AUTOMATIC FLIGHT CONTROL

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15 Νοε 2013 (πριν από 3 χρόνια και 9 μήνες)

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AUTOMATIC FLIGHT CONTROL
SYSTEM (
AFCS
)


The
AFCS

incorporates several
features to
reduce pilot workload:


stability
augmentation,


control
augmentation,


manual, auto trim facilities and various
autopilot modes
.


The digital electronics Automatic Flight
Control System (
AFCS
) is fully coupled in four
axes (pitch, roll, yaw & collective) to enhance
handling qualities



The
AFCS

has
duplex lanes
in pitch, roll and
collective axes and has a simplex lane in yaw
axis.


Each
lane
is built
around one Automatic Flight
Control Computer (
AFCC
).


The system consists of


two
AFCS

computers
,



two Attitude Heading Reference Systems


(
AHRS
), two Air Data Units
,



one Vertical Gyro,


one
Pilot Control Unit (
PCU
)
and Reconfiguration
switches.


The
system is integrated to avionics equipments such
as



radio altimeter,



weather
radar.


Integrated Architecture Display System (
IADS
),


which provides the cockpit displays


(
AFCS

master warning,
AFCS

annunciation for autopilot mode status and
failure
warning,heading

bug,
AFCS

reference bug for airspeed,
baro
-
altitude and Radio height hold).


Aircraft control inputs are achieved through the
use of series actuators also known as


CSAS

actuators in pitch, roll, yaw and collective.
The
CSAS

actuators are integrated


with the main hydraulic boosters and are electro
-
hydraulic servo actuators


with limited authority
of
±

12% for pitch, roll and
collective axis.


There
is a simplex
CSAS

actuator for
yaw axis with
maximum authority of
±

20%.


Auto trim is achieved through parallel actuators
in pitch, roll, yaw and collective
with 100
%
authority.


The
Trim/Force Feel System (
FFS
) is an
electromechanical
unit interfaced


1.
with the mechanical flight control system and


2.with

the Automatic
Flight Control
system
(
AFCS
).


3.The

Trim/
FFS

can be operated directly through
manual control
as well
as by the
AFCS
.

Auto
trim
actuators provides the
following:


1. Engagement/Disengagement of anchor point by means of a magnetic clutch in
the spring
based actuators. This anchor point can be displaced either automatically
or manually
.


2. Artificial Force Feel about the anchor point generating a predetermined feel
force.
The artificial
feel is proportional to displacement in pitch, roll and collective
trim
actuator as
these trim actuators are spring based. The force feel is a constant
friction in
the yaw
axis.


3. Pilot input detection by means of a dual micro
-
switch.


4. Measure of pilot control displacement from the neutral position.


5. Pilot controlled displacement of the anchor point by means of an electrical
motor (
manual trim function) or by declutching the artificial feels (stick release
function
). “
Manual trim” and “stick release” are operative even when the
AFCS

is
disconnected for
pitch, roll & collective Trim/
FFS

units


6. Automatic displacement of the anchor point so as to maintain the series
actuators around
their center position (auto trim function).


7. Viscous damping of the output shaft rotation when the anchor point is
disengaged
so as
to prevent overshoots or oscillations in spring based Trim/
FFS

unit.

Stability Augmentation:



This
forms the inner loop of the
AFCS
. It is a pre
-
requisite for the AP modes and
is engaged
at all
times (from engagement of
AFCS

to its
disengagement).


The objectives are
as under:
-


-

To provide basic
stabilisation

throughout the
flight envelope.


-

To reduce pilot workload in turbulence.


-

To provide long
-
term attitude retention for
hands
-
off flying and short
-
term
attitude retention
in reversionary mode.

Control Augmentation:



This
is designed to optimize the handling
qualities. This is engaged at all times.

Auto
-
Trim:



Four
trim actuators, one each in pitch, roll,
collective and yaw axes are provided to


enable
AFCS

to provide auto trim facility. This
is designed to ensure that the series


actuators are not saturated and always
operate about their center.


Auto
-
Pilot Modes:



The
system supports the following AP modes:


“A/S” mode: Hold through the pitch axis of the
IAS

existing at the time
of
mode
engagement
.


“ALT” mode: Hold through the pitch axis of the
baro
-
altitude existing at
the time of
mode
engagement
. This function is performed through the
collective axis, when the “A/S
” mode
is active on the pitch axis.



HDG
” mode: Acquisition and hold through the roll axis of the heading
bugged on
the
pilot
or co
-
pilot
HSI

on PFD page as per selection of the
HDG

selection switch on
the main
instrument panel.



NAV
” mode: Acquisition and hold through the roll axis of a
NAV

route
by means of
the
steering
command signal supplied by the
NAV

system.



H.HT

+
HOV
” mode: When selected, this mode holds through the
collective axis,
the
radio
-
height
existing at the time of mode engagement.
Also it acquires and holds
zero
doppler

velocities (longitudinal & lateral)
through the cyclic axes.



AFCS

Computer


The digital
AFCS

computer is responsible for
performance of all functions and features
of the
autopilot
.



Pilot Control Unit (
PCU
)


The function of the
PCU

is to interface the crew
and the
flight control subsystem, for controls and
displays for basic stabilization,
upper modes
, pre
-
flight test, maintenance function, and trim
reconfiguration.
PCU

failure messages
are
displayed on
PCU

window,



Air Data Unit (
ADU
)



The
ADU

is pneumatically connected to the
total and static pressure probes and
is
electrically
connected to the total
temperature probe, the
AHRS

and the
IADS
.
The
ADU

measures
the
anemo
-
barometric
data and transmits these to the user systems
through


two low speed
ARINC

429 digital links.

Attitude Heading Reference System (
AHRS
)



There
are two
AHRSs
. Each
AHRS

comprises the following:


a) Integrated Sensor unit (
ISU
)


b) Magnetometer


The Integrated Sensor unit (
ISU
) consists of 3 Fiber Optic Gyro (FOG)
based rate
gyros and
3 accelerometers in perpendicular directions.


The
ISU

receives magnetic
heading from
external magnetometer,
receives speed (indicated airspeed and true air speed) and


baro

altitude inputs from Air Data Units.
ISU

internally uses TAS
information to
improve the
attitude and rate information
.




ISU

outputs through
ARINC

429 high
-
speed digital link
, the
pitch
and roll attitudes, heading, three axes rates, acceleration in three
axes,
baro

altitude
and indicated airspeed.

Vertical Gyro:



The
Vertical Gyro, which is installed on the
nose equipment platform, provides pitch and


roll attitudes to the
AFCS
, as the back
-
up
source.

Navigation System





IADS

provides the following navigation system
output to
AFCS
:


a. Roll steering command,


b. Longitudinal ground speed (
Vx
)


c. Lateral ground speed (
Vy
)

AFCS

Annunciator

Strip



The
AFCS

annunciator

strip on the
IADS
-
PFD page
indicates failure
and auto pilot modes status indication.


Trouble shooting of
afcs

system can be done
by the following inputs


Pilot
SNAG REPORT


PCU ERROR MESSAGE


PCU MAINTENANCE CODES


PRE
-

TEST ON GROUND

SPECIAL TOOL

DTS COMPUTER