OPERATING MANUAL APU 2A-49-10: General

APU

2A-49-10: GeneralThe purpose of the Gulfstream V Auxiliary Power Unit (APU) system is to supply to theflight crew with an auxiliary source of pneumatic power (bleed air) and electrical powerduring flight, or ground operations.

NOTE:

For operational purposes, in-flight use of the APU isdesignated as the Essential mode, while groundoperation is designated as the Non-essential mode ofoperation.

The RE220(GV) APU is powered by a dedicated non-propulsion, single-shaft, constantspeed gas turbine engine. The system is self-governing with automatic start sequencing,self-monitoring and normal commanded and/or automatic protective shutdownsequences. The APU is guaranteed to start at or below 39,000 feet no later than thesecond start attempt, with starts possible from 39,000 feet to 43,000 feet. The APU willoperate at altitudes up to 45,000 feet. Use of the generator is intended in flight only if oneof the main engine generators has failed, or is off-line.

At high altitudes, main engine bleed air is introduced into the APU air inlet duct to assistAPU starting. The Bleed Air Augmentation Valve (BAAV) opens above 35,000 feet toallow engine bleed air to assist APU starting.

The APU requires only DC power, a fuel supply and control input signals from the aircraftfor operation. When operating limits are exceeded, automatic protective shutdown andstart inhibit functions are controlled by the Electronic Control Unit (ECU). Protectiveshutdown or start inhibit may be overridden if the APU is in the Essential mode.

The APU is installed within a fireproof titanium housing (located directly behind thebaggage compartment) in the aft equipment (tail) compartment. One removable door islocated on the left side of the fuselage, aft of the baggage compartment door. Twoadditional doors to provide maintenance, servicing and inspection are located on the sideof the APU housing, accessible through the tail compartment.

The APU system is divided into the following subsystems:

• 2A-49-20: APU Assembly Description

• 2A-49-30: APU Control and Operation Description

2A-49-20: APU Assembly Description1. General:

The APU assembly is composed of the following subsystems:

• Powerplant System

• Starting and Ignition System

• Fuel Control System

• Lubrication System

• Fire Detection and Warning System

• Electrical Power System

• Bleed Air System

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2. Description of Subsystems, Units and Components:

A. Powerplant System:

(See Figure 1 through Figure 4.)

The APU powerplant system is powered by a single-shaft, constant speedgas turbine engine designed to drive an attached accessory gearbox (towhich the APU generator is mounted) and to provide bleed air as a sourceof pneumatic power.

It is composed of the following units and components:

(1) Air Intake:

The air intake consists of an air inlet door and a composite duct thatroutes external ambient air into the APU and APU enclosure. Airenters the inlet door on the aft side of the top left fuselage and thenflows into the inlet plenum, across the oil cooler and circulatesaround the APU, exiting through the exhaust.

The air inlet door is controlled by the APU Electronic Control Unit(ECU) and operated by an electrical actuator. The inlet door openswhen the APU MASTER switch, located on the Cockpit OverheadPanel (COP), is selected to ON. When the air inlet door has openedand the APU is ready to start, a READY light illuminates on the COP.The air intake is closed by the inlet door when the APU is shut downand the APU MASTER switch is selected off.

There are two sensors inside the APU inlet plenum that are used bythe ECU to detect ambient temperature and pressure. The ECUuses these inputs for control of the APU.

(2) Oil Cooler Air Duct:

The oil cooler air duct routes APU inlet air through the APU oilcooler. Ambient air is drawn across the cooler and is circulatedaround the APU, exiting through the APU exhaust.

(3) Accessory Gearbox:

The accessory gearbox is powered by the APU drive shaft and isused to drive the APU generator and lubrication (lube) pump. TheAPU starter is mounted on the accessory gearbox to rotate the driveshaft during starting. Within the gearbox is an integral 5.25 quart oilreservoir containing an oil heating element, oil level sensor andtemperature sensor.

A speed sensor is installed on the gearbox to provide indicatedspeed to the ECU for on-speed control and overspeed protection forAPU operation.

