Aerodynamics: Instability & Control Surfaces Flashcards
What is Dutch roll?
Dutch roll is an oscillatory instability associated with swept-wing jet aircraft.
It is the combination of yawing and rolling motions. When the aircraft yaws, it will develop into a
roll. The yaw itself is not too significant, but the roll is much more noticeable and unstable. This is so
because the aircraft suffers from a continuous reversing rolling action.
What causes Dutch roll?
Swept wings.
Dutch roll occurs when a yaw is induced either by a natural disturbance or by a commanded or an
uncommanded yaw input on a swept-wing aircraft. This causes the outer wing to travel faster and to
become more straight on to the relative airflow (in effect, decreasing the sweep angle of the wing and
increasing its aspect ratio). Both of these phenomena will create an increased airflow speed over the
wing’s upper surface, which produces more lift and increases its angle of attack. At the same time, the
inner wing will travel slower and, in effect, become more swept relative to the airflow, and both
these phenomena will reduce its lift. Therefore, a marked bank occurs to the point where the outer,
upward moving wing exceeds its critical angle of attack, stalls and loses all lift (remember, a stall is
a function of angle of attack, not speed). Therefore the wing drops, causing a yaw to the dropped wing
and thus leading to the sequence being repeated in the opposite direction. This sequence will continue
and produce the oscillatory instability around the longitudinal axis we know as Dutch roll. Pitch
fluctuations only occur with an extreme degree of Dutch roll.
What is the recovery technique from Dutch roll?
For a pilot to recover an aircraft suffering from Dutch roll, he or she would apply opposite aileron to
the direction of the roll, assuming that the yaw dampers are not serviceable.
Although the root cause of Dutch roll is the yawing motion, application of a correcting rudder input
by the pilot normally would worsen the situation. This is so because the yawing motion in the
oscillatory cycle happens extremely quickly, and the pilot’s reaction would not be quick enough to
catch the yaw, which already has developed into a roll and dissipated.
Therefore, a rudder input to correct the initial yaw (which has since dissipated) would in fact
aggravate the roll effect further into a sideslip.
Aileron control therefore is employed because the roll cycle is of sufficient duration to allow the
pilot to apply the correct opposite aileron control. A severe Dutch roll may require two or three
aileron inputs to dampen the oscillation gradually.
What prevents Dutch roll?
Yaw dampers prevent Dutch roll on swept-wing aircraft.
A basic reason for the Dutch-rolling tendency of an aircraft (apart from the wing sweep, of course)
is the lack of effective fin and rudder area to stop it. The smaller fin and rudder area is a design
compromise that makes the aircraft spirally stable to a degree. Therefore, the effectiveness of the fin
area must be increased in some other way to prevent Dutch roll. This is achieved with yaw dampers.
What is directional stability?
Directional stability is the tendency for an aircraft to regain its direction (heading) after the aircraft
has been directionally disturbed (e.g., an induced yaw) from its straight path. This is achieved
naturally because the fin (vertical tailplane) becomes presented to the airflow at a greater angle of
incidence, which generates a restoring aerodynamic force.
What is spiral stability and instability?
Spiral stability (or a spirally stable aircraft) is defined as the tendency of an aircraft in a properly
coordinated banked turn to return to a laterally level flight attitude on release of the ailerons. Spirally
stable aircraft have dominant lateral surfaces (e.g., wings).
Spiral instability or a spirally unstable aircraft will see a banked turn increase fairly quickly,
followed by the nose falling into the turn, leading to the aircraft entering into a spiral dive when the
ailerons are released in a coordinated turn. Spirally unstable aircraft have dominant (too large)
vertical surfaces (e.g., tailplane). What happens is that as the aircraft starts to slip into the turn on
release of the ailerons and before the rolling moment due to the sideslip can take effect, the rather
dominant fin jumps into play. This is so because the fin/tailplane area (outside) becomes exposed to
the relative airflow, which exerts two forces on the aircraft:
1. Around the vertical axis, which straightens the aircraft directionally
2. Around the longitudinal axis, which increases the bank
This accelerates the outer (upper) wing and causes the bank to be increased further. The increased
bank causes another slip, which the fin again straightens. This sequence repeats, and the turn is thus
made steeper. Once the bank angle exceeds a given type-specific amount (say, 30°), the nose falls into
the turn, the speed increases as the roll increases, and the aircraft enters into a spiral dive.
