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• Lubrication
A. Basic Powerplant:
The APU powerplant consists of a single-stage compressor and singlestage
turbine mounted on a common rotating shaft. The compressor is
surrounded by an air chamber (plenum) that is connected to the air intake
duct. The turbine surround receives pressurized air from the compressor,
routing some to the combustor where air is mixed with fuel, ignited and
directed onto the turbine blades. Some compressed air is reserved for
OPERATING MANUAL
PRODUCTION AIRCRAFT SYSTEMS 2A-49-00
Page 1
October 11/02
Revision 8
turbine cooling, ßowing around the turbine and joining the exhaust ßow
prior to venting overboard. See the illustration in Figure 1.
A planetary gearbox is attached to and driven by the rotating compressor
section. The gearbox converts the high RPM of the compressor to lower
values for driving the oil pump, fuel control and pump and the AC
alternator. The gearbox also provides a mounting for the APU starter.
Air for the APU is drawn through an intake door located on the top of the
airplane near the vertical stabilizer fairing (with an air scoop on Serial
Number (SN) 1034 and 1156 and subsequent). Rotation of the compressor
draws in outside air (supplemented by ram pressure for those airplanes
equipped with an air scoop). The air ßow is accelerated by the rotating
compressor and directed through a diffuser that converts air velocity to
pressure. Pressurized air is routed to the turbine plenum. The mechanical
design of the plenum directs a precise amount of air into the combustion
liner where it is mixed with fuel and ignited. The remaining pressurized air
is used to cool the APU exhaust and/or extracted through the load control
valve for engine starting or air conditioning.
A surge valve, mounted on the turbine section, pneumatically unloads the
APU during starting to aid acceleration, and opens in ßight (controlled by
the nutcracker system) to maintain stable APU RPM. Air bled off by the
surge valve is vented into the APU exhaust.
Spent exhaust gases from the APU turbine section are directed through a
tailpipe exhaust assembly overboard, venting beneath the right engine
nacelle. The tailpipe shroud has a larger diameter than the APU turbine
exhaust exit opening. As exhaust gases pass through the shroud, ambient
air in the APU compartment is drawn into the tailpipe assembly by the
ejector pump action of the gas ßow. Ambient air ßow decreases the
temperature of the APU exhaust and creates a lower pressure within the
compartment, causing additional airßow that cools APU components and
accessories.
ModiÞcations have been made to the APU ducting and exhaust
components to enhance APU component cooling and to moderate the
effects of exhaust impingement on the airplane exterior:
• Airplanes SN 1133 and subsequent have a check valve in the
alternator cooling duct that draws in air from the aft equipment bay
for cooling
• Airplanes SN 1310 and subsequent have additional cooling ducts
piped from the air intake manifold to enhance airßow and prevent
soot buildup on APU internal components
• Airplanes SN 1310 and subsequent, and prior airplanes having part
three (3) of ASC 390 have a venturi in the APU bleed air duct that
limits the initial volume of air extracted from the APU for engine start
or air conditioning. Limiting the amount of air that may immediately
be removed from the compressor diffuser prevents temperature
spikes associated with the loss of cooling air in the turbine section.
• Airplanes SN 1311 (sic) and subsequent and prior airplanes having
part two (2) of ASC 390 have an exhaust deßector that directs the
gases away from the right engine nacelle.
OPERATING MANUAL
2A-49-00 PRODUCTION AIRCRAFT SYSTEMS
Page 2
October 11/02
Revision 8
B. Starting and Ignition:
The APU start sequence is initiated by crew activation of the switchlight
controls on the APU panel on the cockpit overhead. When the APU
MASTER switchlight is selected ON, the APU air inlet opens, the APU OIL
PRESS LOW circuit is armed and the APU fuel shutoff valve is opened.
When the APU air inlet door is fully open, the APU START switchlight is
armed. Pressing the APU START switch powers the APU starter from the
airplane batteries or external DC power source, and initiates the automatic
start sequence controlled by the Electronic Control Unit or ECU (see the
description of the start sequence in the Controls and Indications topic of
this section).
The APU starter, mounted on the gearcase, is a DC powered electric motor
　
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