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

Automatic Flight -

System Description

Boeing 737 Operations Manual

CWS R with a CMD engage switch illuminated can be used to capture a selected

radio course while the VOR/LOC or APP mode is armed. Upon intercepting the

radial or localizer, the F/D and A/P annunciations change from CWS R to

VOR/LOC engaged, and the A/P tracks the selected course.

Autothrottle System

The A/T system provides automatic thrust control from the start of takeoff through

climb, cruise, descent, approach and go-around or landing. In normal operation,

the FMC provides the A/T system with N1 limit values.

The A/T moves the thrust levers with a separate servo motor on each thrust lever.

Manually positioning the thrust levers does not cause A/T disengagement unless

10 degrees of thrust lever separation is exceeded during a dual channel approach

after FLARE armed is annunciated. Following manual positioning, the A/T may

reposition the thrust levers to comply with computed thrust requirements except

while in the THR HLD and ARM modes.

The A/T system operates properly with the EECs ON or in ALTN. In either case,

the A/T uses the FMC N1 limits. During A/T operation, it is recommended that

both EECs be ON or both be in ALTN, as this produces minimum thrust lever

separation.

A/T Engagement

Moving the A/T Arm switch to ARM, arms the A/T for engagement in the N1,

MCP SPD or FMC SPD mode. The A/T Arm switch is magnetically held at ARM

and releases to OFF when the A/T becomes disengaged.

A general summary of A/T mode engagement is as follows:

• A/T SPD or N1 modes automatically engage when AFDS command pitch

modes become engaged

• engaging LVL CHG or VNAV climb modes automatically engages the

A/T N1 mode

• engaging LVL CHG or VNAV descent modes automatically engages the

A/T in RETARD and then ARM when thrust is at idle

• if not in a VNAV mode, engagement of ALT ACQ or ALT HOLD

automatically engages the A/T in the MCP SPD mode; otherwise the A/T

remains in FMC SPD

• engagement of G/S capture automatically engages the A/T in the MCP

SPD mode.

Autothrottle Disengagement

Any of the following conditions or actions disengages the A/T:

• moving the A/T Arm switch to OFF

• pushing either A/T Disengage switch

4.20.8

Automatic Flight -

System Description

Boeing 737 Operations Manual

• an A/T system fault is detected

• two seconds have elapsed since landing touchdown

• thrust levers become separated more than 10 degrees during a dual

channel approach after FLARE armed is annunciated

• significant thrust difference along with control wheel roll input of 10

degrees or more, and flap position up through 10.

A/T disengagement is followed by A/T Arm switch releasing to OFF and flashing

red A/T Disengage lights. The A/T Disengage lights do not illuminate when the

A/T automatically disengages after landing touchdown.

Automatic Flight Operations

The phases of flight for automatic flight operations are:

• Takeoff and climb

• Approach and landing

• Enroute

• Go-around

Automatic Flight Takeoff and Climb

Takeoff is a flight director only function of the TO/GA mode. Flight director pitch

and roll commands are displayed and the autothrottle maintains takeoff N1 thrust

limit as selected from the FMC. The autopilot may be engaged after takeoff.

[Option - Flight director commands wings level on takeoff]

Both F/Ds must be ON to engage the takeoff mode prior to starting the takeoff.

The F/D takeoff mode is engaged by pushing the TO/GA switch on either thrust

lever. The FMAs display FD as the A/P status, TO/GA as the pitch mode, and

blank for the roll mode.

[Option - Flight director commands HDG SEL on takeoff]

Both F/Ds must be ON to engage the takeoff mode prior to starting the takeoff.

The F/D takeoff mode is engaged by pushing the TO/GA switch on either thrust

lever. The FMAs display FD as the A/P status, TO/GA as the pitch mode, and

HDG SEL as the roll mode.

During takeoff, pushing a TO/GA switch engages the autothrottle in the N1 mode.

The A/T annunciation changes from ARM to N1 and thrust levers advance toward

takeoff thrust.

The F/D can also be engaged in the takeoff mode with the F/D switches off. If a

TO/GA switch is pushed after 80 knots below 2000 feet AGL and prior to 150

seconds after lift-off, the F/D command bars automatically appear for both pilots.

