AFCS Flashcards
General
Dual system electronically and hydraulically independent from each other, each with 100% control authority
Faults causing links to have hardcovers can easily be over ridden by moving flight controls in opposite direction
AFCS in conjunction with the Flight Director can provide VOR and LOC from and back course capture and track, ILS capture and track, VOR approach, automatic go around, vert speed hold, and heading hold
Provides signals for control of 3 additional actuators:
- Differential Airspeed Hold (DASH) actuator…provides forward cyclic displacement proportional to aircraft forward speed
- Cockpit Control Driver Actuator (CCDA)…responds to FD steering coupled through No. 1 AFCS and moves collective control to maintain altitude or adjust climb/descent
- Longitudinal Cyclic Trim actuators (LCTs)…one attached to each rotor head, reduces fuselage negative angle of attach as forward speed is increased, which reduces fuselage drag, also reduces blade flapping which reduces lower stresses on rotor shafts
AFCS Computer
Located in cabin electronic equipment compartment
Computers receive signals from its own internal sensors as well as external sensors and process these signals into control commands for pitch, roll, and yaw e links, DASH actuators, CCDA, and LCTs
Internal sensors:
- Rate gyro
- Vertical accelerometer
- Static atmospheric pressure
- Side slip transducers
- Airspeed transducer
External sensors:
- Vert gyro (No. 1 gyro for No. 1 AFCS, No. 3 gyro for No. 2 AFCS)
- Directional gyro
- Control position transducers (pitch, roll, and yaw)
- Pitot/static system (above 40 knots only)
- Yaw ports
No. 1 AFCS computer controls:
- No. 1 pitch, roll, and yaw e links (bottom section)
- DASH actuator, top section
- Forward LCT
- Collective CCDA
No. 2 AFCS computer controls:
- No. 2 pitch, roll, and yaw e links (top section)
- DASH actuator (bottom section)
- Aft LCT
Control Position Transducers
Senses cockpit pitch, roll, and yaw control movement from pilots for AFCS computers in order to prevent system from counter acting pilot induced altitude changes
Each CPT consists of 2 linear variable differential transformers (LVDT) contained within adjustable housing, it develops a signal proportional to control movement
Pitch CPT develops signal proportional to cyclic movement, gets processed as a function of airspeed demand
Roll CPT senses lateral cyclic stick movements and supplies signal to AFCS, which creates a control signal to lateral e link, causing movement of link. This in conjunction with mechanical control demand enhances lateral control response.
Yaw CPT similar to roll CPT in response to pilot commands
AFCS Control Panel
Axis Shutoff panel has push button switches to de activate e link in each axis of AFCS channel. 7th button for HEADING HOLD function provided as well for both channels
AFCS must be engaged manually by pilot via two AFCS switches. Selecting switch to ON applies power to AFCS solenoid valves which causes hydraulic pressure to be applied to respective e links.
Only occurs if 115VAC and 28VDC is applied to AFCS computers, synchronizer clock is operating, and vert gyro is functioning normally for pitch and roll
If either system becomes defective, the remaining channel in failed axis will continue to provide control, though total AFCS authority will be reduced by 50% and gain for electrical input all be increased by 75% when compared to normal ops
Squat switch disables longitudinal cyclic stick position signal to DASH, preventing dash actuator response when cyclic stick is moved
Pitot/Static System
Consists of:
- Side slip ports
- Pitot tubes
- Static system ports
No. 1 pitot tube supplies pitot info to No. 1 system and LH pilots gauges, pitot static isolation switch allows isolation of LH pilots instruments from No. 1 AFCS
12 static ports are on the aircraft, separated in groups of 3:
3 on each side of fuselage and 3 in each forward landing gear well. 2 outboard static ports on each side of aircraft are anti iced
4 side slip ports installed in pairs on either side of nose compartment, unequal air pressure at side slip ports on ether side of nose applies differential pressure to side slips transducer in AFCS computer when aircraft is out of trim. Transducers then develop a signal to move yaw e link until pressure is equalized between ports
AFCS also receives pitot/static inputs from pitot tubes and static ports to control DASH and LCTs. Barometric altitude signal for controlling LCTs and collective CCDA is developed in each AFCS unit by altitude transducer
Vert Gyros
No. 1 (copilots) and No. 2 (pilots) attitude gyros provide pitch and roll info to No. 1 and 2 AFCS units respectively
Gyros also provide gyro validity signal to each unit
Directional Gyro
One directional gyro provides signal to both AFCS units, and are powered by 115VAC as No. 1 AFCS
Collective Control Driver Actuator (CCDA)
Controlled by No. 1 AFCS unit and CP BRAKE TRIGGER switch on collectives
Actuator operates in stabilizing mode or synchronizing mode.
