Auxiliary Power Units (15.18) PDF

Auxiliary Power Units (15.18) Learning Objectives 15.18.1 Summarise the purpose and the operational bene ts of an aircraft having an APU (Level 2). 15.18.2 Describe key APU installation features, sub-systems and components and identify how each component contributes to the operation o...

Auxiliary Power Units (15.18) Learning Objectives 15.18.1 Summarise the purpose and the operational bene ts of an aircraft having an APU (Level 2). 15.18.2 Describe key APU installation features, sub-systems and components and identify how each component contributes to the operation of the APU (Level 2). 15.18.3 Describe the purpose, con guration and operation of APU protective systems (Level 2). 2022-08-24 B1-15c Gas Turbine Engine Page 7 of 104 CASA Part 66 - Training Materials Only  Auxiliary Power Units (APUs) Introduction to Auxiliary Power Units Turbine-powered transport aircraft require large amounts of power for starting and operation. For example, substantial electrical power is sometimes needed for passenger amenities such as lighting, entertainment and food preparation. In addition, engine starting and ground air-conditioning require a high-volume pneumatic air source that frequently is not available at remote airports. To meet these demands for ground power when the aircraft engines are not running, most large turbine aircraft are equipped with auxiliary power units, or APUs. Common APU location A typical APU consists of a small turbine power plant driving an electric generator identical to those mounted on the aircraft’s engines. In addition, an APU’s compressor or a separate load compressor supplies bleed air to the aircraft’s pneumatic system for heating, cooling, anti-ice and engine starting. As with any other gas turbine engine, bleed air loads generally place the greatest demand on an APU. A typical small APU uses a single centrifugal compressor to supply air for combustion and bleed air. However, axial ow and centri-axial ow con gurations are also used in larger units. 2022-08-24 B1-15c Gas Turbine Engine Page 8 of 104 CASA Part 66 - Training Materials Only  APU drives an AC generator to provide electrical power An APU is typically started using its own electric starter motor and aircraft battery power. With fuel supplied from one of the aircraft’s main fuel tanks, an APU can start, provide electric power, heat or cool the cabin, and start the main engines without the aid of any ground or portable power source. APUs are designed as constant-speed engines with no manual control. Once started, APUs run at a rated speed regardless of the electrical or pneumatic load demands. They have their own automatic control and safety devices and their own re protection system. 2022-08-24 B1-15c Gas Turbine Engine Page 9 of 104 CASA Part 66 - Training Materials Only  Typical APU tail installation Auxiliary Power Unit (APU) 2022-08-24 B1-15c Gas Turbine Engine Page 10 of 104 CASA Part 66 - Training Materials Only  Single compressor and single turbine Fuel ow is adjusted depending on demand to keep the APU running at constant speed. A typical system utilising mechanical and pneumatic control is shown. However, modern APU control systems are electronic. 2022-08-24 B1-15c Gas Turbine Engine Page 11 of 104 CASA Part 66 - Training Materials Only  APU control systems Fuel for APU operation is drawn from aircraft fuel tanks. Typically, a fuel pump is dedicated to supplying the APU. Fuel pump capacity exceeds the ow required for APU operation, so it is necessary to bypass some of the fuel. Once APU start is initiated, a fuel solenoid opens when oil pressure is sensed and at an engine rpm determined by an electronic control unit. 2022-08-24 B1-15c Gas Turbine Engine Page 12 of 104 CASA Part 66 - Training Materials Only  APU Control Functions Automatic control of the APU governs two functions: Engine rpm Exhaust Gas Temperature (EGT). The fuel control unit (FCU) governor is a series of yweights held in place by a spring. The position of the yweights determines the position of the fuel metering valve, shown at the bottom of the governor unit. The governor controls the scheduling of fuel from 95-100% RPM. The position of the valve determines how much fuel bypasses out of the governor back to the pump inlet and how much fuel ows through the solenoid to the APU combustor. As rpm increases, the yweights y outwards under centrifugal force. As they do this, the linkages are forced against the spring and the valve opens. In this way, the yweight governor controls and limits the rpm and progressively schedules fuel when the APU is starting. When the APU is running on-speed, the valve position ensures equilibrium