TM 1-1520-238-23
1-252.2
Change 5
1.75.
APU OPERATING INSTRUCTIONS – USING EXTERNAL POWER – continued
WARNING
APU operation exceeds acceptable safe noise levels. Personnel working near or operating
will wear approved ear protection to protect their hearing. Failure to comply could result in
permanent hearing loss.
To prevent accidental APU start, ensure that the APU circuit breaker in the AFT avionics bay
and the APU HOLD circuit breaker on the pilot center circuit breaker panel are open when
battery or external electrical power is attached to the helicopter and unqualified personnel
are in and around the pilot crewstation.
APU fires may go unnoticed by crew/operators in crewstations. Post a fire guard in continu-
ous communication by the intercommunication system (ICS) with the crew/operators during
APU run-up.
CAUTION
Do not attempt to use the 95% NORM switch when outside ambient air temperature is above -18 C
(0 F). The viscosity of the hydraulic fluid and APU oil varies with surrounding climatic conditions of
the region. If the 95% NORM switch is used in warm climatic conditions, extreme mechanical stress
is placed on the utility hydraulic accumulator shaft. Excessive wear loads are also placed on the
power takeoff (PTO) clutch during sudden engagement which occurs during 95% norm operation,
causing a decrease in duty life of the dry friction clutch.
If APU does not start, wait 30 seconds for combustion chamber to drain before attempting APU
restart.
NOTE
APU RUN switch is spring loaded from START to RUN. Automatic starting will shut down if switch is
operated improperly.
y. Have assistant check vicinity of helicopter and warn personnel that APU is about to start. Wait for
assistant to advise CLEAR FOR APU START through ICS.
z. Have assistant take position 10 feet to side of engine No. 2 exhaust with fire extinguisher.
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