,000'

Altitude Intervention switch

[Option - With VNAV ALT]

Whenever the airplane levels off at an MCP altitude that is not in the FMC, VNAV

ALT engages. In the illustration above, FMC cruise altitude is FL250 and the

clearance altitude, FL190, is set in the MCP. Pitch maintains altitude and thrust

maintains FMC target speed. In the illustration above, the speed after the

temporary level off would be ECON CLB SPEED.

[Option - With VNAV ALT]

To resume the climb, put the clearance altitude into the MCP altitude window and

push the altitude intervention switch. VNAV SPD engages. Pitch maintains FMC

speed and thrust increases to the climb limit. In the example, the airplane climbs

to FMC CRZ ALT and then levels off in cruise.

The altitude intervention switch may be used during climb or descent to delete

altitude restrictions between the current altitude and the MCP altitude. When level

at a restriction altitude, and cleared to a higher altitude prior to crossing the

restriction waypoint, reset the MCP altitude to the new clearance altitude and push

the altitude intervention switch.

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Boeing 737 Operations Manual

MCP altitude reset and

BCD

Altitude Intervention

T/C

switch pushed.

FL250

FL190

10,000'

CROSS

BCD

Altitude Restriction

AT FL190'

In the illustration above, the current altitude restriction is deleted and the airplane

continues VNAV climb to the cruise altitude. T/C moves to match the new climb

profile.

The altitude intervention switch may be used to increase cruise altitude. When

level at a cruise altitude, and then cleared to a higher cruise altitude, reset the MCP

altitude to the new cruise altitude and push the altitude intervention switch.

MCP altitude reset and

T/C

FL290

Altitude Intervention

switch pushed.

FL250

In the illustration above, the cruise altitude is increased and the airplane enters a

VNAV cruise climb at the economy cruise speed.

Altitude intervention cannot be used to decrease cruise altitude. Setting a lower

altitude then pushing the altitude intervention switch causes the FMC to enter an

early descent in the selected descent mode.

Cruise

At cruise altitude, the FMC sets cruise speed at the default or pilot entered speed

until reaching the top-of-descent (T/D) point. Alternate cruise speed options are:

• long range (LRC)

• flight crew entered speed.

October 15, 2001

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Boeing 737 Operations Manual

Cruise thrust is set as required to maintain level flight at the target speed, with the

autothrottle engaged. The FMC uses maximum range cruise speed if cost index is

set to zero.

Fuel and ETA predictions are based on a constant altitude cruise unless a step

climb altitude is entered.

Step Climb

If a step climb altitude is entered in the CRZ page STEP altitude, the FMC

calculates the point where the step climb should begin.

The distance and ETA to the next step point are shown on the CRZ and

PROGRESS pages. The next step point is shown on the map as a green open circle

with the label S/C.

MCP Speed Intervention

[Option]

VNAV PTH

CRZ

VNAV PTH

Altitude

VNAV PTH

Restriction

Altitude

Restriction

Speed Intervention Pitch Modes

The above illustration shows VNAV mode for each phase of flight during speed

intervention.

With VNAV engaged, pushing the speed intervention switch enables speed

intervention. Speed intervention allows the flight crew to change airplane speed

with the IAS/MACH selector.

In a path descent, VNAV PTH changes to VNAV SPD during speed intervention.

In all other phases, the pitch mode remains the same. In VNAV PTH mode, thrust

controls speed; in VNAV SPD mode, pitch controls speed.

In approach phase during speed intervention, the pitch mode remains in VNAV

PTH after speed intervention is exited. The FMC shall remain in the current

vertical mode regardless of IAS MACH selector changes.

[Option - FMC U10.4 and later]

When speed intervention is exited the descent mode will switch back to path

mode.

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Boeing 737 Operations Manual

Descent

VNAV can perform a descent in either of two modes - path descent or speed

descent. During a path descent, the FMC uses idle thrust and pitch control to

maintain a vertical path, similar to a glideslope in three dimensions. During a

speed descent, the FMC uses idle thrust and pitch control to maintain a target

descent speed, similar to a level change descent.

Top Of Descent (T/D)

The point where the cruise phase changes to the descent phase is the top of

descent. The T/D point is shown on the map as a green open circle with the label

T/D. T/D is calculated from an end of descent (E/D) point.

Intermediate T/D points show on the map as green open circles with the label

T/D-XXXXX (altitude). Intermediate T/D points exist when path segments

between altitude restricted waypoints produce a level path segment. The

intermediate T/D point shows where the descent will resume.

End of Descent (E/D)

The FMC calculates a descent path based on airspeed restrictions, altitude

restrictions and the end of descent (E/D) point. The E/D point is shown on the map

as a green open circle with the label E/D. The E/D is the last of the following

which is not preceded by a lateral discontinuity:

• the runway threshold for approaches with a runway waypoint on the RTE

LEGS page, or

• the missed approach point for approaches not showing a runway waypoint

on the RTE LEGS page, or

• the lowest “at” altitude restriction if no arrival procedure is entered.

Entering an instrument arrival procedure provides an E/D point.

