2A-27-50: Horizontal Stabilizer System:
1. General Description:
The purpose of the horizontal stabilizer system is to provide to the flight crew a
means of automatically or manually controlling the position and the movement of
the horizontal stabilizer. This movement is provided to counteract the pitch
moment associated with flap movement.
The horizontal stabilizer pivots to change angle of incidence as flaps are extended
or retracted. The horizontal stabilizer actuator, located in the vertical stabilizer, is
electrically controlled and actuated to position the horizontal stabilizer. In addition,
the horizontal stabilizer may be used to provide pitch trim in the Emergency
Stabilizer (EMER STAB) mode.
Horizontal stabilizer position is displayed on the Primary Flight Display (PFD) and
the FLIGHT CONTROLS synoptic page.
2. Description of Subsystems, Units and Components:
A. Flap/Horizontal Stabilizer Control Unit (FCU):
The Flap/Horizontal Stabilizer Control Unit (FCU), located in the Baggage
Compartment Electronic Equipment Rack, performs control and system
health functions of the Flap/Stabilizer systems. It contains two independent
control channels: Channel A and Channel B. Each channel communicates
with the other by comparing inputs to the two horizontal stabilizer actuator
AC motors (one per channel) and outputs from the horizontal stabilizer
position resolver (one output per channel). A malfunctioning channel will be
inhibited and the remaining operating channel will continue to control the
flaps and stabilizer.
The FCU provides system protection by completing operational status tests
with Built-In-Test (BIT) and functional fault monitoring. The FCU performs
the tests and monitoring and communicates the data on the Aeronautical
Radio Incorporated (ARINC 429) data bus to the Crew Alerting System
(CAS) through the Fault Warning Computer (FWC) and to the Maintenance
Data Acquisition Unit (MDAU). The FCU accepts input information on the
ARINC 429 for computed airspeed and Weight-On-Wheels (WOW).
B. Horizontal Stabilizer Actuator:
The Horizontal Stabilizer Actuator (HSA) is a electrically powered ballscrew
actuator that normally moves in coordination with the flap position. The
HSA is driven by two AC electric motors controlled by the two channels of
the FCU. The HSA can also be used as a means of adjusting pitch trim in
the Emergency Stabilizer (EMER STAB) mode. The HSA is mounted inside
the upper forward section of the vertical stabilizer.
C. Emergency Stabilizer (EMER STAB) Switch:
(See Figure 11.)
The EMER STAB switch, located on the cockpit center pedestal, allows the
flight crew to enable the Emergency Stabilizer (EMER STAB) mode by
selection of the switch to ARM. In the EMER STAB mode, the horizontal
stabilizer can be trimmed to any position (within the limits of the HSA) to
help counteract excessive control forces in the unlikely event of a jammed
elevator or loss of both hydraulic systems by providing additional pitch trim
and control capability. Once in the EMER STAB mode, the stabilizer can be
positioned anywhere between +1.50° to -4.60°, regardless of flap position.
OPERATING MANUAL
PRODUCTION AIRCRAFT SYSTEMS 2A-27-00
Page 27
May 22/01
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