DAFCS Flashcards
DAFCS
Digital Advanced Flight Control System
DAFCS Description
(1) With DAFCS the aircraft is both stable and agile, reducing pilot workload and fatigue which increases pilot situational awareness.
(2) The result is improved capability, mission effectiveness, operational readiness, operational tempo, and reduced maintenance costs.
(3) The Digital Advanced Flight Control System augments the mechanical Flight Control System and add features that tremendously increase capabilities.
Without the DAFCS the CH-47F is not stable and a challenge to fly.
Dual Channel, Limited Authority System Provides
basic aircraft stability functions, and desirable handling characteristics.
(1) The pilot selectable higher augmentation modes add additional advanced features.
(2) The limited authority of the system ensures that the pilot can always over-ride the system if necessary.
Despite the complete loss of one channel, if the other channel is healthy, DAFCS functionality is largely retained.
(1) The pilot can select either channel or both channels of the DAFCS (1, 2, or both).
(2) The AFCS panel enables coupling of the Flight Director and either manual or automatic control of the Longitudinal Cyclic Trim (LCT) actuators.
The DAFCS provides these features when on.
(1) Damping of angular rates of motion in the pitch, roll, and yaw axes.
(2) Control response augmentation in the pitch, roll, and yaw axes.
(3) Longitudinal static stability and airspeed stability above 40 knots CAS.
(4) Pitch attitude stabilization.
(5) Bank angle and heading hold.
(6) Directional static stability and automatic turn coordination above 40 KCAS and limited automatic turn coordination below 40 knots.
(7) Roll axis Attitude Command/Attitude Hold (ACAH) response type below 40 knots CAS.
(8) Selectable Velocity mode (VEL) below 40 knots groundspeed (with accompanying Forward Flight Velocity (aka Roll-to-Wings-Level) mode (Level) above 40 knots groundspeed).
(9) Selectable Position Hold (PSN) mode below 65 knots groundspeed (through accompanying DECEL mode).
(10)Selectable Altitude Hold using inertial (ALT-INRT) or radar (ALT-RAD) altitude reference. Selectable Decent mode (DESCENT) below 65 knots groundspeed (used in conjunction with DECEL mode).
(11)Selectable automatic longitudinal cyclic trim scheduling.
(12)Coupling with Avionics Management System (AMS) Flight Director.
Rate Command (RC)
1) Most used control mode in a basic helicopter is rate-command/response type in all axes.
2) In other words, control inputs produce corresponding angular or vertical rates.
3) The stick/pedal deflection commands an equal roll/pitch/yaw response.
4) This is the control method used most often when the pilot has good visibility and can react immediately to the numerous visual cues in a Good Visual Environment (GVE).
5) This mode is needed because it gives the pilot the quickest response and most maneuverability.
Attitude Command (AC)
1) With (AC), the DAFCS assist pilot by stabilizing the attitude of the helicopter in one or more axes, allowing the pilot to focus on control of the aircraft’s position.
2) This assistance is necessary due to the loss of the visual cues in a Degraded Visual Environment (DVE).
3) In an Attitude Command control mode, the pilot provides attitude commands to (DAFCS) though control movement.
4) The DAFCS system then attempts to achieve the commanded attitude while maintaining the stability in the axis it’s controlling.
5) This reduces the pilot workload since DAFCS takes control of certain axes relieving the pilot of the extra attention to aircraft control that resulted with the loss of visual cues.
6) This Attitude Command feature is needed and highly effective in a degraded visual environment (DVE).
LINEAR ACCELERATION COMMAND (LAC)
1) With DAFCS 3.3 (LAC) replaces (TRC).
2) Consider (LAC) as an advanced version of (TRC) that is activated by selecting Velocity (VEL) on the Thrust.
3) In this mode, direction and groundspeed is proportional to cyclic stick displacement (like TRC) but, when controls are relaxed, the aircraft holds a constant groundspeed while locking the aircraft onto a ground track.
4) Both groundspeed and direction are adjustable by pilot stick or AFCS Trim Switch deflection.
Disturbance Responses Description
(a) Disturbance Responses are the control modes DAFCS switches to when the pilot is not moving the controls.
(b) Disturbance Responses can occur in a single or multiple axes.
1) Example: You will see Longitudinal Attitude Stability, Lateral Attitude Stability and Roll Attitude Stability described in the operator’s manual.
(c) A HOLD is different from stability.
1) Example: Directional Stability will resist the disturbance that caused the aircraft to (yaw in this example) but will not return the aircraft to the previous state (heading).
2) Directional Hold, however, stored the previous state (heading in this example) and will return the aircraft to the stored heading that was present prior to the disturbance.
ARMED State:
Means a feature is turned on and ready to operate when the parameters required by the feature to operate are met (usually airspeed or groundspeed or distance).
This state is displayed in white on the Vertical Situation Display (VSD) if displayed
CAPTURED (Active) State
Means the feature is ARMED and is now within the parameters required for the feature to operate and the feature is now engaged or is activated.
This state is displayed in green on the VSD if displayed.
CYCLIC DETENT
Means the cyclic is not in motion (in a steady state), with no pressure applied to control centering springs (remove spring pressure by bumping the CD-REL switch) and the centering device release switch (CD REL) is not being depressed.
1) At this time DAFCS establishes an electronically/pseudo centered position. This electronically centered position is the cyclic detent.
2) It is not necessarily for the cyclic to be in the mechanical centered position.
