•B 737 NG Ground School.See the aircraft study guide at www.theorycentre.comThe information contained here is for training purposes only. It is of a general nature it isunamended and does not relate to any individual aircraft. The FCOM must be consulted forup to date information on any particular aircraft.B 737 NG Ground School.
APUAllied Signal 131-9(B)Altitude Operational LimitsThe 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 (3,048meters). Pneumatic power alone is available up to 17,000 feet (5,183 meters).The APU can be started at 41,000 feet or below.
IntroductionThe auxiliary power unit (APU) is a self–contained gasturbine engine installed within a fireproof compartmentlocated in the tail of the airplane.The APU supplies bleed air for engine starting or airconditioning. An AC electrical generator on the APUprovides an auxiliary AC power source.
APU OperationThe APU starts and operates up to the airplanemaximum certified altitude.The APU supplies bleed air for both air conditioningpacks on the ground or one pack in flight.Both transfer busses can be powered on the ground orin flight.
Electronic Control Unit (ECU)An electronic control unit (ECU) monitors and controls the APU. Automatic shutdownprotection is provided for overspeed conditions, low oil pressure, high oil temperature,APU fire, fuel control unit failure, EGT exceedance, and other system faults monitoredby the ECU.The ECU automatically controls APU speed through the electronic fuel control. Ifspeed 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 EGTabove acceptable levels during engine starting, electrical load shedding occurs prior toreducing bleed air.When electrical load and air extraction raise the EGT above acceptable levels otherthan during engine starting, the inlet guide vanes move toward a closed position,reducing bleed air extraction while maintaining electrical load.
Electronic Control Unit (ECU)• Electronic Control Unitcontrols the APU duringstart, steady stateoperations & shutdown• Protective shutdownprotection provided for:• Overspeed conditions• Low oil pressure (<35 psi)• High oil temperature• APU fire• Fuel control unit failure• EGT exceedance• Other systems faultsThe ECU receives 28v dc power from the 28v dcswitched hot battery bus.
• INDICATOR LIGHTS lights• Illumination of the MAINT lightmeans a maintenance problemexists - APU may be operated• Illumination of any amber warninglight indicates automatic shutdown• All lights disarmed when APU switchis in the OFF position• All light remain on for 5 minutes ifilluminated when the switch is set to OFFEGT indicator remains powered for 5minutes after APU is switched OFF.
INDICATOR LIGHTSAPU Maintenance (MAINT) LightIlluminated (blue) – APU maintenance problemexists:• APU may be operated• light is disarmed when APU switch is in OFF.
INDICATOR LIGHTSAPU LOW OIL PRESSURE LightIlluminated (amber) –• during start until the APU oil pressure is normal• oil pressure is low causing an automaticshutdown (after start cycle is complete) (<35 psi)• if light is illuminated when APU switch is placedto OFF, light extinguishes after 5 minutes• light is disarmed when APU switch is in OFFposition.
INDICATOR LIGHTSAPU FAULT LightIlluminated (amber) –• a malfunction exists causing APU to initiate anautomatic shutdown• if light is illuminated when APU switch is placedto OFF, light extinguishes after 5 minutes• light is disarmed when APU switch is in OFFposition.
APU FAULT LightThese are the protective shutdowns that cause a fault light:APU fuel valve shutdownDC power loss shutdownECU shutdownFire shutdownInlet door shutdownInlet overheat shutdownLoss of EGT shutdownLoss of speed shutdownNo acceleration shutdownNo APU rotation shutdownNo flame shutdownOil filter shutdownOil temperature shutdownOvertemperature shutdownReverse flow shutdownSensor failureUnderspeed shutdown
INDICATOR LIGHTSAPU OVERSPEED LightIlluminated (amber) –• APU RPM limit has been exceeded resulting inan automatic shutdown• overspeed shutdown protection feature hasfailed a self–test during a normal APU shutdownThe ECU does the overspeed circuit test during every normal APU shutdown.An overspeed shutdown occurs if the APU speed is more than 106 percent.
When the ECU does a protective shutdown, the mastercaution and APU annunciator light also come on.