(4) Engine:

The APU engine is typical of the modern gas turbine engine,consisting of an inlet, compressor section, combustion section,turbine section and exhaust.

(5) Surge Control Valve:

A surge control valve is installed to control APU surge potentialwhen operating under electrical loads. When open, somecompressor section discharge air is discharged overboard near theexhaust through a separate port.

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PRODUCTION AIRCRAFT SYSTEMS2A-49-00January 28/03

ECU logic commands the surge control valve open when sensedaltitude reaches approximately 16,500 feet. The valve iscommanded closed when main engine start or ECS pack air isselected.

(6) Exhaust:

The APU exhaust gathers and directs exhaust gases overboard. Anexhaust pipe extends through the right side of the aircraft structureand has a metal silencer to reduce exhaust noise. External walls ofthe exhaust pipe are cooled through internal passages withventilating air from the APU enclosure and eductor system. Theexhaust pipe is oversized which creates a venturi effect while theAPU is operating, which draws air across the oil cooler.

B. Starting and Ignition System:

(See Figure 2 and Figure 3.)

(1) Starting System:

The electrical starting system supplies mechanical energy to turnthe rotational components of the APU from initial start throughlightoff. Once the APU has reached a self-sustaining speed, theECU commands starter cutout. Electrical power for the starter isprovided by the aircraft 28V DC power system. Starter duty cyclelimitations are given in Section 2A-49-30, APU Control andOperation Description.

NOTE:

Provided starter duty cycle limitations are observed,the starter is also capable of an immediate restart onspool-down when APU RPM is at or below 7%.

(2) Bleed Air Augmentation Valve:

The Bleed Air Augmentation Valve (BAAV) allows heated mainengine bleed air to enter the APU air inlet duct during high altitudestarts. The BAAV function is altitude dependent and completelyautomatic. During APU starting at altitudes of 35,000 feet andhigher, the ECU will command the BAAV open for fifteen secondsprior to starter engagement. After the fifteen seconds elapses, theAPU starter is energized and the start cycle begins. At 90% APURPM, the ECU closes the BAAV.

(3) Ignition System:

The ignition system for the APU supplies an intermittent, highvoltage spark to burn fuel in the combustion chamber during theAPU ignition/starting sequence. It consists of a dual output ignitionunit, two ignition leads and two igniter plugs.

Operation of the ignition system is fully automatic and controlled bythe ECU. During ground starting (non-essential mode), ignition isterminated at 50% APU RPM. During air starting (essential mode),ignition is terminated at 98% APU RPM. Should an APU engineflameout occur during operation, the ignition unit will automaticallystart ignition through the Auto-Relight function of the ECU.

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C. Fuel Control System:

(See Figure 2, Figure 3 and Figure 6.)

The APU Fuel Control system provides and controls fuel to the APU. It is aclosed-loop control system which receives control inputs from the ECU.The ECU calculates the appropriate control signals using inputs ofatmospheric pressure, temperature, load, RPM and other conditions. Withinput from the ECU, APU operation is fully automatic.

Pressurized fuel from the left main fuel boost pump (controlled by the LMAIN PUMP switch on the COP) is routed to the APU fuel shutoff valvelocated on the left wing rear beam structure. Passing through the APU fuelshutoff valve (controlled by the APU MASTER switch and the ECU), fueltravels to the APU fuel control unit.

NOTE:

The right main fuel boost pump, controlled by the RMAIN PUMP switch on the COP, can be used tosupply pressurized fuel to the APU if the fuel systemcrossflow valve (controlled by the X FLOW switch onthe COP) is open.

The fuel control unit combines the fuel pump, fuel filter, fuel pressureregulator and fuel metering valve into a single line replaceable unit. Itsprimary function is to supply appropriately pressurized fuel to the APU,based on ECU inputs. The ECU also provides inputs to energize the fuelshutoff solenoid; opening a normally closed valve. The fuel is delivered tothe APU engine combustion section, where it is atomized for mixture withcompressor section air.

