5-1

Introduction

This chapter focuses on the flight control systems a pilot uses 

to control the forces of flight, and the aircraft’s direction and 

attitude. It should be noted that flight control systems and 

characteristics can vary greatly depending on the type of 

aircraft flown. The most basic flight control system designs 

are mechanical and date back to early aircraft. They operate 

with a collection of mechanical parts such as rods, cables, 

pulleys, and sometimes chains to transmit the forces of the 

flight deck controls to the control surfaces. Mechanical flight 

control systems are still used today in small general and 

sport category aircraft where the aerodynamic forces are not 

excessive. [Figure 5-1]

Flight Controls

Chapter 5
5-2

Anti-torque pedals

Cyclic stick

Collective lever

Cyclic

Cyclic

Cyclic

Cyclic

Yaw

Yaw

Collective

Collective

Figure 5-3. Helicopter flight control system.

Hydraulic pressure

Hydraulic return

Pivot point

LEGEND

Elevator (UP)

Control stick (AFT—Nose up)

Control cables

Power cylinder

Neutral

Neutral

Control valves

Power disconnect linkage Neutral

Figure 5-2. Hydromechanical flight control system.

Elevator

Control stick

Cable

Pulleys

Push rod

Figure 5-1. Mechanical flight control system.

As aviation matured and aircraft designers learned more about 

aerodynamics, the industry produced larger and faster aircraft. 

Therefore, the aerodynamic forces acting upon the control 

surfaces increased exponentially. To make the control force 

required by pilots manageable, aircraft engineers designed 

more complex systems. At first, hydromechanical designs, 

consisting of a mechanical circuit and a hydraulic circuit, 

were used to reduce the complexity, weight, and limitations 

of mechanical flight controls systems. [Figure 5-2] 

As aircraft became more sophisticated, the control surfaces 

were actuated by electric motors, digital computers, or fiber 

optic cables. Called “fly-by-wire,” this flight control system 

replaces the physical connection between pilot controls and 

the flight control surfaces with an electrical interface. In 

addition, in some large and fast aircraft, controls are boosted 

by hydraulically or electrically actuated systems. In both 

the fly-by-wire and boosted controls, the feel of the control 

reaction is fed back to the pilot by simulated means.

Current research at the National Aeronautics and Space 

Administration (NASA) Dryden Flight Research Center 

involves Intelligent Flight Control Systems (IFCS). The goal 

of this project is to develop an adaptive neural network-based 

flight control system. Applied directly to flight control system 

feedback errors, IFCS provides adjustments to improve 

aircraft performance in normal flight as well as with system 

failures. With IFCS, a pilot is able to maintain control and 

safely land an aircraft that has suffered a failure to a control 

surface or damage to the airframe. It also improves mission 

capability, increases the reliability and safety of flight, and 

eases the pilot workload.

Today’s aircraft employ a variety of flight control systems. 

For example, some aircraft in the sport pilot category rely on 

weight-shift control to fly while balloons use a standard burn 

technique. Helicopters utilize a cyclic to tilt the rotor in the 

desired direction along with a collective to manipulate rotor 

pitch and anti-torque pedals to control yaw. [Figure 5-3]

For additional information on flight control systems, refer 

to the appropriate handbook for information related to the 

flight control systems and characteristics of specific types 

of aircraft.

Flight Control Systems 

Flight Controls

Aircraft flight control systems consist of primary and 

secondary systems. The ailerons, elevator (or stabilator), and 

rudder constitute the primary control system and are required to 

control an aircraft safely during flight. Wing flaps, leading edge 

devices, spoilers, and trim systems constitute the secondary 

control system and improve the performance characteristics of 

the airplane or relieve the pilot of excessive control forces.

Primary Flight Controls

Aircraft control systems are carefully designed to provide 

adequate responsiveness to control inputs while allowing a 
5-3

Figure 5-1. Airplane controls, movement, axes of rotation, and 

type of stability.

Lateral axis 

(longitudinal

stability)

Aileron—Roll

Elevator—Pitch

Rudder—Yaw

Longitudinal 

axis (lateral 

stability)

Vertical axis

(directional 

stability)

Aileron Roll Longitudinal Lateral

Rudder Yaw Vertical Directional

Elevator/

Stabilator Pitch Lateral Longitudinal

Primary

Control

Surface

Airplane 

Movement

Axes of 

Rotation

Type of 

Stability

Figure 5-4. Airplane controls, movement, axes of rotation, and 

type of stability.

