Ata 49 - Apu Garrett-r2

Training Manual ATA 49– APU

Boeing 737-300/400/500-CFM56-3 ATA 104 - Level 3 B1/B2

FOR TRAINING PURPOSES ONLY 1

ATA 49 APU-Garrett

Revision 2/ January 2013




For training purposes only. Copyright by Jat Tehnika/Solinair Technical Training. Jat Tehnika/Solinair is the owner of all rights to training documents. Any use outside the training measures, especially reproduction and/or copying of training documents − also extracts thereof −in any format all (photocopying, using electronic systems or with the aid of other methods) is prohibited. Passing on training material to third parties for the purpose of reproduction and/or copying is prohibited without the express written consent of Jat Tehnika/Solinair . Legal requirements under copyright and criminal law, apply.




TABLE OF CONTENTS

49−00 APU GENERAL ……………………………. 4 APU GENERAL SPECIFICATION PANEL DESCRIPTION 49-12 ACCESS DOOR ……………………………. 15 SHROUD SHROUD DRAINS 49-13 APU ENGINE MOUNTS …………………… 19 APU REMOVAL AND INSTALLATION 49-15 AIR INLET ................................................. 21 AIR INLET DOOR ADJUSTMENT 49- 00 APU ENGINE-INTRODUCTION ………… 27 49-10 EXHAUST DUCT …………………………… 31 49-91 LUBRICATION SYSTEM DESCRIPTION .. 33 OIL TANK OIL PUMP OIL SYSTEM INDICATION 49-30 ENGINE FUEL AND CONTROL ………… . 41 TIMED ACCEL. FCU (TAFCU) APU FUEL SHUTOFF VLV FUEL HEATER FUEL PUMP AND FUEL CONTROL UNIT 49-40 IGNITION AND START SYSTEM ............. 55 APU START SEQUENCE START CIRCUIT STARTER IGNITION EXCITER IGNITER PLUG

49-52 AIR SYSTEM ................................................................ 64 COOLING AIR VALVE BLEED AIR VALVE PROPORTIONAL CONTROL VALVE THREE−WAY SELECTOR SOLENOID VALVE SURGE BLEED VALVE DIFFERENTIAL AIR PRESSURE REGULATOR COOLING AIR VALVE ELECTRONIC TEMPERATURES CONTROL (ETC) 49-60 APU CONTROLS – GENERAL .................................... 80 APU CONTROL UNIT 49-70 INDICATING SYSTEM ................................................. 82 TACHOMETER GENERATOR SPEED SWITCH EGT ( AC WITH ETC) EGT INDICATION (A/C WITH ETC) ELECTRONIC TEMPERATURE CONTROL ELAPSED TIME INDICATOR 49-40 APU START SEQUENCE ……………………………….. 93 APU NORMAL SHUTDOWN APU AUTO SHUTDOWN




49−00 APU GENERAL

The Auxiliary Power Unit (APU) is a gas turbine engine. It has a single shaft and operates at a constant speed. It has a gas turbine consisting of a two stage centrifugal compressor directly coupled to a single stage inward flow turbine. The shaft is geared to the accessory drive section and provides power for driving the engine accessories and the generator. The APU drives the AGB through a reduction gear to maintain 6000 RPM at the generator pad. The APU air inlet provides the compressor inlet air and cooling air to the APU engine. The APU air system consists of pneumatic and mechanical components, which function automatically, to regulate the rate and maximum amount of bleed air that can be drawn from the APU for use in the airplane pneumatic system. A fan driven by the accessory drive section circulates the cooling air to the AC electric generator, lubricating oil cooler and engine accessories. The APU engine fuel system delivers fuel from the airplane tank No. 1 to the APU engine where it is metered and delivered to the combustion chamber. The fuel system regulates the fuel flow to maintain a constant speed. One temperature sensing probe is mounted in the exhaust for fuel control and indication. Another temperature sensing probe is mounted in the air inlet for fuel control. The APU exhaust is a sound reducing system that directs the APU exhaust gases overboard through an air cooled exhaust duct.




Figure 1 –APU Position




Figure 2 –APU Position




Figure 3 –APU Engine




Figure 4 –APU without ETC Block Diagram




Figure 4a –APU with ETC Block Diagram




APU GENERAL SPECIFICATION The Auxiliary Power Unit (APU) is a gas turbine engine. It has a single shaft and operates at a constant speed. The APU is used for providing following: • Electrical power to the airplane electrical system on the ground or in-flight. • Compressed air for engine starting. • Compressed air to the air conditioning system on the ground or in-flight. MODEL DESIGNATION: • Allied Signal - Garrett GTCP 85−129 RATINGS AT SL: • Rated Speed No Loads − Steady State 41 700 RPM Output Speed 6000 RPM at Generator Drive • Fuel Flow Max Load at Standard Day 340 PPH APU TEMPERATURES AT S.L.: • EGT − Sustained Maximum 650°C − Starting or Transient Condition 760°C • Oil − Maximum 140°C

BASIC ALTITUDE LIMITATION: • Sea Level to 10,000 FT (3050 M) − Both electrical and pneumatic power can be used • 10,000 to 17,000 FT (3050 to 5200 M) − Either electrical or pneumatic power can be used • 17,000 to 35,000 FT (5200 to 10,700 M) − Electrical power only can be used UNIT WEIGHT: • 313 LBS/142 Kg




Figure 5 –APU Characteristics




CC

Figure 5 a. – APU ETC




The APU system contains following subsystems: - APU power plant (49-10) - APU engine (49-20) - APU fuel system (49-30) - APU ignition and start system (49-40) - APU bleed air system (49-50) - APU controls (49-60) - APU indicating system (49-70) - APU exhaust system (49-80) -APU lubrication system (49-90). APU engine components: - Compressor section - Turbine section - Accessory gear box (AGB). The power section has a single-stage centrifugal compressor and a single-stage radial inflow turbine. The AGB provides gear reduction for the high speed torque of the power section to the accessories on the gearbox. APU Fuel System The APU fuel system supplies fuel to the combustion chamber. APU Ignition and Start System The APU ignition and start system turn the APU engine to starting speed and to ignite the fuel-air mixture in the combustion chamber. APU Exhaust System The APU exhaust system sends the APU exhaust gases through the APU muffler. The muffler sends the APU exhaust out the tail cone. APU Lubrication system The APU lubrication system lubricates and cools the APU bearings and the AGB.

APU Bleed Air System The APU bleed air system supplies pressurized air to the airplane pneumatic system. The cooling air system for the APU supplies the air to control the temperature of the APU electrical generator and APU lubrication system. A surge control valve vents unnecessary bleed air overboard through the exhaust. A bleed air valve (BAV) isolates the APU air system from the pneumatic system. APU Controls The APU can supply pneumatic bleed air and electrical power to the airplane while the airplane is on the ground or in flight. An APU control unit controls the APU functions. The APU control unit monitors APU functions and controls these lights: - LOW OIL QUANTITY (BLUE) - LOW OIL PRESSURE LIGHT (AMBER) - HIGH OIL TEMPERATURE(AMBER) - OVERSPEED LIGHT (AMBER). APU Indicating System The APU indicating system consist of these components: - EGT gauge - Elapse time indicator. The APU indications are on the P5 panel.




Figure 6– APU Control Components




PANEL DESCRIPTION APU MASTER CONTROL SWITCH • OFF− Normal position when APU is not running. When positioned to OFF with APU running initiates normal shutdown. • ON − Normal position with APU running. • START (Momentary) − When APU switch is moved from OFF to START and released to ON, causes automatic start sequence to be initiated. APU LOW OIL QUANTITY LIGHT (blue) • ILLUMINATED − APU oil quantity is insufficient for extended operation. Light is armed only when APU switch is not in OFF. APU LOW OIL PRESSURE LIGHT (amber) • ILLUMINATED − APU oil pressure is low, causing the APU to initiate an automatic shutdown (after the start cycle is complete). Light is illuminated during start until APU oil pressure is normal. Light is armed only when APU switch is not in OFF. APU HIGH OIL TEMPERATURE LIGHT (amber) • ILLUMINATED − APU oil temperature is excessive, causing APU to initiate an automatic shutdown. Light is armed only when APU switch is not in OFF. NOTE: MASTER CAUTION light and the APU annunciator will illuminate together with any of the above described amber caution lights (Exception: Low oil pressure light during APU start).

APU OVERSPEED LIGHT (amber) • ILLUMINATED − APU speed excessive causing APU to initiate an automatic shutdown. Light will illuminate if APU start is aborted prior to reaching governed speed but will extinguish during a normal restart. APU EXHAUST TEMPERATURE INDICATOR • Displays APU exhaust gas temperature. APU GENERATOR AC AMMETER • Display APU generator load current APU BLEED AIR SWITCH • ON -Opens the APU Bleed Air Valve if the APU is Operating. • OFF- Closes the APU Bleed Air Valve.




Figure 7 – APU Panel Location




Figure 7.a – APU Panel Location




49-12 ACCESS DOOR Access to the APU is through an access door, completely enclosing the APU compartment. The access door is hinged on the right side and latched on the left side. It consists of skin, ribs and stringers forming the outside contour of the fuselage. On the inside are two door open holding rods, two drain cups and cooling air exhaust. On the outside, is a drain mast connected to the drain cups. The door can be opened for normal APU servicing, or it can be removed by unlatching the hinges on the right side. The drain cups should be inspected periodically for cleanliness. The exhaust ducting is cooled by four air inlets around the aft end of the exhaust fairing. 49-12 SHROUD The APU shroud is a titanium compartment that fully protects the APU power plant. The APU shroud gives the protection from an APU fire and decrease the noise from the APU power plant. The APU shroud includes the upper shroud and the lower shroud. The upper shroud attaches to the airplane structure and holds the forward end of the APU exhaust duct. The upper shroud and the lower shroud connect to one another with the shroud latches on the lower shroud. Upper Shroud The upper shroud is installed in the APU compartment. When the APU is removed from the airplane, the upper shroud will stay in the APU compartment. The upper shroud has openings for the fuel lines, air inlet ducts and bleed air duct. Three mounting brackets for the APU engine attach to the shroud rings.