D. Lubrication System:


The APU lubrication system provides a means of storage, delivery andindication of lubricating oil for the APU. The two main areas in the APU thatrequire lubrication are the gearbox and turbine bearing. The APUlubrication system uses a lube pump to lubricate and cool the gearbox,bearings and shafts of the powerplant. External air is routed into the APU tocool the oil system from the oil cooler air duct.

A thermostat-controlled oil heater is provided in the gearbox reservoir toheat the engine oil for the purpose of ensuring high altitude startcapabilities. The oil heater functions independent of the APU system. Foraircraft SN 501-560 without ASC 67A, the heater receives power from theEssential AC Bus on the ground and the Right Main AC Bus in flight, ascontrolled by the Weight-On-Wheels (WOW) relay. For aircraft 561 andsubsequent and SN 501-560 with ASC 67A, the heater receives powerfrom the Right Main AC Bus during all phases of flight.

An internal thermostat actuates the heater when the APU oil temperature isat or below 21°C and deactivates when the temperature reaches 43°C.Should the oil temperature exceed 149°C for 10 seconds, the APU will shutdown if on the ground. However, this automatic shutdown protection isinhibited while in flight (essential mode).

Checking the oil level and/or oil servicing can be accomplished by two

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different methods. It can be done manually by removing the gravity fill capon the oil tank and visually inspecting the oil level. If oil is needed, it can beslowly poured into the tank until oil flows over the cap lip.

Oil level verification and servicing can also be accomplished automatically.The APU oil level is electronically displayed on the oil replenisher, locatedon the left side of the tail compartment, adjacent to the tail compartmentdoor. It is also displayed in the cockpit on the GROUND SERVICE PANELsynoptic page. Automatic servicing is accomplished using the oilreplenisher. The oil replenisher receives power from the 28V DC GroundService Bus. For more information on oil servicing and oil replenisheroperation, see Section 09-02-30: APU Oil Servicing.

E. Fire Detection and Warning System:

(See Figure 1.)

Fire detection and warning for the APU is provided by a detector assemblyconsisting of a stainless steel loop with a permanently attached sensorelement. The assembly is filled with helium gas and hermetically sealed.

Detector operation is based upon Boyle’s Gas Law, which states that if avolume of gas is held at a constant pressure, an increase in pressure willoccur as a result of a rise in temperature. Thus, when heat is applied to thedetector assembly, the corresponding rise in pressure will trigger pressureswitches in the sensor element. The switches in turn will causeannunciations to be prompted for display as described in the followingscenarios:

• High temperature increases above predetermined values occurringwithin a small section of the detector assembly are considered to befire scenarios.

• Overall (ambient) temperature increases above predeterminedvalues occurring over large sections of the detector assembly areconsidered to be overheat scenarios.

The ECU will automatically initiate an immediate protective shutdown if afire condition exists, bypassing the cool down mode. Automatic APUshutdown is inhibited, however, during FIRE TEST checks. This allowsAPU fire detection system testing with the APU operating.

Escape of the helium gas contained within the detector assembly (and thusthe proportional decrease in pressure) will trigger a pressure switchcausing a fault annunciation to be displayed.

The fire detection and warning system is self-resetting. Return of normalpressures within the detector assembly will result in cancellation ofannunciations.

F. Services to the Aircraft:

(1) Electrical Power System:


A 40 kVA generator is mounted on the APU gearbox for use on theground, or in flight when a primary electrical power source is notavailable. It produces 3-phase, 400 Hz, 115V AC power. Once theAPU operating speed exceeds 99% for at least two seconds, theECU ready-to-load signal is sent to the aircraft via AeronauticalRadio Incorporated (ARINC) 429 data bus and an electrical load

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may be applied. The APU generator is cooled and lubricated by oilfrom the APU gearbox.

The APU generator is selected on and off using a control switch(labeled APU GEN) located on the COP. For more information aboutthe APU generator, see Section 2A-24-20, AC Electrical PowerSystem.

(2) Bleed Air System:


The APU bleed air system provides bleed air for engine starting anthe ground and in flight. It also provides bleed air for air conditioningon the ground only.

