ensures that STAB AUG is engaged before any
The amplifier/computer (ANC) receivres
various error and control signals. These weak
other mode of the AFCS.
signals are coupled (summed), modified, and
Attitude Hold
amplified to develop the control surface command
signal to drive the electrohydraulic actuators. The
The attitude hold mode is the basic, hands-
actuator moves a certain direction for a specific
distance, depending on the command signal's
off mode of operation. With attitude hold mode
polarity and magnitude.
engaged, the AFCS maintains aircraft attitude at
the time of engagement in pitch and roll. This
The flight control surfaces (rudder, aileron,
particular mode is governed by the actual degrees
and elevator) are mechanically linked to the
of bank or pitch of the aircraft. For example, a
electrohydraulic actuator. As the control surface
typical attitude hold mode will release when the
moves, a position transmitter synchro develops
aircraft exceeds 60 degrees in pitch or 70
a feedback signal having the opposite polarity to
degrees in roll.
the error signal. The magnitude of the feedback
signal increases as the control surface displace-
ment increases. When the error signal and
Altitude Hold
feedback signal are equal and opposite in
With altitude hold engaged, the aircraft will
magnitude and polarity, the control surface will
maintain the altitude at the time of engagement.
no longer move.
If the aircraft is climbing or diving at engagement,
Movement of the control surface causes the
the aircraft returns to the altitude that existed at
aircraft to displace about its axis (or reference).
This movement corrects the original error signal
engagement. The AFCS receives control signals
for this mode from the air data computer.
as sensed by the sensors. Without any error
signal inputs to the control channel of the
Heading Hold
amplifier/computer, only the feedback signal is
present. The feedback signal is opposite in polarity
and magnitude to the original error signal. This
With heading hold engaged, the AFCS
drives the electrohydraulic actuator an equal
maintains aircraft heading at the time of
distance in the opposite direction and brings the
engagement. If the pilot is flying a heading of 180
degrees and engages the heading hold, the AFCS
control surface back to the null or reference
maintains the heading of 180 degrees.
position.
AFCS MODES
Control Stick Steering/Control
Wheel Steering
This part of the chapter contains a brief
description of the various modes available in a
The control stick steering/control wheel
typical automatic flight control system. The pilot
steering mode lets the pilot manually (moving the
selects one of these modes on the AFCS control
stick/wheel) change the attitude of the aircraft
with the AFCS engaged without disengaging it.
panel by moving the control stick or using knobs
After achieving the new attitude, the pilot releases
on some instruments. Because the circuitry is
complex and varies among the different weapons
the stick/wheel, and the AFCS resumes control
systems, no specific mode will be diagramed.
of the aircraft.
Stability Augmentation Mode
Heading Select
The stability augmentation (STAB AUG)
In the heading select mode, the aircraft will
mode provides improved control of the aircraft
automatically turn to a course selected by the
by automatically damping oscillations about the
pilot. Upon engagement, the aircraft will assume
pitch, roll, and yaw axes. Signals from rate
a fixed maximum roll and turn to the selected
heading.
gyroscopes command control surface movement
through electrohydraulic actuators. In some
high-speed aircraft, STAB AUG is considered
Mach Hold
critical to safe flight. For this reason, the STAB
AUG engagement switch connects in series with
The Mach hold function maintains the Mach
number existing at the time of Mach hold
all other modes of the AFCS. This arrangement