Lower Shroud The lower shroud attaches to the upper shroud with ten shroud latches. The lower shroud has the openings to the filler port for the oil tank and to the overboard exhaust for the APU cooling air. Also, there is an access panel that is removed to read the elapsed time indicator. When the lower shroud is removed, you can get access to do maintenance on the APU.




Figure 8 – APU Shroud




SHROUD DRAINS

The APU drains provide draining unburned fuel from the engine and oil from the shroud in event of leakage inside the shroud. At the aft end of the shroud, a drain cup with stand pipe picks up unburned fuel discharged through the turbine exhaust port. Two fuel drain lines are manifold into a fitting on the engine. This fitting mates with a drain cup in the drain line. The drain line terminates at a fitting at the forward end of the shroud. Adjacent to the drain line terminal end fitting is another fitting. Through this fitting fuel or oil is drained from the shroud. Both fittings at the forward end of the shroud mate with drain cups in the APU compartment access door. The drain cups in the access door are connected to a drain mast. NOTE: The drains require checking for evidence of deterioration due to corrosion. The drain lines and cups can be cleaned by blowing compressed air through them.




Figure 8 a. – APU Drain




49-13 APU ENGINE MOUNTS The APU engine mounts attach the APU engine to the shroud rings. The APU engine mounts give the APU maximum support in all directions and isolate the airplane from vibrations of the APU engine. The three engine mounts are located inside the upper shroud. Two are on the right side and, one is on the left side. Each engine mount consists of two brackets with vibration isolator and cap. The caps are used for quick removal and installation of the APU. The vibration isolator consists of a resilient material bonded to a metal core and enclosed in a tubular metal case. REMOVAL AND INSTALLATION The auxiliary power unit is removed from the airplane by a cradle assembly and two hoist assemblies. The cradle assembly is a stable base for transportation and storage of the APU. The cradle assembly consists of a base made from aluminium angles with an aluminium tube welded to the base. Attach plates on the ends of the tube provide attachment points for the hoist assemblies. The APU attaches to the cradle assembly by three quick-release pins. Two pins are on the forward mount and one pin at the aft mount base. The APU hoist assembly is of a tubular-type construction. The large end of the tube mounts a hand driven gearbox and cable drum. The opposite end has a clevis and pulley assembly. The clevis connects the hoist to one of the two eye bolts in the APU cavity. Cable from the hoist assembly connects to the cradle assembly.

Removal To remove the APU, attach a cradle assembly to the APU, install two fish pole hoist to the airplane structure, and attach the fish pole hoist cables to the cradle assembly. The fish pole hoist lowers the APU out of the airplane. To prepare for removal, open the APU access door and remove the lower shroud. Remove equipment, electric connectors, and hoses. Disconnect the three engine mounts after the cradle assembly and fish pole hoist take up the weight of the APU. Lower the APU from the airplane. Installation The installation is the reverse of the removal. NOTE: Avoid any sideways movement of the APU over the last one inch of travel. This could cause the cooling air inlet duct seat to be misaligned with the cooling fan flange and impair the cooling air system.




RH




E

LH




Figure 9– APU Installation




49-15 AIR INLET

The air supply for the APU engine operation and accessory cooling comes through the inlet duct. The air inlet door and duct are on the right side of the aft fuselage. The air inlet door consists of these components: - Vortex generator and flap - Inlet door and supply duct - Actuator - Actuator push rod - Door position switch Access to the actuator and door position switch is through an access door on the left side of the fuselage, forward of the APU compartment. Power for the door operation is 28V DC. The APU switch on the P5 panel and circuits in the APU control unit control the open and close of the door. When the actuator has power, the pushrods move the door to the open position. Movement of the door deflects the flap on the vortex generator for improvement of air flow into the inlet. When the door is fully open, the door switch operates. The switch operation is part of the APU start cycle.

OPERATION

When the APU master switch is moved to the START position, the APU control unit transmits a 28V DC signal through the ON position of the APU master switch. This 28V DC signal goes to the APU fuel shutoff valve, through the APU control unit. The 28V DC signal energizes the open coil for the APU fuel shutoff valve and opens the APU fuel shutoff valve. When the APU fuel shutoff valve is opened, the 28V DC signal energizes the open coil for the inlet door actuator and opens the air inlet door. After the air inlet door opens the APU engine will start. When the APU master switch is moved to the OFF position, the APU control unit transmits a 28V DC signal through the OFF position of the APU master switch. This 28V DC signal goes to the APU fuel valve, through the APU control unit. The 28V DC signal energizes the close coil for the APU fuel shutoff valve and closes the APU fuel shutoff valve. When the APU fuel shutoff valve is closed, the APU engine stops and the 28V DC signal energizes the close coil for the inlet door actuator. The air inlet door closes.




Figure 10– APU Inlet Door




The air inlet door actuator opens and closes the air inlet door. When the APU switch is the ON position, 28V DC goes to the fuel valve. When the fuel valve is fully open, power goes to the door actuator. Limit switches remove power from the actuator motor when the door is fully open. The door closes when any of these occurs: • APU start switch to OFF position • Fire detection • Fire switch P8 • Fire switch P28. Automatic shutdowns will close the door: • 90-second time delay (starting) • Low oil pressure • High oil temperature • Over speed • Fire detection.




Figure 11– APU Inlet Door Cycles




49-15 AIR INLET DOOR ADJUSTMENT

The APU air inlet door has these four adjustments: Adjustment 1 - Flap deflection with door open. Adjustment is made by adjustment of the length of the control rod and the adjustable flap stop. Adjustment 2 - Door alignment. Adjustment is made by adjustment of the length of the two actuator push rods. Adjustment 3 - Door open switch, door full open position. Adjustment is made by two nuts on the switch. Adjustment 4 - Actuator travel limits, full open to full closed. Adjustment is made by two screws on the actuator.

Figure 12– APU Inlet Door Adjustment




Figure 13– APU Inlet Door Actuator




49- 00 APU ENGINE INTRODUCTION The APU engine supplies the bleed air for pneumatic power and the shaft power to turn the APU electrical generator. The APU engine is a gas turbine. The APU engine has a compressor section, turbine section and gearbox section. Compressor Section The compressor section is the source of the compressed air for the turbine. The compressor section has a two stage compressor and the compressor plenum. The compressor is a centrifugal compressor. The two compressor stages are connected by crossover ducts. One impeller and one diffuser make a compressor stage. The first stage impeller is a double entry impeller. It has two impellers that are back to back. The second stage impeller is a single entry impeller. Both of these impellers have a common shaft. The shaft is installed on two bearings that are pressure lubricated. The gearbox end of the shaft connects with a quill shaft in the gearbox. Seals keep the lubrication system isolated from the turbine gases. Turbine Section The turbine section burns the air fuel mixture and supplies the mechanical power from the combustion. The turbine section has a turbine wheel, torus assembly, nozzle assembly, turbine plenum, and combustion chamber. The turbine wheel is a radial type wheel. The turbine wheel connects to the shaft that turns the compressor impellers. The turbine plenum gets the compressed air from the compressor impellers. Also, the turbine plenum is a heat shield for the aft end of the APU.

The torus assembly connects with the aft end of the combustion chamber. The torus assembly moves the combustion gases to the nozzle assembly. The nozzle assembly moves the combustion gases from the torus assembly to the turbine wheel. A single combustion chamber is made with the inner part of the turbine plenum and torus assembly. The combustion chamber has a combustion liner and cap. The combustion chamber liner supplies the combustion area. The combustion chamber liner has holes to control the location and rate of combustion. The igniter plug and fuel atomizer are installed on the cap of the combustion chamber. Gearbox Section The gearbox section changes the high turbine speed of the APU to a lower speed to turn the necessary components of the APU. The gearbox section is found at the forward end of the APU. The gearbox section has the accessory drive. The accessory drive has a gear train that is installed in a housing. The gear train is turned by a quill shaft. The quill shaft is connected to the shaft that turns the compressor impeller and turbine wheel. The gears in the gearbox section are lubricated by spray from the oil in the gearbox.




Figure 14– APU Engine




OPERATION Static air or ram air goes into the first stage impeller from the compressor plenum. The first stage impeller compresses the air. The compressed air moves through diffuser and crossover ducts to the second stage compressor. The second stage compressor also compresses the air. This compressed air moves through the second diffuser to the turbine plenum. Some of the air flows to the combustion chamber. The remaining air is available for bleed air. The fuel atomizer supplies fuel to the combustion chamber. The igniter plug supplies a spark for the ignition of the fuel air mixture. Ignition is only necessary to start the APU. From the combustion chamber, the hot combustion gases move through the torus assembly where a series of nozzles turns the gases to the turbine wheel. The hot gases turn the turbine wheel. The turbine wheel changed the energy of combustion to mechanical power. The APU uses the mechanical power to turn the compressors, APU electrical generator and other necessary components. The hot gases from the turbine wheel come out of the APU through the exhaust port.