APU bleed air may be used immediately after APU operating speedexceeds 99% for at least two seconds. If the APU was started undercold conditions, APU bleed air loading will be delayed for 60seconds unless attempting a main engine start in flight. The ECUautomatically controls operating speed at 100% during loading andunloading, and at approximately 16,500 feet, a surge control valveopens to provide surge protection. The surge control valve will closeif main engine starting is commenced above 16,500 feet(approximate).

The use and operational logic of APU bleed air is as follows:

(a) ECS Air:

ECS air is utilized only on the ground. Selection of the APUBLEED AIR switch to ON opens the load control valve andopens the isolation valve. APU bleed air is now available toboth ECS packs. Under normal conditions, the load controlvalve is prevented from opening in flight as a function ofWeight-On-Wheels (WOW). If the APU BLEED AIR switch isleft in the ON position prior to takeoff, the valve willautomatically close when WOW shifts to the AIR mode.

NOTE:

During ground operations, the right ECS pack isautomatically inhibited when either the MASTERCRANK or MASTER START switches, located on theCOP, are selected to ON. Also, when the L ENGSTART or R ENG START switch is selected, the leftECS pack is inhibited until the engine has completedthe start cycle.

(b) Main Engine Starting Air:

APU bleed air for main engine starting is available from theground to 30,000 feet MSL. Once the APU is running above99% for greater than two seconds, APU bleed air is madeavailable by selection of the APU BLEED AIR switch to ON orby selection of either the MASTER CRANK or MASTERSTART switches to ON. The APU load control valve opensand bleed air is available for main engine starting. When anengine start sequence has begun, the APU ECU determines

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and schedules ECU-directed main engine starting air cutout.Under normal conditions, engine-directed starter cutoutautomatically occurs prior to the operating speed determinedby the ECU. However, if the main engine does reach theECU-determined operating speed, the ECU will automaticallyclose the load control valve to ensure main engine bleed airbackflow does not occur.

After one engine is running, APU bleed air can still be utilizedto start the second engine. Under normal conditions, APUbleed air pressure is greater than the operating engine bleedair pressure, thus APU bleed air would be used to assist inthe second engine start. From this point, the startingprocedure and logic are the same as the first engine start.

(c) APU Bleed Air Interlock:

An engine bleed air interlock ensures that backflow from anengine to the APU does not occur. Both ground and in-flightavailability of APU bleed air is interlocked with the mainengine bleed air supply, depending upon the position of theengine bleed air switches (L ENG / R ENG BLEED AIR) asshown in the following table. Backflow of main engine bleedair into the APU is also prevented by a check valve installedin the APU bleed air supply duct.

Aircraft Location: L ENG and/or R ENGBLEED AIR SwitchPosition:

Result:

On Ground ON The ECU will automaticallyclose the load control valvewhen the main engine reaches80% HP RPM.

On Ground OFF Interlock is not activated.In Flight ON Selection of APU bleed air will

be inhibited by the ECU. (SeeNOTE below.)

In Flight OFF Interlock is not activated. (SeeNOTE below.)

NOTE:

In order to use APU bleed air for starter-assistedairstarts in flight, both ECS pack control switches (LPACK / R PACK) and both wing anti ice controlswitches (L WING / R WING ANTI ICE) must also beselected OFF, in addition to the L ENG / R ENGBLEED AIR switches.

3. Controls and Indications:

See Section 2A-49-30, APU Control and Operation Description for APU systemcontrols and indications.

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4. Limitations:

See Section 2A-49-30, APU Control and Operation Description for APU systemlimitations.

APU Compartment General ArrangementFigure 1

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APU Components (Front View)Figure 2

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APU Components (Aft View)Figure 3

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APU System Simplified Block DiagramFigure 4

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APU Bleed Air System Simplified Block DiagramFigure 5

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2A-49-30: APU Control and Operation Description1. General:

(See Figure 7.)

