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amber while the reverser is moving. When the reverser is stowed and locked, the
REV indication is removed. The Thrust levers cannot be moved forward until the
Reverse Thrust levers are fully down.
The thrust reversers are protected against deploying inadvertently. If a reverser
unlocks and deploys inadvertently, the reverser system applies bleed air to stow
and lock the reverser.
Airborne Vibration Monitoring System
The airborne vibration monitoring system monitors engine vibration levels. The
vibration indications display on the secondary engine display. The vibration
source indication also displays. If the vibration monitoring system cannot
determine the source (fan, N1, or N2), broadband (BB) displays for the affected
engine. Broadband vibration is the average vibration detected.
Certain engine malfunctions can result in airframe vibrations from the
windmilling engine. As the airplane transitions from cruise to landing, there can
be multiple, narrow regions of altitudes and airspeeds where the vibration level
can become severe. In general, airframe vibrations can best be reduced by
descending and reducing airspeed. However, if after descending and reducing
airspeed, the existing vibration level is unacceptable, and if it is impractical to
further reduce airspeed, the vibration level may be reduced to a previous, lower
level by a slight increase in airspeed.
October 1, 2000
747 Operations Manual
Engines, APU Chapter 7
APU System Description Section 30
Copyright © The Boeing Company. See title page for details.
D6-30151-425 7.30.1
Introduction
The auxiliary power unit (APU) is a self–contained gas turbine engine located in
the airplane tail cone.
The APU can be started on the ground and when left running for takeoff can be
operated in flight up to 20,000 feet.
The APU drives two generators capable of supplying the entire electrical load of
the airplane for normal ground operations. Electrical power is not available in
flight. The APU also provides air to the pneumatic system for operation of
components which require bleed air. The APU has bleed air capacity to run all air
conditioning packs except during engine start. APU bleed air is available in flight
for one pack up to 15,000 feet.
Refer to the following chapters for additional information:
• Chapter 2, Air Systems, for a description of APU bleed air operation
• Chapter 6, Electrical, for a description of APU electrical operation
• Chapter 8, Fire Protection, for a description of the APU fire protection
system
• Chapter 12, Fuel, for a description of the APU fuel system
APU Operation
APU Start
The APU DC electric starter is powered by an APU start transformer rectifier (TR)
whenever galley bus 2 is powered. Galley bus 2 may be powered from either an
external or internal AC power source through the synchronous bus. If galley bus 2
is not powered, starter power is supplied by the 24 volt APU battery. The battery
has a charger which disconnects during APU starter engagement. The APU
battery powers the inlet door, APU controller, DC fuel pump, and APU fire
detection system. The main battery supplies power for the APU fire extinguisher,
APU fuel valve, and standby power for the APU controller. During a battery start
sequence, the APU starter is powered by the APU battery and all APU components
except the starter are powered by the main battery while the APU starter is
engaged.
Rotating the APU selector momentarily to START begins the start sequence. The
APU fuel valve and inlet door open simultaneously. Starter engagement occurs
when the inlet door is fully open. The start sequence continues with ignition,
lightoff, and engine acceleration to rated speed.
APU start cycle restrictions are:
April 01, 2001
747 Operations Manual
Copyright © The Boeing Company. See title page for details.
7.30.2 D6-30151-425
Engines, APU -
APU System Description
If the TR should overheat with the start source switch in TR, starting power is
transferred to the battery and the start continued on battery power. Any further
start attempts with an overheated TR are inhibited.
A failure of the TR, other than an overheat, does not provide automatic switching
to the APU battery. Under these conditions, moving APU Start Source switch to
BATTERY removes the TR from the starting circuit and allows APU starting on
battery power.
APU Run
The EICAS memo message APU RUNNING is displayed when the APU selector
is ON and APU N1 RPMexceeds 95% N1.
APU Shutdown
Rotating the APU selector to OFF begins the shutdown cycle by closing the APU
bleed air valve. The APU continues running unloaded for a sixty second cooldown
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