Figure 5-5. Adverse yaw is caused by higher drag on the outside 

wing, which is producing more lift. Lift

Drag

Lift

Drag

Figure 5-2. Adverse yaw is caused by higher drag on the outside 

wing, which is producing more lift.

Adverse yaw

natural feel. At low airspeeds, the controls usually feel soft 

and sluggish, and the aircraft responds slowly to control 

applications. At higher airspeeds, the controls become 

increasingly firm and aircraft response is more rapid.

Movement of any of the three primary flight control surfaces 

(ailerons, elevator or stabilator, or rudder), changes the airflow 

and pressure distribution over and around the airfoil. These 

changes affect the lift and drag produced by the airfoil/control 

surface combination, and allow a pilot to control the aircraft 

about its three axes of rotation. 

Design features limit the amount of deflection of flight control 

surfaces. For example, control-stop mechanisms may be 

incorporated into the flight control linkages, or movement 

of the control column and/or rudder pedals may be limited. 

The purpose of these design limits is to prevent the pilot from 

inadvertently overcontrolling and overstressing the aircraft 

during normal maneuvers.

A properly designed airplane is stable and easily controlled 

during normal maneuvering. Control surface inputs cause 

movement about the three axes of rotation. The types of 

stability an airplane exhibits also relate to the three axes of 

rotation. [Figure 5-4]

Ailerons

Ailerons control roll about the longitudinal axis. The ailerons 

are attached to the outboard trailing edge of each wing and 

move in the opposite direction from each other. Ailerons are 

connected by cables, bellcranks, pulleys and/or push-pull tubes 

to a control wheel or control stick.

Moving the control wheel or control stick to the right causes 

the right aileron to deflect upward and the left aileron to deflect 

downward. The upward deflection of the right aileron decreases 

the camber resulting in decreased lift on the right wing. The 

corresponding downward deflection of the left aileron increases 

the camber resulting in increased lift on the left wing. Thus, 

the increased lift on the left wing and the decreased lift on the 

right wing causes the airplane to roll to the right.

Adverse Yaw

Since the downward deflected aileron produces more lift as 

evidenced by the wing raising, it also produces more drag. This 

added drag causes the wing to slow down slightly. This results 

in the aircraft yawing toward the wing which had experienced 

an increase in lift (and drag). From the pilot’s perspective, the 

yaw is opposite the direction of the bank. The adverse yaw 

is a result of differential drag and the slight difference in the 

velocity of the left and right wings. [Figure 5-5] 

Adverse yaw becomes more pronounced at low airspeeds. 

At these slower airspeeds aerodynamic pressure on control 

surfaces are low and larger control inputs are required to 

effectively maneuver the airplane. As a result, the increase in 

aileron deflection causes an increase in adverse yaw. The yaw 

is especially evident in aircraft with long wing spans.
5-4

Differential aileron

Aileron deflected up 

Aileron deflected down

Figure 5-6. Differential ailerons.

Figure 5-4. Frise-type ailerons.

Lowered

Neutral

Raised

Drag

Figure 5-7. Frise-type ailerons.

Application of rudder is used to counteract adverse yaw. The 

amount of rudder control required is greatest at low airspeeds, 

high angles of attack, and with large aileron deflections. Like 

all control surfaces at lower airspeeds, the vertical stabilizer/

rudder becomes less effective, and magnifies the control 

problems associated with adverse yaw.

All turns are coordinated by use of ailerons, rudder, and 

elevator. Applying aileron pressure is necessary to place 

the aircraft in the desired angle of bank, while simultaneous 

application of rudder pressure is necessary to counteract the 

resultant adverse yaw. Additionally, because more lift is 

required during a turn than when in straight-and-level flight, 

the angle of attack (AOA) must be increased by applying 

elevator back pressure. The steeper the turn, the more elevator 

back pressure is needed.

As the desired angle of bank is established, aileron and rudder 

pressures should be relaxed. This stops the angle of bank from 

increasing, because the aileron and rudder control surfaces are 

in a neutral and streamlined position. Elevator back pressure 

should be held constant to maintain altitude. The roll-out 

from a turn is similar to the roll-in, except the flight controls 

are applied in the opposite direction. Aileron and rudder are 

applied in the direction of the roll-out or toward the high wing. 