The APU control and operation system performs all functions necessary to safelyand reliably operate the APU. It is composed of the following components:

• Electronic Control Unit (ECU)

• APU Control Panel

• APU Generator (APU GEN) Switch

• APU BLEED AIR Switch

APU Fuel System Simplified Block DiagramFigure 6

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• APU Start Inhibit Circuit (SN 680 & Subs)

2. Description of Subsystems, Units and Components:

(See Figure 8.)

A. Electronic Control Unit (ECU):

(1) General Description:

The APU ECU is located in baggage compartment ElectronicEquipment Rack (EER). It executes control of the APU and providesan interface with all APU subsystems. In addition, the ECUgenerates ARINC 429 data that is provided to the flight crew andmaintenance personnel.

The ECU receives 28V DC power from two sources: the RightEssential DC Bus (Primary) and the Left Battery Bus (Alternate).The power source is automatically selected by the ECU based onsensed voltages. This provides redundancy and prevents APUshutdown due to a loss of a single power source.

The ECU latches power when the APU MASTER switch is selectedON. Power is held until five minutes after APU RPM drops below 5%during a commanded shutdown, allowing the ECU to transmitExhaust Gas Temperature (EGT) and RPM signals, and ARINC 429data to the flight crew.

During both loaded and unloaded phases of APU engine operation,the ECU monitors ambient temperature, ambient pressure, enginespeed, EGT, oil pressure, oil temperature and components ofelectrical system for over-current conditions. Selected parametersare monitored for operational limits and if the limits are exceeded,the ECU will automatically shutdown the APU on the ground (non-essential mode) and in some cases, in flight (essential mode).

The ECU receives data from both Data Acquisition Units (DAUs) 1and 2. It transmits data on ARINC 429 digital databus # 1. If ARINC429 is lost, the APU can still be started and electrically loaded,however, the APU cannot provide a pneumatic load and cannotcommunicate failures.

(2) Operational Logic:

(a) Power-Up:

NOTE:

The flight crew will need to energize the left mainboost pump as part of the APU starting procedure.

On power-up (APU MASTER ON), the ECU performs thefollowing:

• Illuminates the ON legend in the APU MASTER switch

• Performs initialization and APU prestart Built-In Test(BIT)

• Checks APU oil temperature

• Commands APU inlet air door open

• Commands APU fuel shutoff valve open

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If all prestart conditions are passed, ECU commands a signalto illuminate the READY light (within 10 to 16 seconds),indicating the APU is ready to start. During prestart BIT, if afailure is detected that would critically effect mission success,the ECU inhibits APU starting (except low oil temperature),sends a Crew Alerting System (CAS) message and reportsthe fault(s) to the Maintenance Data Acquisition Unit (MDAU).

An APU start under a cold condition can still be attemptedeven though the READY light is not illuminated. Whenilluminated, the READY light extinguishes when APU speedreaches 12% RPM.

(b) During Start:

The start sequence is commenced when the APU STARTswitch is depressed. When the ECU receives the startcommand, the following events occur:

• The ON legend in the APU START switch isilluminated.

• The APU start sequence is initiated.

• If the ECU is in idle mode (READY light illuminated),the ECU transitions to start mode and sends acommand to engage the starter for 12 seconds. If theECU senses oil temperature below -23.3° C (-10° F)(READY light not illuminated), the starter motor maybe engaged for up to 50 seconds. If the APU was incool down mode, the ECU proceeds to ramp APUspeed back up to 100% RPM.

• When the APU reaches 5% RPM, the ECU opens theAPU fuel control unit, energizes the ignition systemand begins to control fuel flow. The ECU controlslightoff acceleration speed by modulating fuel flowbased on ambient temperature and EGT. Ignition isturned off at 50% RPM (on the ground) as the APUaccelerates through turbine operation. In flight, theECU commands ignition cutout at 99% RPM and APUcombustion continues without electrical ignition.

• During the start cycle, the starter cutout is controlledby the ECU. At sea level, cutout of the starter isapproximately 46% RPM. At altitude, the starter cutoutmay be as high as 60% RPM to ensure a positivestart. Since altitude start cycles may be longer,electrical power is available to the starter for a longerperiod of time, based on starter speed.

