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

Flight Controls -

System Description

Boeing 737 Operations Manual

Flight Control Surfaces

Pitch control is provided by:

• two elevators

• a movable horizontal stabilizer.

Roll control is provided by:

• two ailerons

• eight flight spoilers.

Yaw control is provided by a single rudder. During takeoff, the rudder becomes

aerodynamically effective between 40 and 60 knots.

TE flaps and LE flaps and slats provide high lift for takeoff, approach and landing.

[Option: 737-800 with Blended Winglets]

Blended winglets provide enhanced performance, extended range and increased

fuel efficiency.

In the air symmetric flight spoilers are used as speed brakes. On the ground

symmetric flight and ground spoilers destroy lift and increase braking efficiency.

Flight Control Surfaces Location

FLIGHT

SPOILERS

GROUND

LE FLAPS

SPOILERS

RUDDER

LE SLATS

TE FLAPS

STABILIZER

ELEVATORS

AILERONS

9.20.2

Flight Controls -

System Description

Boeing 737 Operations Manual

Roll Control

The roll control surfaces consist of hydraulically powered ailerons and flight

spoilers, which are controlled by rotating either control wheel.

Ailerons

The ailerons provide roll control around the airplane’s longitudinal axis. The

ailerons are positioned by the pilots' control wheels. The A and B FLT CONTROL

switches control hydraulic shutoff valves. These valves can be used to isolate each

aileron, as well as the elevators and rudder, from related hydraulic system

pressure.

The Captain’s control wheel is connected by cables to the aileron power control

units (PCUs) through the aileron feel and centering unit. The First Officer’s

control wheel is connected by cables to the spoiler PCUs through the spoiler

mixer. The two control wheels are connected by a cable drive system which allows

actuation of both ailerons and spoilers by either control wheel. With total

hydraulic power failure the ailerons can be mechanically positioned by rotating

the pilots' control wheels. Control forces are higher due to friction and

aerodynamic loads.

Aileron Transfer Mechanism

If the ailerons or spoilers are jammed, force applied to the Captain’s and the First

Officer’s control wheels will identify which system, ailerons or spoilers, is usable

and which control wheel, Captain’s or First Officer’s, can provide roll control. If

the aileron control system is jammed, force applied to the First Officer’s control

wheel provides roll control from the spoilers. The ailerons and the Captain’s

control wheel are inoperative. If the spoiler system is jammed, force applied to the

Captain’s control wheel provides roll control from the ailerons. The spoilers and

the First Officer’s control wheel are inoperative.

Aileron Trim

Dual AILERON trim switches, located on the aft electronic panel, must be pushed

simultaneously to command trim changes. The trim electrically repositions the

aileron feel and centering unit, which causes the control wheel to rotate and

redefines the aileron neutral position. The amount of aileron trim is indicated on

a scale on the top of each control column.

If aileron trim is used with the autopilot engaged, the trim is not reflected in the

control wheel position. The autopilot overpowers the trim and holds the control

wheel where it is required for heading/track control. Any aileron trim applied

when the autopilot is engaged can result in an out of trim condition and an abrupt

rolling movement when the autopilot is disconnected.

9.20.4

Flight Controls -

System Description

Boeing 737 Operations Manual

Flight Spoilers

Four flight spoilers are located on the upper surface of each wing. Each hydraulic

system, A and B, is dedicated to a different set of spoiler pairs to provide isolation

and maintain symmetric operation in the event of hydraulic system failure.

Hydraulic pressure shutoff valves are controlled by the two flight SPOILER

switches.

Flight spoiler panels are used as speed brakes to increase drag and reduce lift, both

in flight and on the ground. The flight spoilers also supplement roll control in

response to control wheel commands. A spoiler mixer, connected to the aileron

cable-drive, controls the hydraulic power control units on each spoiler panel to

provide spoiler movement proportional to aileron movement.

The flight spoilers rise on the wing with up aileron and remain faired on the wing

with down aileron. When the control wheel is displaced more than approximately

10°, spoiler deflection is initiated.

