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or retracts to maintain airspeed for a given stick position.
2-5-8. Bank Angle Hold.
Bank angle trim without cyclic stick movement is provided
through left or right position of the cyclic stick AFCS
trim switch. Bank angle hold is disengaged anytime a
CENTERING DEVICE RELEASE switch is pressed, a
cyclic stick is moved laterally, or the HDG switch is ENGAGED.
Bank angle hold cannot be reengaged until the
roll is less than 1.5 per second.
2-5-9. Heading Hold.
The directional gyro provides an input to each AFCS
which signals the yaw ILCA to maintained heading within
5 degrees. Heading hold is disengaged if the swivel
switch is set to STEER or UNLOCK, a CENTERING DEVICE
RELEASE switch is pressed, or the directional pedals
are moved. Also, heading hold will be disengaged at
airspeed above 40 knots anytime lateral trim is used, the
stick is moved laterally, or HDG switch is ENGAGED.
Heading hold will not resume until yaw rate is less than
1.5 per second at an airspeed above 40 knots with a
bank angle of less than 1.5.
2-5-10. Airspeed Hold.
The airspeed hold feature provides a constant airspeed
and pitch attitude relative to cyclic stick position at airspeeds
above 40 knots. Airspeed and pitch can be set
with the AFCS trim switch on the cyclic stick or by displacing
the cyclic stick until the desired airspeed is achieved
then pressing the CENTERING DEVICE RELEASE
switch. Refer to Chapter 8 AFCS Off Flight Characteristics.
2-5-11. Altitude Hold.
Two methods of altitude hold can be selected. They are
radar altitude hold or barometric altitude hold.
a. Radar Altitude Hold. Radar altitude hold will
maintain a more precise altitude in hover or over water
flight than barometric altitude hold. Maximum altitude for
the use of radar altitude hold is 1,500 feet AGL.
An error signal, caused by radar altitude deviations, is derived
from the pilot radar altimeter receiver-transmitter and
is processed by the No. 1 AFCS computer. The processed
error signal is applied to the THRUST CONT LEVER CCDA
which drives the THRUST CONT levers in the direction
necessary to null the error signal.
b. Barometric Altitude Hold. Barometric altitude
hold is used in forward flight over terrain. It uses error
signals produced within the No. 1 AFCS computer.
These error signals are in response to static pressure
changes and are proportional to altitude changes. The signal
is processed by the AFCS computer and applied to the
THRUST CONT LEVER CCDA which drives the THRUST
CONT levers in the direction necessary to null the error
signal.
2-5-12. Heading Select.
Heading select is engaged when the HDG switch on the
AFCS panel (fig. 2-5-3) is pressed and the ENGAGED
light illuminates. The heading bug on the selected HSI is
the referenced heading. Rotating the HDG knob of the
HSI to set the bug at a new referenced heading produces
an error signal which is processed by the AFCS computers
and applied to the roll ILCA. The roll ILCA than moves
to produce a standard rate turn up to a maximum bank
angle of 20 until the selected heading is captured.
Heading select can only be selected at airspeed above
40 knots. Heading select is disengaged anytime a CENTERING
DEVICE RELEASE switch is pressed, the HDG
switch on the AFCS panel is disengaged, or when the
opposite CMD SEL switch on the HSI MODE SELECT
panel is pressed.
2-5-13. Longitudinal Cyclic Trim System.
Longitudinal cyclic trim (LCT) control is part of AFCS.
LCT reduces fuselage nose down attitude as forward
airspeed is increased, thus reducing fuselage drag. The
system also reduces rotor blade flapping which results in
lower stresses on the rotor shafts. The LCT actuators are
installed under the swashplates. Signals are transmitted
to these actuators either automatically by AFCS or
manually by CYCLIC TRIM switches drive the actuators
to GND (ground) operating position on ground contact.
2-5-14. Controls and Indicators.
2-5-15. AFCS CONTROL Panel.
The AFCS control panel (fig. 2-5-3) is on the canted
console. It consists of the heading and altitude select,
SYSTEM SEL (select), and CYCLIC TRIM sections.
a. Heading and Altitude Select Switches. The legend
on these switches will dim when the PLT INST rotary
control switch is placed out of the OFF detent.
TM 1-1520-240-10
2-5-5
(1) HDG Switch. The HDG (heading) switch is
used in conjunction with the CMD SEL switch either HSI
MODE SELECT panel and the heading bug on either HSI
to select coupled turns. The switch can be used only
when airspeed is above 40 knots. When the switch is
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