Aoa 737ngx Groundwork Apu Handout

7/17/2019 Aoa 737ngx Groundwork Apu Handout

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AuxiliaryPower Unit9www.flyaoamedia.com

s e c t i

o n



Auxiliary Power Unit www.flyaoamedia.comPage 9-1Rev 1.0 APR 12

The material covered in this document is based off information obtained from

the original manufacturers’ Pilot and Maintenance manuals. It is to be used

for simulation purposes only.

Copyright © 2011 by Angle of Attack Productions, LLC

All rights reserved




Table of Contents Table of Illustrations

APU Overview 3

APU Engine Primary Components 4

APU Fuel Supply 7

APU Start 9

APU Operational Modes 11

APU Altitude Operational Limits 14

APU Shutdown 15

APU Normal Shutdown 15

APU Protective Shutdown 15

APU Automatic Load Shedding 17

Figure 9-1. Auxiliary Power Unit Diagram 6

Figure 9-2. Fuel Supply Diagram 8

Figure 9-3. Inlet Guide Vanes 13




APU Overview

The Auxiliary Power Unit, or APU, is a gas turbine engine

capable of providing electrical and pneumatic serviceson the ground and in the air. It allows the aircraft to be self-

sufcient on the ground without the need for ground power.

The 737NG uses the AlliedSignal, now Honeywell, 131-9B

APU.

The 131-9B is able to start and operate up to the

aircraft’s maximum certied altitude of 41,000 feet.The APU is installed within a reproof compartment in the

tail of the aircraft.

A rewall isolates the APU compartment from the aircraft

fuselage and the horizontal stabilizer assembly.

The APU air inlet door is located on the right side of the aft

fuselage and is automatically controlled. This is a NACA

type inlet, a concept originally developed by the US

National Advisory Committee for Aeronautics in 1945.

It is a low drag inlet, designed to allow air to ow into the

duct in ight. There is an inlet ow deector that modies

the airow into the intake to ensure that it is laminar and

appropriate for ingestion into the APU.

After combustion, the APU exhausts gases through a mufer

and out of the tailcone.

The high speed ow of the APU exhaust forms a low

pressure area inside the APU compartment which pulls

outside air in through a second hole in the tailcone. This

is called the eductor inlet, and draws outside air into the

APU compartment, cooling the APU oil.

This is an efcient means of cooling and removes the needfor a separate cooling fan, eliminating another moving part.

An Electronic Control Unit, or ECU, continuously monitors

and controls the APU from start to shutdown. It also

provides shutdown protection in the event that any one of

several parameters goes out of limits.

Shutdown protection is discussued in more detail later.




APU Engine Primary Components

The APU is very different to the two CFM56 engines on the

737. It has three primary engine components (see fgure 8.1) : ● The power section.

● The load compressor.

● The accessory gearbox.

The power section drives the load compressor and the

accessory gearbox. The power section consists of:

● A single stage centrifugal compressor. ● A combustion chamber.

● A two stage axial ow turbine.

Air enters the APU through the air inlet, and is directed

into the centrifugal compressor which throws it outwards,

compressing it.

The compressed air is directed into the combustion

chamber where it is mixed with fuel and ignited. Ignition and

expansion of the gas in the combustion chamber forces it

through the turbines, spinning them.

The turbines are connected to a single shaft, which in turn

is connected to the centrifugal compressor. Also attached

to this same shaft are a starter-generator, gearbox, and the

pneumatic load compressor.

The purpose of the pneumatic load compressor is to

supply bleed air to aircraft systems that require it, such as

air conditioning, pressurization, ice protection, and for

engine start.

The key difference here is that the two main engines supply

bleed air from the power section, while the APU has a

dedicated compressor for the job.

Because the pneumatic load compressor is attached to

the same shaft as the engine compressor, they both spin at

the same RPM.

In order to vary the amount of bleed air taken from the

APU, the ECU opens and closes Inlet Guide Vanes in the

load compressor inlet. These control the amount of air that

enters the load compressor, and consequently the amount

of air taken from the APU for aircraft systems.

The Inlet Guide Vanes move from 15 degrees to 110

degrees as bleed air demand changes.

The accessory gearbox is also mounted to the APU shaft.




APU Eng.Prim. Components (Cont.)

