B737NG Auxiliary Power Unit Overview

1. •B 737 NG Ground School.
See the aircraft study guide at www.theorycentre.com
The information contained here is for training purposes only. It is of a general nature it is
unamended and does not relate to any individual aircraft. The FCOM must be consulted for
up to date information on any particular aircraft.
B 737 NG Ground School.

2. APU

3. APU
Allied Signal 131-9(B)
Altitude Operational Limits
The APU generator can supply 90 KVA electrical power up to 32,000 feet (9,754 meters)
and 66 KVA to 41,000 feet (12,500 meters).
Electrical and pneumatic power is available at the same time up to 10,000 feet (3,048
meters). Pneumatic power alone is available up to 17,000 feet (5,183 meters).
The APU can be started at 41,000 feet or below.

4. Introduction
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.

5. APU Operation
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.

6. Electronic Control Unit (ECU)
An electronic control unit (ECU) monitors and controls the APU. Automatic shutdown
protection is provided for overspeed conditions, low 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.

7. Electronic Control Unit (ECU)
• Electronic Control Unit
controls the APU during
start, steady state
operations & shutdown
• Protective shutdown
protection provided for:
• Overspeed conditions
• Low oil pressure (<35 psi)
• High oil temperature
• APU fire
• Fuel control unit failure
• EGT exceedance
• Other systems faults
The ECU receives 28v dc power from the 28v dc
switched hot battery bus.

8. Indicator lights
EGT Indicator
APU Switch

9. • INDICATOR LIGHTS lights
• Illumination of the MAINT light
means a maintenance problem
exists - APU may be operated
• Illumination of any amber warning
light indicates automatic shutdown
• All lights disarmed when APU switch
is in the OFF position
• All light remain on for 5 minutes if
illuminated when the switch is set to OFF
EGT indicator remains powered for 5
minutes after APU is switched OFF.

10. INDICATOR LIGHTS
APU Maintenance (MAINT) Light
Illuminated (blue) – APU maintenance problem
exists:
• APU may be operated
• light is disarmed when APU switch is in OFF.

11. INDICATOR LIGHTS
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) (<35 psi)
• 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.

12. INDICATOR LIGHTS
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.

13. APU FAULT Light
These are the protective shutdowns that cause a fault light:
APU fuel valve shutdown
DC power loss shutdown
ECU shutdown
Fire shutdown
Inlet door shutdown
Inlet overheat shutdown
Loss of EGT shutdown
Loss of speed shutdown
No acceleration shutdown
No APU rotation shutdown
No flame shutdown
Oil filter shutdown
Oil temperature shutdown
Overtemperature shutdown
Reverse flow shutdown
Sensor failure
Underspeed shutdown

14. INDICATOR LIGHTS
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
The ECU does the overspeed circuit test during every normal APU shutdown.
An overspeed shutdown occurs if the APU speed is more than 106 percent.

15. When the ECU does a protective shutdown, the master
caution and APU annunciator light also come on.

16. APU switch to ON
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.
The start power sources are 115v ac transfer bus number1 if available or
28v dc from the battery or batteries.

17. APU Start
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.

18. APU Air Inlet door with Vortex Generator.
Air Inlet
The APU air inlet is on the right side of the aft fuselage.
It is forward and below the horizontal stabilizer.

19. APU 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.
Illumination of the APU GEN OFF BUS light indicates that the APU generator may
be used to supply the transfer busses.
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.

20. The APU is a gas turbine engine. It has a single shaft and operates at a constant speed. It drives
an electric generator, pneumatic load compressor, and a reduction gearbox.

22. APU Engine and Cooling Air
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.
Eductor air inlet for cooling.
APU Engine Exhaust.

23. •APU switch to OFF
• Trips generator off the busses &
closes the APU bleed air valve
• APU continues to run at 100% for 1 minute
• APU air inlet door automatically
closes at approx 30% RPM during shutdown

24. APU Shutdown
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.

25. An immediate shutdown is accomplished by pulling the APU Fire Switch

26. Completion of the Shutdown
During shutdown of the APU, these steps occur:
At 30 percent speed, the APU air inlet door starts to close (closes immediately for APU fire).
At less than 7 percent speed, an APU restart can be initiated.
Securing Aircraft.
The APU fuel shutoff valve and air inlet door will close for a normal or protective shutdown.
Wait 40 seconds after the EGT goes below 300 C for the inlet door and fuel shutoff valve to
close before you move the battery switch to the OFF position.

27. APU Fuel Supply
Fuel to start and operate the APU comes from the left side of the fuel manifold when
the AC fuel pumps are operating.
If the AC fuel pumps are not operating, fuel is suction fed from the No. 1 tank. During
APU operation.
Some aircraft are equipped with a DC APU Boost Pump.
The DC boost pump operates when the APU switch is in the ON or START position and
the fuel pressure from the ac pumps that could provide fuel to the APU are low. When
the ac boost pump pressure goes high, the dc boost pump goes off.
The left centre tank pump can be used to pressurise the left fuel manifold. Normal
procedure requires at least one person be present on the flight deck if a centre tank
pump is operating for safety reasons.

