adjacent to the STBY RUD switch, shown in Figure 9. The yaw damper is
normally engaged even if the autopilot is not operating. (The DISENG
legend in the switch will be illuminated amber if the yaw damper is not
engaged.) If the autopilot is engaged, the yaw damper must be engaged,
since autopilot rudder commands use the yaw damper circuits to displace
the rudder.
The yaw damper function is a redundant dual-channel installation. Yaw
damper channel #1 is controlled by the autopilot function hosted in
processor modules in Modular Avionics Unit (MAU) #1, channel #2 by MAU
#2. Each channel has a dedicated Electro-Hydraulic Servo Valve (EHSV)
internal to the rudder actuator. The autopilot processor detects an
uncommanded yaw displacement by monitoring data from the Inertial
Reference Units (IRUs). The autopilot processor signals a rudder
displacement to counter the aircraft yaw through an Actuator Input/Output
Processor (AIOP) module via ARINC-429 bus connection to the EHSV on
the rudder actuator. The amount of rudder displacement necessary is a
function of airspeed / Mach number, and the AIOP uses information from
the Air Data Application (ADA) in the MAU to determine the amount of
rudder to apply. A feedback loop from a Rotary Variable Displacement
Transducer to the AIOP confirms the rudder position. The maximum
amount of rudder displacement available to the yaw damper is five degrees
(5°).
Since only one yaw damper channel is necessary for rudder control, the
active channel alternates on each flight segment (a function of weight-on-
wheels) to prolong system life. If the active channel fails, the standby
channel will automatically assume yaw damper control.
The yaw damper function also provides a rudder input for aircraft turn
coordination provided the flaps are not set to thirty degrees (30°) or more.
The yaw damper will add up to five degrees (5°) of rudder in the direction
of turn without pilot rudder input.
Autopilot control of the rudder is the same functional process as the yaw
damper, but the amount of rudder displacement available to the autopilot is
greater (up to the 22° limit). Larger rudder inputs are necessary for the
lower airspeeds associated with coupled approaches or during single
engine operations.
B. Rudder Trim:
The rudder is trimmed by manual inputs from the trim wheel mounted on
the cockpit aft center pedestal (see Figure 10). There is no trim tab on the
rudder, rather the whole rudder panel moves in response to trim input. The
rudder may be displaced up seven and a half degrees (7½°) left or right
with trim commands. Rotation of the trim wheel moves a cable linkage to a
drum mounted adjacent to the rudder hydraulic actuator. Movement of the
trim wheel rotates the drum and a linkage attached to the drum moves the
shaft of the rudder actuator. The rudder actuator hydraulically positions the
rudder by the commanded amount of deflection, and the linkage from the
drum establishes the trimmed rudder position as a new neutral setting for
the actuator. Manual or yaw damper / autopilot rudder deflections are then
summed to the existing trimmed rudder displacement. (The autopilot does
not have a separate trim input to the rudder.)
OPERATING MANUAL
PRODUCTION AIRCRAFT SYSTEMS2A-27-00
Page 24
August 14/03
To Next Page
Loading...