As the angle of bank decreases, the elevator back pressure 

should be relaxed as necessary to maintain altitude.

In an attempt to reduce the effects of adverse yaw, 

manufacturers have engineered four systems: differential 

ailerons, frise-type ailerons, coupled ailerons and rudder, 

and flaperons. 

Differential Ailerons

With differential ailerons, one aileron is raised a greater 

distance than the other aileron is lowered for a given 

movement of the control wheel or control stick. This produces 

an increase in drag on the descending wing. The greater drag 

results from deflecting the up aileron on the descending wing 

to a greater angle than the down aileron on the rising wing. 

While adverse yaw is reduced, it is not eliminated completely. 

[Figure 5-6]

Frise-Type Ailerons

With a frise-type aileron, when pressure is applied to the 

control wheel or control stick, the aileron that is being raised 

pivots on an offset hinge. This projects the leading edge of 

the aileron into the airflow and creates drag. It helps equalize 

the drag created by the lowered aileron on the opposite wing 

and reduces adverse yaw. [Figure 5-7]

The frise-type aileron also forms a slot so air flows smoothly 

over the lowered aileron, making it more effective at high 

angles of attack. Frise-type ailerons may also be designed 

to function differentially. Like the differential aileron, the 

frise-type aileron does not eliminate adverse yaw entirely. 

Coordinated rudder application is still needed wherever 

ailerons are applied.

Coupled Ailerons and Rudder

Coupled ailerons and rudder are linked controls. This is 

accomplished with rudder-aileron interconnect springs, which 

help correct for aileron drag by automatically deflecting 

the rudder at the same time the ailerons are deflected. For 

example, when the control wheel or control stick is moved 

to produce a left roll, the interconnect cable and spring pulls 

forward on the left rudder pedal just enough to prevent the 

nose of the aircraft from yawing to the right. The force applied 
5-5

Figure 5-5. Coupled ailerons and rudder.

Rudder/Aileron interconnecting springs

Rudder deflects with ailerons

Figure 5-8. Coupled ailerons and rudder.

Figure 5-9. Flaperons on a Skystar Kitfox MK 7.

Nose up

Tail down

Figure 5-6. The elevator is the primary control for changing the 

pitch attitude of an airplane.

Control column

Aft

Up elevator

Downward

aerodynamic force

CG

Figure 5-10. The elevator is the primary control for changing the 

pitch attitude of an airplane.

to the rudder by the springs can be overridden if it becomes 

necessary to slip the aircraft. [Figure 5-8]

Flaperons

Flaperons combine both aspects of flaps and ailerons. In 

addition to controlling the bank angle of an aircraft like 

conventional ailerons, flaperons can be lowered together 

to function much the same as a dedicated set of flaps. The 

pilot retains separate controls for ailerons and flaps. A mixer 

is used to combine the separate pilot inputs into this single 

set of control surfaces called flaperons. Many designs that 

incorporate flaperons mount the control surfaces away from 

the wing to provide undisturbed airflow at high angles of 

attack and/or low airspeeds. [Figure 5-9]

Elevator

The elevator controls pitch about the lateral axis. Like the 

ailerons on small aircraft, the elevator is connected to the 

control column in the flight deck by a series of mechanical 

linkages. Aft movement of the control column deflects 

the trailing edge of the elevator surface up. This is usually 

referred to as up “elevator.” [Figure 5-10] 

The up-elevator position decreases the camber of the elevator 

and creates a downward aerodynamic force, which is greater 

than the normal tail-down force that exists in straight-andlevel flight. The overall effect causes the tail of the aircraft 

to move down and the nose to pitch up. The pitching moment 

occurs about the center of gravity (CG). The strength of the 

pitching moment is determined by the distance between 

the CG and the horizontal tail surface, as well as by the 

aerodynamic effectiveness of the horizontal tail surface. 

Moving the control column forward has the opposite effect. 

In this case, elevator camber increases, creating more lift 

(less tail-down force) on the horizontal stabilizer/elevator. 

This moves the tail upward and pitches the nose down. Again, 

the pitching moment occurs about the CG.

As mentioned earlier in the coverage on stability, power, 

thrustline, and the position of the horizontal tail surfaces 

on the empennage are factors in elevator effectiveness 

controlling pitch. For example, the horizontal tail surfaces 

may be attached near the lower part of the vertical stabilizer, 

at the midpoint, or at the high point, as in the T-tail design.
5-6

Figure 5-7. Airplane with a T-tail design at a hign angle of attack 

and an aft CG.

