What is automatic flight control system?
The main purpose of the Automatic Flight Control System is to
incoparate several features to reduce the pilots workload. Some of the features are stability augmentation, control
augmentation and manual/ auto trim facilities and various kinds of
autopilots. Another purpose is to carry
out various manoeuvres that the pilot
isn’t unable to perform either due to the length of time over which it is
necessary to carry out the task accuracy required or unable to perform due to
the accuracy required or due to the lack of visual cues etc. Some examples of
this might be: precise altitude holding at cruising heights for long periods, ILS-coupled
approaches in extremely low visibility, and hover at night or over water.
The Automatic Flight Control Systems (AFCS) consist of
various independent systems. They are the Digital Flight Control System, auto-throttle system and yaw damper system. These various systems
provide automatic airplane stabilization about the 3 axes, pitch, roll and yaw.
They also control the aircraft with selective guidance from various inputs such
as heading, radio, flight management computer, and air data computer inputs.
The AFCS uses duplex lanes in pitch, roll and collective axes
and has a simplex lane in yaw axis. Duplex is when both ends of the communication can
send and receive signals at the same time. Full-duplex communication is
the same thing as duplex. The only difference in Half-duplex is that
although its bidirectional communication, its signals can only flow in one
direction at a time. Simplex is a communication channel that transmits
information in one direction only. Each lane is built around one Automatic
Flight Control Computer (AFCC).
The Digital Flight Control System (DFCS) system is a dual-axis system which operates in the pitch and roll axis
which are controlled by the elevators and ailerons. It automatically maintains
altitude, airspeed and guides the aircraft to its designated locations and makes
automatic landings without the pilot having to do much. The Control functions
are also translated into the flight director commands for display on the
pilot’s Electronic Attitude Director Indicator and thereby providing the
pilots’ flight attitude inputs during manual operation or allowing the pilots
to monitor autopilot operation. Automatic stabiliser trimming relieves the
various sustained elevator loads which might be incurred due to fuel burnoff.
The yaw damper systems operate the rudders to
correct any kind of periodic yaw oscillations or dutch rolls.
An auto-throttle system will automatically maintain the
selected airspeed or Mach number during cruising conditions and maintains
inputed engine thrust settings when making the flight director controlled
take-off or autopilot/flight director controlled landing approaches by
adjusting engine thrust levers.
The yaw damper is a full-time system, series-connected,
stability augmentation system. Aircrafts Dutch rolls are detected with a rate
sensor in a yaw damper computer system. The rudder is displaced at its proper
time to dampen out any Dutch roll before it can cause any significant effects
on the flight path of the aircraft. The yaw damper system and actuators are
connected such that there is no rudder feedback applied to the rudder pedals. Therefore,
allowing the system to operate independently without getting involved with the pilots
initiated rudder commands.
In a normal aircraft, the yaw damper is a part of the
automatic flight control system. It can normally be engaged independently without
the rest of the autopilot system. On the other hand, most autopilot systems
require the yaw damper to be activated when the rest of the autopilot is activated.
Basically, in layman terms, you can have yaw damper without autopilot, but not
autopilot without the yaw damper.
Now another part of the Integrated Digital autopilot system
is modern yaw dampers.
They use rate sensors and accelerometers to determine the
aircraft’s motion. It then runs the inputs and numbers through special
algorithms and formulas to determine what rudder inputs needed to be made in
order to damp any kind of Dutch roll and to coordinate any type of turn. It also
provides those rudder commands to a servo mechanic or hydraulic system which
operates the rudder of the aircraft.
This is an automated system that uses the yawing
rate of the aircraft. This is done via the sensor the aircraft is
equipped with. This sensor is to help command the rudder. This is done via an
actuator that is capable of deflecting the rudder on its own without pilot
intervention. With this, yaw oscillations will be damped faster than naturally.
Majority of the
aircrafts are naturally stable around the yaw axis but such natural stability
might not be too damped. It means that if uncontrolled, oscillations all around
the yawing axis would continue for a some time before dying off. The yaw damper
purpose to reduce the duration of these oscillations.
The use of this yaw
damper will help provide a better flight for passengers by preventing the
uncomfortable rolling oscillation and yawing. On certain aircraft’s it is
mandatory for the yaw damper to be always operational at all times during
flight above a certain specified altitude.
If the yaw damper’s
channel is integrated with the system then it is called a “parallel
If the yaw damper channel is separate, then it is known as a “series
system” as it is separate from autopilot.
In the parallel system the yaw inputs derived from the autopilot are
resolved with those from the yaw damper to achieve a combined response.
Aircraft that are certified under JARs for flight only with a serviceable yaw
damper will have at least two independent systems.
The automatic yaw axis control in an aircraft with
mechanical flight controls, implemented in an automatic flight control system
of the aircraft, said aircraft including:
yaw actuator adapted to control the orientation of a yaw control surface,
said yaw actuator being also adapted to deliver a measured value of the
torque generated on the yaw control surface,
yaw trim actuator adapted to drive movement of a yaw trim control surface
for limiting the force to be applied by the yaw actuator to produce the
required orientation of the yaw control surface, and
set of sensors for supplying an estimate of a lateral yaw force Ny to
which the aircraft is subjected in flight.