What is lateral stability?
Lateral stability is the tendency for an aircraft to return to a laterally level position around the
longitudinal axis on release of the ailerons in a sideslip.
There are two principal features that make an aircraft naturally laterally stable, namely,
1. Wing dihedral. The airflow due to a sideslip causes an increase in the angle of attack (lift) on the
lower (leading) wing and a decrease in angle of attack on the raised wing because of the dihedral
angle. The lower wing thus produces an increase in lift because of the increased angle of attack,
and the raised wing produces less lift. The difference in lift causes a rolling moment that tends to
restore the wing to its laterally level position.
2. Side loads produced on the keel surface. When the aircraft is sideslipping, a side load will be
produced on the keel surface, particularly the fin. This side load will produce a moment to roll the
aircraft laterally level, which in general terms is stabilizing. The magnitude of this effect depends
on the size of the fin, but regardless, its effect is small compared with other laterally stabilizing
effects.
What are the primary/main flight controls?
Elevator. Controls the motion around the lateral axis, known as pitch/pitching.
Ailerons. Control the motion around the longitudinal axis, known as roll/rolling.
Rudder. Controls the motion about the normal/vertical axis, known as yaw/yawing.
What is longitudinal stability?
Longitudinal stability is an aircraft’s natural ability to return to a stable pitch position around its
lateral axis after a disturbance.
When an aircraft is in equilibrium, the tailplane in general will be producing an up or down load
to balance the moments about the center of gravity. (It is assumed that throughout the elevator remains
in its original position during any disturbance in pitch.) If the aircraft is disturbed in pitch (say, noseup), there will be a temporary increase in the angle of attack. The increase in tailplane angle of attack
produces an increase in tailplane lift, which will cause a nose-down pitching moment. (The tailplane
is thus able to produce a stabilizing moment due to a displacement in pitch as long as the center of
gravity remains within its limits.) The wings also experience this increase in angle of attack, resulting
in the wings producing an increase in lift. The moment and the direction of the moment produced by
this lift will depend on the relative positions of the center of pressure and the center of gravity.
Describe stability at high altitudes.
Longitudinal, lateral, directional, and oscillatory stability in general are reduced at high altitudes, in
terms of dynamic stability, mainly because aerodynamic damping decreases with altitude.
The aircraft will feel and is less stable except for spiral stability, which improves with altitude,
whereas oscillatory stability deteriorates very rapidly with altitude. This is so because for a constant
indicated airspeed (IAS), the fin suffers a smaller angle of incidence and therefore has a smaller
restoring force the higher the altitude. Therefore, the fin is less dominant, which is detrimental to
oscillatory stability but as a consequence means that the aircraft’s lateral surfaces (wings) become
more dominant. This improves the aircraft’s spiral stability qualities (spiral stability always opposes
oscillatory stability, and vice versa).
What is the elevator, and how does it work?
The conventional elevator is a hinged control surface at the rear of the horizontal tailplane (stabilizer)
that is controlled by the pilot’s control column.
As the elevator control surface is deflected, the airflow and thus the aerodynamic force around the
elevator (horizontal tailplane) changes.
Moving the control column back deflects the elevator up, causing an increase in the airflow speed
and thus reducing the static pressure on the underside of the elevator control surface. In addition, the
topside of the elevator faces more into the relative airflow, which causes an increase in the dynamic
pressure experienced. These effects create an aerodynamic force on the elevator (horizontal
tailplane) that rotates (pitches) the aircraft about its lateral axis. That is, back control column
movement moves the elevator control surface upward, producing a downward aerodynamic force that
pitches the aircraft up. Thus the opposite is also true: Forward control column movement moves the
elevator control surface downward, producing an upward aerodynamic force that pitches the aircraft
down
What are ailerons, and how do they work?
Ailerons are control surfaces located at the trailing edges of the wings that control the aircraft’s
motion around its longitudinal axis, known as roll. The ailerons are controlled by left and right
movement of the control column, which commands the ailerons in the following manner: Moving the
control column to the left commands the left aileron to be raised, which reduces the lift on the wing,
and the right aileron is lowered, which increases the lift generated by this wing, thereby rolling the
aircraft into a banked condition, which causes a horizontal lift force (centripetal force) that turns the
aircraft. The ailerons normally are powered (hydraulically) powered on heavy/fast aircraft because
of the heavy operating forces experienced at high speeds.