CG

Figure 5-11. Airplane with a T-tail design at a high AOA and an 

aft CG.

T-Tail

In a T-tail configuration, the elevator is above most of the 

effects of downwash from the propeller as well as airflow 

around the fuselage and/or wings during normal flight 

conditions. Operation of the elevators in this undisturbed air 

allows control movements that are consistent throughout most 

flight regimes. T-tail designs have become popular on many 

light and large aircraft, especially those with aft fuselagemounted engines because the T-tail configuration removes 

the tail from the exhaust blast of the engines. Seaplanes and 

amphibians often have T-tails in order to keep the horizontal 

surfaces as far from the water as possible. An additional 

benefit is reduced vibration and noise inside the aircraft.

At slow speeds, the elevator on a T-tail aircraft must be moved 

through a larger number of degrees of travel to raise the nose 

a given amount than on a conventional-tail aircraft. This is 

because the conventional-tail aircraft has the downwash from 

the propeller pushing down on the tail to assist in raising the 

nose.

Since controls on aircraft are rigged so that increasing control 

forces are required for increased control travel, the forces 

required to raise the nose of a T-tail aircraft are greater than 

those for a conventional-tail aircraft. Longitudinal stability of 

a trimmed aircraft is the same for both types of configuration, 

but the pilot must be aware that the required control forces are 

greater at slow speeds during takeoffs, landings, or stalls than 

for similar size aircraft equipped with conventional tails.

T-tail airplanes also require additional design considerations 

to counter the problem of flutter. Since the weight of the 

horizontal surfaces is at the top of the vertical stabilizer, 

the moment arm created causes high loads on the vertical 

stabilizer which can result in flutter. Engineers must 

compensate for this by increasing the design stiffness of the 

vertical stabilizer, usually resulting in a weight penalty over 

conventional tail designs.

When flying at a very high AOA with a low airspeed and 

an aft CG, the T-tail aircraft may be susceptible to a deep 

stall. In a deep stall, the airflow over the horizontal tail 

is blanketed by the disturbed airflow from the wings and 

fuselage. In these circumstances, elevator or stabilator control 

could be diminished, making it difficult to recover from the 

stall. It should be noted that an aft CG is often a contributing 

factor in these incidents, since similar recovery problems 

are also found with conventional tail aircraft with an aft CG. 

[Figure 5-11] 

Since flight at a high AOA with a low airspeed and an aft 

CG position can be dangerous, many aircraft have systems 

to compensate for this situation. The systems range from 

control stops to elevator down springs. An elevator down 

spring assists in lowering the nose of the aircraft to prevent a 

stall caused by the aft CG position. The stall occurs because 

the properly trimmed airplane is flying with the elevator in a 

trailing edge down position, forcing the tail up and the nose 

down. In this unstable condition, if the aircraft encounters 

turbulence and slows down further, the trim tab no longer 

positions the elevator in the nose-down position. The elevator 

then streamlines, and the nose of the aircraft pitches upward, 

possibly resulting in a stall.

The elevator down spring produces a mechanical load on the 

elevator, causing it to move toward the nose-down position 

if not otherwise balanced. The elevator trim tab balances the 

elevator down spring to position the elevator in a trimmed 

position. When the trim tab becomes ineffective, the down 

spring drives the elevator to a nose-down position. The nose 

of the aircraft lowers, speed builds up, and a stall is prevented. 

[Figure 5-12]

The elevator must also have sufficient authority to hold the 

nose of the aircraft up during the roundout for a landing. In 

this case, a forward CG may cause a problem. During the 

landing flare, power is usually reduced, which decreases the 

airflow over the empennage. This, coupled with the reduced 

landing speed, makes the elevator less effective.

As this discussion demonstrates, pilots must understand and 

follow proper loading procedures, particularly with regard 

to the CG position. More information on aircraft loading, as 

well as weight and balance, is included in Chapter 9, Weight 

and Balance.

Stabilator

As mentioned in Chapter 2, Aircraft Structure, a stabilator 

is essentially a one-piece horizontal stabilizer that pivots 
5-7

Figure 5-8. When the aerodynamic efficiency of the horizontal

tail surface is inadequate due to an aft center of gravity condition,

an elevator down spring may be used to supply a mechanical load 

to lower the nose.

