RUDDER TRIM COMPUTATION - DESCRIPTION AND OPERATION

** ON A/C NOT FOR ALL

1. General

Rudder Trim Function ** ON A/C NOT FOR ALL

The rudder trim function ensures:

In manual control:
. The accomplishment of the pilot trim orders from the manual trim control (control and reset)
. The accomplishment of the deflection orders from the ELACs (engine failure recovery) (Provision only).
In automatic control:
. The accomplishment of the autopilot orders (autotrim on the yaw axis)
. The generation and the accomplishment of the engine failure recovery function.
The rudder trim actuation is described in 27-22-00.

** ON A/C NOT FOR ALL

2. System Description

A. Composition
The system consists of:

An electro-mechanical actuator which comprises two three-phase asynchronous motors connected to a reduction gear by rigid linkage.
Two Flight Augmentation Computers (FAC 1 and FAC 2).
Four transducer units (RVDT) configured in such a way that a single failure would not affect all the units at the same time.
Two engage FLT CTL/FAC pushbutton switches (with FAULT/OFF legend) common to the yaw damper and to the rudder trim (one per FAC).
A rudder-trim control switch located on the RUD TRIM control panel on the center pedestal for manual trim control.
A RUD TRIM/RESET pushbutton switch (this pushbutton switch is not mechanically held), on the RUD TRIM control panel.
A rudder trim indicator with liquid-crystal display located on the left of the rudder-trim control switch.

B. Architecture

Rudder Trim Architecture

The system operates using the changeover technique.
When the two channels are engaged:

The channel 1 has priority
The channel 2 is synchronized on the position of the actuator through the mechanical linkage.

The standby channel is not energized (no enable signal, no power-enable signal).
The control order is not interrupted.
Each channel is duplicated and monitored:

The COMMAND side of the FAC slaves the position of the system
The MONITOR side monitors the system.

The monitoring orders are equalized on the command orders to reduce dispersions due to the use of integrators in the loop.

** ON A/C NOT FOR ALL

3. Operation

A. Principle

Rudder Trim Changeover - Logic

The rudder trim function ensures a trimmed value of the rudder. This value is reproduced at the rudder pedals.
This trim is obtained either manually or automatically.
The value appears:

On the rudder trim indicator
On the display unit of the ECAM system.

The resulting deflection is maintained even in case of total loss of the function. This permits to have a stabilized value, for example in the event of AP loss when an engine failure occurs.
When the changeover principle is retained:

The side 1 has priority through the side 1 signal
Interruption of the actuator enable signals on the standby channel
Automatic engagement of the standby channel upon loss or disengagement of the channel 1.

B. Structure of Rudder Trim Control-Law

Rudder Trim Actuator - Generation of the Control Order

The control law generates a deflection order to control the rudder trim actuator through an integrator which memorizes the required position.
This order is generated:

From the position of the position feedback in synchronization
From the control order of the pilot trim (the reset is obtained through the unloading of the trim integrator)
From the ELAC deflection order (Provision only)
From the long-term turn-coordination order for autotrim on the yaw axis
From the generation of an engine failure detection and its accomplishment.

The engine failure is detected from the lateral acceleration and from the yaw rate through a given threshold. This detection is confirmed by the engine thrust information.
Detection is performed by the command and monitoring sides. The first side which is triggered sends a signal to the opposite side in order to lower its threshold and thus ensure synchronized detection.
The correction signal (fixed deflection values) is then applied, depending on engine failed.
As soon as the engine failure compensation reaches a predetermined threshold, the control law is modified:

The engine failure compensation is performed directly by the FG command order and the delta p compensation is boostered.
The monitoring side is equalized on the command side to reduce the dispersion of the integrators.

C. Operating Logic

(1) General
The operation of the rudder trim function depends on:

The engagement status of the FAC pushbutton switch
The status of the engaged AP signal for the automatic mode
The monitoring specific to the function:
. Computation comparators
. Power comparators
The global monitoring of the computer.

There are two modes of operation:

The manual mode
The automatic mode.

As the automatic mode has priority, the pilot trim is not possible in AP-engaged configuration.

(2) Manual mode
As this mode is not under the control of the ADIRS, it remains available for the pilot trim part even in case of total failure of these peripherals.
Specific operating logics are introduced:

(a) Control order
This order operates if:

The normally closed position of a contact, and
The normally open position of the other contact are activated simultaneously.

If this is not the case the trim order is inhibited. This provides a means of avoiding contact failure or mechanical jamming.

(b) Reset order
This order operates if:

The normally closed position of a contact, and
The normally open position of the other contact are activated simultaneously.

If this is not the case, the signal is not validated.
This signal is processed as a pulse signal : even when no longer applied, the order is taken into account until its accomplishment on condition that the reset order has been validated.
A priority logic is included between the command and the reset orders to avoid any possible jamming:

1 When a control order is applied after the introduction or the non-accomplishment of a reset order, the control order has priority.

2 When a reset order is applied after a control order and even if the latter is being executed, the reset order has priority.

(c) ELAC order (provision only)
In case of loss of the roll normal mode from the ELAC, the accomplishment of the order by the rudder trim is interrupted.

(3) Automatic mode
There is loss of the automatic mode:

If the AP engaged signal is not validated
Or if the status of the peripherals does not allow the achievement of the function.

In addition to this loss:

The AP disconnects
The system returns to the manual mode without FAC disconnection.

The AP also provides signals which validate the detection of engine failure as a function of the engine rating.