9.20.5

Flight Controls -

System Description

Boeing 737 Operations Manual

Pitch Control

The pitch control surfaces consist of hydraulically powered elevators and an

electrically powered stabilizer. The elevators are controlled by forward or aft

movement of the control column. The stabilizer is controlled by autopilot trim or

manual trim.

Elevators

The elevators provide pitch control around the airplane’s lateral axis. The

elevators are positioned by the pilots’ control columns. The A and B FLT

CONTROL switches control hydraulic shutoff valves for the elevators.

Cables connect the pilots’ control columns to elevator power control units (PCUs)

which are powered by hydraulic system A and B. The elevators are interconnected

by a torque tube. With loss of hydraulic system A and B the elevators can be

mechanically positioned by forward or aft movement of the pilots’ control

columns. Control forces are higher due to friction and aerodynamic loads.

Elevator Control Column Override Mechanism

In the event of a control column jam, an override mechanism allows the control

columns to be physically separated. Applying force against the jam will breakout

either the Captain’s or First Officer’s control column. Whichever column moves

freely after the breakout can provide adequate elevator control.

Although total available elevator travel is significantly reduced, there is sufficient

elevator travel available for landing flare. Column forces are higher and exceed

those experienced during manual reversion. If the jam exists during the landing

phase, higher forces are required to generate sufficient elevator control to flare for

landing. Stabilizer trim is available to counteract the sustained control column

force.

Elevator Feel System

The elevator feel computer provides simulated aerodynamic forces using airspeed

(from the elevator pitot system) and stabilizer position. Feel is transmitted to the

control columns by the elevator feel and centering unit. To operate the feel system

the elevator feel computer uses either hydraulic system A or B pressure,

whichever is higher. When either hydraulic system or elevator feel pitot system

fails, excessive differential hydraulic pressure is sensed in the elevator feel

computer and the FEEL DIFF PRESS light illuminates.

9.20.7

Flight Controls -

System Description

Boeing 737 Operations Manual

Mach Trim System

A Mach trim system provides speed stability at the higher Mach numbers. Mach

trim is automatically accomplished above Mach .615 by adjusting the elevators

with respect to the stabilizer as speed increases. The flight control computers use

Mach information from the ADIRU to compute a Mach trim actuator position.

The Mach trim actuator repositions the elevator feel and centering unit which

adjusts the control column neutral position.

Stabilizer

The horizontal stabilizer is positioned by a single electric trim motor controlled

through either the stab trim switches on the control wheel or autopilot trim. The

stabilizer may also be positioned by manually rotating the stabilizer trim wheel.

Stabilizer Trim

Stabilizer trim switches on each control wheel actuate the electric trim motor

through the main electric stabilizer trim circuit when the airplane is flown

manually. With the autopilot engaged, stabilizer trim is accomplished through the

autopilot stabilizer trim circuit. The main electric and autopilot stabilizer trim

have two speed modes: high speed with flaps extended and low speed with flaps

retracted. If the autopilot is engaged, actuating either pair of stabilizer trim

switches automatically disengages the autopilot. The stabilizer trim wheels rotate

whenever electric stabilizer trim is actuated.

The STAB TRIM MAIN ELECT cutout switch and the STAB TRIM

AUTOPILOT cutout switch, located on the control stand, are provided to allow

the autopilot or main electric trim inputs to be disconnected from the stabilizer

trim motor.

Control column actuated stabilizer trim cutout switches stop operation of the main

electric and autopilot trim when the control column movement opposes trim

direction. When the STAB TRIM override switch is positioned to OVERRIDE,

electric trim can be used regardless of control column position.

Manual stabilizer control is accomplished through cables which allow the pilot to

position the stabilizer by rotating the stabilizer trim wheels. The stabilizer is held

in position by two independent brake systems. Manual rotation of the trim wheels

can be used to override autopilot or main electric trim. The effort required to

manually rotate the stabilizer trim wheels may be higher under certain flight

conditions. Grasping the stabilizer trim wheel will stop stabilizer motion.

Stabilizer Trim Operation with Forward or Aft CG

In the event the stabilizer is trimmed to the end of the electrical trim limits,

additional trim is available through the use of the manual trim wheels. If manual

trim is used to position the stabilizer beyond the electrical trim limits, the stabilizer

trim switches may be used to return the stabilizer to electrical trim limits.