[Option - Flight director commands wings level on takeoff]

During takeoff, prior to 60 KIAS:

• the pitch command is 10 degrees nose down

• the roll command is wings level

• the autothrottle is engaged in the N1 mode

4.20.9

Automatic Flight -

System Description

Boeing 737 Operations Manual

• thrust levers advance until the engines reach takeoff thrust

• the FMAs display N1 for the autothrottle mode, TO/GA for the pitch

mode, and blank for the roll mode.

[Option - Flight director commands HDG SEL on takeoff]

During takeoff, prior to 60 KIAS:

• the pitch command is 10 degrees nose down

• the roll command is HDG SEL

• the autothrottle is engaged in the N1 mode

• thrust levers advance until the engines reach takeoff thrust

• the FMAs display N1 for the autothrottle mode, TO/GA for the pitch

mode, and HDG SEL for the roll mode.

At 60 knots, the F/D pitch commands 15 degrees nose up.

At 84 knots, the A/T mode annunciates THR HLD.

At lift-off:

• the pitch command continues at 15 degrees until sufficient climb rate is

acquired. Pitch then commands MCP speed (normally V2) plus 20 knots

• if an engine failure occurs during takeoff, the pitch command target speed

is:

• V2, if airspeed is below V2

• existing speed, if airspeed is between V2 and V2 + 20

• V2 + 20, if airspeed is above V2 + 20

[Option - Flight director commands wings level on takeoff]

• the roll command maintains wings level.

[Option - Flight director commands HDG SEL on takeoff]

[Option - Honeywell -904/-905 FCC]

• the roll command maintains HDG SEL. Bank angle is limited to 8 degrees

until 200 feet AGL.

After lift-off:

• the A/T remains in THR HLD until 800 feet RA. A/T annunciation then

changes from THR HLD to ARM and reduction to climb thrust can be

made by pushing the N1 switch

[Option - Without automatic thrust reduction after takeoff]

• automatic thrust reduction to climb power occurs when VNAV, ALT ACQ

or ALT HOLD is engaged. Until 2 1/2 minutes after liftoff, automatic

thrust reduction is inhibited when engaging LVL CHG or V/S modes

4.20.10

Automatic Flight -

System Description

Boeing 737 Operations Manual

[Option - Automatic thrust reduction after takeoff, FMC update 10.1 and later]

• automatic reduction to climb thrust occurs upon reaching the selected

thrust reduction altitude which is shown on the FMC CDU TAKEOFF

REF page 2/2 during preflight, or when the airplane levels off in ALT

HOLD or VNAV PTH. Pilot entries can be made to override the default

value. Allowable entries are 800 feet to 9999 feet

• flight director engaged status is terminated by engaging an autopilot in

CMD (CMD replaces FD in A/P status display)

• pitch engages in LVL CHG and pitch mode FMA is MCP SPD unless

another pitch mode has been selected

• MCP IAS/Mach display and airspeed cursor change to V2 + 20 knots

• roll mode engages in HDG SEL unless another roll mode has been

selected.

To terminate the takeoff mode below 400 feet RA, both F/D switches must be

turned OFF. Above 400 feet RA, selection of another pitch mode or engaging an

autopilot will terminate the takeoff mode; other F/D roll modes can be also

selected.

4.20.11

Automatic Flight -

System Description

Boeing 737 Operations Manual

Automatic Flight Takeoff Profile

[Option - EFIS/MAP, flight director commands wings level on takeoff]

Engage A/P

• A/P engages in

MCP SPD

LNAV

CMD

• Pitch mode

engages in

MCP SPD

TO/GA

LNAV

Thrust Reduction

• A/T N1 mode

engages at

CLB power

ARM

TO/GA

LNAV

800 ft RA -

• A/T ARM

mode engages

Above 400 ft RA

THR HLD

TO/GA

LNAV

• Select roll

mode

84 knots -

THR HLD

TO/GA

• A/T THR

HLD mode

engages

Push TO/GA

Switch

TO/GA

(both FD ON and

A/T armed)

4.20.12

Automatic Flight -

System Description

Boeing 737 Operations Manual

[Option - PFD/ND, flight director commands HDG SEL on takeoff]

Engage A/P

HDG SEL

MCP SPD

• A/P engages in

CMD

• Pitch mode

engages in

MCP SPD

HDG SEL

TO/GA

Thrust Reduction

• A/T N1 mode

engages at

CLB power

ARM

HDG SEL

TO/GA

800 ft RA -

• A/T ARM

mode engages

84 knots -

THR HLD

HDG SEL

TO/GA

• A/T THR

HLD mode

engages

Push TO/GA

HDG SEL

TO/GA

Switch

(both FD ON and

A/T armed)

Automatic Flight En Route

The autopilot and/or the flight director can be used after takeoff to fly a lateral

navigation track (LNAV) and a vertical navigation track (VNAV) provided by the

FMC.

