automatic flight control system (afcs) - AVIONICA-ALA-22

• 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
• 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, baroaltitude 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 electrohydraulic 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%
• 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
• 3.The Trim/FFS can be operated directly through
manual control as well as by the AFCS.
Auto trim actuators provides the
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
Stability Augmentation:
• This forms the inner loop of the AFCS. It is a prerequisite for the AP modes and is engaged at all
times (from engagement of AFCS to its
• 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.
• 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
• 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, preflight 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
• 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
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

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