TM 1-1520-238-T-7
11–66
11–7.
SYSTEM DESCRIPTION (cont)
11–7
(c) Automatic mode (fig. 11–40) is operational when electrical power is applied. SCUs receive 28 VDC
from the STAB AUTO DC circuit breaker (CB3) and 115 VAC from the STAB AUTO AC circuit breaker (CB2). The
SCUs conduct a self-test, if circuit logic is correct, automatic mode programming of the stabilator starts at 30
knots forward airspeed. SCU inputs which determine circuit logic are airspeed information received from the
ADSS, the respective airspeed transducer, collective stick position from the collective servocylinder LVDT,
NOE/APRCH switch position on the pilot’s ASE panel, pitch rate input from the respective pitch rate gyro, and the
validity of the opposite SCU (reason check). When the SCUs determine that circuit logic is correct, the AUTO
MAN relay (K1-1/2) energizes. When relay K1-1/2 energizes, power is supplied to the respective actuator motor to
drive the stabilator to the position determined by the SCUs. The motor position is fed back to the respective SCU
through a 10K ohm potentiometer for comparison by the other SCU. If actuator motor positions disagree by more
than 10, SCU logic generates a fail condition. When a fail condition occurs, the SCUs de-energize relay K1-1/2.
The MAN STAB indicators on the pilot and CPG caution/warning panels light via a ground received by relay
K1-1/2. SCUs generate and send a SCU fail signal to the audio warning system through relay K1-8 for SCU 2 and
relay K1-7 for SCU 1 (not shown), enabling the stabilator audio warning tone. Pressing the pilot’s or CPG’s
automatic operation/audio warning RESET pushbutton on the stabilator manual control panel disables the audio
warning tone and attempts to re-engage the automatic mode. If the fault is not cleared, the stabilator system
remains in manual mode. If the fault is cleared, the automatic mode reengages and the MAN STAB indicators go
out. If the NOE/APRCH switch is selected and airspeed is above 80 knots, automatic mode operates normally. If
airspeed falls below 80 knots, the stabilator is driven to 25 down. The stabilator remains at 25 down until
airspeed is increased above 80 knots.