9.20.8

Flight Controls -

System Description

Boeing 737 Operations Manual

Stabilizer Position Indication and Green Band

Stabilizer position is displayed in units on two STAB TRIM indicators located

inboard of each stabilizer trim wheel. The STAB TRIM indicators also display the

TAKEOFF green band indication.

The trim authority for each mode of trim is limited to:

• Main Electric Trim

[737-600]

• flaps retracted 4.10 to 14.5 units

[737-700]

• flaps retracted 4.30 to 14.5 units

[737-800]

• flaps retracted 3.95 to 14.5 units

[737-900]

• flaps retracted 3.90 to 14.5 units

• flaps extended 0.05 to 14.5 units

• Autopilot Trim 0.05 to 14.5 units

• Manual Trim -0.20 to 16.9 units.

The green band range of the STAB TRIM indicator shows the takeoff trim range.

An intermittent horn sounds if takeoff is attempted with the stabilizer trim outside

the takeoff trim range.

Speed Trim System

The speed trim system (STS) is a speed stability augmentation system designed to

improve flight characteristics during operations with a low gross weight, aft center

of gravity and high thrust when the autopilot is not engaged. The purpose of the

STS is to return the airplane to a trimmed speed by commanding the stabilizer in

a direction opposite the speed change. The STS monitors inputs of stabilizer

position, thrust lever position, airspeed and vertical speed and then trims the

stabilizer using the autopilot stabilizer trim. As the airplane speed increases or

decreases from the trimmed speed, the stabilizer is commanded in the direction to

return the airplane to the trimmed speed. This increases control column forces to

force the airplane to return to the trimmed speed. As the airplane returns to the

trimmed speed, the STS commanded stabilizer movement is removed.

STS operates most frequently during takeoffs, climb and go-arounds. Conditions

for speed trim operation are listed below:

• Airspeed between 100 KIAS and

• N1 above 60%

Mach 0.68

• Autopilot not engaged

•

10 seconds after takeoff

• Sensing of trim requirement

•

5 seconds following release of

trim switches

9.20.9

Flight Controls -

System Description

Boeing 737 Operations Manual

Stall Identification

Stall identification and control is enhanced by the yaw damper, the Elevator Feel

Shift (EFS) module and the speed trim system. These three systems work together

to help the pilot identify and prevent further movement into a stall condition.

During high AOA operations, the SMYD reduces yaw damper commanded rudder

movement.

The EFS module increases hydraulic system A pressure to the elevator feel and

centering unit during a stall. This increases forward control column force to

approximately two times normal feel pressure. The EFS module is armed

whenever an inhibit condition is not present. Inhibit conditions are: on the ground,

radio altitude less than 100 feet and autopilot engaged. However, if EFS is active

when descending through 100 feet RA, it remains active until AOA is reduced

below approximately stickshaker threshold. There are no flight deck indications

that the system is properly armed or activated.

As airspeed decreases towards stall speed, the speed trim system trims the

stabilizer nose down and enables trim above stickshaker AOA. With this trim

schedule the pilot must pull more aft column to stall the airplane. With the column

aft, the amount of column force increase with the onset of EFS module is more

pronounced.

9.20.11

Flight Controls -

System Description

Boeing 737 Operations Manual

Yaw Control

Yaw control is accomplished by a hydraulically powered rudder and a digital yaw

damper system. The rudder is controlled by displacing the rudder pedals. The yaw

damping functions are controlled through the stall management/yaw damper

(SMYD) computers.

Rudder

The rudder provides yaw control about the airplane’s vertical axis. The A and B

FLT CONTROL switches control hydraulic shutoff valves for the rudder and the

standby rudder.

[737 modified rudder - not installed]

Each set of rudder pedals is mechanically connected by cables to the input levers

of the main and standby rudder PCUs. The main rudder PCU is powered by

hydraulic system A and B. The standby rudder PCU is powered by the standby

hydraulic system. At speeds above approximately 135 kts, hydraulic system A

pressure to the rudder PCU is reduced. This function limits full rudder authority

in flight after takeoff and before landing.

