TM 1-2840-252-23-3
DECU TROUBLESHOOTING
SECTION I. INTRODUCTION
G-1 GENERAL
a. This Appendix provides fault isolation instructions for the Model EMC-32T Full Authority Digital Electronic
Control (FADEC) supplied by Chandler Evans, Control Systems Division, Coltec Industries, Inc., Charter Oak
Blvd., Box 330651, West Hartford, CT 06133-0651.
b. In subsequent revisions to this manual, changes or additions to the text, tables or illustrations will be indicated
by a vertical line in the left margin of affected material.
G-2 DESCRIPTION
a. The EMC-32T Full Authority Digital Electronic Control (FADEC) includes all control functions required for
proper and complete control of the Textron Lycoming T55 turboshaft engine. The EMC-32T version operates
the T55-L-714 engine on the MH47E helicopter.
b. Fuel Control System. The control system consists of a digital electronic control unit (DECU) and a
hydromechanical assembly (HMA) manufactured by Chandler Evans, and Lycoming-supplied sensors and
electrical harnesses. The system requires signal inputs from airframe components and utilizes airframe
harnesses for inter- connection between HMA, DECU, engine, and airframe components (see figure 201). The
system provides two operating modes: primary and reversionary. The primary mode is the normal mode of
operation. In the event of a primary failure. the system automatically switches to the reversionary backup
mode. Reversionary mode can also be selected by the pilot.
c.
Hydromechanical Assembly (HMA). The HMA consists of two units: The hydromechanical metering unit
(HMU) and the fuel pumping unit (FPU). The HMU includes all fuel metering components to support both
primary and reversionary fuel metering, a self-contained alternator for powering the electronics, both primary
and reversionary compressor bleed air control, and redundant core speed sensing. The FPU includes a jet-
induced boost into a main stage gear section. When operating in primary mode, the HMU receives actuation
signals from the DECU that operate the primary stepper motor. The stepper motor controls the position of the
fuel metering valve. A potentiometer connected to the rotary portion of the metering valve provides closed-
loop feedback to the DECU. When operating in reversionary mode, the primary stepper motor is de-
energized, and fuel flow is deter- mined by the hydromechanical Wf/P3 reversionary control. The scheduled
Wf/P3 is multiplied by P3 to give altitude-sensitive control of metered fuel flow. A mechanical P3 transducer,
mechanical Wf/P3 times P3 multiplier, Wf/P3 servomechanism and reversionary stepper motor comprise this
function. The output of the mechanical multiplier drives the fuel metering valve to the required flow.
d. Digital Electronic Control Unit (DECU). The DECU includes a microcomputer-based primary control section and
an independent reversionary section, controlled through an independent microcomputer for backup. In
primary mode, the DECU provides engine load matching through either shaft torque or engine temperature as
selected by the pilot. The DECU sends signals to the HMU to control fuel flow based on the required engine
load match. The DECU's primary section also provides automatic start logic, surge detection and recovery,
torque sharing and management, control of the bleed valve actuator, and self-diagnostic capabilities. In the
event of a serious failure within the primary section, the system automatically switches to the redundant
electronic reversionary control. Engine load anticipation is provided using rotor thrust lever position, with
trimming provided by beep up and beep down switches. The DECU's reversionary section operates with the
hydromechanical Wf/P3 manual system by controlling the reversionary stepper motor and changeover
G-1
