YAW DAMPER COMPUTATION - DESCRIPTION AND OPERATION

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1. General

Yaw Damper Function

The yaw damper function ensures:

In manual control, the accomplishment of the yaw orders from the elevator aileron computer (ELAC) (stabilization and manual turn coordination).
It also provides a yaw-damping degraded law in the event of ELAC failure (alternate law).
In automatic control, the accomplishment of the autopilot orders from the Flight Management and Guidance Computer (FMGC) for turn coordination and guidance (align and roll out).
It also ensures, in automatic flight, assistance in engine failure recovery and yaw stability.
The yaw damper actuation is described in 27-26-00.

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2. System Description

A. Composition
The system consists of:

Two electro-hydraulic servo-actuators (1 per FAC) centered to the neutral position by an external spring device. Each servo-actuator includes a feedback position transducer (Linear Variable Differential Transducer: LVDT)
Two Flight Augmentation Computers (FAC 1 and FAC 2)
A feedback position transducer unit located on the output shaft common to both servo-actuators (Two Rotary Variable Differential Transducers: RVDT)
Two FLT CTL/FAC pushbutton switches common to the RUD TRIM and RTL functions (for FAC engagement).

B. Architecture

FAC - Architecture of Yaw Damper

All the computations specific to this function (laws, logic and engagement) are duplicated in each FAC.
The system operates using the changeover technique : when both the yaw damper 1 and the yaw damper 2 are engaged, the channel 1 has priority.
The channel 2 is synchronized on the position of the other channel and its associated servo-actuator is depressurized. This depressurization is performed by two solenoid valves. Each solenoid valve drives a by-pass valve. Only one solenoid valve is required to depressurize the servo-actuator. A pressure switch monitors the status of the solenoid valves.
If the two servo-actuators are not pressurized, the rudder is centered to the neutral position (zero or the trimmed value).
The rudder receives the yaw damper orders but these are not reproduced at the rudder pedals.
The Green hydraulic system supplies the servo-actuator No. 1 associated with the FAC 1.
The Yellow hydraulic system supplies the servo-actuator No. 2 associated with the FAC 2.
A current amplifier in the FAC delivers the orders to slave the servo-actuator in position. A servovalve then executes these orders.
The slaving order is never interrupted even when a failure is detected :
the servo-actuator is neutralized through action on the electrovalves.
Each solenoid valve is under the control of an independent logic (C and M).
The C1 and C2 transducers (LVDT) serve for the slaving. The S1 and S2 transducers (RVDT) permit to monitor this slaving.
Each FAC generates the priority order in the form of a hard-wired discrete. The fluctuations of the 26 V/400 Hz power are compensated.

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3. Interface

Interconnections between FAC and Yaw Damper Servo-Actuator

The figure given below shows the interconnections between the FAC and the yaw damper servo-actuator.

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4. Operation

A. Principle

(1) Manual mode
In AP-disengaged configuration, the yaw damper function is linked to the ELAC.

In normal mode, on the roll axis:
The ELAC generates a lateral deflection law which integrates the control of the rudder (stabilization and turn coordination).
The yaw damper carries out this law and indicates the correct achievement of this function through a hard-wired discrete.
If necessary, the ELAC must operate in degraded law on the roll axis.
In degraded mode indicated by the ELAC:
The FAC computes the yaw damper function and generates a simplified law of Dutch roll damping (alternate law).
This law, which has a fixed and limited authority plus or minus 5 deg., only uses gains function of the selected positions of the flaps and slats.

(2) Automatic mode
As soon as the AP is engaged, the yaw damper operates in the mode given below:

Dutch roll damping except in approach phase
Turn coordination to reduce the sideslip in turn.

These two orders are inhibited during the landing phase and accomplished directly in the AP guidance orders.

Assistance in engine failure recovery from a lateral acceleration signal through a threshold
Accomplishment of the guidance orders : align and roll out.

B. Structure of Yaw Damper Control-Law

Yaw Damper Control-Law

The control law generates a deflection order to control the yaw damper servo-actuator:

From the position of the position feedback in synchronization
From the ELAC deflection order
From an alternate law based on a wash-out yaw-rate term with a gain function of the flap and slat configuration. The whole law is limited to a safety value (plus or minus 5 deg.).
From the aileron deflection order on the AP for turn coordination
From the landing guidance order on the yaw axis delivered by the AP
From a Dutch-roll damping law. This law uses a wash-out yaw-rate term and a phase advance term applied in clean configuration.
From a term of assistance in engine failure recovery. This term uses a lateral acceleration term through a threshold.

