ATA 27 - FLIGHT CONTROLS
Rudder Travel Limiting
Rudder Travel Limiting
The rudder-travel limiting system limits the control inputs to the servocontrols to change the rudder travel in relation to the airspeed (Vc).
The limitation is such that the maximum deflection of the rudder remains lower than the deflection which would induce limit loads on the structure, throughout the flight envelope.
The system is made of a travel limitation unit which is under the control of the
Flight Augmentation Computers (FACs) (Ref. ch. 22).
A simplified emergency control system (open loop) brings back automatically the
stops to the "low-speed" position when the two FACs are failed and slats are extended.
General
Figure 8: Travel Limitation Unit” on page 13
The mechanical design of the TLU is such that a single mechanical failure (rupture
or disconnection) cannot cause the loss of the travel limitation function.
The TLU has two brushless electric motors separately controlled by an electronic
assembly.
Each motor drives two screws via a reduction gear and permits the symmetrical
linear displacement of two nuts used as adjustable stops. (The two nuts have opposite
pitches).
A non-locking rotary stop limits the stroke of one of the screw/nut assemblies
which are irreversible.
There are two levers on each connection shaft; one is connected to the input rod
and the other is used as a punctual stop.
The movement of each screw is transmitted to a transducer unit (RVDT) via a reduction
gear which permits to indicate the position of the variable stop.
Two rigging pins are used to set the two levers to the zero position (mid stroke).
Figure 9: Component Location” on page 13
Normal control circuit
Figure 10: TLU Motor Control” on page 14
The motor itself is a 3-phase asynchronous motor with no brushes:
• the associated electronic set supplies the motor windings with variable voltage
and variable frequency which are function of the input signal (representative of
the position error),
• the system permits to get torque/speed characteristics for the motor equivalent
to those obtained with a DC motor,
• a relay isolates the motor windings from the electronic set output when its coil
is not energized by the enable signal.
Moreover, in this situation, the output signals are sent back to the monitoring circuits
for comparison with fixed thresholds
• the command signal that the computer dedicated to each electronic set sends
is a DC current which varies between plus or minus 8 mA into a 500-ohm resistor
(for the computer, the motor is equivalent to a standard servovalve),
• another relay upstream of the actuator and controlled by the computer supplies
the 28VDC power.
To prevent icing, there is a heating system which includes two coils and their regulating
thermostats.
Emergency control circuit
Figure 10: TLU Motor Control” on page 14
When the two FACs can no longer achieve normal control (for example if the two
electronic power sets of the TLU are failed), an emergency control brings back the
stops to the low speed configuration (maximum possible deflection of the rudder)
when slats are extended.
For this, the motor is used as a 2-phase asynchronous motor energized by 26 V
400 Hz power.
This control mode is achieved when the coil of a specific relay is energized for a
period of 30 s approx (this time is greatly sufficient to bring back the stops to the
low speed configuration).
The rudder-travel limiting system limits the control inputs to the servocontrols to change the rudder travel in relation to the airspeed (Vc).
The limitation is such that the maximum deflection of the rudder remains lower than the deflection which would induce limit loads on the structure, throughout the flight envelope.
The system is made of a travel limitation unit which is under the control of the
Flight Augmentation Computers (FACs) (Ref. ch. 22).
A simplified emergency control system (open loop) brings back automatically the
stops to the "low-speed" position when the two FACs are failed and slats are extended.
General
Figure 8: Travel Limitation Unit” on page 13
The mechanical design of the TLU is such that a single mechanical failure (rupture
or disconnection) cannot cause the loss of the travel limitation function.
The TLU has two brushless electric motors separately controlled by an electronic
assembly.
Each motor drives two screws via a reduction gear and permits the symmetrical
linear displacement of two nuts used as adjustable stops. (The two nuts have opposite
pitches).
A non-locking rotary stop limits the stroke of one of the screw/nut assemblies
which are irreversible.
There are two levers on each connection shaft; one is connected to the input rod
and the other is used as a punctual stop.
The movement of each screw is transmitted to a transducer unit (RVDT) via a reduction
gear which permits to indicate the position of the variable stop.
Two rigging pins are used to set the two levers to the zero position (mid stroke).
Figure 9: Component Location” on page 13
Normal control circuit
Figure 10: TLU Motor Control” on page 14
The motor itself is a 3-phase asynchronous motor with no brushes:
• the associated electronic set supplies the motor windings with variable voltage
and variable frequency which are function of the input signal (representative of
the position error),
• the system permits to get torque/speed characteristics for the motor equivalent
to those obtained with a DC motor,
• a relay isolates the motor windings from the electronic set output when its coil
is not energized by the enable signal.
Moreover, in this situation, the output signals are sent back to the monitoring circuits
for comparison with fixed thresholds
• the command signal that the computer dedicated to each electronic set sends
is a DC current which varies between plus or minus 8 mA into a 500-ohm resistor
(for the computer, the motor is equivalent to a standard servovalve),
• another relay upstream of the actuator and controlled by the computer supplies
the 28VDC power.
To prevent icing, there is a heating system which includes two coils and their regulating
thermostats.
Emergency control circuit
Figure 10: TLU Motor Control” on page 14
When the two FACs can no longer achieve normal control (for example if the two
electronic power sets of the TLU are failed), an emergency control brings back the
stops to the low speed configuration (maximum possible deflection of the rudder)
when slats are extended.
For this, the motor is used as a 2-phase asynchronous motor energized by 26 V
400 Hz power.
This control mode is achieved when the coil of a specific relay is energized for a
period of 30 s approx (this time is greatly sufficient to bring back the stops to the
low speed configuration).