(4) Warnings
The loss of the rudder trim function is indicated on the display unit of the ECAM system:

Loss of one channel:
RUD TRIM 1 or 2 amber warning
Total loss:
RUD TRIM SYS amber warning + chime

The RUD TRIM indication on the FLT CTL page of the ECAM system is flagged (2 crosses XX appear) if this item of information is not available.

D. Monitoring of the Rudder Trim Function

Monitoring of Rudder Trim - 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 ADCs on Vc parameter compared two by two (Ref. AMM D/O 22-65-00-00)
This ensures the availability of the automatic function in the event of a single detected or undetected failure. The manual law is independent of the ADC function
Monitoring of the acquisitions of the ELAC and FMGC peripherals

Limitation in amplitude (+ or - 20 deg. of rudder) and in speed (1 deg./s in manual control and approximately 1.7 deg./s in automatic control)

Limitation in amplitude (+ or - 25 deg. of rudder) and in speed (1 deg./s in manual control and approximately 2.8 deg./s in automatic control)

Limitation in amplitude (+ or - 20 deg. of rudder) and in speed (1 deg./s in manual control and approximately 1.8 deg./s in automatic control)

Equalization of the monitoring channel on the command channel to reduce permanent deviations
Monitoring of the computation through a comparator between the command and the monitoring integrator (C1 comparator)
Monitoring of the power channel through C2 comparator between the deflection order and the position feedback
Monitoring of the transducers (Ref. AMM D/O 22-65-00-00)
Monitoring of the manual trim by the priority logics.

** ON A/C NOT FOR ALL

4. Interface with Controls

A. Interface with Rudder-Trim Control Switch

Rudder-Trim Control Switch ** ON A/C NOT FOR ALL

Interconnections between FAC and Rudder-Trim Control Switch

The rudder-trim control switch located on the center pedestal enables manual control of the rudder trim.
On the RUD TRIM control panel, an arrow and a placard indicate the direction (L or R).
The signals given below are used:

Ground on the normally open contact = control
Open circuit on the normally closed contact = control.

Each FAC receives:

Two signals (ground and open circuit) for trim control to the left
Two signals (ground and open circuit) for trim control to the right.

B. Interface with RUD TRIM/RESET Pushbutton Switch

Interconnections between FAC and RUD TRIM/RESET Pushbutton Switch

The RUD TRIM/RESET pushbutton switch located on the center pedestal enables the pilot to move the rudder to the neutral position.
This pushbutton switch is not mechanically held. While operated (pushed in), it closes two contact stages. The FAC memorizes this action and then achieves the reset (even after the pushbutton switch is released).
Each FAC receives two reset signals:

One signal of the normally closed contact from one stage
One signal of the normally open contact from the other stage.

Reset signal active:

Normally closed contact : open circuit
Normally open contact : ground.

C. Interface with Rudder Trim Actuator

(1) Description of rudder trim actuator

Rudder Trim Actuator - Principle

The rudder trim actuator which comprises two motors enables the accomplishment of the rudder trim order.
These motors are squirrel-cage, three-phase asynchronous motors. They are supplied with variable voltage and frequency according to the position error signal delivered by the FAC.
A power electronic set in the actuator permits to achieve this transformation.
It allows to obtain torque/speed characteristics equivalent to those obtained with a DC motor.
The actuator is power supplied with 26 V/400 Hz and 28 V directly from the FAC circuit breakers.
A control relay controlled by the FAC command logic can isolate the windings of the motor. In addition, an external relay cuts off the power to the motor through the FAC monitoring logic. This disables the motor in the event of a failure.
The slaving feedback units are brushless inductive potentiometers with Rotary Variable Differential Transformers (RVDT).
The output shaft of the actuator drives an irreversible screw through a torque shaft.

(2) Interface

Interconnections between FAC and Rudder Trim Actuator

The figure given below shows the interconnections between the FAC and the rudder trim actuator.

** ON A/C NOT FOR ALL

5. Test Procedure
When the FAC initiates the self-test upon energization, an actuator internal monitoring enables to check:

The capability of the actuator electronic set for correct slaving according to a predetermined order
The triggering capability of the monitoring if this slaving is not achieved
The correct reception of the enable signal.

To this end the FAC delivers stimuli during the test procedure.
The monitoring logic utilizes a specific stage of the winding isolation relay to test the enable signal.
The result of the monitoring is available on the test output. It is to be noted that during the test no inputs are applied to displace the rudder.
The test therefore uses:

The inactive stage of the winding isolation relays which send back the actuator control orders to the monitoring circuits
Another stage of the relay to check that the control relay moves to the working position
Monitoring circuits which take into account the slaving order compared to a predetermined threshold.

The test procedure is as follows:

PHASE 1: The FAC sends a null advisory value and does not validate the activation signal.
As the input value does not correspond to the fixed threshold value, the monitoring function must be triggered. The test signal changes to an open circuit (= warning). This validates the non-blocked status of the monitoring.
PHASE 2: The FAC sends an advisory value which is the one expected by the actuator and the activation signal is not valid. This advisory value is set to half the travel.
If the control order is generated correctly, the monitoring must not be trigged (feedback value equal to the threshold) and the test signal returns to good status (ground).
This validates the electronic section which ensures the slaving and tapping of the monitoring signals.
PHASE 3: The FAC sends again a null advisory value but validates the activation signal. This puts the actuator into service without displacement of the rudder.
A specific stage detects the closing of the control motor. Its signal serves for forcing the monitoring logic to the good status.

During this procedure initiated at FAC energization, the activation signal of the monitoring logic which puts into service the actuator 28 V power, must always be validated.
This relay is tested to check its capability to open, in a specific phase of the automatic test.

[Rev.10 from 2021] 2026.04.01 05:10:47 UTC