Figure 15– APU Engine With ETC/ Without ETC




49-10 EXHAUST DUCT The APU exhaust system sends the exhaust gases overboard from the APU engine. The APU exhaust system also decreases the noise level of the exhaust gases. Exhaust Duct The exhaust duct has a heat shield and the exhaust duct muffler. The exhaust duct muffler is installed in the heat shield. There is a cylindrical air space between the exhaust duct muffler and the heat shield. A felt metal liner is installed on the inner surface of the exhaust duct muffler to decrease the noise level of the exhaust gases. Two fire detection elements attach to the exhaust duct. One fire detection element goes along the forward end of the exhaust duct muffler. The second fire detection element goes laterally along the heat shield. This second fire detection element is attached to the top and the bottom of the heat shield. The forward end of the heat shield is held by the APU shroud. A drip pan attaches to the heat shield to protect the heat shield from all hydraulic leaks. A bonding jumper is attached to the heat shield. The bonding jumper supplies a positive electrical ground to the airplane structure. The exhaust duct muffler does not touch the exhaust port of the APU turbine section. There is a gap between the exhaust port of the APU turbine section and the exhaust duct muffler. The aft end of the exhaust duct is held by the aft fairing.

Aft Fairing The aft fairing holds the aft end of the exhaust duct. It has a machined casting that moves with the airplane tail cone. The aft fairing has two short cylindrical ducts. The outer duct connects to the heat shield of the exhaust duct. The inner duct connects to the exhaust duct muffler. The aft fairing has holes to decrease the temperature of the air intake for the exhaust duct. The aft fairing attaches to the airplane tail cone and to the heat shield of the exhaust duct with bolts. Operation The high velocity exhaust gases from the APU engine flow overboard through the exhaust duct during the APU operation. The exhaust duct temperature is decreased by aspiration. The exhaust gas flow in the exhaust duct muffler causes a negative pressure. This negative pressure occurs at the gap between the exhaust port of the APU turbine section and the forward end of the exhaust duct muffler. The negative pressure pulls the ambient air through the holes in the aft fairing. This ambient air goes to the aft end of the turbine exhaust port where it mixes with the exhaust gases. This mixture of the ambient air and the exhaust gases goes overboard.




Figure 16 – APU Exhaust

Muffler




49-91 LUBRICATION SYSTEM DESCRIPTION Lubrication of the APU engine is by a self contained, positive pressure, dry sump system. It cools and lubricates all gears and bearings in the engine by providing a pressurized supply of oil. Oil is taken from the tank by the pressure pump and delivered through the filter to the accessory gear drive to cool and lubricate the gears and to the bearings on the compressor and turbine sections of the APU. It is then scavenged by the scavenge pump and sent to the oil cooler which uses air for cooling of the oil. The oil is then returned to the tank. Oil pressure is maintained at a constant 90 psi by the pressure relief valve. If the filter becomes blocked, a bypass valve opens at 50 psid permitting a flow through the system. The scavenge cavity and the oil tank are vented to the APU exhaust. The oil tank is on the left side of the compressor section. The indication lights are on the P5 panel in the flight compartment. The lubrication system consist of the following components: - Oil tank - Oil level switch - Oil pump - Oil filter - Oil cooler - Oil pressure switches (2) - Oil temperature switch - Distribution tubing - Indication lights.

System General Operation

The oil from the oil tank moves to the oil distribution system by the main oil pump. The scavenge pump removes the oil through the oil cooler before the oil goes to the oil tank.




Figure 17– Oil System




49-91 OIL TANK

The oil tank contains the supply of oil for the APU lubrication system. The tank is equipped with a filler port with cap, drain plug and two oil lines. Low Oil Quantity Switch An oil quantity level switch is used to transmit low oil quantity signal. The switch is of a float type enclosed in a housing. When the oil quantity in the tank decreases below 1,5 quarts, the switch closes and the blue LOW OIL QUANTITY light illuminates on P5 panel. Oil Quantity Float Switch An oil quantity level switch is used to transmit low oil quantity signal. When the oil quantity is below 6 quarts, the ADD switch closes: • ADD light on the lube service panel illuminates and • Oil tank solenoid valve opens. During refill, when the oil level reaches 8 quarts the full switch closes. • The oil tank solenoid valve closes and • FULL light illuminate.

Oil Tank Servicing (Manual) Servicing of the oil sump is accomplished by opening the APU access door. The oil level is checked by removing the filler cap and using the dipstick. If the oil level is below the FULL mark, add oil until the level is at the FULL mark on the dipstick. Oil Tank Servicing (remote oil servicing) The external service panel provides a means of replenishing the oil supply for the APU oil tank. The oil tank is replenished by connecting the oil supply to the pressure fill connection and pumping oil into the tank until the green light illuminates. Required oil supply pressure is 35 psi. CAUTION: Do not mix oils of different type or brand names, some oils will chemically change when you mix them. This can cause damage to the APU.




Figure 18 – Oil System Components




49-91 OIL PUMP The oil pump supplies oil under pressure for lubrication of APU gearbox and bearings and scavenges the oil back to the tank. The oil pump assembly consists of a separate pressure and scavenge pump, pressure regulating valve, oil filter and a filter bypass valve. The pressure and scavenge pumps are driven by a common shaft. During APU operation, the pressure pump draws oil from the tank and supplies it through the filter for the lubrication of the APU engine. Oil is returned to the oil tank by the scavenge pump.

Oil Filter The oil filter provides the cleaning action for the oil flowing from the pressure pump. Oil Cooler The oil cooler provides the necessary cooling for engine oil during APU operation. The cooler is a cylindrical unit constructed of bonded aluminum tubes housed in a shell. Oil is delivered by the scavenge pump and passes around the tubes before returning to the tank. Cooling air supplied by the fan flows through the tubes and removes the heat from the oil. The cooling air exhausts into the APU shroud.

Oil temperature switch The switch senses the temperature of oil at the pressure pump outlet. When the oil temperature reaches 140°C (285°F), the switch closes and provides power (28 VDC) for an amber HIGH OIL TEMP light on P5 panel and shuts down the APU.

Low oil pressure switch The switch senses oil pressure at the pump outlet. A diaphragm operated snap action switch opens at 55 psig and closes at 45 psig oil pressure. During APU start, with increasing oil pressure, the switch opens at 55 psig, removing power from the LOW OIL PRESSURE light on P5 panel. If during APU operation, the pressure decreases to 45 psig, the switch closes and provides power for the LOW OIL PRESSURE light and shuts down the APU. Sequencing oil pressure switch The sequencing oil pressure switch ensures that combustion cannot be initiated during an APU start until a minimum lubricating oil pressure has been built up. During APU start, with increasing oil pressure, a diaphragm operated snap action switch closes at 4 psig oil pressure and provides 28 volt DC power to the fuel solenoid valve and the ignition exciter.




Figure 19 – Oil System Pump




49-94 OIL SYSTEM INDICATION NORMAL SEQUENCE Low Oil Quantity Switch The oil quantity switch closes when the oil tank level decreases to 1,5 quarts. The blue LOW OIL QUANTITY light illuminates on P5 panel if the APU switch is in the ON position.

Low Oil Pressure Switch The low oil pressure switch opens at 55 psig during the APU start. The amber LOW OIL PRESSURE light on P5 panel extinguishes. (with the light illuminated up to 55 psig, the MASTER CAUTION and APU annunciator are not illuminated). During normal APU shutdown, low oil pressure indication is disconnected by the APU switch in the OFF position.

Backup Sequence

Low Oil Pressure Switch During APU operation, when the oil pressure decreases to 45 psig, the oil pressure switch closes, fuel solenoid valve closes and APU shuts down. The LOW OIL PRESSURE, MASTER CAUTION and APU annunciator lights illuminate. A 10 second time delay eliminates transient variations in oil pressure due to flight maneuvers. The lights are reset by placing APU switch to OFF. High Oil Temperature Switch During APU operation, when the oil temperature reaches 140°C (285°F), the temperature switch closes, relay K1-2 energizes, fuel solenoid valve closes and APU shuts down. The HIGH OIL TEMPERATURE, MASTER CAUTION and APU annunciator lights illuminate and remain illuminated even if oil temperature decreases to below 140°C ( 285°F ). This is achieved by relay K1-2. The lights are reset by placing APU switch to the OFF position




Figure 20 – Oil System Indication

A 10 second time delay eliminates transient variations in oil pressure due to flight maneuvers. The lights are reset by placing APU switch to OFF.




49-30 ENGINE FUEL AND CONTROL

The APU fuel system supplies fuel from the airplane fuel system to the APU engine and automatically meters the flow rate to match speed and loading requirements. Fuel from the tank is delivered to the fuel control unit through the fuel valve. The fuel control unit supplies and regulates the fuel flow to the combustion chamber. The regulated fuel flow controls the acceleration of the engine during the starting operation. When a load is applied to the APU, the fuel control unit meters the fuel flow to maintain a near constant speed and a safe operating temperature of the unit. When the APU is operating above 95% RPM the 3-way solenoid valve closes and APU speed is controlled by the flyweight governor. The governor regulates fuel flow to keep a constant APU speed during different load conditions. Whenever the APU is shut down the fuel solenoid valve is deenergized. The fuel shutoff valve closes only during normal shutdown or by the action of the fire detection system and fire handles operation

Operation Sequence All of the components of the APU fuel system are automatically controlled. When the APU MASTER SWITCH is moved to the START and ON position, the APU fuel shutoff valve opens, fuel from the tank flows to the pump, (if installed) heater and low pressure filter to the timed acceleration fuel control unit. During the start sequence, when the sequencing oil pressure switch closes, the fuel solenoid valve opens allowing fuel flow to the combustion chamber. The acceleration limiter references the control air pressure (compressor discharge pressure) to fuel pressure.




Figure 21a. – APU Fuel System/ Engine Without ETC




49-31 ENGINE FUEL AND CONTROL A/C WITHOUT ETC

The APU fuel system supplies fuel from the airplane fuel system to the APU engine and automatically meters the flow rate to match speed and loading requirements. Fuel from the tank is delivered to the fuel control unit through the fuel valve. The fuel control unit supplies and regulates the fuel flow to the combustion chamber. The regulated fuel flow controls the acceleration of the engine during the starting operation. When a load is applied to the APU, the fuel control unit meters the fuel flow to maintain a near constant speed and a safe operating temperature of the unit. When the APU is operating above 95% RPM, the 3-way solenoid valve closes and APU speed is controlled by the flyweight governor. The governor regulates fuel flow to keep a constant APU speed during different load conditions. Whenever the APU is shut down the fuel solenoid valve is deenergized. The fuel shutoff valve closes only during normal shutdown or by the action of the fire detection system and fire handles operation.