This reduces the high rotational speed of the shaft to a

lower speed for the accessories mounted on the gearbox.The gearbox turns the APU starter-generator, and other

components.

The starter-generator is used when starting the APU and

generates electrical power once it is running.

Notes


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OIL COOLER

FROM EDUCTOR

INLET

COMPRESSOR

AIR INLET

EDUCTOR INLET

SURGE CONTROLVALVE

APU BLEED AIRVALVE

FROM FUEL

SYSTEM

FCU

STARTER

GENERATOR

Figure 9-1. Auxiliary Power Unit Diagram



APU Fuel Supply

Fuel supply to the APU is controlled by the APU Fuel

Control Unit. The Fuel Control Unit regulates the fuel supplyfor different running conditions, and uses a motor driven by

the accessory gearbox.

The APU uses the same fuel supply as the two main engines.

Fuel piping is arranged such that the APU normally takes

fuel from the left side of the fuel system. (see fgure 8.2)

The APU is capable of drawing fuel without positivepressure from the fuel pumps. When no fuel pumps are

operational, fuel is suction fed from Main Tank 1 using the

Fuel Control Unit’s own motor.

Operating without the assistance of a fuel pump can

reduce the service life of the Fuel Control Unit motor

however. To address this, an automatically operated DC

Fuel Boost Pump is installed. This pump draws fuel from Main

Tank 1 when the APU Fuel Control Unit senses low fuel

pressure. This provides positive pressure and preserves the

service life of the Fuel Control Unit.

The DC Fuel Boost Pump is usually used during APU startup

when no AC power source is available to power the AC

Pumps.

Under normal conditions once the APU is running, an AC

Fuel Pump is used to pressurize the system. There are twoAC Pumps for each fuel tank.

The fuel system features a Crossfeed Valve that effectively

isolates each side of the fuel system from the other. With the

Crossfeed Valve closed, any of the three AC pumps on the

left side of the system can supply the APU. This includes the

two Main Tank 1 pumps and the left Center Tank pump.

The Main Tank 1 Aft Fuel Pump is normally used to feed the

APU on the ground. If the APU will be run for an extended

period, the left Center Tank pump may be used to prevent

a fuel imbalance.

With the Crossfeed Valve open, fuel may also be fed from

Main Tank 2. Operation of the DC Fuel Boost Pump is

automatic, but the AC Pumps must be manually selected

ON or OFF on the Forward Overhead Panel. When an

AC pump is used and pressurizes the system, the DC pump

automatically shuts off.

APU fuel consumption is roughly 225 pounds per hour

running both packs. This is ver y much a ballpark gure – fuel

consumption varies depending on a variety of conditions.


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TO APU

Figure 9-2. Fuel Supply Diagram



APU Start

APU start is controlled by the ECU, Electronic Control Unit.

It is a fully automatic start sequence.

The Battery Switch on the Forward Overhead Panel must

be set ON before the APU can be started.

The APU Fire Switch on the Aft Electronics Panel must

also be IN , and the APU Fire Control Handle in the main

landing gear wheel well must be in the UP position .

Controls and indications for the APU are located on the

Forward Overhead Panel.

The start sequence is commenced by holding the APU

switch momentarily to START . The switch is spring loaded

back to the ON position , and will return there when

released. When the switch is selected to START , the

Electronic Control Unit opens the APU Fuel Shut-off Valve

and the APU Air Inlet Door.

Either 28v DC power from the battery or 115v AC power

from AC Transfer Bus #1 may be used to start the APU. This

passes through the Start Power Unit which converts it to

270v DC power.

The Start Power Unit forwards this to the Start Converter

Unit which converts it to AC power for the starter-generatoron the APU gearbox.

As the name implies, the starter-generator performs two

main functions:

● It supplies the initial rotation of the APU during the start

cycle.

● And provides a source of electrical power for aircraft

systems once the APU is running.

If starting on the battery, there will be a signicant

amperage draw indicated on the AC/DC Metering Panel

when the starter-generator kicks in. This is usually in the

region of a 400 amps draw – it takes a lot of power to get

that APU turning.

Additionally to the negative amps indication, the BAT

DISCHARGE light will illuminate.

The APU draws power from the Main Battery for startup, so

the Auxiliary Battery is automatically isolated during APU

start.