[737 modified rudder- installed]

Each set of rudder pedals is mechanically connected by cables to the input levers

of the main and standby rudder PCUs. The main PCU consists of two independent

input rods, two individual control valves, and two separate actuators; one for

Hydraulic system A and one for Hydraulic system B. The standby rudder PCU is

controlled by a separate input rod and control valve and powered by the standby

hydraulic system. All three input rods have individual jam override mechanisms

that allows input commands to continue to be transferred to the remaining free

input rods if an input rod is hindered or jammed.

[737 modified rudder - installed]

At speeds above approximately 135 kts, both hydraulic system A and B pressure

are reduced within the main PCU. This function limits full rudder authority in

flight after takeoff and before landing.

[737 modified rudder - installed]

The main rudder PCU contains a Force Fight Monitor (FFM) that detects opposing

pressure (force fight) between A and B actuators. This may occur if either system

A or B input is jammed or failed. The FFM output is used to automatically turn on

the Standby Hydraulic pump pressurizing the standby PCU.

The standby rudder PCU is powered by the standby hydraulic system. The standby

hydraulic system is provided as a backup if system A and/or B pressure is lost.

With the standby PCU powered the pilot retains adequate rudder control

capability. It can be operated manually through the FLT CONTROL switches or

automatically. (Refer to Chapter 13, Hydraulics, Standby Hydraulic System)

9.20.12

Flight Controls -

System Description

Boeing 737 Operations Manual

[737 modified rudder- installed]

An amber STBY RUD ON light illuminates when the standby rudder hydraulic

system is pressurized. STBY RUD ON light illumination activates Master Caution

and Flight Control warning lights on the Systems Annunciation Panel.

Rudder Trim

The RUDDER trim control, located on the aft electronic panel, electrically

repositions the rudder feel and centering unit which adjusts the rudder neutral

position. The rudder pedals are displaced proportionately. The RUDDER TRIM

indicator displays the rudder trim position in units.

Yaw Damper

The yaw damper system consists of a main and standby yaw damper. Both yaw

dampers are controlled through Stall Management/Yaw Damper

(SMYD)

computers. The SMYD computers receive inputs from both ADIRUs, both control

wheels and the YAW DAMPER switch. SMYDs provide yaw damper inputs to the

main rudder power control unit (PCU) or standby rudder PCU, as appropriate.

Either yaw damper is capable of providing dutch roll prevention, gust damping

and turn coordination. Yaw damper operation does not result in rudder pedal

movement. Only main yaw damper inputs are shown on the yaw damper indicator.

The pilot can override either main or standby yaw damper inputs using either the

rudder pedals or trim inputs.

During normal operation the main yaw damper uses hydraulic system B and the

SMYD computers provide continuous system monitoring. If the SMYD senses a

system fault, the YAW DAMPER Switch is automatically moved to the OFF

position, the amber YAW DAMPER light illuminates, the yaw damper is

disconnected and the YAW DAMPER switch cannot be reset to ON. If hydraulic

system B pressure is lost, the YAW DAMPER switch remains in the ON position

until the B FLT CONTROL switch is positioned to OFF or STBY RUD. Then the

YAW DAMPER switch disengages, the amber YAW DAMPER light illuminates

and the YAW DAMPER switch cannot be reengaged.

During manual reversion flight (loss of hydraulic system A and B pressure), both

FLT CONTROL switches are positioned to STBY RUD. In this case, the YAW

DAMPER switch can be reset to ON and the standby hydraulic system powers the

standby yaw damper.

9.20.13

Flight Controls -

System Description

Boeing 737 Operations Manual

[737 modified rudder- installed]

During manual reversion flight (loss of hydraulic system A and B pressure), both

FLT CONTROL switches are positioned to STBY RUD. In this case, the YAW

DAMPER switch can be reset to ON and the standby hydraulic system powers the

standby yaw damper. During Standby Yaw Damper operation, movement of the

control wheel sends a signal to the standby rudder PCU to move the rudder. This

gives rudder assist to help turn the airplane when control of the ailerons is through

manual reversion.