The method in accordance with the invention
includes steps of calculating:
setpoint value for the position of the yaw control surface sent to the
yaw actuator and determined by means of the estimate of the lateral force
estimated torque of the yaw actuator determined by means of the measured
value of the torque of the yaw actuator and a measured position of the
yaw control surface, the estimated torque calculated in this way having a
lower oscillation dynamic range than the measured torque,
command for activating and deactivating the yaw trim actuator determined
by means of the estimated torque.
Pictures and block diagrams of yaw dampers that are available
in the commercial market.
Pitch Control of a A320
Fig B: Yaw
Control of a A330 and A320
Fig C: Yaw
Control Schematic Diagram For A320/A330
Rudder Pressure Limiter For Boeing 737
Block Diagram of the Yaw Damper in a Boeing 737
What is an Auto Throttle System?
The auto throttle system is made to provide and give really precise and
smooth engine target parameter settings for control of airspeed. This system will
deliver significant flight performance and various safety benefits resulting in
a few things. Such as reduced crew
workload, increased situational awareness and passenger will be able to relax
The auto throttle system will provide smooth and comfortable speed
management by managing and controlling the aircraft’s thrust levers, which
helps keep the pilot be constantly in the loop. This system will help enable exact
management and control of speed and thrust during takeoff, climbing, cruising,
approach to runway, maneuvering in the air, descent, landing on the runway
The autothrottle is known as the automatic throttle and it would help
allow the pilot to control the power settings of the engines of the aircraft by
specifying a desired flight value, compared to manually controlling the fuel
There are two parameters that an autothrottle can try to maintain. They
are to attain speed and thrust.
In speed mode, the autothrottle is placed to attain a specific
targeted speed. This mode helps control the aircraft speed within safe
operating limits. For example, if the officer selects a targeted speed which is
slower than the aircrafts stalling speed or a speed faster than the
aircrafts maximum speed, this system will maintain
a speed which is closest to the targeted speed that is within the range of safe
In thrust mode, the engine is kept at a specific power setting
according to the different flight phases for each aircraft. For example, when
taking off, auto throttle maintains constant takeoff power until takeoff is
totally completed. During ascend, auto throttle maintains constant ascend
power. Same thing happens in descent, auto throttle retards the
throttle to IDLE position, and so on. When auto throttle is working in thrust
mode, speed is controlled by pitch, and is not protected by auto throttle.
modes of operation of the auto throttle system.
Mode Control Panel Speed (MCP)
Flight Management System Speed (FMS)
Landing Flare Retard
Appendix A on how each operation mode is used.
What is a Trim System?
A ‘secondary’ flight control system is the Trim
When an aircraft trims, it adjusts the various
aerodynamic forces acting on the control surfaces of the aircraft and so that
the aircraft keeps to the set attitude and altittude without any pilot input.
When all the axes of rotation are changed due to aerodynamic forces, not
all aircrafts can be trimmed in all three axes. Theoretically, all aircraft
designs will have some kind of pitch axis trimmer and most of the various
aircrafts have provisions for trimming in the yawing axis. There are many
various kinds of trim systems available in the commercial market.
The most common trim system is the trim tab which is installed
in either fixed or flight adjustable configurations. Anti-servo tabs and
adjustable springs on a stabilator of
an aircraft are the other various types of trim systems. A stabilator is known
as an all-moving joint plate, is a fully movable aircraft horizontal
Elevator Trim Control System Of a Cessna
Fig B: Trim
Tab Operation (Nose Up)
Fig C: Trim
Tab Operation (Nose Down)
Automatic Trim Control System
Automatic trim control system usually refers to the
aircraft’s pitch trim systems. Pitch trim system’s will help to maintain
longitudinal stability of the aircraft without the need of pilot to constantly
deflect the elevators using the control column. The aircraft’s stabiliser angle
must be changed and trimmed in accordance to the changes in the CG and
aerodynamic lift. Trimming of the stabiliser can also be done via the pilot
manually using the trim switch on the control wheel or automatically when
autopilot is engaged.
The automatic trim system is able to produce trim commands
for transferring trim from the elevator to the stabilizer such that 3 major
things can happen.
(1) Almost all of the autopilot elevator authority remains available
(2) Various nuisance trim activities are avoided
(3) The so called “bump” which is produced upon deactivation
of the autopilot is minimized.