The LOW OIL PRESS light will illuminate during the start


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process, and will extinguish once APU oil pressure reaches

normal levels.

The APU’s Exhaust Gas Temperature indication may

uctuate throughout its entire range during start prior to

normal EGT rise. This is normal, and has no adverse effect

on starting the APU.

Note that there are no limitation indications on the EGT

gauge – EGT is monitored automatically by the ECU, andthe APU will be shut down automatically if it exceeds limits.

It is therefore not necessary to monitor EGT during APU

start.

The ECU commands ignition and fuel injection during

startup automatically as the APU reaches the appropriate

speeds.

The start cycle will terminate automatically after 120

seconds if the APU has not yet reached the required RPM

to disengage the starter.

The start cycle may therefore take as long as 120 seconds,

and the APU should be run for a further minute after start

before it is used as a bleed air source.

This minute of idle running is intended as a stabilization

period to extend the service life of the APU. Although thestart cycle itself takes a minute or so, if powered from the

battery at this point it uses the equivalent of approximately

7 minutes of battery life.

Once the start cycle is complete, and the APU has

reached 95% speed, the ECU gives a ‘Ready to Load’

signal to other aircraft systems. This signals that the APU is

ready to accept pneumatic and electrical loads.

The electrical system indicates this to the crew by way of

the APU GEN OFF BUS light, which illuminates blue when

the APU is capable of powering an AC bus but is not yet

doing so.

There is no direct equivalent indication for the air system.

APU Start (Cont.)



Rev 1.0 APR 12

APU Operational Modes

We have already stated that the APU should be run

for at least one minute after start before it is used as ableed air source. Taking bleed air from the APU places a

considerable load on it, far more demanding than taking

electrical power from the starter-generator.

The ECU selects from four bleed air modes depending on

demand from aircraft systems:

● No bleed mode

● Duct pressurization mode

● Main engine start mode

● Air conditioning system mode

The ‘no bleed mode’ is set when there is no bleed air

demand from the pneumatic system and the APU Bleed Air

Valve is closed.

When the pilot selects the APU Bleed Air switch OFF on

the Forward Overhead Panel, the APU Bleed Air Valve

closes.

The ECU closes the Inlet Guide Vanes to 15 degrees.

Even without any bleed air demand, the load compressor

will still be spinning as it is attached to the shaft along with

the APU power section components.

To keep the load compressor cool, the Inlet Guide Vanes

do not close fully, even in ‘no bleed mode’ with no bleed

air demand. They close only as far as 15 degrees.

The ECU sets ‘Duct pressurization mode’ when the APU

Bleed Air Valve is open, but there is no actual demand

from the air system. In this case, the Inlet Guide Vanes open

further to allow the load compressor to pressurize thepneumatic system air ducts.

‘Main engine start mode’ opens the Inlet Guide Vanes

as needed to meet the high airow requirement of main

engine start.

Air conditioning system, or ACS mode sets the Inlet Guide

Vane position as necessary to supply air to the air

conditioning system.

The air conditioning system itself has four modes of

operation:

● One pack inight.

● One pack ground.



Rev 1.0 APR 12

APU Operational Modes (Cont.) ● Two packs, normal.

● Two packs, high.

The ECU opens the Inlet Guide Vanes to the appropriate

position for each of these modes so as to supply the

required airow.

APU fuel consumption is considerably greater when

operating a single pack than when operating both.

A single pack must work much harder than two packs to

cool the cabin to a given temperature. The APU must

therefore supply higher pressure bleed air to allow the

single pack to function.

To supply higher pressure bleed air, the APU Inlet Guide

Vanes must open further than they would otherwise have

to to supply both packs. The further open the Inlet Guide

Vanes are, the greater the torque required to keep the

APU rotating at a constant speed (see fgure 8.3) .

This requires the Fuel Control Unit to inject more fuel,

increasing fuel consumption.

Additionally, although a single pack requires greater

pressure than two packs would, it requires less actual

quantity of airow.

There is therefore a considerable excess of bleed air

produced that is not required.

This excess bleed air is exhausted through a Surge

Control Valve, which ducts it through the APU exhaust. This

increases exhaust gas temperatures, and the additional

airow through the exhaust can increase the noisesignature of the APU by approximately 2 decibels.

Running both packs on the ground therefore reduces noise,

reduces fuel consumption and extends the life of the APU

hot section. This is the recommended practice.



