or no-signal position. Some aircraft do not use
A radio navigation aid can also supply heading
an altitude controller. In place of an altitude
information to the AFCS. If the pilot desires to
controller, the AFCS uses signals from the ADC.
fly to a selected ground station, the radio reciever
Figure 8-22, view A, shows a three-quarter
develops a signal to produce the desired ground
view of a barometric altitude control. The outside
track directly to the station.
AIR DATA INFORMATION. --Changes in
the speed of an aircraft also affect the effective-
ness of the control surfaces. At a given altitude,
slow speeds require more control surface move-
ment than high speeds to accomplish the same
maneuver. The pilot maintains (or changes) the
altitude by reference to the altimeter. The AFCS
can also maintain a constant altitude. To
accomplish this task the AFCS uses altitude data
supplied by an air data sensor or air data
computer (ADC) as the reference altitude.
Airspeed. --Control surface signals are
modified by a gain control unit to compensate for
changes in airspeed. This unit uses the difference
between ram pressure and static pressure. A
mechanical schematic of the gain control unit is
shown in figure 8-21. Here, you can see that as
airspeed increases (ram air pressure increases), the
bellows causes the spring to become more
compressed. This allows the armature to move
each potentiometer's sliding arm to modify the
control surface signals an amount representative
of the change in airspeed of the aircraft. When
airspeed decreases, the armature moves to the
right, selecting a different amplifier gain. The
opposite occurs for increases in airspeed.
Altitude. --The AFCS includes an altitude
control feature to maintain the aircraft at a fixed
altitude. The altitude controller consists of an
aneroid, a mechanism for transmitting and
magnifying the motion of the aneroid, and a
solenoid-operated clutch. It also includes a
synchro transmitter and a centering device for
returning the synchro transmitter rotor to the null
Figure 8-22.-(A) Barometric altitude control; (B) Internal
parts; (C) Simplied schematic.
Figure 8-21.-Mechanical schematic of a gain control unit.
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