Elevator

Down spring

Pivot points

Bell crank

Figure 5-12. When the aerodynamic efficiency of the horizontal 

tail surface is inadequate due to an aft CG condition, an elevator 

down spring may be used to supply a mechanical load to lower 

the nose.

Figure 5-9. One piece horizontal tail surface that pivots up and 

down about a central hinge point.

Antiservo tab

Balance weight

Stabilator

Pivot point

Figure 5-13. The stabilator is a one-piece horizontal tail surface 

that pivots up and down about a central hinge point.

Figure 5-14. The Piaggio P180 includes a variable-sweep canard 

design, which provides longitudinal stability about the lateral 

axis.

from a central hinge point. When the control column is 

pulled back, it raises the stabilator’s trailing edge, pulling 

the airplane’s nose up. Pushing the control column forward 

lowers the trailing edge of the stabilator and pitches the nose 

of the airplane down. 

Because stabilators pivot around a central hinge point, they 

are extremely sensitive to control inputs and aerodynamic 

loads. Antiservo tabs are incorporated on the trailing edge to 

decrease sensitivity. They deflect in the same direction as the 

stabilator. This results in an increase in the force required to 

move the stabilator, thus making it less prone to pilot-induced 

overcontrolling. In addition, a balance weight is usually 

incorporated in front of the main spar. The balance weight 

may project into the empennage or may be incorporated on 

the forward portion of the stabilator tips. [Figure 5-13]

Canard

The canard design utilizes the concept of two lifting surfaces, 

the canard functioning as a horizontal stabilizer located in 

front of the main wings. In effect, the canard is an airfoil 

similar to the horizontal surface on a conventional aft-tail 

design. The difference is that the canard actually creates lift 

and holds the nose up, as opposed to the aft-tail design which 

exerts downward force on the tail to prevent the nose from 

rotating downward. [Figure 5-14]

The canard design dates back to the pioneer days of aviation, 

most notably used on the Wright Flyer. Recently, the canard 

configuration has regained popularity and is appearing on 

newer aircraft. Canard designs include two types–one with a 

horizontal surface of about the same size as a normal aft-tail 

design, and the other with a surface of the same approximate 

size and airfoil of the aft-mounted wing known as a tandem 

wing configuration. Theoretically, the canard is considered 

more efficient because using the horizontal surface to help 

lift the weight of the aircraft should result in less drag for a 

given amount of lift.

Rudder

The rudder controls movement of the aircraft about its vertical 

axis. This motion is called yaw. Like the other primary control 

surfaces, the rudder is a movable surface hinged to a fixed 

surface, in this case to the vertical stabilizer, or fin. Moving 

the left or right rudder pedal controls the rudder.

When the rudder is deflected into the airflow, a horizontal 

force is exerted in the opposite direction. [Figure 5-15] By 

pushing the left pedal, the rudder moves left. This alters the 

airflow around the vertical stabilizer/rudder, and creates a 
5-8

Aerodynamic force

Figure 5-11. The effect of left rudder pressure.

Left rudder

CG Left rudder forward

Yaw

Figure 5-15. The effect of left rudder pressure.

Figure 5-16. Beechcraft Bonanza V35.

sideward lift that moves the tail to the right and yaws the nose 

of the airplane to the left. Rudder effectiveness increases with 

speed; therefore, large deflections at low speeds and small 

deflections at high speeds may be required to provide the 

desired reaction. In propeller-driven aircraft, any slipstream 

flowing over the rudder increases its effectiveness.

V-Tail

The V-tail design utilizes two slanted tail surfaces to perform 

the same functions as the surfaces of a conventional elevator 

and rudder configuration. The fixed surfaces act as both 

horizontal and vertical stabilizers. [Figure 5-16]

The movable surfaces, which are usually called ruddervators, 

are connected through a special linkage that allows the control 

wheel to move both surfaces simultaneously. On the other 

hand, displacement of the rudder pedals moves the surfaces 

differentially, thereby providing directional control.

When both rudder and elevator controls are moved by the 

pilot, a control mixing mechanism moves each surface the 

appropriate amount. The control system for the V-tail is 

more complex than that required for a conventional tail. In 

addition, the V-tail design is more susceptible to Dutch roll 

tendencies than a conventional tail, and total reduction in 

drag is minimal.