All these control orders are limited in speed and in amplitude.
The control law generates a deflection order to control the yaw damper servo-actuator:

From the position of the position feedback in synchronization
From the ELAC deflection order
From an alternate law based on a wash-out yaw-rate term with a gain function of the flap and slat configuration. The whole law is limited to a safety value (plus or minus 5 deg.).
From the aileron deflection order on the AP for turn coordination
From the landing guidance order on the yaw axis delivered by the AP
From a Dutch-roll damping law. This law uses a wash-out yaw-rate term and a phase advance term applied in clean configuration.
From a term of assistance in engine failure recovery. This term uses a lateral acceleration term through a threshold. This threshold is zero.

All these control orders are limited in speed and in amplitude.

C. Operating Logic
The activation of the yaw damper function depends on:

The engagement status of the FLT CTL/FAC pushbutton switch
The logic of the modes (AP engaged or not, ELAC in normal mode or not, status of the ADIRS etc.)
The monitoring specific to the function:
computation comparators and power comparators
The global monitoring of the computer.

The correct operation of the mode is checked:

For the ELAC: if the normal law is not executed. The ELAC then turns to the standby law on the roll axis.
For the AP:
if the acquisition of the AP-engaged signal is not correct
or if the status of the peripherals does not allow the achievement of the function (dual failure of the ADIRS).
The AP disconnects and the system returns to the manual mode without FAC disconnection.

The loss of the yaw damper function is indicated on the display unit of the ECAM system:

Loss of one channel:
YAW DAMPER 1 or 2 amber warning
Total loss:
YAW DAMPER 1 + 2 amber warning + chime.

D. Monitoring of Yaw Damper Function

Monitoring of Yaw Damper - Block Diagram

The block diagram given below shows the organization of this function and the various types of monitoring which are integrated.
These are:

Monitoring of the IRS function through a vote on the yaw rate and lateral acceleration parameters (Ref. AMM D/O 22-65-00-00).
This ensures the availability of the manual and automatic functions in the event of a single detected or undetected failure. It also ensures the availability of the alternate law upon a second detected failure.
Monitoring of the ELAC and FMGC peripherals at the level of the ARINC buses and the hard-wired discretes of engagement of these peripherals

Limitation in amplitude (+ or - 20 deg.) and in speed (40 deg./s in manual control and 30 deg./s in AP).

Limitation in amplitude (+ or - 20 deg.) and in speed (40 deg./s in manual control and 30 deg./s in AP). In clean configuration, speed is 15 deg./s limited in manual control and in AP.

Limitation in amplitude (+ or - 25 deg.) and in speed (40 deg./s in manual control and 30 deg./s in AP). In clean configuration, speed is 15 deg./s limited in manual control and in AP.

Monitoring of the computation through duplication and vote of the mid value of the deflection order among three values:
. command deflection order
. monitoring deflection order
. null order (stability order).
This ensures the passivation of any erratic value and its elimination from the vote.
The voter circuit includes comparators:
. C3 comparator between the deflection order generated by one channel and the value finally voted. This identifies the faulty channel.
. C1 comparator between the command and the monitoring voter. This comparator monitors the digital section of the computer.
Monitoring of the power channel through comparison (C2) between the deflection order and the position feedback.
In-flight monitoring of the pressurization status of the hydraulic systems.
Monitoring of the transducers (Ref. AMM D/O 22-65-00-00).

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5. Test Procedure

A. Computer
At power rise, during the safety tests, the sections specific to the yaw damper and mainly the hard-wired engage logic, are validated.

B. Servo-Actuator
This test is introduced to minimize the time of risk of hidden failures which can affect the standby channel.
This test is used to check:

The electrical continuity of the current amplifier up to the servovalve by introduction of a non-executed fixed order (servo-actuator not validated)
The pressure switch between the electrovalves (capability to trigger) (which can be tested only with the hydraulic pressure applied).

The pressure switch is automatically tested with the hydraulic pressure coming from the engine (use of ENGINE RUNNING data to launch the test).

[Rev.10 from 2021] 2026.04.01 05:10:44 UTC