What is the rudder and how does it work?
The rudder is a hinged control surface at the rear of the fin (vertical tailplane) that is controlled by the
pilot’s rudder pedals.
As the rudder control surface is deflected, the airflow and thus the aerodynamic force around the
rudder (vertical tailplane) changes. Moving the left rudder pedal deflects the rudder to the left,
causing an increase in the airflow speed and thus reducing the static pressure on the right-hand side of
the rudder control surface. In addition, the left side of the rudder faces more into the relative airflow,
which causes an increase in the dynamic pressure experienced. These effects create an aerodynamic
force to the right on the rudder (vertical tailplane) that rotates (yaws) the aircraft about its
vertical/normal axis at its center of gravity point to the left.
How does the effectiveness of the control surfaces vary with speed?
The control surfaces become more effective at higher speeds. This results in a requirement for large
control movements at low speeds and smaller control movements at high speeds to produce the same
control force.
What is elevator reversal?
Elevator reversal occurs at high speeds when the air loads/forces are large enough to cause a twisting
moment on the deflected elevator surface to either a neutral or opposite position that results in sudden
reversal of the aircraft’s pitch attitude.
What is adverse yaw?
Adverse yaw is a yawing motion opposite to the turning/rolling motion of the aircraft.
Adverse yaw is caused by the drag on the down-going aileron being greater than that on the upgoing aileron. This imbalance in drag causes the yawing motion around the normal/vertical axis.
Since this yaw is adverse (i.e., in a banked turn to the left, the yaw is to the right), it is opposing the
turn, which is detrimental to the aircraft’s performance.
Adverse (aileron) yaw is corrected in the design by the use of either differential ailerons or Frise
ailerons.
What is aileron reversal (adverse), and when is it likely to occur?
Aileron reversal occurs at high speeds when the air loads/forces are large enough that they cause an
increase in lift. Because most of this lift is centered on the down-going aileron at the rear of the wing,
a nose-down twisting moment at the wing will be caused. This will result in a decrease in the
incidence of the wing to the extent that the loss of lift due to the wing twisting cancels the lift gained
from the aileron. At this point the aileron causes no rolling moment, and if the wing twisting is
exaggerated (which a down-going aileron can do), the rolling motion around the longitudinal axis can
be reversed; hence an adverse rolling motion.
What is a yaw-induced adverse rolling motion, and when is it likely to occur?
The rudder inducing the aircraft to yaw one way can cause another form of adverse rolling motion in
the opposite direction. This happens at high speeds (above VMO/MMO) because the deflected rudder
experiences a sideways force that causes the aircraft to roll in the opposite direction; i.e., right rudder
experiences a sideways force from right to left, causing a rolling moment to the left.
What are spoilers, and how do they work?
Spoilers consist of opening panels that extend from the upper surface of the wing and have the effect
of spoiling/disturbing the airflow over the wing (drag), thereby reducing the lift.
For roll control:
The spoilers are raised on one wing and not the other, which creates an imbalance of lift values that
produces a rolling moment. The spoilers are connected to the normal aileron controls, and they work
in tandem with each other for roll control. Spoilers are in fact a more efficient roll-control surface
than ailerons. The disadvantage of roll-control spoilers is that they cause an overall loss of lift, which
may cause a loss of height and is particularly undesirable when flying close to the ground.
As air speed brakes:
The spoilers are raised symmetrically on both wings to a flight detent position (using the speed brake
lever), which causes a large increase in drag that slows down the speed of the aircraft.
Note: Buffet is usually experienced with spoiler (speed brake) deployment.
As ground lift dumpers:
The spoilers are raised systematically on both wings to the ground detent position (greater angle than
the flight detent position), which causes a large increase in drag that (1) decreases lift over the wing,
causing the aircraft to sink to the ground, and (2) acts as ground speed brakes to slow down the speed
of the aircraft
What are the three purposes of spoilers?
- Roll control (usually in combination with the ailerons). Note that the primary purpose of spoilers is
roll control. - Air speed brakes
- Ground lift dumpers
Describe differential and nondifferential spoilers.