3) The cyclic detent is used by Position Hold (PH) Velocity (VEL) and other DAFCS features.
4) You will notice that DAFCS features usually have two airspeeds/groundspeeds associated with them.
a) Example: The Low speed Regime uses 45 knots accelerating and 35 knots decelerating.
b) If one speed was used, the feature would toggle on and off at that speed. Using the two different speeds prevents this toggling.
Core Features Description
Core features are features that are operating as soon as the DAFCS selector switch is placed in the 1, 2 or both positions.
(a) These features activate automatically when within the parameters programed for them to operate.
(b) The core DAFCS is always active unless failures are present, or the system is selected off.
(c) Core Feature Explanation: DAFCS is ON with no pilot-selectable features engaged.
Core Features
Rate Damping
Control Rate Augmentation
Longitudinal Static Stability and Airspeed Stability
Pitch Attitude Stabilization
Roll and Yaw Hold
Automatic Turn Coordination and Lateral-Directional Trim
Roll axis Attitude Command/Attitude Hold (ACAH)
Damping of angular rates of motion in pitch roll, and yaw axes (also called rate damping).
(a) Damping is a method of decreasing oscillations by resisting rates of motion.
(b) DAFCS resists all aircraft rates of motion not caused by pilot input in the pitch roll, and yaw axes.
(c) DAFCS uses EGI angular rate feedback to move the ILCA Extensible Link Actuator (ELA) in the pitch, roll, and yaw axes for this feature.
(d) When airborne, this feature is always active.
Control response augmentation in the pitch, roll and yaw axes (also called Control Response Quickening).
(a) (Inertia) More effort is required to start a movement than is required to continue a movement, so DAFCS augments pilot input to start the motion then ‘washes out’ the extra input once the motion has started.
(b) This feature also counters sluggishness that is sometimes caused by rate damping.
Longitudinal static stability and airspeed stability above 40 KCAS.
(a) Below 40 KCAS, rotor-wash prevents accurate KCAS from the Air Data Computers (ADCs), so the DAFCS uses the DASH Actuator to hold the pitch attitude equal to the cyclic position.
(b) DAFCS always dampens pitch rates using the pitch extensible link but works with the DASH actuator below 40 KCAS to maintain pitch attitude hold below 40 knots.
(c) Above 40 knots, the ADCs now provide accurate KCAS and DAFCS now sees cyclic position as a defined KCAS.
(d) This is airspeed hold above 40 KCAS (like cruise control).
Pitch attitude stabilization
(a) Pitch attitude stabilization is accomplished by the pitch ILCA ELA and DASH actuator using a combination of EGI pitch rate and pitch attitude.
(b) Pitch attitude stabilization is active when airborne regardless of airspeed, or whether the CD REL button is pressed.
(c) This feature simply holds the pitch attitude stable. It is not attempting to acquire or hold an airspeed as above.
Roll and yaw axes holds
(a) Roll and Yaw axes are using attitude holds.
(b) Roll attitude hold is accomplished by the roll ILCA ELA using a combination of EGI roll rate and roll attitude.
(c) Yaw (Heading hold) is accomplished by the yaw ILCA ELA using EGI true heading. It is accurate to ± 3°.
(d) Holds are different from stabilization.
Automatic Turn Coordination and Lateral-Directional Trim
(a) The automatic turn coordination and automatic trimming of the roll and yaw axes.
(b) If the CD REL button is released while the trim ball is floating within the inner goalposts, DAFCS will automatically center, the trim ball
(c) If the CD REL button is released while the ball is displaced far enough to just to touch the inner goalpost or more, then DAFCS will hold that position.
1) Example: Holding the aircraft out of trim for smoke and fume elimination.
2) It is not possible to trim the pedals for small sideslip angles producing less than 3/16 ball displacement.
(d) The DAFCS automatic turn coordination feature is active when accelerating above 40 kts airspeed.
1) If already in a turn when decelerating below 40 knots, then DAFCS provides low speed limited turn coordination down to 15 knots forward groundspeed.
2) The low-speed limited turn coordination will hold a maximum bank angle of 15 deg.
3) Once the aircraft has rolled out of the turn while below 40 knots, it will not go back into turn coordination until the airspeed is above 40 knots.
Roll axis Attitude Command/Attitude Hold (ACAH) response type below 40 knots CAS.
(a) Within five seconds after decelerating below 35 knots airspeed, the DAFCS provides attitude-command attitude-hold (ACAH) in the roll axis using the roll ILCA and rate-command with heading-hold in the yaw axis using the yaw ILCA.
Pilot-Selectable features (description)
(a) These features must be selected by the pilot when desired.
(b) The features activate when within the parameters programed for their operation.
(c) The pilot selectable modes add functionality to the core DAFCS but never remove it.
Pilot-Selectable features
Forward Flight Velocity Mode (RWL)
Selectable Position Hold (PSN) and DECEL to Hover Modes
Selectable Altitude Hold
Descent Mode in conjunction with the DECEL mode
Selectable automatic longitudinal cyclic trim (LCT) scheduling
DAFCS Coupling to FD
Velocity (VEL) mode
Velocity (VEL) mode and Position Hold (PSN) mode are disarmed at power-up and cannot be used at the same time.
One of the uses for Velocity mode is for pacing moving vehicles and matching their speed. This is a low-speed sub mode that activates below 35 KGS). The other mode is Forward Flight Velocity mode.