Other roll modes available are:

• VOR course (VOR/LOC)

• heading select (HDG SEL).

Other pitch modes available are:

• altitude hold (ALT HOLD)

• level change (MCP SPD)

• vertical speed (V/S).

4.20.13

Automatic Flight -

System Description

Boeing 737 Operations Manual

Automatic Flight Approach and Landing

The AFDS provides guidance for single A/P non-precision approaches. The

VOR/LOC switch arms the AFDS for VOR or localizer tracking. Descent may be

accomplished using VNAV, LVL CHG, or V/S. VOR/LOC, LNAV, or HDG SEL

may be used for the roll mode.

The AFDS provides guidance for single or dual A/P precision approaches. The

approach mode arms the AFDS to capture and track the localizer and glideslope.

Approach (APP) Mode Dual A/Ps

Approach mode allows both A/Ps to be engaged at the same time. Dual A/P

operation provides fail passive operation through landing flare and touchdown or

an automatic go-around. During fail passive operation, the flight controls respond

to the A/P commanding the lesser control movement. If a failure occurs in one

A/P, the failed channel is counteracted by the second channel such that both A/Ps

disconnect with minimal airplane maneuvering and with aural and visual

warnings to the pilot.

One VHF NAV receiver must be tuned to an ILS frequency before the approach

mode can be selected. For a dual A/P approach, the second VHF NAV receiver

must be tuned to the ILS frequency and the corresponding A/P engaged in CMD

prior to 800 feet RA.

Localizer and Glideslope Armed

After setting the localizer frequency and course, pushing the APP switch selects

the APP mode. The APP switch illuminates and VOR/LOC and G/S annunciate

armed. The APP mode permits selecting the second A/P to engage in CMD. This

arms the second A/P for automatic engagement after LOC and G/S capture and

when descent below 1500 RA occurs.

The localizer can be intercepted in the HDG SEL, CWS R or LNAV mode.

[Option - G/S capture inhibited before LOC capture]

Glideslope (G/S) capture is inhibited prior to localizer capture.

Localizer Capture

[Option - EFIS/MAP]

The LOC capture point is variable and depends on intercept angle and rate of

closure, but does not occur at less than 1/2 dot. Upon LOC capture, VOR/LOC

annunciates captured, 1 CH is annunciated for A/P status, the previous roll mode

disengages and the airplane turns to track the LOC.

4.20.14

Automatic Flight -

System Description

Boeing 737 Operations Manual

[Option - PFD/ND]

The LOC capture point is variable and depends on intercept angle and rate of

closure, but does not occur at less than 1/2 dot. Upon LOC capture, VOR/LOC

annunciates captured, SINGLE CH is annunciated for A/P status, the previous roll

mode disengages and the airplane turns to track the LOC.

Glideslope Capture

[Option - G/S capture inhibited before LOC capture]

Glideslope capture is inhibited prior to localizer capture.

The G/S can be captured from above or below. Capture occurs at 2/5 dot and

results in the following:

• G/S annunciates captured

• previous pitch mode disengages

• APP light extinguishes if localizer has also been captured

• airplane pitch tracks the G/S

• GA displayed on thrust mode display (N1 thrust limit).

After VOR/LOC and G/S are both captured, the APP mode can be exited by:

• pushing a TO/GA switch

• disengaging A/P and turning off both F/D switches

• retuning a VHF NAV receiver.

After LOC and G/S Capture

Shortly after capturing LOC or G/S and below 1500 feet RA:

• the second A/P couples with the flight controls

• test of the ILS deviation monitor system is performed and the G/S or LOC

display turns amber and flashes

• FLARE armed is annunciated

[Option - EFIS/MAP]

• the 1 CH annunciation extinguishes

[Option - PFD/ND]

• the SINGLE CH annunciation extinguishes

• A/P go-around mode arms but is not annunciated.

[Option - CWS deactivated on approach]

Note: After localizer and glideslope capture during a dual autopilot approach,

CWS cannot be engaged by manually overriding pitch and roll. Manual

override of autopilots causes autopilot disengagement.

The A/Ps disengage and the F/D command bars retract to indicate an invalid ILS

signal.