Secondary Flight Controls

Secondary flight control systems may consist of wing flaps, 

leading edge devices, spoilers, and trim systems.

Flaps

Flaps are the most common high-lift devices used on aircraft. 

These surfaces, which are attached to the trailing edge of 

the wing, increase both lift and induced drag for any given 

AOA. Flaps allow a compromise between high cruising speed 

and low landing speed, because they may be extended when 

needed, and retracted into the wing’s structure when not 

needed. There are four common types of flaps: plain, split, 

slotted, and Fowler flaps. [Figure 5-17]

The plain flap is the simplest of the four types. It increases 

the airfoil camber, resulting in a significant increase in the 

coefficient of lift (CL) at a given AOA. At the same time, 

it greatly increases drag and moves the center of pressure 

(CP) aft on the airfoil, resulting in a nose-down pitching 

moment.

The split flap is deflected from the lower surface of the airfoil 

and produces a slightly greater increase in lift than the plain 

flap. More drag is created because of the turbulent air pattern 

produced behind the airfoil. When fully extended, both plain 

and split flaps produce high drag with little additional lift. 

The most popular flap on aircraft today is the slotted flap. 

Variations of this design are used for small aircraft, as well 

as for large ones. Slotted flaps increase the lift coefficient 

significantly more than plain or split flaps. On small aircraft, 

the hinge is located below the lower surface of the flap, and 

when the flap is lowered, a duct forms between the flap well 

in the wing and the leading edge of the flap. When the slotted 

flap is lowered, high energy air from the lower surface is 

ducted to the flap’s upper surface. The high energy air from 

the slot accelerates the upper surface boundary layer and 

delays airflow separation, providing a higher CL. Thus, the 

slotted flap produces much greater increases in maximum 

coefficient of lift (CL-MAX) than the plain or split flap. While 

there are many types of slotted flaps, large aircraft often 

have double- and even triple-slotted flaps. These allow the 

maximum increase in drag without the airflow over the flaps 

separating and destroying the lift they produce.
5-9

Figure 5-17. Five common types of flaps.

Plain flap

Basic section

Split flap

Slotted flap

Fowler flap

Slotted Fowler flap

Figure 5-18. Leading edge high lift devices.

Leading edge cuff

Leading edge flap

Movable slot

Fixed slot

Leading Edge Devices

High-lift devices also can be applied to the leading edge of 

the airfoil. The most common types are fixed slots, movable 

slats, leading edge flaps, and cuffs. [Figure 5-18]

Fixed slots direct airflow to the upper wing surface and delay 

airflow separation at higher angles of attack. The slot does 

not increase the wing camber, but allows a higher maximum 

CL because the stall is delayed until the wing reaches a 

greater AOA. 

Movable slats consist of leading edge segments, which move 

on tracks. At low angles of attack, each slat is held flush 

against the wing’s leading edge by the high pressure that forms 

at the wing’s leading edge. As the AOA increases, the highpressure area moves aft below the lower surface of the wing, 

allowing the slats to move forward. Some slats, however, are 

Fowler flaps are a type of slotted flap. This flap design not 

only changes the camber of the wing, it also increases the 

wing area. Instead of rotating down on a hinge, it slides 

backwards on tracks. In the first portion of its extension, it 

increases the drag very little, but increases the lift a great deal 

as it increases both the area and camber. As the extension 

continues, the flap deflects downward. During the last portion 

of its travel, the flap increases the drag with little additional 

increase in lift.
5-10

Figure 5-19. Spoilers reduce lift and increase drag during descent 

and landing.

tabs, balance tabs, antiservo tabs, ground adjustable tabs, and 

an adjustable stabilizer.

Trim Tabs

The most common installation on small aircraft is a single 

trim tab attached to the trailing edge of the elevator. Most trim 

tabs are manually operated by a small, vertically mounted 

control wheel. However, a trim crank may be found in some 

aircraft. The flight deck control includes a trim tab position 

indicator. Placing the trim control in the full nose-down 

position moves the trim tab to its full up position. With 

the trim tab up and into the airstream, the airflow over the 

horizontal tail surface tends to force the trailing edge of the 

elevator down. This causes the tail of the airplane to move 

up, and the nose to move down. [Figure 5-20]

If the trim tab is set to the full nose-up position, the tab moves 

to its full down position. In this case, the air flowing under 

the horizontal tail surface hits the tab and forces the trailing 

edge of the elevator up, reducing the elevator’s AOA. This 

causes the tail of the airplane to move down, and the nose 

to move up.