Fuel System Schematic (A/C WITHOUT ETC) All of the components of the APU fuel system are automatically controlled. When the APU master switch is moved to the START and ON position, the APU fuel shutoff valve opens, fuel from the tank flows to the pump, (if installed) heater and low pressure filter to the fuel control unit. During the start sequence, when the sequencing oil pressure switch closes, the fuel solenoid valve opens allowing fuel flow to the combustion chamber. The engine acceleration is controlled by the action of the acceleration limiter valve. The valve references the control air pressure ( compressor discharge pressure ) to fuel pressure. Continued acceleration to operating speed is controlled by the governor within the fuel control unit. The governor regulates fuel flow to match the power demands.




Figure 21 – Fuel System – APU without ETC




Figure 21.b – FCU Acceleration

Page 76 for adjustment

Page 76 for adjustment




49-52 CONTROL THERMOSTAT (A/C WITHOUT ETC)

The control thermostat limits fuel flow during acceleration and ensures that the maximum amount of bleed air is extracted from the APU without overloading. The control thermostat operates as an acceleration and a load control thermostat. The thermostat is installed in the turbine exhaust port. The thermostat consists of a spring loaded ball valve and a thermostatic core in a housing with openings for the passage of exhaust gas. Connected to the thermostat is a pneumatic line. The valve opens at 630°C exhaust gas temperature. Operation The thermostat is connected through the 3-way solenoid valve to the acceleration limiter valve. The 3-way solenoid valve is usually open. During the acceleration of the APU engine to 95% speed, the thermostat is an acceleration control thermostat. The thermostat discards a specified quantity of control air to prevent a turbine overspeed and a high EGT. When the APU speed is more than 95%, the 3-way solenoid valve closes. This changes the thermostat to the load control thermostat. When the EGT is 650°C (1200°F), the APU control thermostat opens to bleed air from the bleed air valve. This will permit the bleed air valve to move to the closed position and lower the EGT. Whenever a High EGT is present ( during acceleration or normal operation), the thermostat opens and bleeding off the control pressure from the acceleration control ( FCU ) or the bleed air valve.

Adjustment •The APU must be operating fully loaded. If the EGT is not within 620−650° C range, the thermostat requires adjustment. Each index mark on the adjustment head equals 2°C, rotating the adjustment collar clockwise increases the temperature setting. CAUTION: Do not use a wrench on thermostat adjustment collar or body when connect the pneumatic line. This can cause the thermostat adjustment to change witch can permit the APU to become to hot. The thermostat has a housing, spring-loaded ball valve, thermostatic core, and pneumatic line connection.




Figure 21.c – APU Control Thermostat/ Engine Without ETC




Fuel Control Unit (A/C without the ETC) The FCU is a hydro mechanical fuel control whose governing action controls the speed of the APU. The acceleration of the engine is controlled by the action of the acceleration limiter valve. The valve references the control air pressure (compressor discharge pressure) to fuel pressure. The pressure actuated diaphragm in the acceleration limiter valve operates a bypass valve to bypass more or less fuel to maintain the required ratio of fuel flow to control pressure. The acceleration limiter valve controls the quantity of fuel for acceleration during the APU start. The acceleration limiter valve has a diaphragm, piston and guide assembly, bypass valve, spring, adjustment screw, nut, and bypass filter. Air supplied by the compressor controls the acceleration limiter valve. The air is supplied from an orifice tee. The orifice tee is found forward of the combustor unit on the lower right side of the diffuser housing. During acceleration, if the EGT exceeds 620°C to 650°C, the thermostat opens and bleeds some of the control air from the acceleration limiter valve. This increases the bypass, reducing fuel flow to the combustion chamber and thereby reduces EGT. When the APU is operating above 95% RPM the 3-way solenoid valve closes and APU speed is controlled by the fuel governor. The governor regulates fuel flow to match the power demands. The governor controls the fuel flow to the fuel atomizer during the APU usual operation. The governor supplies the fuel to keep a constant APU speed during different load conditions.

If the speed decreases due to loading, the governor bypasses less fuel allowing more fuel to the combustion chamber to restore the speed. If the speed increases due to reduction in load the governor increases tile bypass, reducing the fuel flow to the combustion chamber. The governor has a shaft and sleeve assembly, governor cage, flyweights, spring, adjustment screw, and nut. High pressure fuel is on all sides of the fly weights. The flyweights move the sleeve to cover the shaft ports for control of the bypass of the fuel. This will give the correct quantity of fuel to the fuel atomizer. Whenever APU is shut down the fuel solenoid valve is deenergized. The fuel shutoff valve closes only during normal shutdown or by the action of the fire detection system and fire handles operation.




TIMED ACCEL. FCU (TAFCU) The Timed Acceleration Fuel Control Unit (TAFCU) is a hydro- mechanical fuel control. Supplies and regulates the fuel flow for acceleration and to maintain a near constant operating speed and a safe operating temperature of the APU to match the power demands. Acceleration Limiter The first 20% of acceleration of the engine is controlled by the action of the acceleration limiter. The acceleration limiter references the control air pressure (compressor discharge pressure CDP) to fuel pressure ( Chamber A ). The pressure actuated diaphragm in the acceleration limiter operates a bypass valve to bypass more or less fuel to maintain the required ratio of fuel flow to control pressure. At 20% RPM the CDP pressure is high enough to close the acceleration limiter valve, witch closes the bypass to the fuel pump inlet. Further acceleration control is transferred to the speed governor. Continued acceleration to operating speed is controlled by the timed acceleration of the governor within the fuel control unit. If during acceleration up to 95% RPM the EGT reaches the EGT temperature schedule, the ETC opens the proportional valve and bleeds some of the control air from the acceleration limiter. This increases the bypass, reducing fuel flow to the combustion chamber and thereby reduces EGT.

Speed Governor The governor assembly consists of a flyweight carrier and governor flyweights that turn within a chamber of the governor assembly fuel control housing. The chamber is full of fuel. The timer shaft attaches to the flyweight carrier that also turns within chamber A but extends into chambers B and C. During engine operation, the fuel pump delivers fuel under pressure to the TAFCU, in to chamber C and through the Δ p check valve and high pressure filter into chamber A. The governor metering valve is free to slide up or down on the timer shaft. Fuel goes through the metering orifice M from chamber A to bypass. The fuel returns to the pump inlet as governor fuel bypass. High pressure fuel from the pump goes through the Δp valve and high pressure filter, through chamber A and on to the combustion chamber. When The APU is not running, the piston return spring pushes the accelerator piston up which draws fuel through the return check valve and fills chamber B. The timer shaft moves downward during acceleration at a controlled rate until 100 percent speed. When it reaches the 100 percent, the piston bottoms out on an adjustment shoulder.




Figure 22 – Fuel System - APU with ETC




Figure 22.a – Fuel System –APU with ETC




Figure 23. – Fuel System Function




Figure 24 – APU Fuel System Components/ Engine Without ETC




APU FUEL SHUTOFF VLV

The fuel shutoff valve allows fuel supply from the tank to the APU engine. The valve is located on the rear spar of fuel tank No.1 in the left main wheel well. The valve consists of an actuator assembly and a valve body assembly. An electric motor ( 28V DC) operates the valve. The valve has a manual override handle.

The APU switch on the P5 panel and the protection circuits in the APU control unit control the operation of the shutoff valve. When the APU switch is in the ON position, the shutoff valve opens. The shutoff valve close when any of these conditions occurs: - APU start switch to OFF position - Fire detection - Fire switch P8 - Fire switch P28

Automatic shutdowns close the fuel shutoff valve: - 90-second time delay (starting) - Low oil pressure - High oil temperature - Over speed - Fire detection

NOTE: The valve can be operated manually without electrical power by the override handle. NOTE: The valve actuator assembly can be removed separately from the valve body. To remove the valve body mechanic need to enter the tank No.1. Boost pump The pump supplies pressurized fuel during starting of the APU. The pump consists of a motor driven vanes, bypass valve, inlet and outlet ports. It is held in position by four boats through flanges on pump body. The pump uses 28 VDC power.

The pump operates only during APU start if: •the APU Master Switch is in ”ON” and •Boost Pump relays from tank No.1 are ”OFF” and •APU RPM is below 95%.




Figure 24.a – APU DC Booster Pump




Figure 25 – APU SOV




FUEL HEATER

The fuel heater prevents ice in the fuel filter. The fuel heater and control valve with a temperature sensor is on the inside surface of the upper shroud. The heater is a tubular fuel/air heat exchanger. The control valve has a bimetallic element to sense the fuel temperature and let bleed air go through the heater. When the fuel temperature that goes through the valve decreases to 3°C (37°F), the valve opens. This lets hot air from the APU bleed duct go through the heater and into the shroud. When the fuel temperature increases to 18°C (64°F), the valve closes. A check valve in the fuel line upstream of the heater keeps pressure in the fuel line. This makes sure of a positive prime for starting. A second check valve in the air line keeps fuel from the bleed air duct.




