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.
9.20.6 D6-27370-866-EGP
Pitch Control
The pitch control surfaces consist of hydraulically powered elevators and an
electrically powered stabilizer. The elevators are controlled by forward or aft
movement of the control column. The stabilizer is controlled by autopilot trim or
manual trim.
Elevators
The elevators provide pitch control around the airplane’s lateral axis. The
elevators are positioned by the pilots’ control columns. The A and B FLT
CONTROL switches control hydraulic shutoff valves for the elevators.
Cables connect the pilots’ control columns to elevator Power Control Units
(PCUs) which are powered by hydraulic system A and B. The elevators are
interconnected by a torque tube. With loss of hydraulic system A and B the
elevators can be mechanically positioned by forward or aft movement of the
pilots’ control columns. Control forces are higher due to friction and aerodynamic
loads.
Elevator Control Column Override Mechanism
In the event of a control column jam, an override mechanism allows the control
columns to be physically separated. Applying force against the jam will breakout
either the Captain’s or First Officer’s control column. Whichever column moves
freely after the breakout can provide adequate elevator control.
Although total available elevator travel is significantly reduced, there is sufficient
elevator travel available for landing flare. Column forces are higher and exceed
those experienced during manual reversion. If the jam exists during the landing
phase, higher forces are required to generate sufficient elevator control to flare for
landing. Stabilizer trim is available to counteract the sustained control column
force.
Elevator Feel System
The elevator feel computer provides simulated aerodynamic forces using airspeed
(from the elevator pitot system) and stabilizer position. Feel is transmitted to the
control columns by the elevator feel and centering unit. To operate the feel system
the elevator feel computer uses either hydraulic system A or B pressure,
whichever is higher. When either hydraulic system or elevator feel pitot system
fails, excessive differential hydraulic pressure is sensed in the elevator feel
computer and the FEEL DIFF PRESS light illuminates.
September 28, 2006
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