In spite of the opposing directional movement of the trim 

tab and the elevator, control of trim is natural to a pilot. If 

the pilot needs to exert constant back pressure on a control 

column, the need for nose-up trim is indicated. The normal 

trim procedure is to continue trimming until the aircraft is 

balanced and the nose-heavy condition is no longer apparent. 

Pilots normally establish the desired power, pitch attitude, 

and configuration first, and then trim the aircraft to relieve 

control pressures that may exist for that flight condition. 

Any time power, pitch attitude, or configuration is changed, 

expect that retrimming will be necessary to relieve the control 

pressures for the new flight condition.

pilot operated and can be deployed at any AOA. Opening a 

slat allows the air below the wing to flow over the wing’s 

upper surface, delaying airflow separation.

Leading edge flaps, like trailing edge flaps, are used to 

increase both CL-MAX and the camber of the wings. This type 

of leading edge device is frequently used in conjunction with 

trailing edge flaps and can reduce the nose-down pitching 

movement produced by the latter. As is true with trailing edge 

flaps, a small increment of leading edge flaps increases lift to 

a much greater extent than drag. As greater amounts of flaps 

are extended, drag increases at a greater rate than lift.

Leading edge cuffs, like leading edge flaps and trailing edge 

flaps are used to increase both CL-MAX and the camber of 

the wings. Unlike leading edge flaps and trailing edge flaps, 

leading edge cuffs are fixed aerodynamic devices. In most 

cases leading edge cuffs extend the leading edge down and 

forward. This causes the airflow to attach better to the upper 

surface of the wing at higher angles of attack, thus lowering 

an aircraft’s stall speed. The fixed nature of leading edge 

cuffs extracts a penalty in maximum cruise airspeed, but 

recent advances in design and technology have reduced this 

penalty. 

Spoilers

Found on many gliders and some aircraft, high drag devices 

called spoilers are deployed from the wings to spoil the smooth 

airflow, reducing lift and increasing drag. On gliders, spoilers 

are most often used to control rate of descent for accurate 

landings. On other aircraft, spoilers are often used for roll 

control, an advantage of which is the elimination of adverse 

yaw. To turn right, for example, the spoiler on the right wing 

is raised, destroying some of the lift and creating more drag on 

the right. The right wing drops, and the aircraft banks and yaws 

to the right. Deploying spoilers on both wings at the same time 

allows the aircraft to descend without gaining speed. Spoilers 

are also deployed to help reduce ground roll after landing. By 

destroying lift, they transfer weight to the wheels, improving 

braking effectiveness. [Figure 5-19]

Trim Systems

Although an aircraft can be operated throughout a wide range 

of attitudes, airspeeds, and power settings, it can be designed 

to fly hands-off within only a very limited combination of 

these variables. Trim systems are used to relieve the pilot of 

the need to maintain constant pressure on the flight controls, 

and usually consist of flight deck controls and small hinged 

devices attached to the trailing edge of one or more of the 

primary flight control surfaces. Designed to help minimize 

a pilot’s workload, trim systems aerodynamically assist 

movement and position of the flight control surface to which 

they are attached. Common types of trim systems include trim 
5-11

Figure 5-17. An antiservo tab attempts to streamline the 

control surface and is used to make the stabilator less sensitive 

by opposing the force exerted by the pilot.

Pivot point

Stabilator

Antiservo tab

Figure 5-21. An antiservo tab attempts to streamline the control 

surface and is used to make the stabilator less sensitive by opposing 

the force exerted by the pilot.

Figure 5-22. A ground adjustable tab is used on the rudder of many 

small airplanes to correct for a tendency to fly with the fuselage 

slightly misaligned with the relative wind.

Figure 5-16. The movement of the elevator is opposite to the 

direction of movement of the elevator trim tab.

Nose-down trim

Nose-up trim

Tab up—elevator down

Tab down—elevator up

Trim tab

Elevator

Trim tab

Elevator

Figure 5-20. The movement of the elevator is opposite to the 

direction of movement of the elevator trim tab.