The difference between differential and nondifferential spoilers is in how they provide lateral roll
control when already extended as speed brakes.
Nondif erential spoilers. When already partly extended as a speed brake, the spoilers will
extend further on one side but will not retract on the other side in response to a roll command.
When already fully extended as a speed brake, both sides remain in the extended speed brake
position, and therefore, the spoilers do not provide any roll control.
Dif erential spoilers. When already partly extended as a speed brake, the spoilers will extend
further on one side and retract on the other side in response to a roll command. When already
fully extended as a speed brake, the spoilers will remain extended on one side and retract on the
other side in response to a roll command.
Give six reasons for spoilers.
- The ailerons are limited in size and therefore effectiveness.
- On a thin swept wing, ailerons that are too large will experience a high degree of air loading/lift,
resulting in the wing twisting at high speeds that can produce aileron reversal (removes aileron
roll control), which is very detrimental. (See Q: What is aileron reversal (adverse), and when is
it likely to occur? page 36.) - Ailerons tend to lose effectiveness at high speeds due to the spanwise diagonal airflow across the
aileron, which is less effective than a perpendicular airflow. - High-speed swept-winged aircraft cause a strong rolling moment with yaw, known as adverse
rolling moment with yaw. (See Q: What is yaw-induced adverse rolling moment, and when is it
likely to occur? page 37.)
Other than roll control, the spoilers are needed to counteract (brake) the aircraft’s high speed in
the air and on the ground: - Because the aircraft has low drag and the engines have a slow lag response rate, there is a need for
high-drag devices in flight to act as a brake when the aircraft is required to lose speed and/or
height quickly. This is achieved by the use of the spoilers on both wings being raised
simultaneously to the flight detent position, which creates a drag force opposing thrust and
therefore reduces the aircraft’s speed and/or height. - On landing or during a rejected takeoff, there is a need to dump the lift off the wing and onto the
wheels to assist in stopping the aircraft. This is achieved by the use of the spoilers on both wings
being raised simultaneously to the ground or up detent position in a similar manner as the inflight
speed brake. This position has a greater angle of deployment than the flight detent and/or uses more
spoiler panels, therefore creating a greater drag force.
What limits the use of spoilers, and why do spoilers blow back?
Spoilers are limited by very high speeds (VDF/MDF), which cause them to blow back.
At very high speeds, the spoilers will be blown back to or near to their fully retracted position.
This occurs because the high air loads experienced on the spoilers’ surfaces at high speeds are
greater than their design limit. Obviously, the force experienced is a function of airspeed and angle of
deflection
How is spoiler blowback prevented?
Spoilers are designed not to blow back in the normal operating speed range of the aircraft. Therefore,
correct speed management of the aircraft will prevent the spoilers from blowing back
How do you correct for spoiler blowback?
In flight, reduce speed by reducing thrust to a speed where the spoilers will operate normally, and
then recycle the speed brake lever.
Note: Spoiler blowback will only occur when the aircraft’s speed is excessive (i.e., VDF/MDF),
which itself should be experienced only in a nonnormal flight condition, e.g., spiral dive, etc., when
the recovery drill incorporates reducing speed by closing the throttles.
What do leading-edge slats do?
Leading-edge slats increase the wing’s chamber area and mean aerodynamic chord (MAC), thereby
increasing its coefficient of lift (CL
) maximum, which reduces the aircraft’s stall speed.
What are Krueger flaps?
Krueger flaps are leading-edge wing flaps used to increase the wing chamber and therefore increase
the coefficient of lift maximum.
What are Fowler flaps?
Fowler flaps are trailing-edge wing flaps (usually triple slotted) used to increase the wing area and
chamber, which increases the coefficient of lift maximum for low flap settings, e.g., 1 to 25°. High
flap settings increase drag predominately more than lift and therefore are used to lose speed and/or
height, most commonly during an approach to land.
What is the primary use of flaps on a jet aircraft?
The primary use of flaps, especially on a jet aircraft, is to increase lift by extending the geometric
chord line of the wing, which increase its chamber and area.
What are the effects of extending flaps in flight?
Lowering the flaps in flight generally will cause a change in the pitching moment. The direction and
degree of the change in pitch depend on the relative original position of the center of pressure and the
center of gravity.
