Boeing B737 NG - Systems Summary [APU]
The auxiliary power unit (APU) is a self–contained gas turbine engine installed within a fireproof compartment located in the tail of the airplane.
The APU supplies bleed air for engine starting or air conditioning. An AC electrical generator on the APU provides an auxiliary AC power source.
COOLING AIR
INLET
EXHAUST
OUTLET
EXHAUST
MUFFLER
APU FUEL
LINE
APU BLEED
AIR DUCT
AIR INLET DOOR
APU DUCT
AIR DIFFUSER
DUCT
VORTEX
GENERATOR
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Boeing B737 NG - Systems Summary [APU]
The APU starts and operates up to the airplane maximum certified altitude.
The APU supplies bleed air for both air conditioning packs on the ground or one pack in flight. Both transfer busses can be powered on the ground or in flight.
[Option - APU DC fuel boost pump]
Fuel to start and operate the APU comes from the left side of the fuel manifold when the AC fuel pumps are operating. A DC operated APU fuel boost pump is installed to ensure positive fuel pressure to the APU fuel control unit. During APU start and operation, the pump operates automatically when the APU fuel control unit senses low fuel pressure. The pump shuts off automatically when an AC fuel pump pressurizes the fuel manifold. If the AC and DC fuel pumps are not operating, fuel is suction fed from the No. 1 tank. During APU operation, fuel is automatically heated to prevent icing.
APU engine air routes to the APU through an automatically operated air inlet door located on the right side of the fuselage. APU exhaust gases discharge overboard through an exhaust muffler.
Air for APU cooling enters through a cooling air inlet above the APU exhaust outlet. This air circulates through the APU compartment, passes through the oil cooler and vents through the exhaust outlet.
APU operation requires the following:
• APU fire switch on the overheat/fire panel must be IN
• APU fire control handle on the APU ground control panel must be IN
• battery switch must be ON.
Electrical power to start the APU comes from No. 1 transfer bus or the airplane battery(ies). With AC power available, the starter generator uses AC power to start the APU. With no AC power, the starter generator uses battery power to start the
APU.
Moving the battery switch to OFF on the ground or in the air automatically shuts down the APU because of power loss to the electronic control unit.
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Boeing B737 NG - Systems Summary [APU]
The automatic start sequence begins by moving the APU switch momentarily to
START. This initiates opening of the air inlet door. When the APU inlet door reaches the full open position the start sequence begins. After the APU reaches the proper speed, ignition and fuel are provided. When the APU is ready to accept a bleed air or electrical load the APU GEN OFF BUS light illuminates.
Note: When the APU is started using battery power only, there is no indication on the electrical metering panel that the APU generator has come on line and is ready to be selected. Both the frequency and voltage readings are zero until the APU generator is placed on line.
Note: During the APU start cycle, the APU EGT indication may fluctuate from
0º to 1100º C prior to normal EGT rise and the LOW OIL PRESSURE light may cycle on and off several times. These indications have no adverse effect on starting the APU. It is not necessary to monitor EGT during start.
If the APU does not reach the proper speed with the proper acceleration rate within the time limit of the starter, the start cycle automatically terminates. The start cycle may take as long as 120 seconds. Automatic shutdown occurs in the event of EGT exceedance.
If the start fails or the APU GEN OFF BUS light fails to illuminate by the end of the start cycle, a system failure has occurred and the FAULT light illuminates.
Operate the APU for one full minute before using it as a bleed air source. This one minute stabilization is recommended to extend the service life of the APU.
Operate the APU for one full minute with no bleed air load prior to shutdown. This cooling period is recommended to extend the service life of the APU. When the
APU switch is moved to OFF, this time delay is met automatically.
Moving the APU switch to OFF trips the APU generator, closes the APU bleed air valve and extinguishes the APU GEN OFF BUS light. Shutdown occurs automatically after 60 seconds. When the APU speed decreases sufficiently during shutdown, the fuel valve and inlet door close. If the fuel valve does not close, the FAULT light will illuminate after approximately 30 seconds. An immediate shutdown can be accomplished by pulling the APU fire switch.
