737 MAX Flight Crew Operations Manual
Flight Controls -
System Description
MN-FLT-OH-201
9.20.15
Yaw Control
Yaw control is accomplished by a hydraulically powered rudder and a digital yaw
damper system. The rudder is controlled by displacing the rudder pedals. The yaw
damping functions are controlled through the Stall Management/Yaw Damper
(SMYD) computers.
Rudder
The rudder provides yaw control about the airplane’s vertical axis. The A and B
FLT CONTROL switches control hydraulic shutoff valves for the rudder and the
standby rudder.
Each set of rudder pedals is mechanically connected by cables to the input levers
of the main and standby rudder PCUs. The main PCU consists of two independent
input rods, two individual control valves, and two separate actuators; one for
Hydraulic system A and one for Hydraulic system B. The standby rudder PCU is
controlled by a separate input rod and control valve and powered by the standby
hydraulic system. All three input rods have individual jam override mechanisms
that allows input commands to continue to be transferred to the remaining free
input rods if an input rod or downstream hardware is hindered or jammed.
With both engines operating above approximately 135 kts, both hydraulic system
A and B pressure are each reduced within the main PCU by approximately 25%
each. This function limits full rudder authority in flight after takeoff and before
landing. It operates the same in the air and on the ground. It will disengage if a
difference in engine speed is detected.
The main rudder PCU contains a Force Fight Monitor (FFM) that detects opposing
pressure (force fight) between A and B actuators. This may occur if either system
A or B input is jammed or disconnected. The FFM output is used to automatically
turn on the Standby Hydraulic pump, open the standby rudder shutoff valve to
pressurize the standby rudder PCU, and illuminate the STBY RUD ON, Master
Caution, and Flight Control (FLT CONT) lights.
The standby rudder PCU is powered by the standby hydraulic system. The standby
hydraulic system is provided as a backup if system A and/or B pressure is lost.
With the standby PCU powered the pilot retains adequate rudder control
capability. It can be operated manually through the FLT CONTROL switches or
automatically. (Refer to Chapter 13, Hydraulics, Standby Hydraulic System)
An amber STBY RUD ON light illuminates when the standby rudder hydraulic
system is pressurized. The standby rudder system can be pressurized with either
Flight Control switch, automatically during takeoff or landing (Refer to Chapter
13, Hydraulics, Standby Hydraulic System) or automatically by the Force Fight
Monitor. The STBY RUD ON light illumination activates Master Caution and
Flight Control warning lights on the Systems Annunciation Panel.
March 1, 2021