Yaw Control Schematic

[737 modified rudder - not installed]

YAW DAMPER

YAW

DAMPER

L/ADIRU

OFF

RUDDER

TRIM

R/ADIRU

RUDDER FEEL

STALL MANAGEMENT

AND CENTERING

YAW DAMPER

UNIT

COMPUTERS

RUDDER

MAIN

YAW DAMPER

MAIN

RUDDER PCU

STANDBY

YAW DAMPER

STANDBY

RUDDER PCU

RUDDER

SHUTOFF

VALVE

9.20.14

Flight Controls -

System Description

Boeing 737 Operations Manual

Speed Brakes

The speed brakes consist of flight spoilers and ground spoilers. Hydraulic system

A powers all four ground spoilers, two on the upper surface of each wing. The

SPEED BRAKE lever controls the spoilers. When the SPEED BRAKE lever is

actuated all the spoilers extend when the airplane is on the ground and only the

flight spoilers extend when the airplane is in the air.

The SPEEDBRAKES EXTENDED light provides an indication of spoiler

operation in-flight and on the ground. In-flight, the light illuminates to warn the

crew that the speed brakes are extended while in the landing configuration or

below 800 feet AGL. On the ground, the light illuminates when hydraulic pressure

is sensed in the ground spoiler shutoff valve with the speed brake lever in the

DOWN position.

In-Flight Operation

Operating the SPEED BRAKE lever in flight causes all flight spoiler panels to rise

symmetrically to act as speed brakes. Caution should be exercised when

deploying flight spoilers during a turn, as they greatly increase roll rate. When the

speed brakes are in an intermediate position roll rates increase significantly.

Moving the SPEED BRAKE lever beyond the FLIGHT DETENT causes

buffeting and is prohibited in flight.

Ground Operation

During landing, the auto speed brake system operates when these conditions

occur:

• SPEED BRAKE lever is in the ARMED position

• SPEED BRAKE ARMED light is illuminated

• radio altitude is less than 10 feet

• landing gear strut compresses on touchdown

Note: Compression of any landing gear strut enables the flight spoilers to

deploy. Compression of the right main landing gear strut enables the

ground spoilers to deploy.

• both thrust levers are retarded to IDLE

• main landing gear wheels spin up (more than 60 kts).

The SPEED BRAKE lever automatically moves to the UP position and the

spoilers deploy.

If a wheel spin-up signal is not detected, when the air/ground system senses

ground mode (any gear strut compresses) the SPEED BRAKE lever moves to the

UP position and flight spoiler panels deploy automatically. When the right main

landing gear strut compresses, a mechanical linkage opens the ground spoiler

shutoff valve and the ground spoilers deploy.

9.20.16

Flight Controls -

System Description

Boeing 737 Operations Manual

If the SPEED BRAKE lever is in the DOWN position during landing or rejected

takeoff, the auto speed brake system operates when these conditions occur:

• main landing gear wheels spin up (more than 60 kts)

• both thrust levers are retarded to IDLE

• reverse thrust levers are positioned for reverse thrust.

The SPEED BRAKE lever automatically moves to the UP position and spoilers

deploy.

After an RTO or landing, if either thrust lever is advanced, the SPEED BRAKE

lever automatically moves to the DOWN detent and all spoiler panels retract. The

spoiler panels may also be retracted by manually moving the SPEED BRAKE

lever to the DOWN detent.

9.20.17

Flight Controls -

System Description

Boeing 737 Operations Manual

Flaps and Slats

The flaps and slats are high lift devices that increase wing lift and decrease stall

speed during takeoff, low speed maneuvering and landing.

LE devices consist of four flaps and eight slats: two flaps inboard and four slats

outboard of each engine. Slats extend to form a sealed or slotted leading edge

depending on the TE flap setting. The TE devices consist of double slotted flaps

inboard and outboard of each engine.

TE flap positions 1-15 provide increased lift; positions 15-40 provide increased

lift and drag. Flap positions 30 and 40 are normal landing flap positions. Flaps 15

is used for some non-normal landing conditions.

To prevent excessive structural loads from increased Mach at higher altitude, flap

extension above 20,000 feet should not be attempted.