4.20.15

Automatic Flight -

System Description

Boeing 737 Operations Manual

800 Feet Radio Altitude

The second A/P must be engaged in CMD by 800 feet RA to execute a dual

channel A/P approach. Otherwise, CMD engagement of the second A/P is

inhibited.

400 Feet Radio Altitude

The stabilizer is automatically trimmed an additional amount nose up. If the A/Ps

subsequently disengage, forward control column force may be required to hold the

desired pitch attitude.

If FLARE is not armed by approximately 350 feet RA, both A/Ps automatically

disengage.

Flare

The A/P flare maneuver starts at approximately 50 feet RA and is completed at

touchdown:

• FLARE engaged is annunciated and F/D command bars retract

• the stabilizer is automatically trimmed an additional amount nose up at 50

feet RA.

• the A/T begins retarding thrust at approximately 27 feet RA so as to reach

idle at touchdown. A/T FMA annunciates RETARD.

• the A/T automatically disengages approximately 2 seconds after

touchdown.

• the A/P must be manually disengaged after touchdown. Landing rollout is

executed manually after disengaging the A/P.

Approach (APP) Mode Single A/P

A single A/P ILS approach can be executed by engaging only one A/P in CMD

after pushing the APP mode select switch. Single A/P approach operation is the

same as dual, with the following exceptions:

• full automatic flare and touchdown capability is not available. FLARE is

not annunciated and stabilizer trim bias is not applied

[Option - EFIS/MAP]

• A/P status of 1 CH is annunciated for the entire approach after localizer

capture

[Option - PFD/ND]

• A/P status of SINGLE CH is annunciated for the entire approach after

localizer capture

• an A/P go-around is not available.

4.20.16

Automatic Flight -

System Description

Boeing 737 Operations Manual

Automatic Flight Approach Profile

[Option - EFIS/MAP]

MCP SPD

ALT HLD

HDG SEL

CMD

G/S

VOR/LOC

Select APP

Second A/P CMD

LOC capture

MCP SPD

ALT HLD

VOR/LOC

CMD

Set heading

G/S

1 CH

G/S Capture

• GA displayed

on Thrust

Mode Display

MCP SPD

G/S

VOR/LOC

CMD

1 CH

MCP SPD

G/S

VOR/LOC

CMD

FLARE

Below 1500 ft

MCP SPD

FLARE

VOR/LOC

CMD

• Go-around

mode armed

but not

RETARD

FLARE

VOR/LOC

CMD

annunciated

50 ft RA

• FLARE

maneuver begins

• F/D bars retract

from view

27 ft RA

Touchdown

• A/T retards to

• Manually disconnect A/P

idle

• A/T disconnects 2 seconds after touchdown

4.20.17

Automatic Flight -

System Description

Boeing 737 Operations Manual

[Option - PFD/ND]

MCP SPD

HDG SEL ALT HLD

VOR/LOC

G/S

CMD

Select APP

Second A/P CMD

MCP SPD

VOR/LOC

ALT HLD

LOC capture

G/S

Set heading

SINGLE

G/S Capture

• GA displayed

MCP SPD

VOR/LOC

G/S

on Thrust

Mode Display

SINGLE

MCP SPD

VOR/LOC

G/S

FLARE

CMD

MCP SPD

VOR/LOC

FLARE

Below 1500 ft

CMD

• Go-around

mode armed

but not

RETARD

VOR/LOC

FLARE

annunciated

CMD

50 ft RA

• FLARE

maneuver begins

• F/D bars retract

from view

27 ft RA

Touchdown

• A/T retards to

• Manually disconnect A/P

idle

• A/T disconnects 2 seconds after touchdown

4.20.18

Automatic Flight -

System Description

Boeing 737 Operations Manual

Go-Around

Go-Around (GA) mode is engaged by pushing either TO/GA switch. An A/P

go-around requires dual A/P operation and is armed when FLARE armed is

annunciated. If both A/Ps are not operating, a manual F/D go-around is available.

With the A/T Arm switch at ARM, the A/T go-around mode is armed:

• when descending below 2000 feet RA

[Option - A/P auto disengages for TO/GA above 2000 feet RA]

[Option - Honeywell -708 FCC]

• when above 2000 feet RA with flaps down or G/S captured

• with or without the AFDS engaged.

A/P Go-Around

The A/P GA mode requires dual A/P operation and is available after FLARE

armed is annunciated and prior to the A/P sensing touchdown.