• The ON legend in the APU START switch isextinguished at starter cutout: 46% on the ground andup to 60% in flight.

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• At 99% APU RPM, the start sequence is completed.

NOTE:

When performing an APU start in flight, the ECU willautomatically sample inlet pressure to determineaircraft altitude. If above 35,000 feet, the ECU willcommand the Bleed Air Augmentation Valve (BAAV)open to heat the APU inlet for 15 seconds prior tostarting.

(c) Commanded (Normal) Shutdown:

The APU is shut down by selection of the APU STOP switchto OFF. When the ECU receives the shutdown command, thefollowing events occur:

• The ECU commands a cooldown period. If the APUwas loaded at the time of shutdown, the loads areshed within two seconds entering the cooldown mode.If on the ground and up to 20,000 feet, the ECUdecreases APU RPM on a linear scale for 60 seconds(to 70% RPM), then shuts down the APU. If above20,000 feet, the ECU will maintain APU RPM for 60seconds, then shut down the APU. If the APU STARTswitch is depressed at any time during the 60 secondtime period, the APU is commanded to 100% RPM. Atany time when the APU MASTER switch is selectedoff, the APU will immediately shut down (APU fuelshutoff valve commanded closed).

• When the APU speed decreases to 63%, the ECUcommands the APU air inlet door to 10% open. Whenthe door stops at 10% open, the ECU examines APUspeed and waits until the speed drops below 40%RPM and then the door is commanded to close.

• After the cooldown period is completed, the ECUperforms a shutdown by testing the overspeedprotection circuitry within the ECU. At this point, theonly remaining load is the ARINC 429 databus. Anyfaults detected during shutdown testing are sent toCAS and/or the MDAU. The ECU then starts a fiveminute timer to supply power to the EGT indicator,RPM indicator and for closing the APU air inlet door.The ECU will cause an advisory (blue) APU MASTERON message to be displayed on CAS until the APUMASTER switch is selected off. If the APU MASTERswitch is selected off before the 5 minute timerexpires, the APU MASTER ON message will not bedisplayed. Finally, 28V DC power is unlatched.

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• Recycling the APU MASTER switch off and then to ONwill reboot the ECU to begin another APU cycle.

NOTE:

The starter is capable of an immediate restart onspool-down when the APU is at or below 7% RPM.

NOTE:

The cooldown mode is needed to stabilize turbinetemperatures and cool components. Selecting theAPU MASTER to OFF prior to completion of thecooldown period could damage the APU.

(3) Failure Protection and Shutdown:

The ECU has the authority to inhibit starting of the APU, and toautomatically shut down the APU, when BIT indicates that starting orcontinued operation of the APU may cause damage.

The two operational modes of the APU are flight (essential mode)and ground (non-essential mode). When in the essential mode,some protective shutdowns are disabled to allow continuedoperation. Although the shutdown is disabled, fault messages arestill transmitted to the MDAU and CAS. If the APU is operating in theessential mode prior to landing, the APU will continue to assume theaircraft is in essential mode until the APU MASTER switch isselected off.

The following table lists conditions for inhibition of starting and forautomatic shutdown in the essential and/or non-essential modes.“SI” refers to “Starting Inhibited”; “N-ESS” refers to “Non-essential”;“ESS” refers to “Essential”.

Condition:APU Mode:

Cause:SI: N-ESS: ESS:

ECU Failure X X ECU InternalFailure

HardwareOverspeed X X

OverspeedDetected: 106%RPM

SoftwareOverspeed X X

OverspeedDetected: 106%RPM

Loss of Speed X X X Both MonopoleSensors Failed

Loss of OverspeedProtection X X X ECU Overspeed

Protection LostAPU Fire X X X APU Fire SignalSlow Start 1

X X

Lightoff Detected,RPM Greater Than5%, Starter DoesNot Cut Out Within30 Seconds

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Condition:APU Mode:

Cause:SI: N-ESS: ESS:

Slow Start 2

X X

Lightoff and StarterCutout Detected,RPM Does NotReach 95% Within30 Seconds

No Flame

X X

No EGT DetectedWithin 17 Secondsof Fuel SolenoidOpen Signal

No Acceleration

X X

Acceleration LessThan 0.05% ofNormal 2.5% PerSecond

No Crank X X APU Starter FailureLoss of ElectricalDC X X X Loss of 28V DC for

50 MillisecondsDoor Failed Closed1 X X X

Door OpenCommand NotReceived in 30Seconds

Door Failed Closed2

X X

Speed GreaterThan 95%, or AtCooldown Speed(70%) and InletDoor Closes to 5%Open

FallbackX X

APU Drops Below25% Speed AfterStarter Cutout

Overtemperature X EGT LimitExceeded

Reverse FlowX

Inlet TemperatureExceeds LimitsWith BAAV Closed

Loss of EGT SignalX X

Both EGTThermocouplesFailed

Low Oil Pressure X Low Oil PressureLimit Exceeded

High OilTemperature X

Oil TemperatureLimit Exceeded(On-Speed or Cool-Down)

Underspeed

X

Speed Less ThanReference Speed(On-Speed or Cool-Down)

Low Oil PressureSwitch Fail X

Oil Pressure SwitchFailed Open andRPM Less Than7%.

A. APU Control Panel:

(See Figure 9.)

The APU control panel is located on the Cockpit Overhead Panel (COP)

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and is labeled APU. It provides all controls and the majority of indicationsnecessary for APU operation. It is composed of the following controls andindicators, shown and described in the figures that follow:

• EGT Indicator

• RPM Indicator

• MASTER Switch

• START Switch

• STOP Switch

• READY Light

• FIRE EXT Switch

• FIRE Annunciator Light

• R COWL OPEN Annunciator Light (SN 680 & Subs)

B. APU Generator (APU GEN) Switch:

A brief description of the APU Generator (APU GEN) switch is given inFigure 7. Additional details are presented in Section 2A-24-20, ACElectrical Power System.

C. APU BLEED AIR Switch:

A brief description of the APU BLEED AIR switch is given in Figure 7.Additional details are presented in Section 2A-36-20, PneumaticsDistribution and Indication System.

D. APU Start Inhibit Circuit (SN 680 & Subs):

On airplanes SN 680 and subsequent, an APU start inhibit circuit isincorporated. The purpose of this circuit is to prohibit APU starting whenthe airplane is on the ground and the right engine lower fan cowl door isopen, thus preventing possible damage to the door by impingement of hightemperature APU exhaust on the door. This is accomplished by interruptingthe APU start signal from the APU START switch to the APU ECU.

When the airplane is on the ground and the right engine lower fan cowldoor is closed, a ground is applied through an unused, normally opencontact on each of the two (2) fan cowl door position sensing switches.From the sensing switches the ground is routed through the CombinedWeight on Wheels (CWOW) relay to the negative contact of the APU startinhibit relay, where it waits for the 28V DC power needed to energize therelay. When the APU MASTER siwtch is selected ON, 28V DC is applied tothe positive contact of the relay and the relay is energized. When the APUSTART switch is selected ON, 28V DC power is routed through the now-energized relay to the APU ECU. From this point, the APU ground startinglogic and sequence is the same as those airplanes not having the startinhibit circuit.

As noted in the previous paragraph, there are two (2) door position sensingswitches installed on the right engine lower fan cowl door. The APU startinhibit circuit uses the normally open contact on each of the switches; thenormally closed contacts are used by the R COWL OPEN annunciator lighton the APU control panel shown in Figure 9. These contacts are connectedin parallel to prevent inadvertent interruption of the start circuit due to asingle switch failure or improper switch adjustment. Either switch in theopen position will trigger the R COWL OPEN annunciator light, whereas

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both switches must be in the open position to prohibit APU starting.

When the airplane is airborne, the fan cowl door position sensing switchesare removed from the APU start inhibit circuit by virtue of the CWOW relayhaving shifted to the AIR mode. The APU airborne starting logic andsequence is the same as those airplanes not having the start inhibit circuit.