Flap and Slat Sequencing

LE devices and TE flaps are normally extended and retracted by hydraulic power

from system B. When the FLAP lever is in the UP detent, all flaps and LE devices

are commanded to the retracted or up position. Moving the FLAP lever aft allows

selection of flap detent positions 1, 2, 5, 10, 15, 25, 30 or 40. The LE devices

deployment is sequenced as a function of TE flaps deployment.

When the FLAP lever is moved from the UP position to the 1, 2, or 5 position, the

TE flaps extend to the commanded position and the LE:

• flaps extend to the full extended position and

• slats extend to the extend position.

When the FLAP lever is moved beyond the 5 position the TE flaps extend to the

commanded position and the LE:

• flaps remain at the full extended position and

• slats extend to the full extended position.

The LE devices sequence is reversed upon retraction.

Mechanical gates hinder inadvertent FLAP lever movement beyond flaps 1 for

one engine inoperative go-around and flaps 15 for normal go-around.

Indicator lights on the center instrument panel provide overall LE devices position

status. The LE DEVICES annunciator panel on the aft overhead panel indicates

the positions of the individual flaps and slats.

Flap Load Relief

The flaps/slat electronics unit (FSEU) provides a TE flap load relief function

which protects the flaps from excessive air loads. This function is operative at the

flaps 30 and flaps 40 positions only. The FLAP lever does not move, but the flap

position indicator displays flap retraction and re-extension.

9.20.19

Flight Controls -

System Description

Boeing 737 Operations Manual

When the flaps are set at 40, the TE flaps:

• retract to 30 if airspeed exceeds 163 knots

• re-extend when airspeed is reduced below 158 knots.

When the flaps are set at 30, the TE flaps:

• retract to 25 if the airspeed exceeds 176 knots

• re-extend when airspeed is reduced below 171 knots.

[Option]

The FLAP LOAD RELIEF light illuminates when the TE flap load relief function

is activated.

Autoslats

At flap positions 1, 2 and 5 an autoslat function is available that moves the LE slats

to full extended if the airplane approaches a stall condition.

The autoslat system is designed to enhance airplane stall characteristics at high

angles of attack during takeoff or approach to landing. When TE flaps 1 through

5 are selected, the LE slats are in the extend position. As the airplane approaches

the stall angle, the slats automatically drive to the full extended position prior to

stick shaker activation. The slats return to the extend position when the pitch angle

is sufficiently reduced below the stall critical attitude.

The autoslat system is designed to enhance airplane stall characteristics at high

angles of attack during takeoff or approach to landing. When TE flaps 1 through

5 are selected, the LE slats are in the extend position. As the airplane approaches

the stall angle, the slats automatically begin driving to the full extended position

prior to stick shaker activation. The slats return to the extend position when the

pitch angle is sufficiently reduced below the stall critical attitude.

Autoslat operation is normally powered by hydraulic system B. An alternate

source of power is provided by system A through a power transfer unit (PTU) if a

loss of pressure is sensed from the higher volume system B engine driven pump.

The PTU provides system A pressure to power a hydraulic motorized pump,

pressurizing system B fluid to provide power for the autoslat operation. (Refer to

Chapter 13, Hydraulics, Power Transfer Unit)

Alternate Extension

In the event that hydraulic system B fails, an alternate method of extending the LE

devices and extending and retracting the TE flaps is provided.

9.20.20

Flight Controls -

System Description

Boeing 737 Operations Manual

The TE flaps can be operated electrically through the use of two alternate flap

switches. The guarded ALTERNATE FLAPS master switch closes a flap bypass

valve to prevent hydraulic lock of the flap drive unit and arms the alternate flaps

position switch. The ALTERNATE FLAPS position switch controls an electric

motor that extends or retracts the TE flaps. The switch must be held in the DOWN

position until the flaps reach the desired position. No asymmetry or skew

protection is provided through the alternate (electrical) flap drive system.

When using alternate flap extension the LE flaps and slats are driven to the full

extended position using power from the standby hydraulic system. In this case the

ALTERNATE FLAPS master switch energizes the standby pump and the

ALTERNATE FLAPS position switch, held in the down position momentarily,

fully extends the LE devices.

Note: The LE devices cannot be retracted by the standby hydraulic system.

9.20.21

Boeing 737 - 600/700/800/900. Operations Manual (1997 year) - page 40