The factors that contribute to this are
1. The increase in lift created by the increased wing area and chamber will lead to a pitch-up moment
if the center of pressure remains in front of the center of gravity.
2. If the associated rearward movement of the center of pressure is behind the center of gravity, then
this will produce a nose-down pitch.
3. The flaps will cause an increase in the downwash, and this will reduce the angle of attack of the
tailplane, giving a nose-up moment.
4. The increase in drag caused by the flaps will cause a nose-up or nose-down moment depending on
whether the flaps are above or below the lateral axis.
The overall change and direction in the pitching moment will depend on which of these effects is
predominant. Normally, the increased lift created by extending the wing chord line when the flaps are
extended is dominant and will cause a nose-up pitching tendency because the center of pressure
normally remains in front of the center of gravity.
What are the effects of raising flaps in flight?
The raising of flaps in flight, if not compensated for by increasing speed and changing attitude, will
result in a loss of lift.
How do flaps affect takeoff ground run?
Flaps set within the takeof range. A higher flap setting, within the takeoff range, will reduce the
takeoff ground run for a given aircraft weight. The use of flaps increases the maximum coefficient of
lift of the wing due to the increased chord line for a low drag penalty, which reduces the stall speed
(VS
) and consequently the rotation (VR
) and takeoff safety (V2
) speeds. This provides good
acceleration until it has sufficient kinetic energy to reduce the takeoff ground run. Typically, various
flap settings from the first to the penultimate flap setting are available for takeoff (i.e., takeoff range).
The higher the flap setting within this range, the less is the takeoff run required because the drag is not
significantly increased because the angle of attack is low. However, the drag increment is higher
when the aircraft is in flight and out-of-ground effect because of the aircraft’s angle of attack is much
higher.
Note: Initial and second-segment climb performance thus will be reduced with a high takeoff flap
setting. (See Q: How does the use of flaps af ect the aircraft’s takeof performance? page 197.)
Flaps set outside the takeof range. A high flap setting outside the takeoff range will result in a
large drag penalty that will reduce the aircraft’s acceleration, and therefore, the takeoff run will be
greatly increased before VR
is attained. No or a very low flap setting outside the takeoff range on
takeoff will result in a low coefficient of lift produced by the wing for a given speed, and thus a
higher unstick (VR
) speed is required to create the required lift for flight. Therefore, an increased
takeoff run to attain the higher VR
is required.
What are the purposes of yaw and roll dampers, and how do they work?
The purpose of a yaw damper is to: (1) prevent Dutch roll and (2) coordinate turns. The purpose of a
roll dampers is to: (1) damp/remove Dutch roll, (2) provide roll damping in turbulence, and (3)
provide spiral stability.
The yaw damper’s main purpose is to prevent Dutch roll. When the fin area is insufficient to
provide a natural oscillatory stability, the effective fin area must be increased in some other way.
This is accomplished on power-operated rudders with the yaw damper. (See Q: What causes Dutch
roll? page 31.)
A yaw damper is a gyro system that is sensitive to changes in yaw, and it feeds a signal into the
rudder, which applies opposite rudder to the yaw before the roll occurs, thus preventing Dutch roll.
The roll damper works through the aileron controls and can be used to
1. Supplement a yaw damper for Dutch roll control by damping out the roll once it has been
established (especially common on oscillatory unstable aircraft).
2. Purely for roll damping in turbulence.
3. Control spiral stability. (See Q: What is spiral stability and instability? page 32.)
A roll damper normally is associated with providing spiral stability.
Describe parallel yaw dampers.
Parallel yaw dampers (early type) apply rudder control through the same control run as the pilot, and
their activity is reflected in the rudder bar activity (moves the rudder pedals). While this provides a
visual indication of the yaw dampers’ serviceability, it does increase the rudder loads experienced
by the pilot. To prevent this making matters worse in the event of an engine out failure on takeoff or a
crosswind landing, the damper can be switched off for takeoffs and landings. Since this damper in
effect parallels the pilot’s actions, it became known as the parallel yaw damper.
Describe series yaw dampers.
The series yaw damper is a development of the parallel yaw damper and is found commonly in
modern jet aircraft. This system is attached to the rudder control circuit at the back of the aircraft, and
as such, it does not move the pilot’s rudder pedals when it moves the rudder. This means that the
rudder foot forces are not increased and therefore allows the series yaw damper to be used for
engine-out takeoffs and landings.