APU switch to ONON – normal position when APU is running.START (momentary) – positioning APU switch fromOFF to START and releasing it to ON, initiates anautomatic start sequence.The start power sources are 115v ac transfer bus number1 if available or28v dc from the battery or batteries.
APU StartThe automatic start sequence begins by moving the APU switch momentarily toSTART. This initiates opening of the air inlet door.When the APU inlet door reaches the full open position the start sequencebegins. 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 OFFBUS light illuminates.Note: When the APU is started using battery power only, there is no indication onthe electrical metering panel that the APU generator has come on line and isready to be selected. Both the frequency and voltage readings are zero until theAPU generator is placed on line.Note: During the APU start cycle, the APU EGT indication may fluctuate from 0ºto 1100º C prior to normal EGT rise and the LOW OIL PRESSURE light maycycle on and off several times. These indications have no adverse effect onstarting the APU.It is not necessary to monitor EGT during start.
APU Air Inlet door with Vortex Generator.Air InletThe APU air inlet is on the right side of the aft fuselage.It is forward and below the horizontal stabilizer.
APU StartIf the APU does not reach the proper speed with the proper acceleration rate withinthe 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 thestart cycle, a system failure has occurred and the FAULT light illuminates.Illumination of the APU GEN OFF BUS light indicates that the APU generator maybe 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.
The APU is a gas turbine engine. It has a single shaft and operates at a constant speed. It drivesan electric generator, pneumatic load compressor, and a reduction gearbox.
APU Engine and Cooling AirAPU engine air routes to the APU through an automatically operated air inlet doorlocated on the right side of the fuselage. APU exhaust gases discharge overboardthrough 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 andvents through the exhaust outlet.Eductor air inlet for cooling.APU Engine Exhaust.
•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 automaticallycloses at approx 30% RPM during shutdown
APU ShutdownOperate the APU for one full minute with no bleed air load prior to shutdown. Thiscooling 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 airvalve and extinguishes the APU GEN OFF BUS light.Shutdown occurs automatically after 60 seconds. When the APU speed decreasessufficiently during shutdown, the fuel valve and inlet door close. If the fuel valvedoes not close, the FAULT light will illuminate after approximately 30 seconds.An immediate shutdown can be accomplished by pulling the APU fire switch.
An immediate shutdown is accomplished by pulling the APU Fire Switch
Completion of the ShutdownDuring 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 toclose before you move the battery switch to the OFF position.
APU Fuel SupplyFuel to start and operate the APU comes from the left side of the fuel manifold whenthe AC fuel pumps are operating.If the AC fuel pumps are not operating, fuel is suction fed from the No. 1 tank. DuringAPU 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 andthe fuel pressure from the ac pumps that could provide fuel to the APU are low. Whenthe 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. Normalprocedure requires at least one person be present on the flight deck if a centre tankpump is operating for safety reasons.
Fuel SupplyFuel for APU is obtained fromleft side of the fuel manifold• When any left fuel pump isoperating, the fuel is pressurefed to the APU• When no pumps are operating,fuel is suction fed from maintank no.1• Some aircraft are fitted with a DC fuel pumpwhich operates when no main fuel pump isoperating.• Fuel Burn rate approximately 110 Kg/Hr.• APU uses approximately 15% more fuel for1pack operation than for 2 pack!
It is recommended that the APU is operated with the two airconditioning packs when maximum cabin cooling is necessary.The results of the operation of two air conditioning packs will be coolercabin temperatures, lower APU fuel burn, better APU hot section lifeand lower APU noise.
APU LIMITATIONSAirplanes 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.
The APU Electronic control unit (ECU) controls the APU bleedvalve solenoid. The bleed valve opens when the solenoid isenergized from the ECU and pneumatic power is availablefrom the load compressor.The APU ECU energizes the solenoid when these conditionsoccur:* 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 comesfrom the ADIRU (Normal operation)Alternative operation.* Altitude is less than 21,000 ft (6,401 meters), if increasingaltitude and altitude comes from inlet pressure sensor (P2)* Altitude is less than 19,000 ft (5,791 meters), if descendingand altitude comes from inlet pressure sensor (P2).
