737 Flight Crew Operations Manual
Flight Controls -
System Description
Boeing Proprietary. Copyright © Boeing. May be subject to export restrictions under EAR. See title page for details.
D6-27370-866-EGP 9.20.3
Roll Control
The roll control surfaces consist of hydraulically powered ailerons and flight
spoilers, which are controlled by rotating either control wheel.
Ailerons
The ailerons provide roll control around the airplane’s longitudinal axis. The
ailerons are positioned by the pilots' control wheels. The A and B FLT CONTROL
switches control hydraulic shutoff valves. These valves can be used to isolate each
aileron, as well as the elevators and rudder, from related hydraulic system
pressure.
The Captain’s control wheel is connected by cables to the aileron Power Control
Units (PCUs) through the aileron feel and centering unit. The First Officer’s
control wheel is connected by cables to the spoiler PCUs through the spoiler
mixer. The two control wheels are connected by a cable drive system which allows
actuation of both ailerons and spoilers by either control wheel. With total
hydraulic power failure the ailerons can be mechanically positioned by rotating
the pilots' control wheels. Control forces are higher due to friction and
aerodynamic loads.
Aileron Transfer Mechanism
If the ailerons or spoilers are jammed, force applied to the Captain’s and the First
Officer’s control wheels will identify which system, ailerons or spoilers, is usable
and which control wheel, Captain’s or First Officer’s, can provide roll control. If
the aileron control system is jammed, force applied to the First Officer’s control
wheel provides roll control from the spoilers. The ailerons and the Captain’s
control wheel are inoperative. If the spoiler system is jammed, force applied to the
Captain’s control wheel provides roll control from the ailerons. The spoilers and
the First Officer’s control wheel are inoperative.
Aileron Trim
Dual AILERON trim switches, located on the aft electronic panel, must be pushed
simultaneously to command trim changes. The trim electrically repositions the
aileron feel and centering unit, which causes the control wheel to rotate and
redefines the aileron neutral position. The amount of aileron trim is indicated on
a scale on the top of each control column.
If aileron trim is used with the autopilot engaged, the trim is not reflected in the
control wheel position. The autopilot overpowers the trim and holds the control
wheel where it is required for heading/track control. Any aileron trim applied
when the autopilot is engaged can result in an out of trim condition and an abrupt
rolling movement when the autopilot is disconnected.
September 28, 2006
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