28. Fuel Supply
Fuel for APU is obtained from
left side of the fuel manifold
• When any left fuel pump is
operating, the fuel is pressure
fed to the APU
• When no pumps are operating,
fuel is suction fed from main
tank no.1
• Some aircraft are fitted with a DC fuel pump
which operates when no main fuel pump is
operating.
• Fuel Burn rate approximately 110 Kg/Hr.
• APU uses approximately 15% more fuel for
1pack operation than for 2 pack!

29. It is recommended that the APU is operated with the two air
conditioning packs when maximum cabin cooling is necessary.
The results of the operation of two air conditioning packs will be cooler
cabin temperatures, lower APU fuel burn, better APU hot section life
and lower APU noise.

30. APU LIMITATIONS
Airplanes operating under FAA Rules:
In flight - APU bleed + electrical load: max alt 10,000 ft.
APU bleed: max alt 17,000 ft.
APU electrical load: max alt 41,000 ft.
Airplanes operating under FAA Rules:
Ground only - APU bleed + electrical load: max alt 15,000 ft.

31. The APU Electronic control unit (ECU) controls the APU bleed
valve solenoid. The bleed valve opens when the solenoid is
energized from the ECU and pneumatic power is available
from the load compressor.
The APU ECU energizes the solenoid when these conditions
occur:
* APU bleed air switch is in the ON position
* APU is not in cool down cycle
* APU speed is more than 95 percent
•Altitude is less than 17,000 ft (5,200 meters), if altitude comes
from the ADIRU (Normal operation)
Alternative operation.
* Altitude is less than 21,000 ft (6,401 meters), if increasing
altitude and altitude comes from inlet pressure sensor (P2)
* Altitude is less than 19,000 ft (5,791 meters), if descending
and altitude comes from inlet pressure sensor (P2).

32. Non–AFM Operational Information
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.
Doors
The APU access door at the bottom of the APU
compartment is for servicing and maintenance.

33. Non–AFM Operational Information
The following items are not AFM limitations, but are
provided for flight crew information.
The start system consists of, the starter-generator, Start
power unit (SPU), and Start converter unit (SCU) together
these supply initial rotation of the APU. The start power
sources are 28v dc from the aircraft batteries or 115v ac
transfer bus number1.
Start Power Unit
The start power unit changes 115v ac or 28v dc electrical
power to 270v dc power. Transfer bus 1 or the battery
supplies power to the start power unit (SPU).
Start Converter Unit
The start converter unit (SCU) changes the 270v dc power to
AC and sends it to the starter-generator.
The maximum duty cycle for the SPU and SCU is three
starts, one after the other, followed by a 15-minute cool
down period. If you do too many starts one after the other,
the SCU and SPU will get too hot and stop the APU start.

34. Main Engine Start Mode
In main engine start mode, the ECU meets the increased
airflow requirements. This mode has priority over all other
modes.
Air Conditioning System Mode (ACS)
In each ACS mode, the ECU controls airflow as necessary to
supply air to the air conditioning system. These are the four
ACS modes:
* One pack inflight
* One pack ground
* Two pack, normal
* Two pack, high.

35. APU Automatic Load Shedding
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.

36. Questions
After start, when is the APU ready to accept bleed air and electrical loads?

37. Questions
After start, when is the APU ready to accept bleed air or electrical loads?
The APU GEN OFF BUS light (blue) illuminates at 95% of normal governed speed:
At this time the APU is ready to accept electrical load but another one minute is required
(to stabilise) before using APU bleed air.

38. Questions
What will happen to the APU if the battery switch is selected to OFF?

39. Questions
What will happen to the APU if the battery switch is selected to OFF?
Moving the battery switch off on the ground or in flight will cause an immediate APU shutdown.

40. Questions
If the EGT exceeds operating limits, will the APU shutdown automatically?

41. Questions
If the EGT exceeds operating limits, will the APU shutdown automatically?
Yes - the electronic control unit would automatically shut the APU down & illuminate the fault light
EGT limit is 730º C
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.

42. Questions
What is the maximum altitude for APU bleed air load (no electrical load)?

43. Questions
What is the maximum altitude for APU bleed air load (no electrical load)?
17,000 feet

44. Questions
How long is required between selecting the APU to OFF and switching the
main battery switch OFF?

45. Questions
How long is required between selecting the APU to OFF and switching the
main battery switch OFF?
About 2 Minutes.
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.
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.
The APU fuel shutoff valve and air inlet door will close for a normal or protective shutdown.
Wait 40 seconds after the EGT goes below 300 C for the inlet door and fuel shutoff valve to
close before you move the battery switch to the OFF position.

46. 300°C Plus 40 seconds before
switching the Battery switch to OFF!

47. Questions
Electrical power to start the APU comes from which source?

48. Questions
Electrical power to start the APU comes from which source?
Electrical Requirements for APU Operation
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 main battery
or main and auxiliary batteries.
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 from the switched hot battery bus to the electronic control unit.

49. Questions
The APU can support electrical loads and one air-conditioning pack up to what altitude?

50. Questions
The APU can support electrical loads and one air-conditioning pack up to what altitude?

51. Questions
The OVERSPEED light illuminates on the APU panel after the APU has been selected
OFF. This indicates:

52. Questions
The OVERSPEED light illuminates on the APU panel after the APU has been selected
OFF. This indicates:
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

53. •The END of AUTO FLIGHT Part 1
The END of APU.
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