Airplanes having the APU start inhibit circuit have an APU INHIBIT FLIGHTBYPASS momentary switch installed on the system monitor test panel onthe left EER. This switch, when held in the ON position, will enable the APUstart circuit should the CWOW relay or APU start inhibit relay fail.

3. Controls and Indications:

A. Circuit Breakers:

The APU system is protected by the following circuit breakers (CBs):

Circuit Breaker Name: CB Panel: Location: Power Source:APU CONT #1 LEER C-13 L ESS DC BusAPU CONT #2 REER C-11 R BATT BusAPU DOOR LEER C-12 L ESS DC BusAPU PWR #1 LEER C-14 L BATT BUSAPU PWR #2 REER C-10 R ESS DC BusAPU OIL HEATER (GND)(1)

LEER C-11 ESS AC Bus

APU OIL HEATER (AIR) REER C-12 R MN AC Bus

NOTE(S):

(1) Aircraft SN 501-560 without ASC 67A.

A. Crew Alerting System (CAS) Messages:

Area Monitored: CAS Message: Message Color:APU ECU APU ESSENTIAL AmberAPU ECU APU EXCEEDANCE AmberAPU ECU APU STARTER ENGAGE AmberAPU ECU APU DOOR BlueAPU ECU APU MASTER ON BlueAPU ECU APU MAINT REQD Blue

A. Other Indications:

NOTE:

A description of the APU/BLEED synoptic pageindications is provided in Section 2B-03-30: CrewAlerting System Description.

4. Limitations:

A. APU Instrument Markings:

(1) EGT:

No limitations markings.

(2) RPM:

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• 106% and above: Red Arc

• 104% to 105%: Amber Arc

• 0 to 103%: White Arc

B. APU Operation:

(1) General:

The APU can be operated on the ground, during takeoff, in flight andduring landing. In flight it is an optional source of electrical power viathe APU GEN instead of one or both engine-driven generators. TheAPU cannot be used to supply pressurization in flight. The APU maybe used for starter-assisted airstarts below 30,000 ft if required.

(2) Maximum Permissible EGT:

• Start: 1050° C

• Running: 732° C

(3) Maximum Rotor Speed:

The maximum rotor speed for all conditions is 106%

(4) APU Starting Limits:

Continuous operation of the APU starter when powered by airplanebatteries is limited to a maximum of three (3) consecutive startattempts. A one (1) hour cool down period must be observed beforethe next full starter cycle is commenced.

APU start attempts when powered by an external DC cart are limitedto a maximum of three (3) attempts. A fifteen (15) minute cool downis required between start attempts to protect airplane wiring. A one(1) hour cool down period must be observed before the next fullstarter cycle is commenced.

NOTE:

Successful consecutive starts are limited to six (6) atten (10) minute intervals per start.

CAUTION

ALLOW FIFTEEN (15) MINUTES BEFORE ATTEMPT-ING ANOTHER APU START USING EXTERNAL DCPOWER. THIS ALLOWS THE ELECTRICAL FEEDERCABLE FROM THE EXTERNAL POWERRECEPTACLE TO THE APU STARTER TO COOL.

C. APU Generator Electrical Load:

The APU generator can deliver 100% (40 kVA) electrical power on groundor in flight from sea level to 45,000 ft.

D. APU Airstart Envelope:

For APU start limitations, see Figure 10: APU Start and OperatingEnvelope. The APU is guaranteed to start at or below 39,000 ft. APU startsare possible from 39,000 ft to 43,000 ft.

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E. Other Limitations:

There is no provision to manually open the inlet door if the door actuatorfails or electrical power is not available. If the inlet door is in transit andelectrical power is lost, the inlet door will stop in the position where powerwas lost.

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APU Controls andIndications Component

LocationsFigure 7

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APU Electronic ControlUnit (ECU) Block Diagram

Figure 8

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APU Control PanelFigure 9 (Sheet 1 of 3)

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APU Start and Operating EnvelopeFigure 10

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OPERATING MANUAL APU 2A-49-10: General

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