Balance Tabs

The control forces may be excessively high in some aircraft, 

and, in order to decrease them, the manufacturer may use 

balance tabs. They look like trim tabs and are hinged in 

approximately the same places as trim tabs. The essential 

difference between the two is that the balancing tab is coupled 

to the control surface rod so that when the primary control 

surface is moved in any direction, the tab automatically 

moves in the opposite direction. The airflow striking the tab 

counterbalances some of the air pressure against the primary 

control surface, and enables the pilot to move more easily 

and hold the control surface in position. 

If the linkage between the balance tab and the fixed surface is 

adjustable from the flight deck, the tab acts as a combination 

trim and balance tab that can be adjusted to any desired 

deflection. 

Antiservo Tabs

Antiservo tabs work in the same manner as balance tabs 

except, instead of moving in the opposite direction, they move 

in the same direction as the trailing edge of the stabilator. 

In addition to decreasing the sensitivity of the stabilator, an 

antiservo tab also functions as a trim device to relieve control 

pressure and maintain the stabilator in the desired position. 

The fixed end of the linkage is on the opposite side of the 

surface from the horn on the tab; when the trailing edge of the 

stabilator moves up, the linkage forces the trailing edge of the 

tab up. When the stabilator moves down, the tab also moves 

down. Conversely, trim tabs on elevators move opposite of 

the control surface. [Figure 5-21]

Ground Adjustable Tabs

Many small aircraft have a nonmovable metal trim tab on the 

rudder. This tab is bent in one direction or the other while on 

the ground to apply a trim force to the rudder. The correct 

displacement is determined by trial and error. Usually, small 

adjustments are necessary until the aircraft no longer skids 

left or right during normal cruising flight. [Figure 5-22]
5-12

Figure 5-24. Basic autopilot system integrated into the flight 

control system.

Figure 5-23. Some airplanes, including most jet transports, use an 

adjustable stabilizer to provide the required pitch trim forces.

Figure 5-19. Some airplanes, including most jet transports, 

use an adjustable stabilizer to provide the required pitch trim 

forces. 

Adjustable stabilizer

Nose down

Nose up

Trim motor or trim cable

Jackscrew

Pivot

Adjustable Stabilizer

Rather than using a movable tab on the trailing edge of the 

elevator, some aircraft have an adjustable stabilizer. With this 

arrangement, linkages pivot the horizontal stabilizer about its 

rear spar. This is accomplished by use of a jackscrew mounted 

on the leading edge of the stabilator. [Figure 5-23]

On small aircraft, the jackscrew is cable operated with a trim 

wheel or crank. On larger aircraft, it is motor driven. The 

trimming effect and flight deck indications for an adjustable 

stabilizer are similar to those of a trim tab.

Autopilot

Autopilot is an automatic flight control system that keeps an 

aircraft in level flight or on a set course. It can be directed by 

the pilot, or it may be coupled to a radio navigation signal. 

Autopilot reduces the physical and mental demands on a pilot 

and increases safety. The common features available on an 

autopilot are altitude and heading hold. 

The simplest systems use gyroscopic attitude indicators and 

magnetic compasses to control servos connected to the flight 

control system. [Figure 5-24] The number and location of 

these servos depends on the complexity of the system. For 

example, a single-axis autopilot controls the aircraft about the 

longitudinal axis and a servo actuates the ailerons. A threeaxis autopilot controls the aircraft about the longitudinal, 

lateral, and vertical axes. Three different servos actuate 

ailerons, elevator, and rudder. More advanced systems often 

include a vertical speed and/or indicated airspeed hold mode. 

Advanced autopilot systems are coupled to navigational aids 

through a flight director.

The autopilot system also incorporates a disconnect safety 

feature to disengage the system automatically or manually. 

These autopilots work with inertial navigation systems, 

global positioning systems (GPS), and flight computers to 

control the aircraft. In fly-by-wire systems, the autopilot is 

an integrated component. 

Additionally, autopilots can be manually overridden. Because 

autopilot systems differ widely in their operation, refer to the 

autopilot operating instructions in the Airplane Flight Manual 

(AFM) or the Pilot’s Operating Handbook (POH).

Chapter Summary

Because flight control systems and aerodynamic characteristics 

vary greatly between aircraft, it is essential that a pilot 

become familiar with the primary and secondary flight 

control systems of the aircraft being flown. The primary 

source of this information is the AFM or the POH. Various 

manufacturer and owner group websites can also be a 

valuable source of additional information.