Stabilizing mode…initiated when vert mode on FD MSP is active, causes No. 1 AFCS unit to engage and move CCDA to control helicopter selected vertical rate of climb, descent, or glide slope. Altitude hold circuit will return helicopter to datum altitude on release of collective control
Synchronizing mode…initiated when Vertical Mode is disengaged OR if CP BRAKE TRIGGER is pressed. This causes No. 1 AFCS unit to direct CCDA to follow any changes in altitude without applying corrective movements to flight controls. Lever will return mag brake to reference position.
Longitudinal Cyclic Trim Actuators
LCT in auto mode…No. 1 AFCS automatically moves forward LCT, No. 2 AFCS automatically moves LCT in aft rotor head.
LCT in manual mode…pilot controls each individual LCT with respective EXT/RET switch on canted console
When squat switches sense ground contact, both actuators move to GND position
Aft Landing Gear Proximity Switch
One switch installed on each landing gear, they close when aircraft has landed which tells AFCS aircraft is on the ground.
AFCS then:
- Reduces pitch rate sensitive by approx 50%
- Disables inputs of LCTs and moves both to GND
- Disables inputs to DASH
- Disconnects heading hold (if on)
LH switch disconnects No. 1 AFCS features and RH switch disconnects No. 2 AFCS features. EITHER SWITCH will disable Hobbs meter
Differential Airspeed Hold (DASH)
DASH compensates for positive and negative stick gradient and provides a more linear stick gradient throughout changes in airspeed
Why?
When aircraft accelerates from hover, forward rotor downwash flows into aft rotor disc, requiring more collective pitch in aft rotor system (positive stick gradient) until its max which is around 40 knots
As it continues past 40 knots, the aft rotor now experiences cleaner air and pitch differential between both rotor discs becomes less (negative stick gradient). This is present to a decreasing extent as airspeed continues to increase
Because of all this stated above, more cyclic displacements is required for a given change in airspeed below 40 knots than is required for same amount of control changes in airspeed above 40 knots
Top half of DASH controlled by No. 1 AFCS, lower half by No. 2 AFCS
Flight Director
FLT DIR CPLR button must be depressed to couple any FD mode. Coupler quick release is located on RH cyclic only.
FD receives/processes signals from:
- VOR/LOC receiver
- VHF NAV receiver
- Radio altimeter
- Vert gyro
- Directional gyro
- Normal accelerometer
- Pressure altitude sensor
- Pilots HSI
- Pilots VSI
After processing signals is generates 2 sets of lateral and collective data. One goes to ADI for display to each pilot via command bars, the other goes to AFCS for coupled flight
Both AFCS units receive lateral commands from FD computer and are limited to 20*, command is compared to aircraft to form a bank angle error command. Airspeed must be greater than 40 knots and roll inputs of more than 1.5 degrees per second must not be evident
FD Coupler button must be pressed to couple any vertical mode EXCEPT Altitude Hold. Collective commands only made by No. 1 AFCS
Signal rate is limited to 0.3 inches/second and summed with normal acceleration signal to form a command to Collective CCDA. Barometric commands are created within No. 1 AFCS; vert speed, go around, ILS glide slope are all created by FD computer to No. 1 AFCS
Heading Mode
To activate, select HDG so FD will provide inputs to lateral controls and turn to heading selected by heading bug
In HDG mode the FD will override NAV BC and LS modes
Loss of validity signals from either vert gyro or directional gyros will cause roll steering pointer to bias out of view, and roll channel of AFCS will hold last attitude commanded
Nav Mode
Supplies steering commands for VOR, GPS, and LOC.
If aircraft is out of lateral bracket sensor trip point, roll steering pointer will receive heading steer commands by heading bug, causing NAV ARM and HDG bugs to illuminate. As aircraft reaches sensor point, system switches to VOR; NAV ARM and HDG light go out and NAV CAP illuminates
Capture command is then created to capture and track selected VOR
Overstation flying sensed by detector which removes VOR deviation signal from command until no longer erratic. At this point new course info must be put into HSI
If NAV receiver signals aren’t valid prior to capture point, lateral beam sensor wont trip and system will remain in HDG mode. If after capture the NAV receiver, compass data, or vert gyro show signs of distress, roll steering pointer will bias out of view and roll AFCS will hold last valid attitude command
LOC Mode
LOC mode functions same as VOR mode but with localizer frequency tuned in
Selecting ILS button arms both localizer and glide slope modes
ILS ARM mode will be maintained until vertical bracket sensor is reached, then ILS GS light will illuminate. At capture, commands are generated to intercept GS beam.
Localizer and glide slope are interlocked so localizer must be captured prior to capturing GS
If GS receiver isn’t operating correctly prior to capture, vert beam sensor won’t trip and system will remain in existing VS mode
If glide slope or vert gyro signal become invalid, the system will automatically revert to manual control in AFCS, and collective steering pointer will bias out of view