Figure 26 - Fuel Heat & Flow Control VLV




FUEL PUMP AND FUEL CONTROL UNIT

The fuel pump and fuel control unit controls the fuel flow to the combustion chamber during starting and normal operation. The fuel pump and fuel control unit assembly is on the accessory drive section. The fuel pump and fuel control unit consists of these components: - Engine-driven fuel pump - Filter - Acceleration limiter valve - Timed Acceleration Fuel Control Unit (TAFCU) - Flyweight governor - Fuel inlet and outlet ports - Control air connection. The fuel pump supplies fuel to the combustion chamber through the acceleration limiter valve during starting. The TAFCU controls fuel flow during operation. Adjustment/Test of the Governor Operate the APU with the bleed valve closed and no electrical load. The generator frequency should be 405 to 410 Hz. Adjust for the correct frequency by the governor adjustment. Clockwise rotation increases the speed. Do not adjust the governor speed more than 1/12 of a turn. The adjustment nut has six sides. Turn of the nut 1/6 of a turn is one wrenching flat. 1/12 of a turn is half of a wrenching flat. This is a one time adjustment of the governor. If it is necessary to adjust the governor more than 1/12 of a turn, replace the timed acceleration fuel control unit.

Acceleration Limiting Valve Opening ( Crack ) Pressure Test. • Start the APU and let the APU operate at a stable speed

with no load applied for 5 minutes.

NOTE: This will make sure fuel is available to the APU

•Do the usual shutdown. •Open the APU FUEL BOOST circuit breaker on the P6-5 load control center. •Attach a test pressure gage to the high pressure fuel line from the fuel atomizer. Remove the cap from the control air pressure port of the FCU. The control air pressure compressor discharge pressure (CDP ) vented at the control unit to eliminate the influence of air pressure on control spring force. The APU then motored and the cracking pressure (60 +/- 5 psig). •Adjust pressure to correct value using the screw on the acceleration limiter valve. Clockwise rotation increases the crack pressure. Counterclockwise rotation decreases the crack pressure. AIRPLANES WITH THE ETC If the cracking pressure is more than 65 psig or less than

55 psig, replace the timed acceleration fuel control unit. • Set the APU master switch to OFF.




Figure 27 – FCU Installation




Governor Speed Check Operate APU with the APU bleed air valve closed and no electrical load. The frequency meter should read 405 to 410 Hz. Adjust correct frequency by the governor adjustment screw. Clockwise rotation increases the speed. Counterclockwise rotation decreases the speed. Fuel Filters Two fuel filters are used to prevent contamination of the pump and fuel control unit. The filters are full flow types with replaceable elements housed in filter bowls. The low pressure filter incorporates a bypass valve opening at a differential pressure of 15 psi. Fuel Solenoid Valve The solenoid valve allows fuel flow to the combustion chamber after the initial rotation of the APU. The valve is solenoid operated, open/closed. The valve is energized open (28VDC) during APU start, when the sequencing oil pressure switch sense 4 psi. The valve is deenergized closed whenever APU is shutdown normally or automatically. NOTE: Protection circuits in the APU control unit and the sequencing oil pressure switch control the valve operation.




Figure 28 – FCU Installation




49-40 IGNITION AND START SYSTEM The APU ignition and start system turns the APU engine to the start speed necessary to permit ignition of the fuel-air mixture start in the combustion chamber. The system has following components: - Start switch - Starter - Start relay R5 - Ignition exciter - Igniters plug - Control circuits. The starter, ignition exciter, and igniters plug are on the APU engine. The starter relay R5 is in the electronic equipment compartment on the right aft side of the E3 rack. The starter switch and indication light are on the P5 panel in the flight compartment. The start system has interfaces with these components: - Sequencing oil pressure switch - Speed switch - APU control unit - APU switch - Battery switch. During APU start the starter turns the APU engine. The ignition exciter and fuel solenoid valve operate by the operation of the sequencing oil pressure switch. The starter and ignition circuits go off by the operation of the speed switch during APU engine acceleration. During normal engine operation the exciter stays off.

These components control the APU start and acceleration: - APU control unit - Electronic temperature control (ETC) - TAFCU - Acceleration limiter valve - Electronic speed switch. During the start sequence, the air inlet door moves to the full open position. A signal from the APU control unit goes through the 50% speed switch and energizes the starter. At 10% RPM when the oil pressure gets to 4 psig, the sequencing oil pressure switch closes. This lets the fuel solenoid valve and ignition systems energize. If there is fuel and ignition, the APU accelerates. At 37% RPM when the oil pressure gets to 55 psig, the low oil pressure (LOP) switch opens and turns off the LOP light in the flight deck. At 50% RPM, the starter is de-energized. The APU continues to accelerate on its own to 95% RPM. At 95% RPM these things occur: - APU GEN OFF light is on - Hour meter starts - Selector solenoid valve energizes - APU bleed switch is armed - Low oil pressure (LOP) automatic is armed - Surge bleed valve is armed - ETC temperature is changed. The APU is ready to supply electrical and/or pneumatic power to the airplane.




Figure 29 – APU Starter Installation




APU START SEQUENCE Control Sequence Off The APU control unit receives 28 VDC power from the BATTERY BUS. Start Placing the APU switch momentarily from OFF to the START position and releasing the APU switch to the ON position the start sequence is initiated. After the fuel valve and air inlet door are fully open, power is supplied to arm the ignition circuit, energize 90−second timer and energizes relay R5. Crank With relay R5 energized, battery (or external DC) is connected to the APU starter. The APU rotates, oil pressure rises and at 4 psi the sequencing oil pressure switch closes. 28 V DC power is now supplied to the ignition exciter and the igniter plug. At 50% RPM, the speed switch operates, relay R5 deenergizes and power is disconnected from the APU starter. ( If 50% RPM is not reached within 90 seconds, power is removed from the start and ignition circuits.) At 95% RPM, the speed switch operates and power is disconnected to the ignition exciter.




Figure 30 – APU Start Sequence




START CIRCUIT To start the APU, put the battery switch to the ON position. When the APU switch momentarily goes from the OFF to START position, the starter enable circuit arms. When you release the APU switch to the ON position, the fuel shut-off valve and the air inlet door open. After the fuel shut-off valve and the air inlet door are open, the APU control unit operates the starter enable circuit. The starter enable circuit operates these components: - APU electronic speed switch - 90 second timer - R5 relay. The APU turns when R5 energizes. The starter receives power from the battery or external DC. Oil pressure increases as the APU turns. At 4 psi the sequencing oil pressure switch closes. When the sequencing oil pressure switch closes: - Ignition exciter receives power - Igniters plug operates - Fuel solenoid valve opens. At 50 % RPM, the 50 % speed switch operates. This removes power from the R5 relay and the APU starter. If the APU does not get to 50 p% RPM in 90 seconds, an automatic APU shut-down occurs. The shut-down circuit does these functions: - Opens R5 - Closes the fuel solenoid valve - Removes power from the ignition circuits - Closes the air inlet door - Closes the fuel shut off valve. At 95 % RPM the 95 % speed switch operates and removes power to the ignition exciter. An APU automatic shut-down occurs for a 110 % over speed.

The shut-down circuit for over speed does these functions: - Opens R5 - Closes the fuel solenoid valve - Removes power from the ignition circuits - Closes the air inlet door - Closes the fuel shut off valve -Causes the OVERSPEED light on the P5 panel to come on. NOTE: APU control unit with test PASS/FAIL lights For an over speed shut-down, you can not start the APU again until you do a reset of the APU control unit. Push the over speed reset switch on the APU control unit and put the APU start switch to the OFF position to do a reset of the circuit.

To operate the APU on the ground, the battery switch must be ON.

On the ground the APU stops when you put the battery switch to the OFF position.

In the air the APU does not stop when you put the battery switch to the OFF position.

The APU stops from the operation of the fire detection system or manual operation of the fire handles panels P8 or P28.




Figure 31 – APU Start Circuit




STARTER

The starter provides the initial power to turn the APU compressor, turbine, and engine driven accessories to a speed sufficient to get good airflow for combustion. The starter is on the accessory drive section. The starter consists of these components: - Electrical motor - Internal friction clutch - Engagement mechanism. The power for the starter is 28V DC from the battery or external DC power connection. The starter energizes by the APU start switch after the APU fuel shutoff valve and air inlet door are open. It continues to drive the APU until the 50 % RPM speed switch or a 90 second timing circuit disconnects the power supply to the starter. The DC ammeter on the P5 panel in the flight compartment monitors the starter operation. NOTE: You must identify the electrical leads with the corresponding starter terminals to prevent cross wiring. Crossed wires may result in damage to APU components when you do an APU start.

Figure 32 – APU Starter Installation




IGNITION EXCITER

The ignition exciter provides the high voltage to produce the spark at the igniters plug. The exciter is on the turbine plenum at the 6:00 position. The power to the exciter is 28V DC during APU starting The exciter energizes during APU starting by the sequencing oil pressure switch. The exciter de-energizes when the APU reach 95 % RPM. APU Start Relay R5 The Start Relay R5 transmits the heavy electrical load during the operation of the starter motor. WARNING: CURRENT IN THE IGNITION SYSTEM IS OF VERY HIGH VOLTAGE AND CAN BE FATAL. YOU MUST REMOVE POWER FROM IGNITION SYSTEM FOR A MINIMUM OF THREE MINUTES BEFORE ANY DISCONNECTS ARE MADE. AFTER YOU DISCONNECT THE HIGH VOLTAGE LEAD, MAKE SURE OF COMPLETE DISCHARGE OF CAPACITORS IN IGNITION UNIT BY IMMEDIATELY SHORTING HIGH VOLTAGE LEAD TERMINAL TO GROUND.

IGNITER PLUG

The igniters plug provides the high energy spark to ignite the fuel/air mixture in the combustion chamber. The igniters plug is on the forward side of the combustion chamber. The igniters consists of : - Outer casing - Canter electrode - Ceramic insulator. The power supply to the igniters plug is from the ignition exciter.