Non–AFM Operational InformationThe following items are not AFM limitations, but areprovided 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 avoidengine power above idle.DoorsThe APU access door at the bottom of the APUcompartment is for servicing and maintenance.
Non–AFM Operational InformationThe following items are not AFM limitations, but areprovided for flight crew information.The start system consists of, the starter-generator, Startpower unit (SPU), and Start converter unit (SCU) togetherthese supply initial rotation of the APU. The start powersources are 28v dc from the aircraft batteries or 115v actransfer bus number1.Start Power UnitThe start power unit changes 115v ac or 28v dc electricalpower to 270v dc power. Transfer bus 1 or the batterysupplies power to the start power unit (SPU).Start Converter UnitThe start converter unit (SCU) changes the 270v dc power toAC and sends it to the starter-generator.The maximum duty cycle for the SPU and SCU is threestarts, one after the other, followed by a 15-minute cooldown period. If you do too many starts one after the other,the SCU and SPU will get too hot and stop the APU start.
Main Engine Start ModeIn main engine start mode, the ECU meets the increasedairflow requirements. This mode has priority over all othermodes.Air Conditioning System Mode (ACS)In each ACS mode, the ECU controls airflow as necessary tosupply air to the air conditioning system. These are the fourACS modes:* One pack inflight* One pack ground* Two pack, normal* Two pack, high.
APU Automatic Load SheddingIn flight, if the APU is the only source of electrical power, all galley busses andmain 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 overloadcondition is sensed, the APU sheds galley busses and main busses until the loadis within limits.Manual restoration of galley and main bus power can be attempted by moving theCAB/UTIL Power Switch to OFF, then back ON.
QuestionsAfter start, when is the APU ready to accept bleed air and electrical loads?
QuestionsAfter 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.
QuestionsWhat will happen to the APU if the battery switch is selected to OFF?
QuestionsWhat 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.
QuestionsIf the EGT exceeds operating limits, will the APU shutdown automatically?
QuestionsIf the EGT exceeds operating limits, will the APU shutdown automatically?Yes - the electronic control unit would automatically shut the APU down & illuminate the fault lightEGT limit is 730º CThe ECU automatically controls APU speed through the electronic fuel control. If speed or EGTexceed acceptable levels with the APU providing electrical load only, some electrical load isshed. When electrical load and air extraction raise the EGT above acceptable levels duringengine starting, electrical load shedding occurs prior to reducing bleed air.When electrical load and air extraction raise the EGT above acceptable levels other than duringengine starting, the inlet guide vanes move toward a closed position, reducing bleed airextraction while maintaining electrical load.
QuestionsWhat is the maximum altitude for APU bleed air load (no electrical load)?
QuestionsWhat is the maximum altitude for APU bleed air load (no electrical load)?17,000 feet
QuestionsHow long is required between selecting the APU to OFF and switching themain battery switch OFF?
QuestionsHow long is required between selecting the APU to OFF and switching themain battery switch OFF?About 2 Minutes.Operate the APU for one full minute with no bleed air load prior to shutdown. This coolingperiod 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 andextinguishes 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 toclose before you move the battery switch to the OFF position.
300°C Plus 40 seconds beforeswitching the Battery switch to OFF!
QuestionsElectrical power to start the APU comes from which source?
QuestionsElectrical power to start the APU comes from which source?Electrical Requirements for APU OperationAPU 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 batteryor 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 theAPU because of power loss from the switched hot battery bus to the electronic control unit.
QuestionsThe APU can support electrical loads and one air-conditioning pack up to what altitude?
QuestionsThe APU can support electrical loads and one air-conditioning pack up to what altitude?
QuestionsThe OVERSPEED light illuminates on the APU panel after the APU has been selectedOFF. This indicates:
QuestionsThe OVERSPEED light illuminates on the APU panel after the APU has been selectedOFF. This indicates:APU OVERSPEED LightIlluminated (amber) –• APU RPM limit has been exceeded resulting in anautomatic shutdown• overspeed shutdown protection feature has faileda self–test during a normal APU shutdown
•The END of AUTO FLIGHT Part 1The END of APU.Now take the test at www.theorycentre.comFor more information firstname.lastname@example.org