What is a stabilizer/variable-incidence tailplane?
A stabilizer/variable-incidence tailplane is an all-moving horizontal tailplane control surface (i.e.,
not fixed in one position). (See Q: What is the purpose of a stabilizer? page 42.)
Normally, an all-moving horizontal tailplane is called a stabilizer when it is solely responsible
for longitudinal balancing and it has a separate elevator with its own controls and movement range
for pitch maneuverability. A stabilizer normally is moved by its own independent stab trim system
that can be either a manual or an automatic device. Normally, an all-moving tailplane is called a
variable-incidence tailplane when it does not have an elevator surface. Therefore, the variableincidence tailplane provides pitch maneuverability by control column and longitudinal balancing by
the trim system.
What is the purpose of a stabilizer?
The purpose of the horizontal stabilizer is to provide a longitudinal balancing force to the aircraft.
Thus the elevator range and aircraft’s pitch maneuverability are not compromised and remain
available to be used solely to control the pitch of the aircraft. The stabilizer covers large and small
pitching moments; e.g., a single person who moves from the aft to the forward cabin will displace the
overall weight balance.
What are the four reasons for a variable-incidence tailplane/stabilizer, especially on a jet
aircraft?
The four reasons for use of a variable-incidence tailplane or stabilizer are
1. To provide a balancing force for a large center of gravity range
2. To provide a balancing force for a large speed range
3. To cope (longitudinally) with large trim changes as a result of position changes to the wing leading
and trailing edge high-lift devices (configuration changes)
4. To reduce elevator trim drag to a minimum
While any of these requirements in isolation might not demand a variable-incidence tailplane, in
combination they certainly do. Once the need has been established for one of the requirements, then
advantages also occur in the other areas.
Describe the effects of a stuck stabilizer.
The condition an aircraft experiences from a stuck stabilizer, other than when the aircraft’s
equilibrium is matched to the position of the stuck stabilizer, is a degraded longitudinal balancing
ability, which is due to the backup employment of the less powerful elevator in providing this
balancing force.
This condition has the following effects:
1. The longitudinal balancing force the elevator experiences is much higher than when it is used for
its normal pitch control duties, and this is translated into heavy stick forces felt by the pilot.
2. Because the elevator surface is smaller than the stabilizer, the response to the pilot’s inputs is
slower.
3. The elevator’s pitch-control capability is reduced because the majority of its range is being used to
provide a longitudinal balancing force.
What is the best center of gravity position with a stuck stabilizer, and why?
With a stuck stabilizer, the best center of gravity position is aft. This can be accomplished by moving
passengers to the rear of the aircraft and/or by moving fuel to outer wing tanks if possible.
What is the required action with a stuck stabilizer?
If a stabilizer has stuck while the aircraft is in a substantially trimmed-out condition, then as long as
you maintain the speed at which the tail jammed, you will remain substantially in trim and therefore
stable. However, because the aircraft will have to depart from the cruise speed in preparation to
land, the following steps will reduce the main reasons for having a variable-incidence tailplane in the
first place and also reduce the effects of a stuck stabilizer. (See Q: Describe the ef ects of a stuck
stabilizer? page 43.) This helps to maintain the aircraft’s longitudinal stability and maneuverability
in pitch during its approach and landing. Generally, the following procedure would be followed:
1. Divert to a nearby airfield (so that the center of gravity movement due to the aircraft’s change in
weight is not excessive over a prolonged flight).
2. Move the center of gravity to an aft position. This will
a. Longitudinally balance the aircraft at low speeds and thereby reduce the stick forces.
b. Reduce the elevator demand for the landing flare. (See Q: What is the best center of gravity
position with a stuck stabilizer, and why? page 43.)
3. Reduce speed as late as possible to minimize the length of time a balancing force, with its
associated high stick forces, is required from the elevator.
4. Plan a long final approach, and make configuration changes, gear, and flaps earlier than usual to
give time to sort out the aircraft before the next change is due.
5. Use a reduced flap setting for landing, which will reduce the landing flare required. This allows
you to maintain a higher approach speed, which reduces the divergence of the aircraft from its
cruise trimmed speed, reduces the balancing force required from the elevator, and thus reduces the
stick forces experienced.