Figure 33 – Ignition Plug Installation




49-52 AIR SYSTEM The APU air system functions are: - Cools the APU accessories - Provides bleed air for the airplane pneumatic system - Helps control APU operation. The cooling system is on top of the upper shroud. The bleed air valve is in the bleed duct on the aft side of the APU compartment forward bulkhead. The proportional control valve (APU with ETC) is on the bottom of the turbine case aft of the solenoid selector valve. The surge bleed valve is on the turbine plenum on the right side of the APU. The selector solenoid valve is on the lower right side of the APU below the surge bleed valve. Cooling Valve Operation The cooling air system consists of a cooling duct with a valve that is operated by an actuator. Part of the air that goes into the APU compartment cools the generator and oil. This air goes through the valve which is open when APU operates. A gearbox-driven fan helps the cooling air flow. Bleed Air Valve Operation During APU operation, air from the compressor section supplies air to the pneumatic system through the bleed valve. Proportional Control Valve (APU with ETC) The surge bleed valve prevents compressor surge during APU operation. Selector Solenoid Valve The selector valve connects the proportional control valve to the acceleration limiter valve or to the bleed valve.

COOLING AIR VALVE The cooling air valve lets air flow through the cooling air inlet duct during APU operation for the generator and the oil cooler. The valve also isolates the APU compartment for fire protection. The valve and actuator are in the cooling duct above the upper shroud. The valve has a butterfly valve that operates by a pneumatic actuator. Air supply for the actuator operation is from the bleed duct upstream of the bleed valve through a check valve and filter. Access Access to the valve and actuator is through an access door immediately forward of the APU compartment and through a rectangular opening in the horizontal web above the upper shroud. Operation During APU start when the compressor bleed pressure increases to 7 psi, the actuator operates and moves the butterfly valve to the open position. During APU stop the compressor pressure decreases and the valve closes. NOTE: A viewing tube and cap is downstream of the valve. You must move the horizontal stabilizer to the full up position to get access to the viewing tube cap. You look down the viewing to do a check that the cooling air inlet screen is clean.




When the solenoid is de −energized, control pressure is supplied to chamber D, chamber C is vented and the butterfly closes. Two switches are operated by the bleed valve. One is for the amber DUAL BLEED light on P5 panel and the other is for the APU surge bleed valve.

Figure 34 – APU Bleed Air VLV




49-51 BLEED AIR VALVE DESCRIPTION (A/C WITHOUT ETC) OPERATION When the solenoid is energized, air upstream of the butterfly’ passes through the filter and regulator assembly and switcher valve to chamber C of the valve. Chamber D is vented through the switcher valve. As the butterfly opens, upstream pressure decreases, chamber B pressure decreases faster than chamber A pressure by means of the restrictor. The differential pressure acts on the diaphragm and opens the rate control valve. This partially vent, chamber C, thus decreasing the opening rate of the valve. If high EGT occurs, the thermostat opens, venting partially chamber C, and the valve modulates towards closed position. When the solenoid is de −energized, control pressure is supplied to chamber D, chamber C is vented and the butterfly closes. Two switches are operated by the bleed valve. One is for the amber DUAL BLEED light on P5 panel and the other is for the APU surge bleed valve.




Figure 34 a. – APU Air System / Engine Without ETC

19psi




BLEED AIR VALVE The bleed air valve controls the bleed air flow from the APU to the pneumatic system. The valve is in the bleed air duct on the aft side of the APU compartment forward bulkhead. The valve consists of following parts: - Butterfly valve - Actuator - Rate control valve - Switcher valve and solenoid. NOTE: A solenoid controls the valve. Valve operated pneumatically. Power for the solenoid is 28V DC. The bleed valve is controlled by following components: - APU bleed valve switch on the P5 panel - 95% switch - Proportional control valve (PCV) - Electronic temperature control (ETC). NOTE: Air pressure in the pneumatic duct through a differential air pressure regulator operates the bleed valve.

APU BLEED AIR VALVE SCHEMATIC When the solenoid is energized, air upstream of the butterfly valve goes through the filter and regulator assembly and switch valve to chamber C of the valve. Chamber D vents through the switcher valve. As the butterfly valve opens, upstream pressure decreases and chamber B pressure decreases faster then chamber A pressure because of the restrictor. The differential pressure acts on the diaphragm and opens the rate control valve. If high EGT occurs, the proportional control valve opens. Air partially vents from chamber C. The valve modulates towards the closed position. When the solenoid is de-energized, control air pressure goes to chamber D. This causes chamber C to vent and the butterfly valve closes. The bleed air valve controls the bleed air flow from the APU to the pneumatic system. NOTE: Bleed air valve is in the compartment forward of the APU. Two clamps hold the bleed air valve to the bleed air duct. BlAS with ETC When, the solenoid is energized, air upstream of the butterfly passes through the filter and regulator assembly and switcher valve to chamber C of the valve. Chamber D is vented through the switcher valve. As the butterfly opens, upstream pressure decreases, chamber B pressure decreases faster than chamber A pressure by means of the restrictor. The differential pressure acts on the diaphragm and opens the rate control valve. This partially vents chamber C, thus decreasing the opening rate of the valve. If high EGT occurs, the proportional valve opens, venting partially chamber C, and the valve modulates towards closed position




Figure 34.b – APU Air System/Engine with ETC

19psi




PROPORTIONAL CONTROL VALVE

The proportional control valve limits fuel flow during acceleration and ensures that the APU delivers the maximum amount of bleed air without overloading. The electronic temperature control (ETC) unit controls the valve.

The proportional control valve is on the bottom of the turbine case aft of the solenoid selector valve.

The proportional control valve is normally spring loaded open. When the ETC energizes, by the BIAS closed input to the torque motor driver, closes the valve. As the BIAS is overcome by EGT the valve opens. Air flow through the dual ports on either side flows out with the exhaust.

THREE−WAY SELECTOR SOLENOID VALVE The three−way selector solenoid valve allows the thermostat to be connected to the acceleration Limiter valve or to the bleed valve.

Operation (A/C with the ETC) During acceleration, the solenoid is de −energized, the proportional control valve (PCV) is connected to the acceleration limiter valve. At 95% RPM, the solenoid is energized, the proportional control valve is connected to the bleed valve.

SURGE BLEED VALVE The surge bleed valve prevents compressor surge during APU operation in flight. The surge valve is always closed on the ground. In flight the valve will be open if APU Speed is above 95 % and the bleed valve is closed. The valve will be closed if the bleed valve is open.

NOTE: To check the surge valve operation, the APU must be operating. Air position is simulated by opening LANDING GEAR LIGHTS circuit breaker on P6 panel, this breaker controls AIR/GND relay. Surge valve operation is indicated by an increased air flow from the APU accessory cooling air exhaust.




Figure 35 – APU Bleed Air Components




DIFFERENTIAL AIR PRESSURE REGULATOR The differential air pressure regulator provides constant pressure air (19psi) supply for operation of the bleed valve. The regulator is on the bleed valve. The regulator consists of these parts: - Air inlet - Filter - Spring-loaded diaphragm - Metering valve - Relief valve. The regulator pressure is set at 19 psig.

The APU compressor discharge pressure is supplied to the regulator. The metering valve restricts the output to 19 psig independent of ambient conditions.

Adjustment/Test You measure the output of the regulator with a pressure gauge. While the APU operates, the output pressure should be 19 psig. You can change the pressure by a screw on the regulator.




Figure 36 – Air Pressure Regulator




COOLING AIR VALVE

The cooling air valve lets air flow through the cooling air inlet duct during APU operation for the generator and the oil cooler. The valve also isolates the APU compartment for fire protection.

The valve and actuator are in the cooling duct above the upper shroud. The valve has a butterfly valve that operates by a pneumatic actuator. Air supply for the actuator operation is from the bleed duct upstream of the bleed valve through a check valve and filter.

Access to the valve and actuator is through an access door immediately forward of the APU compartment and through a rectangular opening in the horizontal web above the upper shroud.

During APU start when the compressor bleed pressure increases to 7 psi, the actuator operates and moves the butterfly valve to the open position. During APU stop the compressor pressure decreases and the valve closes. NOTE: A viewing tube and cap is downstream of the valve. You must move the horizontal stabilizer to the full up position to get access to the viewing tube cap. You look down the viewing tube to do a check that the cooling air inlet screen is clean.




Figure 37 – Cooling Air VLV

You must move the horizontal stabilizer to the full up position to get access to the viewing tube cap.




ELECTRONIC TEMPERATURES CONTROL (ETC) The electronic temperature control (ETC) does a check of the exhaust gas temperature directly from the two ETC thermocouple probes T 5 . The ETC also supplies the overload protection for the APU. When an overload condition occurs, the ETC sends a signal to the proportional control valve to close the bleed air valve. This will permit the EGT to decrease to the correct limit. After the EGT decreases to the correct limit, the ETC sends a signal to the proportional control valve to open the bleed air valve. During the APU operation, the ETC sends a signal to the EGT indicator on the P5 forward overhead panel. On some airplanes, the ETC also sends the signal to the EGT indicator after the APU stops. The ETC provides proportional control of EGT during start, and control of EGT when the APU is providing bleed air power. The ETC consists of electronics two process inputs of 28 VDC power, a 28 VDC 95% RPM discrete signal, EGT thermocouple signals (T5 ) and compressor inlet temperature (T2 ) signals and discrete signals from the two main engine start switches. The ETC is activated by selecting the APU master control switch to START/ ON position. Outputs from the ETC consist of: • DC drive signal to the proportional control valve and • Conditioned EGT signal for flight deck indication. ETC Control APU START to 95% RPM SPEED. The highest T5 signal wins from the both thermocouples and are amplified to the EGT temperature schedule and EGT indication. When the EGT reach the EGT schedule a signal output energizes the torque motor of the proportional control valve. Allowing the proportional valve (PCV) to open. This causes a decrease of the control pressure (CDP) in the acceleration limiter. This reduces the fuel rate. Enables the EGT to decrease until the EGT schedule is reached.