If there is no E/D point, FMC predictions assume a computed profile to 1000 feet

above the destination field elevation, at a position which will vary according to

selection of arrival procedures. The FMC will provide a slowdown profile for

approach. VNAV path descent is not available if there is no E/D point.

VNAV Descent and Approach Path

The descent path starts at the calculated top of descent (T/D) point and includes

waypoint altitude restrictions. The path is based on:

• idle thrust

• descent wind speed decreasing

• speedbrakes retracted

with decreasing altitude

• applicable target speed.

After the first “at” or “at or below” restriction, the path angle is level until

intercepting the idle thrust descent path to the next altitude constrained waypoint.

October 15, 2001

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Boeing 737 Operations Manual

[Option - FMC U10.2 and later]

Note: When passing top of descent following high speed cruise operation (within

approximately 6 knots of Vmo/Mmo, cost index of 100 or higher), VNAV

may revert to LVL CHG to prevent overspeed. Reduce airspeed to the

VNAV target descent speed prior to reengaging VNAV.

[Option - With geometric descent path]

After the first “at” or “at or below” restriction, the path angle is constant between

waypoints.

Normally, the target speed is economy speed above the airspeed restriction

altitude and 240 knots below that altitude, until deceleration for approach. VNAV

will not permit descent below the airspeed restriction altitude until the airspeed is

at or below the restricted value plus ten knots. The start and end of the airport

speed restriction deceleration segment is shown on the map as a green open circles

with no labels.

The descent path assumes deceleration to reach the final approach fix (FAF), or

the glideslope intercept point at VREF 40+20 knots.

Target speeds are changed by entries on the LEGS or DESCENT pages. Wind and

thrust assumptions are changed on the DES FORECASTS page.

Deceleration points show on the map as green open circles with the label DECEL.

Deceleration points show prior to:

• airspeed constrained waypoints

• holding patterns

• approach flap extension.

If more than one deceleration segment exists in the flight plan, only the next

deceleration point shows. Deceleration points can also show prior to cruise

holding patterns or other speed reductions.

[Option - With VNAV ALT]

During descent, VNAV ALT engages if the airplane levels at an MCP altitude not

in the FMC.

VNAV Path Descent

An E/D point must be defined in order to accomplish a path descent. It may be

defined manually or by the selection of an arrival procedure.

The FMC defaults to the path descent mode for planning purposes. If the

necessary information for a path descent is not available by the time the airplane

reaches the T/D point, the FMC will revert to the speed descent mode.

The path descent normally begins automatically at the calculated T/D point,

provided the MCP altitude is reset for the descent. If descent is not initiated by the

T/D, a path descent may not be available. At the T/D, the FMC commands idle

thrust and pitch to follow the descent path.

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The descent complies with waypoint altitude restrictions by following the

calculated vertical path.

Note: A path descent uses the target speed for planning purposes only. There is

no attempt to maintain the target speed.

A path descent will automatically revert to a speed descent, or VNAV will

disengage, if all required parameters are not maintained during descent.

Note: When descending in VNAV PTH, the FCC will disengage VNAV and

switch to LVL CHG if actual speed becomes equal to or slightly less than

the minimum speed, denoted by the underspeed limiting symbol in the

MCP IAS/Mach window. This can also happen in turbulence or gusty

conditions when the minimum speed may momentarily increase due to G

loading. See section 4.20, Minimum Speed Reversion.

The FMC uses a special program called “Energy Compensation” at certain times

during an ACT PATH DES. This program goes into effect when the MCP has been

temporarily set to an altitude above the planned descent path. The airspeed cursor

will slowly move toward a slower airspeed while the “TARGET” speed on the

FMC remains constant. The airspeed reduction improves the capability of

recapturing the planned descent path. When the airplane is cleared to resume the

descent, the airspeed will slowly build up to the FMC target speed as the airplane

recaptures the planned descent path.

The CDU message DRAG REQUIRED is displayed if an unexpected tailwind

results in a significant increase in airspeed to maintain path. The CDU message

DES PATH UNACHIEVEABLE is displayed if the FMC determines that the

planned descent profile cannot be accomplished. VNAV disengages if a limit

speed will be exceeded.

[Option - FMC U10.3]

A path descent must be initiated while within the allowable cross-track error for

LNAV, however LNAV may be disengaged during descent while remaining in the

path mode. To maintain a path descent under these conditions, the airplane must

remain within a distance equal to twice the RNP from the LNAV course. If this

distance is exceeded, VNAV will change to speed descent if no vertical angle is

specified for the current leg. VNAV will disengage if there is a vertical angle

specified and LNAV is not engaged.

[Option - FMC U10.4 and later]

A path descent must be initiated while within the allowable cross-track error for

LNAV, however LNAV may be disengaged during descent while remaining in the

path mode. To maintain a path descent under these conditions, the airplane must

remain within a distance equal to twice the RNP from the LNAV course. VNAV

will remain in path regardless of cross-track.

11.31.25

Boeing 737 - 600/700/800/900. Operations Manual (1997 year) - page 58