With the first push of either TO/GA switch:

• A/T (if armed) engages in GA and the A/T Engaged Mode annunciation

on the FMA indicates GA

• thrust advances toward the reduced go-around N1 to produce 1000 to

2000 fpm rate of climb

• pitch mode engages in TO/GA and the Pitch Engaged Mode annunciation

on the FMA indicates TO/GA

• F/D pitch commands 15 degrees nose up until reaching programmed rate

of climb. F/D pitch then commands target airspeed for each flap setting

based on maximum takeoff weight calculations

• F/D roll commands hold current ground track. The Roll Engaged Mode

annunciation on the FMA is blank

• the IAS/Mach display blanks

• the command airspeed cursor automatically moves to a target airspeed for

the existing flap position based on maximum takeoff weight calculations.

Note: If the go-around mode is selected after touchdown and prior to A/T

disengagement, the A/Ps disengage and the A/Ts may command GA thrust.

With the second push of either TO/GA switch after A/T reaches reduced

go-around thrust:

• the A/T advances to the full go-around N1 limit.

TO/GA mode termination from A/P go-around:

• below 400 feet RA, the AFDS remains in the go-around mode unless both

A/Ps and F/Ds are disengaged

• above 400 feet RA, select a different pitch or roll mode.

• if the roll mode is changed first:

4.20.19

Automatic Flight -

System Description

Boeing 737 Operations Manual

• the selected mode engages in single A/P roll operation and is

controlled by the A/P which was first in CMD

• pitch remains in dual A/P control in TO/GA mode.

• if the pitch mode is changed first:

• the selected mode engages in single A/P pitch operation and is

controlled by the A/P which was first in CMD

• the second A/P disengages

• the roll mode engages in CWS R.

• the A/T GA mode is terminated when:

• another pitch mode is selected

• ALT ACQ annunciates engaged.

Note: The pitch mode cannot be changed from TO/GA until sufficient

nose-down trim has been input to allow single channel A/P operation.

This nose-down trim is automatically added by the A/P to reset the trim

input made by the A/P at 400 feet RA and at 50 feet RA during the

approach.

With pitch mode engaged in TO/GA, ALT ACQ engages when approaching the

selected altitude and ALT HOLD engages at the selected altitude if the stabilizer

position is satisfactory for single A/P operation.

• if stabilizer trim position is not satisfactory for single A/P operation:

• ALT ACQ is inhibited

• A/P disengage lights illuminate steady red

• pitch remains in TO/GA.

Note: To extinguish A/P disengage lights, disengage A/Ps or select higher

altitude on MCP.

F/D Go-Around

If both A/Ps are not engaged, a manual F/D only go-around is available under the

following conditions:

• inflight below 2000 feet RA

[Option - A/P auto disengages for TO/GA above 2000 feet RA]

[Option - Honeywell -708 FCC]

• inflight above 2000 feet RA with flaps down or G/S captured

• not in takeoff mode.

With the first push of either TO/GA switch:

• A/T (if armed) engages in GA and advances thrust toward the reduced

go-around N1 to produce 1000 to 2000 fpm rate of climb. The A/T

Engaged Mode annunciation on the FMA indicates GA

• autopilot (if engaged) disengages

• pitch mode engages in TO/GA and the Pitch Engaged Mode annunciation

on the FMA indicates TO/GA

4.20.20

Automatic Flight -

System Description

Boeing 737 Operations Manual

• F/D pitch commands 15 degrees nose up until reaching programmed rate

of climb. F/D pitch then commands target airspeed for each flap setting

based on maximum takeoff weight calculations

• F/D roll commands approach ground track at time of engagement. The

Roll Engaged Mode annunciation on the FMA is blank

• the IAS/Mach display blanks

• the command airspeed cursor automatically moves to a target airspeed for

the existing flap position based on maximum takeoff weight calculations.

With the second push of either TO/GA switch (if A/T engaged and after A/T

reaches reduced go-around thrust):

• the A/T advances to the full go-around N1 limit

TO/GA mode termination from F/D go-around:

• below 400 feet RA, both F/D switches must be turned off.

• above 400 feet RA, select a different pitch or roll mode.

• if the roll mode is changed first:

• F/D roll engages in the selected mode

• the F/D pitch mode remains in TO/GA.

• if the pitch mode is changed first:

• the F/D roll mode automatically changes to HDG SEL

• F/D pitch engages in the selected mode.