ETC Control APU > 95% to 100 % RPM SPEED and APU Bleed Air Operation to full load condition. With the APU Bleed Valve open the APU EGT reach the EGT schedule. A signal output energizes the torque motor of the proportional control valve. Allowing the proportional valve (PCV) to open. This causes a decrease of the control pressure and the APU Bleed Valve moves to the more close position. ( APU Overload Protection). This enables the EGT to decrease until the EGT schedule is reached. The proportional valve closes. The APU Bleed Valve opens again. Main Engine Start ( MES ) When the engine start selector is placed to GND, the ETC allows the increasing temperature schedule when the APU runs with > 95% RPM. This schedule is limited to 690 °C APU EGT. Overtemperature Schedule The overtemperature schedule is activated when the APU EGT exceeds 690 °C, the ETC give a signal output energizes the torque motor of the proportional control valve. Allowing the proportional valve ( PCV ) to open. This causes a decrease of the control pressure and the APU Bleed Valve moves to the close position. This enables the APU EGT to decrease until the EGT schedule is reached. T5 Thermocouples Loss ( APU Starting ) When the APU speed is < 95% RPM (starting) The ETC give a signal output energizes the torque motor of the proportional control valve, allowing the proportional valve ( PCV ) to open. The APU will have a hung start.




Figure 39 – ETC




T5 Thermocouples Loss ( APU > 95% RPM ) When the APU speed is > 95% RPM .The ETC give no signal output energizes the torque motor of the proportional control valve. Allowing the proportional valve ( PCV ) to close. The APU will have no overload protection with APU bleed valve operation. When the APU exceeds 690° C the overtemperature schedule is activated and the APU Bleed Valve moves to the close position. This enables the APU EGT to decrease. Simultaneously the EGT indicator will display zero.

APU Control Unit (M280) is used for starting, operation and shut down. The unit contains circuit cards and relays. On the front of the unit is a circuit breaker and an overspeed reset switch. On Control Unit with the TEST PASS and TEST FAIL lights there is an overspeed RESET/TEST SW on the face of the APU Control unit to do the BITE internal check for the problems. Compressor Inlet Temperature Sensor (T2) –gives the compressor inlet temperature signal to the ETC. It is resistive sensor and he sends signal to ETC when the speed of APU is greater then 95%. This signal is used by ETC to control the EGT during acceleration and loaded conditions.




Figure 40 – APU Control Unit




Electronic Temperature Control (ETC) Adjustment. •Open the aft cargo compartment. Remove the ETC cover. •Disconnect the electrical connector from the T2 Sensor from the APU compressor inlet. Adjust the ETC. •Start and operate the APU at a stable speed without a load applied. •Set the APU BLEED switch on the P5 forward overhead panel to the ON position. •Set the ISOLATION VALVE switch on the P5 forward overhead panel to the OPEN position. •Set the Left PACK and Right PACK switch on the P5 forward overhead panel to the AUTO position. •Stop for 2 minutes. •Set the Left PACK and Right PACK switch on the P5 forward overhead panel to the HIGH position •Add a 100 amp electrical load to the APU generator. •Monitor the APU EGT to make sure it is at the operators specific temperature between 566 °C and 649 °C. •If the APU is not at the correct temperature, adjust the ETC : • Remove the ETC adjustment nut from the ETC. •Turn the ETC adjustment screw clockwise to decrease the EGT. • Turn the ETC adjustment screw counterclockwise to increase the EGT. • NOTE: One full turn of the adjustment screw will change the EGT 3 °C. The adjustment screw is a 25 turn adjustment screw without stops at the minimum and maximum positions. The adjustment screw will continue to turn after 25 turns but the EGT will not change.




Figure 41 – APU ETC




49-60 APU CONTROLS – GENERAL

The APU engine controls are for these functions: - Automatic sequencing of the start cycle - Normal operation - Shut-down protection. The speed switch is in the aft section of the passenger cabin in the ceiling. The APU control unit is in the electronic equipment compartment. The tachometer generator is in the APU accessory section. The electronic temperature control (ETC) is in the aft cargo compartment. The controls consist of these components: - Manual start switch P5 panel - Speed sensing electronic circuits - Input from tachometer generator - Automatic control and protection circuits. The APU engine controls automatically starts these functions: - Fuel injection - Ignition - Termination of the starter operation - Termination of ignition unit operation. The controls allow the use of the generator and bleed air during normal operation and protects the APU against faults.

APU Flight Compartment Controls These are the APU control switches in the flight compartment: - APU switch (P5) - APU fire warning switch (P8) - Battery switch (P5). APU Ground Control Panel (P28) The APU fire control panel P28 is in the right main wheel well aft bulkhead.

Figure 42 – APU Controls




APU CONTROL UNIT

The APU control unit is used for: - Starting - Operation - Shutdown of the APU. The unit contains printed circuit cards. These items are on the front of the unit: - Circuit breaker - Over speed reset button - Test button - Test lights (2). The power for the operation of the circuits is 28 V DC. The control unit is on the E3-3 shelf in the electronic equipment compartment. Test Operation To do a test of the APU control unit, the APU master switch must be in the OFF position. Push the over speed RESET/TEST button and release. The unit does a self test. Twenty seconds later, a red test fail light or a green test pass light comes on. If the red light comes on, remove and replace the unit. It takes approximately 30 seconds for either light to go out after the test is complete.

Figure 43 – APU Control Unit




49-70 INDICATING SYSTEM The indicating system provides means to monitor the operation of the APU engine. The EGT indicator is on the P5 panel. The EGT thermocouple and elapsed time indicator are on the APU. The indicator system consists of an exhaust gas temperature and elapsed time indicator. During APU operation, the thermocouple measures the exhaust gas temperature. It changes to an electrical signal and goes to the electronic temperature control unit. The electronic temperature control unit changes it to a temperature indication on the P5 panel. The elapsed time indicator records time in hours of APU operation.




Figure 44 – APU Tachometer Generator




TACHOMETER GENERATOR The tachometer generator provides APU engine speed (frequency) input to the speed switch. As the APU engine accelerates during the start sequence, the output produced by the generator is proportional to the speed of the APU engine. This frequency is supplied to the speed switch. Electronic speed switch The tachometer generator sends the signal to the electronic speed switch for the APU engine speed. The speed switch controls the starting cycle sequence, normal operation and provides overspeed protection for the APU engine. The speed switch send the 50% starter cutout signal, 95% signal and the 110% overspeed signal for APU monitoring and protection to the APU control unit.

The switch is located in the aft passenger compartment in the lowered ceiling area.




SPEED SWITCH

The speed switch controls these functions: - Starting cycle sequence - Normal operation - Over speed protection. The speed switch is in the aft passenger cabin compartment in the ceiling. The speed switch is in a hermetically sealed case. The case contains solid state electronic circuits and relays.

Figure 45 – APU Speed Switch




EGT ( AC WITH ETC) Thermocouple The thermocouple is four single bead junctions consisting of chromel and alumel in cylindrical shields grouped together in a single unit. Junctions 1 and 3, and 2 and 4 are electrically paired. Balancing Resistor The balancing resistor permits adjustment of the circuit resistance. The resistance consists of a spool of constantan wire, coarse and vernier adjustment terminals and a spare resistance spool. Indicator The indicator is a mill-voltmeter, driven by an output from the ETC, with integral lights in a hermetically sealed case. The dial is calibrated from 0°C to 850°C with green, orange and red operating bands. Power Power supply (0−5 volt dc), is only for the indicator integral illumination

EGT INDICATION (A/C WITH ETC)

The thermocouple senses the temperature at four places across the APU exhaust pipe. Pairs of thermocouple junctions are wired to the electronic temperature control unit. In this unit the highest temperature signal is used for the control and indication functions. The APU exhaust temperature indicator operates from 0°C to 850°C temperature display only when the electronic temperature control unit is powered. This occurs during APU start after inlet door opening causes the K7 relay to close. When a running APU is shut down, the exhaust temperature indication rapidly decreases to 0°C as power is removed from the electronic temperature control unit. This occurs, during APU shutdown, after any kind of shutdown signal enters the APU control unit, or the inlet door moving from the fully open position to closed. Maintenance Practices Circuit Resistance is set to a specific value by selecting the coarse and fine resistors of the resistance unit. The EGT resistor assembly and constantan (CN) wire provides interchangeability with another APU configuration




Figure 47 – APU EGT Indicator




ELECTRONIC TEMPERATURE CONTROL The electronic temperature control (ETC) monitors and controls the exhaust gas temperature (EGT) of the APU. The ETC receives two thermocouple (TC) signals which it uses for system logic. The ETC also receives an inlet air temperature signal (T2) which it uses to modify temperature schedules to find proper APU performance. The EGT (T5) and T2 signals are then used to control EGT during acceleration and load conditions. The ETC bleeds air from the acceleration limiter valve (ALV) through the proportional control valve (PCV) to provide acceleration EGT control. The ETC controls PCV torque motor (T/M) current to maintain a constant EGT during acceleration. After the APU is more than 95%, the solenoid selector valve changes from the ALV to the bleed air valve (BAV). The ETC then controls EGT by control of PCV T/M current to let control air bleed from the BAV actuator to decrease the bleed load on the APU. This decreases EGT. The ETC on-speed control logic provides for two operational modes, environmental control system (ECS)and main engine start (MES). In the ECS mode, the ETC allows small EGT changes for operation of the ECS. In the MES mode, the ETC allows larger EGT changes for aircraft engine start. The T2 sensor is a resistive transmitter in the compressor inlet to provide inlet temperatures to find changes to the ETC temperature control schedule. The signal is used only above 95%. If T2 is not valid, the ETC uses an alternate T5 value of 650°C.