Having exercised these measures in response to this failure, the aircraft will retain enough scope
to maintain a longitudinally stable condition and enough elevator pitch maneuverability to adjust the
aircraft’s approach and landing attitude, which should not produce too many problems for the pilot.
Describe a runaway stabilizer condition and required action.
A stabilizer typically is held in its trimmed position by a series of brakes for both manual and
autopilot modes. If these brakes should fail, then the stabilizer will experience backpressure from the
airflow, which will rotate the stabilizer to its maximum upward or downward mechanical stops,
thereby inducing a marked out-of-trim unstable condition.
A stabilizer runaway, whether auto or manual trim, should not go undetected for too long, and
when detected, decisive action should be employed.
Each aircraft type has its own drill, but in general terms:
1. Hold the control column firmly.
2. Autopilot (if engaged), disengage.
3. Stab trim cutout switches to cut out.
If the runaway continues,
4. Stabilizer trim wheel, grasp and hold.
Continue your flight using manual trim, and adopt early airspeed and configuration conditions.
Describe the effects of a jammed/degraded elevator.
A degraded or jammed elevator will result in less effective elevator maneuverability in pitch control.
What is the best center of gravity position with a jammed/degraded elevator?
The best position for the center of gravity with a degraded/jammed elevator is aft. This can be
accomplished by moving the passengers to the rear of the cabin and/or by moving fuel to the outer
wing tanks if possible. An aft center of gravity position lessens the need for large pitch-control
demands, especially during the approach and landing flare.
What is the required action for a jammed/degraded elevator?
If there is no elevator control (jammed), then the stabilizer trim can be used for pitch control. If the
elevator control is reduced (degraded), then it should be assessed if there is still sufficient elevator
range available to land safely and used to do so if applicable. If not, then the condition should be
regarded as having no elevator control.
With a degraded or jammed elevator, several actions can be taken to minimize the need for major
pitch changes and/or to improve the handling and management of the aircraft:
1. If possible, move the center of gravity rearward; this will reduce the need for large elevator
angles, especially during the landing.
2. Plan a long final approach, and make configuration changes, gear, and flaps earlier than usual to
allow more time to sort out the aircraft before the next change has to be made.
3. Restrict the flap angle for landing to reduce the flare demanded.
Describe the effects of a failure/reduction in elevator feels.
Artificial feel systems (normally duplicated) are employed on powered controls, especially the
elevators. They meet the requirement of progressive feel against control surface deflection at constant
speed and against a constant angle at varying speed based on our old friend
½ R V2
Whenever the feel on an elevator control is significantly reduced, great care must be exercised in
its use. The control must be moved slowly and smoothly over minimum angles to avoid overstressing
the control surface structure but enough to maintain the flight path. Overstressing the control surface
with a lack of feel is a significant problem, and for this reason, turbulence should be avoided.
What is the best center of gravity position with a reduced or failed elevator feel system?
The best position for the center of gravity with a reduced or failed elevator feel system is forward.
This can be accomplished by moving the passengers to the front of the cabin and/or by moving the
fuel into forward tanks if possible.
This increases the aircraft’s natural longitudinal stability and renders the pitch control less
sensitive and feeling heavier, therefore making the aircraft less responsive to small elevator
movements. Thus the chances of the pilot overstressing the elevator control surface are minimized,
although still possible.
Describe the effects of the air loads on a control surface and how these effects are managed?
If a control surface is deflected, the dynamic pressure/aerodynamic loads on it will increase and act
as a lift force through its center of pressure point. When multiplied by the control surface arm, it gives
the size of the moment trying to rotate the control surface back to its neutral position. This moment is
known as the hinge moment or air load force. That is,
Hinge moment (air loads force) = lift force (air load) × arm
Note: Where the lift force is a design product of the size of the control surface and the magnitude
of the lift force experienced depends on (1) airspeed and (2) angle of deflection of the control
surface. That is, the lift force increases dynamically in flight with either an increased control angle of
deflection at a constant speed or at a constant angle of deflection with an increased speed. And where
the arm is a design product of the distance between the center of pressure point and the hinge line.
They produce a hinge moment/air load force that tries to return the surface to its neutral position.
What is a tab surface, and what can it be used for?