Rev 1.0 APR 12

Figure 9-3. Inlet Guide Vanes

12

3

4

5

6

789

10

11

12

13

14

1516



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APU Shutdown

Like the start process, the Electronic Control Unit, or

ECU, controls the shutdown. There are two types of APUshutdowns:

● Normal shutdown

● Protective shutdown

APU Normal Shutdown

The ‘normal shutdown’ is initiated by placing the APU switch

to the OFF position.

This signals the ECU to begin the shutdown process. The

normal shutdown is preceded by a 60 second cool down

period, which begins as soon as the APU switch is set OFF.

When the APU switch is set OFF , the ECU per forms several

actions:

●

It removes the ‘Ready to Load’ signal, thus indicatingto aircraft systems that the APU is no longer ready to

accept pneumatic or electric load.

● It closes the APU Bleed Air Valve.

● Closes the Inlet Guide Vanes to 15 degrees.

● Opens the Surge Control Valve.

● De-energizes the APU starter-generator.

●

Starts the 60 second timer for the cool down period.

The cool down period preserves the life of the APU hot

section, and prevents coke accumulating in the turbinebearing and fuel nozzles.

As the APU speed decreases below 30%, the APU Fuel

Shutoff Valve and inlet door start to close.

If the APU Fuel Shutoff Valve does not close, the FAULT

light will illuminate after approximately 30 seconds.

Below 7%, an APU restart can be initiated if desired by

moving the APU switch back to START .

The APU can be shut down immediately without the 60

second cool down period by pulling the APU Fire Switch .

Clearly this is not standard practice, and should only be

done in an emergency.

APU Protective Shutdown

Under certain conditions the APU will shut down

automatically to prevent damage to itself or other aircraft

components.

There are three different indications in the cockpit that

indicate a protective shutdown; all three are on the APU




APU Shutdown (Cont.)

Panel.

● The LOW OIL PRESSURE light illuminates when oilpressure drops below limits for 20 seconds or more.

○ This causes a protective shutdown.

● The FAULT light illuminates for a large number of

conditions:

○ Fuel shutoff valve not in the commanded position,

○ Inlet door not in the commanded position,

○ Loss of DC power, ○ Electronic Control Unit failure,

○ APU re,

○ APU inlet overheat,

○ Loss of both Exhaust Gas Temperature signals,

○ No APU speed signal,

○ No APU acceleration,

○ No APU rotation,

○ Low Exhaust Gas Temperature after introduction of fuel,

○ Generator lter clogged,

○ High oil temperature,

○ APU overtemperature,

○ Reverse ow through the load compressor,

○ Oil temperature sensor failure,

○ Inlet temperature failure,

○ APU underspeed.

Any one of these will trigger a protective shutdown.

Finally, the OVER SPEED light will illuminate for a further

three conditions:

● Fuel Control Unit solenoid valve fails in the open position,

● Loss of overspeed protection,

● APU overspeed.

Any one of these will trigger a protective shutdown.

The LOW OIL PRESSURE, FAULT and OVER SPEED lights

will extinguish when the APU switch is cycled to OFF , then

back to ON again with APU speed less than 7%. Why 7%?

Because that’s the speed below which the APU can berestarted again.

The MAINT light illuminates when oil pressure drops below

a specied level, or if the starter-generator has a shorted

rotating diode. In both cases the APU may continue to run,

but will require maintenance as soon as possible.



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APU Automatic Load Shedding

The APU is of course only capable of supplying a nite

amount of electrical power to aircraft systems.

When the APU is the only source of AC power, system

logic automatically removes electrical loads to prevent an

overload of the APU. This is called ‘load shedding’, and

may occur both on the ground and in ight.

In ight, if the APU is the only source of electrical power,

all galley busses are automatically shed. If electrical loadstill exceeds design limits, both main AC busses are also

automatically shed.

The APU will also shed load on the ground if it is the only

source of electrical power.

If an overload condition is sensed, the APU sheds the

galley buses rst, then the main buses until the load is within

limits.

The APU can take on more load on the ground than in

the air due to better airow cooling on the ground. It

is therefore capable of handling more demand on the

ground than in the air, so the threshold for load shedding

will be higher.

Notes

Aoa 737ngx Groundwork Apu Handout

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