TM 55-1520-240-T
15-1.2 OVERALL ELECTRICAL CABLING INTERCONNECTIONS OF APU SYSTEM COMPONENTS, "INTERFACED" TO MAIN ENGINE "STARTER" COMPONENTS (Continued)
15-1.2
A.
OVERVIEW - OF APU INTERFACED TO MAIN ENGINE STARTERS
The APU Interfaced to the Main Engine Starters includes the activation of the following components and
interconnections as shown in paragraph 15-1.2.
1. Hydraulic STARTERS on each engine
2. Engine START valves
3. Solenoid-operated pilot valves on the Utility System Pressure Control Modules
4. Start switches and relays
5. Caution lights
6. Ignition LOCK switch
7. Start Fuel Solenoid Valve
8. APU Fuel Solenoid Valve
9. APU Fuel Boost Pump
10. APU Electronic Sequence Unit (ESU)
11. APU FAULT RELAY
12. APU Ignition Exciter
13. APU T62-T-2B Engine
B.
BRIEF OPERATIONAL DESCRIPTION OF MAIN ENGINE START SYSTEM
On helicopters without 74, when the Main Engine START switch is moved to MOTOR, the STARTER ON
caution light comes "ON" and the Engine Start valve "OPENS". The START VALVE applies UTILITY SYS-
TEM PRESSURE from the APU to the Main Engine starter, thus rotating the Engine Starter and Compres-
sor.
On helicopters with 74, when the main ENG START switch is set to 1 or 2, the engine start valve
"OPENS". The START VALVE applies UTILITY SYSTEM PRESSURE from the APU to the Main Engine
starter, thus rotating the Engine Starter and Compressor.
NOTE
For additional information, refer to task 15-5.5 - ENGINE Hydraulic Starting System
Operational Check and tasks 4.4 - Engine Start and Ignition System (Without 74) or
4.10- Engine Start and Ignition System (With 74).
C.
BRIEF OPERATIONAL DESCRIPTION OF APU SYSTEM
The Gas Turbine Auxiliary Power Unit T62-T-2B (APU) is mounted in the aft cabin above the ramp. The
basic components of the APU are the gas turbine engine, hydraulic motor-pump, fuel control, accessory
drive, and ac generator. An APU control box, ESU, which monitors APU operation is on the left side of the
cabin above the ramp. The motor-pump on the APU pressurizes the utility and flight control hydraulic sys-
tems for main engine starting and ground checks. The APU also drives an ac generator which supplies
power to No. 1 and No. 2 electrical systems. The APU receives fuel from the left fuel system thru a
booster pump, a manual fuel shutoff valve, and a solenoid valve.
The APU control switch is on the electrical power panel on the overhead switch panel. It is a three-position
switch marked APU OFF, RUN, and START. The switch is spring loaded from RUN to START. To start the
APU, the switch is moved from OFF to RUN for 3 to 5 seconds, then set to START for 2 seconds, then
released to RUN. APU start is then automatic and controlled by the control box. The APU ON caution light
will come on in about 10 to 12 seconds.
NOTE
For additional information, refer to task 15-1.5, APU Operational Check.
END OF TASK
15-12
