AFCS
Enhances the static stability and handling qualities of the helicopter.
4 Subsystems : SAS, Trim, FPS and Stabilator
SAS
Enhances the dynamic stability through short term rate dampening in the P, R, and Y axis
Each SAS has 5% control authority = 10% max
SAS 1 - Analog, brain is the SAS amplifier in the avionics compartment
Malfunction indicated by erratic tip path plane
Inputs: P - #1 stab amp
R - #2 vertical gyro (pilot)
Y - In SAS 1 amp
A/S - A/S transducer
SAS 2 - Digital, brain in SAS/FPS computer under center console
Malfunction indicated by SAS2 failure advisory light on AFCS switch panel
Inputs: P - #2 stab amp
R - roll rate gyro
Y - yaw rate gyro
A/S - air data transducer
Trim
“Muscle” for FPS
Provides gradient force to maintain position of cyclic and TR controls
Boost required for operation
Consists of: 2 electromechanical trim actuators (R & Y)
1 electrohydromechanical trim actuator (P)
During actuator jam, slip clutches require 80 lbs max (Y) and 13 lbs max (R)
Hardover causes Caution/Advisory lights and could move cyclic ½” or pedals ¼”
FPS
Basic autopilot
The “Brain”
Enhances static stability through long term rate dampening in the P, R, and Y axis
When coupled w/ Trim, has 100% control authority
For FPS to function at 100%, SAS 1 and/or 2, Boost and Trim must be on and operational
* Stabilator helps but is not required
Below 60
60
Above 60
Att Hold
Att Hold
Hdg Hold
P
R
Y
Att Hold / A/S Hold
Att Hold
Hdg Hold / Turn coordination
Stabilator
Variable angle of incidence airfoil that enhance handling qualities and longitudinal control of A/C
* Improves static and dynamic stability about the pitch axis
*AUTO mode positions stab to the best angle of attack for existing flight conditions
Stabilator amplifiers receive inputs from various sensors on the A/C
C - Collective Position Sensor
- located on MMU
- detect pilot collective displacement
L - Lateral Accelerometers
- located in the cabin bulkead over the CE’s stations
- sense out of trim conditions
A - A/S / Air data Transducers
- located by the pilot’s and co-pilot’s pedals
- provide en electronic airspeed signal
P - Pitch Rate Gyros
- located in the stab amps
- sense pitch attitude changes
S - Streamlines w/ MR downwash (a/s / air data transducer)
Streamlines at low a/s (30 KIAS and below) to minimize nose-up attitude due to MR
downwash on the stab
C - Collective Coupling (collective position transducer)
Provides collective coupling to minimize pitch attitude excursions due to collective inputs
* increase in collective would result in the a/c pitching up so stab programs down
* as collective is reduced, stab programs down to prevent nose from pitching down
A - Angle of Incidence (a/s / air data transducer)
Angle of incidence decreases as a/s increases = enhanced static and longitudinal stability
L - Lateral Excursions (lateral accelerometers)
Lateral sideslip to pitch coupling to reduce susceptibility to gusts
* also compensates to downwash on stab and TR efficiency
Right sideslip (left pedal applied): increased downwash and nose pitches up (stab programs down)
* also results in more induced flow thru TF and less lift provided by TR
Left sideslip (right pedal applied): reduced downwash on the stab and nose pitches down (stab up)
* also results in decreased induced flow thru TR and more lift by TR
P - Pitch Rate Feedback (pitch rate gyros)
Improves longitudinal stability and reduces susceptibility to wind gusts.
* pitch rate sensed by pitch rate gyros in stab amps and corrections are made to help
maintain level pitch attitudes during turbulent conditions
Stab also programs as “G” loading increases in turns
* forces acting upon the fuselage tend to pull the nose down in a turn
- stab programs up to prevent nose from dropping
Test circuit verifies proper operation of the fault monitoring system. When pressed, #1 stab actuator
moves, resulting in mis-compare signal and AUTO mode deactivation (test disabled above 60 KIAS)
*Max mis-compare range = 10 at up to 30 KIAS and tapers to 4 at 150 KIAS
Each stab amp processes its own a/s, collective position, lateral acceleration and pitch rate information
At a/s below 80 KIAS, larger of two a/s signals used. Above 80 KIAS, each stab amp uses own a/s signal