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tank boost pump with the crossßow open) pressurizes the fuel line. When
APU RPM reaches 10% during start, the shutoff valve opens and fuel
passes through the fuel line to the APU FCU.
The FCU is a high pressure gear pump attached to and driven by the APU
oil pump. Within the FCU are Þlters, a relief valve, a servo valve to maintain
constant fuel pressure, a metering valve, a shutoff valve and a fuel nozzle.
Fuel drawn from the left wing tank is Þltered, pressurized, metered to
satisfy APU load requirements, passed through the shutoff valve and
sprayed into the combustor through a dual oriÞce nozzle. During APU start,
fuel is initially supplied through the primary oriÞce. As the APU accelerates,
fuel pressure increases with the increasing speed of the gearbox (and oil
pump rotation and pressure) and fuel requirements of the APU increase.
The secondary oriÞce opens to supply additional fuel to support APU
acceleration and maintain full RPM.
The protective functions of the ECU shut down the APU by closing the FCU
shutoff valve (positioned just prior to the fuel nozzle) if normal limitations
are exceeded. Closure of the shutoff valve causes a surge in fuel pressure.
The relief valve opens with the pressure surge and routes the excess fuel
back to the pump inlet. As APU RPM slows from fuel starvation, fuel
pressure decreases and the relief valve closes.
OPERATING MANUAL
2A-49-00 PRODUCTION AIRCRAFT SYSTEMS
Page 4
October 11/02
Revision 8
D. Lubrication:
The oil pump that drives the FCU fuel pump supplies pressure and splash
lubrication to all gears, shafts and bearings within the APU. The oil pump
draws oil from a sump beneath the pump through a suction line. Oil passes
through a pressure regulating valve, an oil Þlter and a low oil pressure
switch before distribution to the APU lubrication points. The oil is then
gravity scavenged back to the oil sump. If APU oil pressure drops below
thirty-one (31) psi, the low oil pressure switch will cause the illumination of
the red OIL PRESS LOW light on the APU control panel. Cooling is
provided by external cooling Þns on the sump that transfer heat to the
surrounding air. The oil sump (or tank) has a high oil temperature switch
and a drain plug with a magnetic chip detector. If cooling air is insufficient
and the APU oil temperature reaches 141¡C - 147¡C, the red OIL TEMP
HIGH light will illuminate on the APU control panel. APU oil level is read on
a dipstick attached to the sump (tank) Þller opening cap. Normal capacity is
2.5 U.S. quarts. An access panel on the APU enclosure must be removed
in order to reach the dipstick / Þller cap.
2. APU Accessories:
The APU is equipped with the following accessories:
• Electrical Alternator
• Bleed Air System
• Fire Detection, Warning and Extinguishing System
A. Electrical Alternator:
The APU alternator is identical and interchangeable with the engine-driven
alternators. It is mounted on the APU gearbox and cooled by an integrated
fan. The alternator delivers 115 V AC, 400 Hz (±20) three phase power at
loads up to 30 kVA, enabling the alternator to power all airplane electrical
systems. Because the APU alternator runs at a constant speed
(approximately 8,000 RPM) it does not require a converter and can power
airplane buses directly.
The alternator can provide electrical power in ßight as well as on the
ground, however the amount of load (kVA) available from the alternator
decreases at higher altitudes due to APU performance degradation with
decreasing air density. For airplanes SN 1000-1155, the available load
decreases linearly from 30 kVA to 15 kVA from 22,000 feet to 30,000 feet.
On airplanes SN 1156 and subsequent the APU air intake is modiÞed with
an air scoop to provide more air to the APU at higher altitudes. For these
airplanes, 30 kVA is available up to 30,000 feet with a reduction to 20 kVA
at 35,000 feet, the maximum APU operating altitude.
An APU ALTNR OFF light (blue) on the APU overhead panel illuminates
when the APU has reached 95% RPM for four (4) seconds, signalling that
the alternator may be selected ON to power airplane systems. The APU
alternator is connected to the airplane electrical system by pressing the
AUX PWR switchlight on the ELECTRIC MASTER section of the
ELECTRIC POWER MONITOR panel to ON. The voltage and frequency of
the APU alternator may then be monitored by pressing the AUX switchlight
next to the AC VOLTS / FREQ display. The AUX PWR % display on the AC
POWER CONTROL should read 100 with the APU alternator powering all
　
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