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Boeing B737 NG - Systems Summary [APU]
An electronic control unit (ECU) monitors and controls the APU.
Automatic shutdown protection is provided for overspeed conditions; low oil oil pressure, high oil temperature, APU fire, fuel control unit failure, EGT exceedance, and other system faults monitored by the ECU.
The ECU automatically controls APU speed through the electronic fuel control. If speed or EGT exceed acceptable levels with the APU providing electrical load only, some electrical load is shed. When electrical load and air extraction raise the
EGT above acceptable levels during engine starting, electrical load shedding occurs prior to reducing bleed air. When electrical load and air extraction raise the
EGT above acceptable levels other than during engine starting, the inlet guide vanes move toward a closed position, reducing bleed air extraction while maintaining electrical load.
[Option - CAB/UTIL Power Switch]
In flight, if the APU is the only source of electrical power, all galley busses and main buses are automatically shed. If electrical load still exceeds design limits, both IFE busses are also automatically shed. On the ground, the APU attempts to carry a full electrical load. If an overload condition is sensed, the APU sheds galley busses and main busses until the load is within limits. Manual restoration of galley and main bus power can be attempted by moving the CAB/UTIL Power
Switch to OFF, then back ON.
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Boeing B737 NG - Systems Summary [APU]
MAINT
APU
LOW OIL
PRESSURE
FAULT
OVER
SPEED
6
8
4
2
0
APU
OFF
ON
START
FORWARD OVERHEAD PANEL
APU Maintenance (MAINT) Light
Illuminated (blue) – APU maintenance problem exists:
• APU may be operated
• light is disarmed when APU switch is in OFF.
APU Exhaust Gas Temperature (EGT) Indicator
Displays APU EGT
EGT indicator remains powered for 5 minutes after shutdown.
APU OVERSPEED Light
Illuminated (amber) –
• APU RPM limit has been exceeded resulting in an automatic shutdown
• overspeed shutdown protection feature has failed a self–test during a normal APU shutdown
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Boeing B737 NG - Systems Summary [APU]
• if light is illuminated when APU switch is placed to OFF, light extinguishes after 5 minutes
• light is disarmed when the APU switch is in OFF position.
APU FAULT Light
Illuminated (amber) –
• a malfunction exists causing APU to initiate an automatic shutdown
• if light is illuminated when APU switch is placed to OFF, light extinguishes after 5 minutes
• light is disarmed when APU switch is in OFF position.
APU LOW OIL PRESSURE Light
Illuminated (amber) –
• during start until the APU oil pressure is normal
• oil pressure is low causing an automatic shutdown (after start cycle is complete)
• if light is illuminated when APU switch is placed to OFF, light extinguishes after 5 minutes
• light is disarmed when APU switch is in OFF position.
APU Switch
OFF – normal position when APU is not running
• positioning switch to OFF with APU running trips APU generator off the bus(es), if connected, and closes APU bleed air valve. APU continues to run for a 60 second cooling period
• APU air inlet door automatically closes after shutdown.
ON – normal position when APU is running.
START (momentary) – positioning APU switch from OFF to START and releasing it to ON, initiates an automatic start sequence.
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Boeing B737 NG - Systems Summary [APU]
[Option - Typical JAA]
APU bleed + electrical load: max alt 10,000 ft.
[Option - Typical FAA]
Inflight - APU bleed + electrical load: max alt 10,000 ft.
[Option - Typical FAA]
Ground only - APU bleed + electrical load: max alt 15,000 ft.
APU bleed: max alt 17,000 ft.
APU electrical load: max alt 41,000 ft.
Non–AFM Operational Information
Note:
The following items are not AFM limitations, but are provided for flight
crew information.
APU bleed valve must be closed when:
• ground air connected and isolation valve open
• engine no. 1 bleed valve open
• isolation and engine no. 2 bleed valves open.APU bleed valve may be open
during engine start, but avoid engine power above idle.
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