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that spins the APU up to a speed sufficient to support ignition and light off,
approximately sixty percent (60%) RPM. A clutch delivers starter torque to
the gearbox. When the APU RPM reaches approximately 60%, DC power
is removed from the starter, the clutch disengages the starter from the
gearbox, and the starter slows to a stop.
It is important that the crew have an indication of APU starter
disengagement. If the starter remains engaged and the APU continues to
accelerate to 100% RPM, damage and/or catastrophic failure of the starter
may occur. In initial production of the GIV airplanes, there was no indicator
monitoring DC power application to the starter. ASC 13, applicable to SN
1000-1155, and incorporated into production assembly with SN 1156 and
subsequent, provided circuitry to the APU START switchlight to illuminate
the ON legend whenever DC power is applied to the starter. Starter cutout
can be monitored by conÞrming that the ON legend extinguishes above
60% RPM. ASC 212 (applicable to airplanes SN 1000-1155 having ASC
13) and ASC 212 AM1 (applicable to airplanes SN 1156 and subsequent)
improved on the reliability of the monitoring circuit by powering the circuit
from the Essential DC bus and adding auxiliary contacts to the circuit. (The
circuitry installed with ASC 13 and incorporated in SN 1156 was powered
by the Battery Tie bus that is de-energized when the APU reaches 60%
RPM, and thus could not provide starter monitoring should the starter
remain powered at higher APU speeds.)
The ECU sequences the start process initiated with the APU START
switchlight. RPM is displayed on the APU control panel as soon as the
starter begins rotating the APU gearbox. At 10% RPM, the fuel control
opens, injecting atomized fuel into the combustor and the igniter is
energized.
The igniter is a high energy capacitive discharge unit consisting of a single
plug with a long shank protected by a heat shield. The plug is mounted on
either the turbine plenum or the left side of the turbine intake (depending on
APU model). The igniter plug produces approximately Þve thousand
(5,000) volts and remains energized until the APU reaches 95% RPM with
an additional four (4) second delay to ensure stable APU idle RPM. There
is no indication of ignitor cut-out. A high-voltage APU ignition system is
installed on airplanes SN 1363 and subsequent, and available for retroÞt
into pervious airplanes as ASC 404. The higher voltage increases service
life and improves in-ßight starting.
OPERATING MANUAL
PRODUCTION AIRCRAFT SYSTEMS 2A-49-00
Page 3
October 11/02
Revision 8
C. Electronic and Fuel Controls:
APU operation is monitored and controlled by the ECU through inputs from
the cockpit control panel, APU speed sensing and temperature elements
and outputs to the Fuel Control Unit (FCU). The ECU is a solid state
electronic control mounted outside of the APU enclosure to minimize
exposure to heat and vibration. On airplanes SN 1436 and subsequent, a
second ECU is installed as a Òcold spareÓ, making a replacement ECU
readily available if needed.
The unit receives RPM information from a speed sensing monopole device
installed on the APU gearbox. The monopole is a cylindrical pointer with a
magnetically charged tip. The tip is closely aligned a rotating gear train in
the gearbox. As the teeth of the gear pass the tip of the monopole, an AC
electric current is generated with a frequency proportional to gear rotation
(and APU) speed. At an APU RPM of 100%, the frequency is approximately
17,600 Hz. The ECU senses this frequency as APU RPM.
The operating temperature of the APU is sensed by a single thermocouple
positioned in the APU exhaust stream aft of the turbine. The thermocouple
generates a voltage proportional to Exhaust Gas Temperature (EGT) and
transmits the voltage to the ECU where it is translated as EGT.
The ECU uses the RPM and EGT inputs to correctly sequence APU
starting, adjust the fuel ßow through the FCU to maintain the APU within
normal operating ranges, and with additional inputs from oil temperature,
oil pressure and amperage readings from system sub-components, protect
the APU with an auto-shutdown feature if abnormal conditions are
detected. The conditions prompting and automatic APU shutdown are
discussed in the Controls and Indications topic of this section.
The ECU controls the speed and temperature of the APU by adjusting the
amount of fuel available for combustion in the APU (ignition is not
necessary above 95% RPM). Fuel for the APU is drawn from the left fuel
tank hopper through a shutoff valve. The left tank boost pump (or the right
　
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