• the A/T GA mode (if engaged) is terminated when:

• another pitch mode is selected

• ALT ACQ annunciates engaged.

Note: Engaging an A/P in CMD automatically engages the A/P and F/Ds in

LVL CHG for pitch and HDG SEL for roll.

Single Engine F/D Go-Around

With a push of either TO/GA switch:

• F/D roll commands hold current ground track. The Roll Engaged Mode

annunciation on the FMA is blank

• pitch mode engages in TO/GA and the Pitch Engaged Mode annunciation

on the FMA indicates TO/GA

• the F/D target speed is displayed on IAS/Mach display

• the F/D target speed is displayed on the airspeed cursor

• F/D pitch commands 13 degrees nose up. As climb rate increases, F/D

pitch commands maintain a target speed.

• if engine failure occurs prior to go-around engagement, then F/D

target speed is the selected MCP speed.

• if engine failure occurs after go-around engagement, then F/D target

speed depends on whether ten seconds have elapsed since go-around

engagement:

4.20.21

Automatic Flight -

System Description

Boeing 737 Operations Manual

• if prior to ten seconds, the MCP selected approach speed becomes

target speed

• if after ten seconds and the airspeed at engine failure is within five

knots of the go-around engagement speed, the airspeed that

existed at go-around engagement becomes target speed

• if after ten seconds and the airspeed at engine failure is more than

five knots above go-around engagement speed, then the current

airspeed becomes target speed.

Note: The target speed is never less than V2 speed based on flap position

unless in windshear conditions.

F/D commanded acceleration cannot occur until a higher speed is selected on the

MCP IAS/Mach display.

4.20.22

Automatic Flight -

System Description

Boeing 737 Operations Manual

Automatic Flight Go-Around Profile

[Option - EFIS/MAP]

Missed Approach

altitude

MCP SPD

ALT HLD

LNAV

CMD

• Pitch mode

engages in

ALT HLD

Approaching

Missed Approach

altitude -

MCP SPD

ALT ACQ

LNAV

CMD

• Pitch mode

engages in

ALT ACQ

• A/T engages

in MCP SPD

Flap retract

TO/GA

LNAV

CMD

altitude

• Retract flaps

on schedule

Above 400 feet

TO/GA

LNAV

CMD

• Select roll

mode

Push TO/GA

switches

TO/GA

CMD

• A/T engages

in G/A

4.20.23

Automatic Flight -

System Description

Boeing 737 Operations Manual

[Option - PFD/ND]

Missed Approach

altitude

MCP SPD

LNAV

ALT HLD

• Pitch mode

engages in

CMD

ALT HLD

Approaching

Missed Approach

MCP SPD

LNAV

ALT ACQ

altitude -

• Pitch mode

CMD

engages in

ALT ACQ

• A/T engages

in MCP SPD

LNAV

TO/GA

Flap retract

CMD

altitude

• Retract flaps

on schedule

LNAV

TO/GA

Above 400 feet

CMD

• Select roll

mode

Push TO/GA

TO/GA

switches

CMD

• A/T engages

in G/A

AFS Operation in Windshear

General

The autopilot and flight director provide positive corrective action to counteract

most windshears. The autothrottle system also aids in windshear recovery by

providing quick response to any increase or decrease in speed. The commanded

levels of power may be beyond what the average pilot considers necessary but, in

fact, are required by the situation.

4.20.24

Automatic Flight -

System Description

Boeing 737 Operations Manual

Takeoff or Go-Around

If windshear is encountered during F/D takeoff or go-around, the F/D pitch

command bar provides commands to maintain V2 + 20 kts until vertical speed

decreases to approximately +600 fpm. At this point, the F/D pitch bar commands

a 15 degree nose-up pitch attitude. If vertical speed continues to decrease, the F/D

continues to command a 15 degree pitch attitude until a speed of approximately

stick shaker is reached. It then commands pitch attitudes which result in

intermittent activation of the stick shaker. As the airplane transits the windshear

condition, the F/D programming reverses. As climb rate increases above

approximately +600 fpm, the F/D commands pitch attitudes which result in

acceleration back to V2 + 20 kts. The A/P and F/D both operate in a similar

manner during A/P or F/D go-around.