Briefly, an aircraft automatic trim system gives out control
signals to transfer trim from the aircraft’s elevators to its stabilizers
during an automatic aircraft control flight mode. The trim system will include
an elevator control authority limit computer which comptates for producing a
signal which represents the available elevator control in the automatic flight
control mode. An elevator demand circuit which produces a signal representative
of actual elevator demand. A ratio signal is also given out, corresponding to
the ratio of elevator demand to available elevator control. A detector sensor generates
and produces the stabilizer trim command signals in response to the ratio
This function will help keep the aircraft in trim at all
times that the autopilot is in use. It senses when the servo is needed to
keep pressure on the controls due to an out-of-trim condition and it will then control
the trim circuits in the servo to provide the proper output to control the
elevator trim until the control pressure is relieved from it.
you command a climb to the aircraft, the system will input the elevators servo
to pull the elevator up. For this to be done, it requires a constant
force on the elevator control column as the aircraft trim has already been set
for levelled flight. With the Auto-Trim, the servo will input into your
aircraft’s electric trim to be adjusted to reduce and relieve the pressure on
the control system. This will drive the trim motor until the aircraft is
trimmed for climbing. Once the aircraft’s reaches the desired altitude
and the aircraft levels off and starts cruising, the aircraft will again be out
of the required trim. The auto trim will again give an input into the
aircraft’s electric trim system to adjust for proper levelled-flight trim.
Take off mode is engaged by pressing either of the take-off go-around
switches on the throttles, with the aircraft on the ground the auto throttles
armed and the desired n1 thrust limit selected
from the flight management system computer display unit. The auto
throttle enunciation changes from armed to n1 and the thrust levers advance
toward take-off thrust. The auto throttle sets the computed n1 value between 60
to 80 knots.
The auto throttle enunciation changes to throttle hold, to indicate the
auto throttle cannot change thrust lever position although they can still be
repositioned manually. After take-off the auto throttles remain in throttle
hold, until 400 feet radio altitude is reached and 18 seconds have elapsed
since lift off. Auto throttle
enunciation then changes from throttle hold to arm.
Reduction to climb thrust can now be made by pressing the N1 switch. Flexible
takeoff or takeoff legs is a facility to allow the auto throttle to calculate
and provide a reduced thrust takeoff by entering an assumed temperature into
the flight management system or the thrust mode limit panel.
The autothrottle maintains thrust at the n1 limit selected from the
flight management system computer display unit. N1 is enunciated with the
autothrottle and the n1 switch illuminates. Pressing the n1 switch changes the
auto throttle mode from n1 to armed.
If an engine fails while the auto throttle is in the n1 mode the thrust
lever of the failed engine would advance forward a few degrees and return to or
below the other thrust lever position. The speed mode is available throughout the
flight once the takeoff phase is completed. Pressing the mode control panel
speed selects which selects the speed mode if compatible with the engaged
automatic flight director system pitch mode.
MCP speed is enunciated for the
auto throttle and the speed mode switch illuminates. The speed or Mach shown in
the mode control panel IAS Mach display is the target speed. the auto throttle
will not set power above the displayed n1 limit, however the auto throttle can
exceed an n1 value that had been manually set by the n1 manual set knob. if an
engine fails while the auto throttle is in a speed mode both thrust levers
advance together to maintain the target speed .
the flight management system speed mode is an auto throttle mode which
is commanded by the flight management system during vertical navigation operation.
when engaged the mode control panel IAS Mach
display is blank. the SB cursors are positioned at the flight management
systems commanded airspeed and the auto throttle maintains this commanded speed.
FMC speed is enunciated in the auto throttle flight mode annunciator. the auto
throttle is limited to the n1 value shown on the n1 gauge on the engine
instruments and crew alerting system display. the autothrottle enunciates arm
when the auto throttle arm switch is at arm and no auto throttle mode is
engaged. the thrust levers can be manually positioned without interference from
the auto throttle system, while arm is enunciated. the auto throttle
automatically transfers to arm from the speed or n1mode, when the mode is be
selected by pressing the respective mode selector switch while the switch is
illuminated .The autothrottle engages and enunciates retard during level change
and vertical navigation descents. retard
changes to arm when the thrust levers reach the aft stop or when manually
prevented from reaching the aft stop. During landing, the retard mode engages reduces
thrust and enunciates retard, shortly after flare mode engagement or at 27 feet
Radio altitude whichever occurs first. During a non-precision or visual approach
with flaps extended to 15 or greater and the automatic flight director system
not in altitude require or altitude hold, the autothrottle retard mode engages
at 27 feet Radio altitude. the autothrottle automatically disengages
approximately two seconds after landing touchdown with the autothrottle armed
switch at arm. The autothrottle go around mode is armed when descending below
2,000 feet radio altitude with or without the automatic flight director system
engaged. Once armed the autothrottle go-around mode can be engaged until two
seconds have elapsed after landing touchdown. Pressing either takeoff go-around
switch engages the autothrottle go-around mode thrust or n1 is enunciated for
the auto throttle and the thrust levers advanced to the reduced go-around
thrust setting. This reduced setting produces a 1,000 to 2,000 feet per minute
rate of climb. After reaching reduced go-around thrust pressing either
takeoff go-around switch the second time, signals the auto throttle to advance
thrust to the full go-around n1 limit. After reaching reduced or full go-around
thrust the auto throttle go-around mode can be terminated by selecting another automatic
flight director system pitch mode or when altitude acquire enunciates engaged.