A tab is a small hinged surface found on a primary flight control surface. Usually the tab forms part of
the trailing edge on a manual control surface. A tab can be used to provide
1. Trimming
2. Control balance
3. Servo operation of a control surface
to reduce/balance the opposing hinge moment (air load force) on the associated control surface.
What is a balance tab?
A balance tab is a form of aerodynamic control balance on a control surface. A control balance tab
balances the main aerodynamic lift force load on a control surface with an opposing force, which
thereby reduces the overall hinge moment (air load force).
Hinge moment (air loads) = lift force × arm
This is reflected by the stick control force that the pilot experiences being reduced to a manageable
level. (See Q: Describe the ef ects of the air loads on a control surface and how these ef ects are
managed, page 46.)
What is a hinge/horn balance?
A setback hinge is another form of aerodynamic control balance on a control surface, whereby the
design of the control surface sets the hinge line back into the control surface, thus reducing the center
of pressure to hinge line arm, which results in reducing the control surface hinge moment.
[Remember, hinge moment (air loads) = lift force × arm.] Thus the stick control force that reflects the
overall hinge moment experienced on the control surface is reduced to a manageable level for the
pilot. (See Q: Describe the ef ects of the air loads on a control surface and how these ef ects are
managed, page 46.)
Another form of aerodynamic control balance on a control surface is a horn balance. A horn
balance is a protruding control surface that produces a balancing lift force in the opposite direction of
the main lift force and reduces the overall hinge moment/air load force. They are common on
elevator/stabilizers. Thus the stick control force that reflects the overall hinge moment experienced on
the control surface is reduced to a manageable level for the pilot.
Note: A horn balance and setback hinge typically are used in tandem on a control surface.
What is a mass balance and what is it used for?
A mass balance is another form of aerodynamic balance control on a control surface. The hinge
moment/air load force experienced by a deflected control surface tries to rotate the control surface
back to its neutral position, but it is balanced by a mass weight that keeps it in its deflected position.
Thus the stick control force that reflects the overall air load force/hinge moment experienced on the
control surface is reduced to a manageable level.
Why are control surfaces hydraulically operated on large aircraft?
On large, fast aircraft, especially modern jets, it is found that the control forces required to move a
control surface are simply beyond the strength of the pilot and are also too great to be controlled by
pure aerodynamic designs, e.g., balance tabs. This is so because the shear sizes and weights of the
control surface arms in question and the aerodynamic airflow lift forces (load) generated on the
deflected control surface are too great.
For modern large, fast jet aircraft, the answer lies in the powered control surface, typically
hydraulic-powered systems, because they generate enough power to cope with the full air load force
(i.e., not balanced) experienced on the control surface.
Why does a powered controlled surface need an artificial feel system?
An artificial feel system is required because power-operated flying controls are irreversible, i.e.,
they do not feed back to the pilot any sensory information about how hard the control surface is and
thus what aerodynamic air forces it is coping with. Therefore, there is a need to give this information
to the pilot so that he or she is aware of the control angles being applied and their effect on the
aircraft, in short, keeping the pilot in the sensory loop, which allows him or her to guard against
overstressing the control surface.
How does an artificial feel system work?
The simplest form of artificial feel consists of a spring box fitted into the control run. This provides a
feel and self-centering action, but the stick forces are constant and therefore are only suitable for
aircraft types with a limited altitude and speed range.
What is Q feel, and where is it used?
Q feel is a sophisticated computer-based artificial feel system based on ½ R V2
that is felt by the pilot
through the control column and rudder pedals and is used commonly on aircraft with powered flight
controls, i.e., elevator, rudder, and ailerons. It meets the requirements of progressive feel to match
variable control surface deflection at a constant speed and/or for a constant angle of deflection at
varying speeds
What are the inputs to Q feel?
- Static and dynamic pressure
- Control surface angle of deflection
What are active controls?
An active control is a surface that moves automatically/actively in response to nondirect inputs. For
example, balance tabs actively/automatically move in response to their associated control surfaces
being moved. Auto slats actively/automatically move to their full extend position to provide a better
coefficient of lift (CL
) on a B737-300 if the aircraft senses a particular flight condition; i.e., trailingedge flaps set 1 to 5 positions, auto slats in normal position, and aircraft close to the stall.