The EGT T/C assembly sends the ETC two EGT signals. The ETC compares the two signals and uses the highest value for EGT control. The EGT probe assembly junction are in parallel, so that loss of a set of TC junctions will not affect operations. Loss of both EGT signals to the ETC causes the ETC to clamp the PCV T/M driver. This lets the PCV open and bleed control air from the ALV or BAV actuator. The PCV is a normally open, electrically closed valve. The ETC decreases the control signal from the PCV T/M driver to let the PCV open and bleed control air from the system if EGT is more than the control limits. The ETC has an external adjustment to decrease the operational set-point of the ECS logic. The set-point controls the EGT when the ETC causes the PCV to bleed control air from the BAV actuator. Total decrease is 83°C (150°F) from the maximum of 649°C (1200°F). Adjustment is by an adjustment screw on the ETC case. NOTE: If the PCV does not close as commanded or if it leaks, slow or hung starts could occur. If it does not open, EGT over temperature could occur. If both sets of EGT junctions fail on the EGT probe assembly during APU start, a hung start may occur. If they fail during ECS or MES modes, the BAV closes and APU bleed air will not be available.




Figure 47 – APU EGT Control




49-81 EXHAUST GAS TEMP. IND. ( AC WITHOUT ETC) The APU engine exhaust gas temperature is used to monitor the operation of the APU. Thermocouple The thermocouple is a temperature sensing device consisting of a chrome and alumel junction in a cylindrical shield. The alumel stud terminal (−) is longer than the chromel stud terminal (+). Balancing Resistor The balancing resistor permits adjustment of the circuit resistance. The resistor consists of a spool of No. 24 constantan wire, coarse and vernier adjustment terminals and a spare resistance spool. Indicator The indicator is a sensitive mill voltmeter with integral lights in a hermetically sealed case. The dial is calibrated from 0°C to 850°C with green, orange and red operating bands. Power Power supply (0−5 volt dc), is only for the indicator integral illumination.

EGT IND. DESCRIPTION (A/C WITHOUT ETC) During APU operation, heat is transferred from the exhaust gas to the thermocouple. Under the influence of heat, the thermocouple generates electrical power proportional to the difference in temperature between the APU exhaust and the ambient temperature at the indicator. The indicator converts the electrical signals to true APU exhaust gas temperature and compensates for changes in ambient temperature at the indicator. Circuit Resistance Adjustment The EGT circuit resistance without the indicator should be 13.50 +0.05 ohms at 20 +5°C (68 +9°F). Circuit resistance can be adjusted by using the resistance spool (decreasing or increasing wire length) and changing connections on the coarse adjustment (A1−A5, 0.15 ohms each) and vernier adjustment (B1−B3, 0.05 ohms each).




Figure 58 – APU EGT/ Engine Without ETC




Figure 46 – APU EGT




ELAPSED TIME INDICATOR The elapsed time indicator records the hours of APU operation. The indicator is an electric clock in a hermetically sealed case. The total operating time is indicated, by a digital counter, in hours and tenths of an hour. A circuit breaker is near the indicator for surge current protection. The indicator uses 28 V DC power and is activated by the 95% switch.

Figure 49 – APU Elapsed Time Indicator




49-40 APU START SEQUENCE Prior to starting the APU, the battery switch is placed to the ON position. This allows battery bus power to the control circuits and master switch. Placing the master switch momentarily to START and releasing it to the ON position, the start sequence is initiated. The fuel shutoff valve opens fully, followed by the air inlet door. ( At the same time the APU fuel boost pump is energized ). When the door is fully open, the battery (or external DC if used) is connected to the starter and APU begins to rotate. At approximately 10% RPM, the oil pressure increases to 4 psig the sequencing oil pressure switch closes, sequencing power to ignition exciter unit and the fuel solenoid valve. The APU accelerates, being assisted by the starter. At 50% RPM, the speed switch operates and the power to starter is disconnected. (The starter operation is limited by the time limiting circuit.) The oil pressure now reaches 55 psig (at approximately 35% RPM) allowing the low oil pressure switch to open, extinguishing amber LOW OIL PRESSURE light on P5 forward overhead panel. At 95% RPM, the speed switch operates electrical power is removed from the ignition system, the hour meter starts, the 3−way solenoid valve is energized and APU is ready to supply electrical and pneumatic power.




Figure 50 – APU Start Sequence




APU NORMAL SHUTDOWN

•The APU is normally shut down by placing the APU switch to the OFF position. This simulates overspeed, thus checking the overspeed circuits but the OVERSPEED light does not illuminating. •The fuel solenoid valve closes. As the APU slows down the 110% RPM switch relaxes, but the fuel solenoid valve remains de−energized. •If during the normal shutdown, relay K1 trip does not energize, the APU still shuts down by closing the fuel shutoff valve. This failure of K1 is indicated by the OVERSPEED light being illuminated.




Figure 51 – APU Shutdown Sequence




APU AUTO SHUTDOWN Overspeed Shutdown When true 110% RPM is sensed by the speed switch, the trip relay energizes and latches. •the fuel solenoid valve closes and APU shuts down. As APU slows down, 110% RPM switch relaxes, but fuel solenoid valve remains de−energized. •The OVERSPEED, MASTER CAUTION and APU annunciator lights illuminate. •The light illuminates and can be reset by a switch on the M280 APU control unit. •When APU has been shut down by the overspeed circuit, it cannot be re-started without resetting the overspeed control circuit. Low Oil Pressure Shutdown During APU operation, when the oil pressure decreases to 45 psig, •the low oil pressure switch closes, •the fuel solenoid valve closes and APU shuts down. A 10 second time delay eliminates transient variations in oil pressure due to flight maneuvers. •The LOW OIL PRESSURE, MASTER CAUTION and APU annunciator lights illuminate. The lights are reset by placing APU switch to OFF. High Oil Temperature Shutdown During APU operation, when the oil temperature reaches 140°C (285°F), •the temperature switch closes, •the fuel solenoid valve closes and APU shuts down. The HIGH OIL TEMPERATURE, MASTER CAUTION and APU annunciator lights illuminate and remain illuminated even if oil temperature decreases to below 140°C (285°F). The lights are reset by placing APU switch to the OFF position.

Fire Shutdown In the event of fire detection, or actuation of either fire handle, the trip relay energizes, •the fuel solenoid valve de −energizes and the APU shuts down. •The fuel shutoff valve and air inlet door closes. Battery Switch Shutdown •Operation of the APU requires that the battery switch must be ON. Placing the battery switch to OFF causes a power loss of the fuel solenoid valve and APU shut down. The APU fuel shutoff valve does not close, as this is controlled by the APU switch. In the air, placing the battery switch to OFF does not shut down the APU




Figure 52 – APU Auto Shutdown Sequence




Acceleration Limiting Valve Opening ( Crack ) Pressure Test. Procedure: • Start the APU and let the APU operate at a stable speed with no load applied for 5 minutes. NOTE: This will make sure fuel is available to the APU. •Do the usual shutdown. (AMM 49-11-00/201 ) •Open the APU FUEL BOOST circuit breaker on the P6-5 load control center. •Attach a test pressure gage to the high pressure fuel line from the fuel atomizer. Remove the cap from the control air pressure port of the FCU. The control air pressure (compressor discharge pressure ) must vented at the control unit to eliminate the influence of air pressure on control spring force. The APU must then motored and the cracking pressure should be 60 +/- 3 psig. •Observe starter duty cycles. ( AMM 49-11-00/201 ) Adjust to read correct pressure using the screw on the acceleration limiter valve. Clockwise rotation increases the crack pressure. Counterclockwise rotation decreases the crack pressure. CLOSE the APU FUEL BOOST circuit breaker on the P6-5 load control center. Governor Speed Check Operate APU is with the APU bleed air valve closed and no electrical load. The frequency meter should read 405 to 410 Hz. Adjust to read correct frequency by the governor adjustment screw. Clockwise rotation increases the speed. Counterclockwise rotation decreases the speed.

The exhaust gas temperature (EGT) is controlled by the APU control thermostat. The APU control thermostat has two functions: (1) The APU control thermostat is an acceleration control thermostat at an APU speed up to 95%. When the EGT is higher than 620-649°C (1150-1200°F), the APU control thermostat opens to bleed control air from the acceleration limiter valve. This permits the bypass valve of the fuel control unit to send fuel back to the inlet fuel port and will decrease the EGT. When the EGT decreases below the specified limit, the APU control thermostat closes. When the APU control thermostat closes, the bleed air from the acceleration limiter valve stops and the bypass valve closes. (2) When the APU speed is more than 95%, the 3-way solenoid valve is energized closed and the APU control thermostat works as a load control thermostat. The APU control thermostat operates as an overload protection for the APU. For a more detailed description of this function of the APU control thermostat, refer to 49-52-00/001. The fuel atomizer sprays the atomized fuel into the combustion chamber in a constant pattern. The constant pattern is not changed by different fuel pressures. Low pressure fuel flows to the small orifice plate for full atomization. When the fuel pressure increases, the flow divider valve opens and permits the fuel to flow to the two orifices plates. This will correctly atomize the high-pressure fuel. When the APU speed is 100%, the governor controls the fuel flow to keep the APU engine speed constant for different load conditions. When the APU speed is more than 100%, the governor flyweights move outward to open the bypass valve. This will decrease the fuel supply to the fuel atomizer. When the APU engine speed decreases too much, the governor flyweights move inward to close the bypass valve. This will increase the fuel flow to the fuel atomizer and let the APU speed increase. When the APU master switch is moved to the OFF position, electrical power is supplied to the overspeed oscillator of the speed switch. The 110% contact in the speed switch closes. This gives a signal that is the same as an overspeed condition. Electrical power is removed from the fuel solenoid valve and the APU fuel valve. The flow of fuel to the fuel atomizer is stopped. The APU slows down and stops. All the unwanted fuel in the combustion chamber drains through the fuel drain.

Ata 49 - Apu Garrett-r2

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