Approach and Landing

If windshear is encountered during an ILS approach, both the F/D and A/P attempt

to hold the airplane on altitude, or on glideslope after glideslope capture, without

regard to angle of attack or stick shaker limitations. Airspeed could decrease

below stick shaker and into a stall if the pilot does not intervene by pushing the

TO/GA switch or disconnecting the A/P and flying manually.

WARNING: Although the F/D, A/P and A/T may be performing as

previously described, severe windshear may exceed the

performance capability of the system and/or the airplane. In

this situation, the flight crew must, if necessary to avoid

ground contact, be prepared to disconnect the autothrottle,

advance thrust levers to the forward stop, disconnect the

autopilot and manually fly the airplane.

Command Speed Limiting and Reversion Modes

AFS command limiting and reversion operation is independent of the stall

warning and mach warning systems.

Command Speed Limiting

The AFS provides speed, pitch and thrust commands to avoid exceeding the

following limit speeds:

• Vmo/Mmo

• wing flap placards

• landing gear placard

• minimum speed.

The commanded speed can be equal to, but does not exceed a limit speed.

4.20.25

Automatic Flight -

System Description

Boeing 737 Operations Manual

Speeds greater than Vmo/Mmo cannot be selected from the MCP. Speeds can be

selected which exceed flap and gear placards or are less than minimum speed.

Minimum speed is based on angle of attack and is approximately 1.3 Vs for the

current flap configuration. It is sensed by the angle of attack vanes, one on either

side of the forward fuselage.

If a speed greater than a placard speed, or less than minimum speed is selected, the

AFS allows acceleration or deceleration to slightly short of the limit, then

commands the limit speed. The overspeed or underspeed limiting symbol appears

in the MCP IAS/Mach display when the commanded speed cannot be reached.

Either pitch or thrust, whichever is engaged in a speed mode, attempts to hold the

limit speed. The commanded limit speed and MCP speed condition symbol,

remain until another speed is selected which does not exceed the limit. A speed 15

knots greater than the minimum speed must be selected to remove the underspeed

limiting symbol.

Reversion Modes

During some flight situations, speed control by the AFDS or A/T alone could be

insufficient to prevent exceeding a limit speed. If this occurs, AFDS or A/T modes

automatically revert to a more effective combination. The reversion modes are:

• placard limit reversion

• minimum airspeed reversion.

Mode reversion occurs slightly before reaching the limit speed. Both the AFDS

and A/T have reversion modes which activate according to the condition causing

the reversion.

Placard Limit Reversion

When one of the placard limit reversions (gear, flap or Vmo/Mmo) is reached, the

overspeed limiting symbol appears in the MCP IAS/Mach display and the

following occurs:

• if not in AFDS or A/T speed control and the A/T is armed, the A/T reverts

to SPEED and controls speed to the placard limit

• if in AFDS or A/T speed control, no reversion is necessary. The AFDS or

A/T, whichever is controlling speed, holds speed slightly below the

placard limit

• if the A/T is not available, no reversion response to gear or flap placard

speeds is available. The AFDS reverts to speed control for Vmo/Mmo

speed limiting.

4.20.26

Automatic Flight -

System Description

Boeing 737 Operations Manual

Minimum Speed Reversion

The AFDS and A/T do not control to a speed which is less than minimum speed

for the current flap configuration. This speed is approximately 1.3 Vs. Minimum

speed, FMC speed, or selected speed, whichever is higher, becomes the AFS

commanded speed. If actual speed becomes equal to or slightly less than the

minimum speed, the underspeed limiting symbol appears in the MCP IAS/Mach

Display, and if operating in the V/S mode, the AFDS reverts to LVL CHG. The

AFDS will also revert to LVL CHG from VNAV PTH, except when flying a level

segment.

The AFS commands a speed 5 knots greater than minimum speed. Reaching a

speed 5 knots greater than minimum speed reactivates normal MCP speed

selection control. The AFDS commands nose down pitch to increase airspeed if

the thrust levers are not advanced. When actual speed becomes 5 knots greater

than minimum speed, the underspeed limiting symbol disappears.

The A/P disengages and the F/D command bars retract when in a LVL CHG climb

with a command speed equal to minimum speed and a minimum rate of climb

cannot be maintained without decelerating.

Minimum speed reversion is not available when the A/T is OFF and the AFDS is

in ALT HOLD or after G/S capture. Minimum speed reversion is also not

available when in VNAV PTH and flying a level segment.

4.20.27

Boeing 737 - 600/700/800/900. Operations Manual (1997 year) - page 31