AUXILIARY POWER UNIT
2.48 AUXILIARY POWER UNIT (APU).
To prevent an accidental APU start, the
APU circuit breaker in the aft avionics
bay and the APU HOLD circuit breaker
on the pilot overhead circuit breaker
panel shall be out when battery or exter-
nal electrical power is connected to the
helicopter and unqualified personnel are
in or around the pilot crew station.
The APU indirectly provides hydraulic, pneumatic, and
electrical power for the operation of helicopter systems
whenever the engines are not driving the main transmis-
sion accessory section. The APU provides the means of
engine starting without the need for an aircraft ground
power unit (AGPU). It is located just inboard of the right
engine nacelle in the aft equipment bay. The APU consists
of a gearbox, a compressor, and a turbine section, togeth-
er with associated fuel, lubrication, and electrical systems.
The APU controller and the APU control panel are
installed separately from the APU. The utility hydraulic ac-
cumulator, located on the deck of the aft equipment bay
below the APU, provides hydraulic pressure to the APU
hydraulic starter through a solenoid-operated hydraulic
start valve. The start valve opens when the APU START
switch is positioned to START and closes automatically at
60% APU speed. The utility hydraulic accumulator is dis-
cussed in more detail in Section VI of this chapter.
2.48.1 Fuel System.
The aft fuel cell provides fuel for
APU operation and is discussed in more detail in Section
IV of this chapter. The APU fuel control automatically reg-
ulates fuel flow. The APU burns approximately
135 pounds of fuel per hour.
2.48.2 Lubrication System.
The APU has a self-con-
tained oil system. An oil filler cap is located on the left side
of the unit. Oil cooling is provided by airflow over cooling
fins at the compressor inlet. The oil level sight gage (fig
2-45) is located on the right side and is an integral part of
the oil sump. The sight gage can be inspected through an
opening just below the right engine nacelle.
2.48.3 Electrical System.
The APU electrical system
requires dc power which is normally delivered by the heli-
copter battery when the APU control switch is set to RUN.
This switch will apply dc power to energize the APU boost
pump, open the APU fuel shutoff valve, and at the same
time, send a signal to open the utility accumulator start so-
lenoid valve to allow hydraulic pressure to turn the starter,
and shut off the SDC inlet throttle valve to unload the SDC
for an APU start. The APU ON caution light on the pilot
caution/warning panel illuminates when the APU is oper-
2.48.4 APU Controller.
The APU controller provides
for automatic start and operation of the APU and power
take-off clutch (PTO) engagement. The controller moni-
tors the APU for loss of thermocouple, overtemperature,
overspeed, overcurrent, low oil pressure, percent rpm,
and exhaust gas temperature. The APU controller trans-
mits a shutdown signal to the APU whenever the APU
control panel switch is set to OFF. The controller also
transmits the shutdown signal automatically for fault
detection of any of the monitored functions. The APU
FAIL caution light on the pilot caution/ warning panel is il-
luminated by the low oil pressure switch when automatic
shutdown occurs. The PTO clutch actuation is also con-
trolled by the APU controller. Normally, PTO clutch en-
gagement occurs at 60% APU speed if the main rotor is
below 90% speed; however, if the main rotor is above
90% speed, it will engage at 95% APU speed. For cold
weather starts below 0 F (18 C), PTO clutch engage-
ment is at 95% APU speed using a cold start switch on the
APU control panel.
2.48.5 APU Control Panel.
The APU control panel (fig
2-41), on the pilot right console, has the following three
a. APU Control Switch.
A three-position APU con-
trol switch allows the pilot to START, RUN, and shut down
(OFF) the APU. This switch is spring-loaded from the
START to RUN position. Pilot pressure at the START
position is required for only one or two seconds to ensure
electrical circuit latching.
Do not use APU 95% cold start switch
when the ambient temperature is above 0
F (18 C). Use of this switch will reduce
the power takeoff (PTO) clutch life drasti-
cally, and could cause premature failure
of the clutch duplex bearing/needle bear-
ing as well as main transmission acces-
sory gear case component failure.