between fuel owing into the FCU and fuel diverted back to the fuel pump inlet, maintaining rpm. The acceleration limiter functions as a pressure-regulating valve whose setting is varied by the Compressor Discharge Pressure (CDP). CDP and spring pressure hold the governor fuel bypass valve closed. The acceleration limiter controls the fuel ow to ensure the correct quantity to accelerate the engine. As the engine accelerates, the fuel ow is constantly increasing. After the engine lights off during the start sequence, CDP then varies with the air ow, rpm and ambient conditions and gives a reasonable indication of the amount of fuel necessary to manage the acceleration.The acceleration limiter controls the scheduling of fuel from 0-95% RPM. APU fuel control unit 2022-08-24 B1-15c Gas Turbine Engine Page 13 of 104 CASA Part 66 - Training Materials Only  APU Over-Temperature Protection System EGT is limited by a ground-adjustable pneumatic thermostat valve positioned in the exhaust duct. If the exhaust temperature exceeds a certain value, the thermostat valve opens, bleeding off some pressurised air from the acceleration limiter. This opens the acceleration limiter, with the result that the fuel ow to the APU decreases and lowers the EGT. The thermostat is a temperature-actuated air valve normally closed and connected pneumatically to the acceleration limiter. Pneumatic thermostat valve 2022-08-24 B1-15c Gas Turbine Engine Page 14 of 104 CASA Part 66 - Training Materials Only  APU Control Units There are two broad types of APU Control Units: Electronic speed-sensing unit Electronic Control Box or Unit (ECB). Electronic speed-sensing units provide start sequencing, fault monitoring and protection while a conventional hydro-pneumatic fuel control governs the loaded and unloaded engine EGT and rpm. Full Authority Digital Engine Control (FADEC) uses an ECB which automatically controls all functions, including speed governing. Engine Control Box (ECB) 2022-08-24 B1-15c Gas Turbine Engine Page 15 of 104 CASA Part 66 - Training Materials Only  Electronic Speed-Sensing Unit The electronic speed-sensing unit controls and monitors the following sequence: Regulates starter motor operation and cut-off Opens the APU fuel re shut-off valve Opens the fuel solenoid valve to start Energises and de-energises the ignition cycle Opens the surge bleed valve Records start cycles or runtime Arms the bleed air system. APU on-speed governing is controlled by the hydro-pneumatic fuel control unit. APU EGT is controlled by the pneumatic thermostat valve. 2022-08-24 B1-15c Gas Turbine Engine Page 16 of 104 CASA Part 66 - Training Materials Only  Electronic Control Box Modern APU controllers are commonly referred to as the auxiliary power unit’s Electronic Control Box, or ECB. The ECB is a full-authority, microprocessor-based digital controller. Once a start is initiated, all further control of the APU is fully automatic and is provided by the ECB. Speci cally, the ECB controls the engine starting sequence, acceleration, governed speed operation, operation within temperature limits, inlet guide vane position, surge valve position, and both normal and protective shutdown sequences. In addition to controlling the APU, the ECB has an integral BITE (Built-In Test Equipment) capability speci cally incorporated to shorten diagnostic operations involved in troubleshooting and corrective maintenance. The ECB continuously monitors and stores shutdown fault information along with failures of certain Line Replaceable Units (LRUs). Fault information is stored in the box for later interrogation, and the box also provides cockpit fault display and diagnostic information from a central maintenance computer interface. ECAM APU screen 2022-08-24 B1-15c Gas Turbine Engine Page 17 of 104 CASA Part 66 - Training Materials Only  Typical Operation When the APU master switch is selected to ON, power is supplied to the electronic speed-sensing unit (APU control box), an indicator ‘APU’ light illuminates and the APU air intake door opens. When the intake door is fully open, power is supplied to the start circuit. After the START button is momentarily pressed or the start switch is moved, the start control relay is energised and remains energised. The electronic speed-sensing unit controls and monitors the following sequence: A light in the START button comes on and the APU starter motor cranks the APU. The APU fuel re shut-off valve opens. The fuel solenoid valve opens at 10% rpm. Ignition is energised. The anti-surge bleed valve opens. At 45%, the starter is de-energised. At 95%, ignition is de-energised. The hour meter starts recording APU runtime. The bleed air system is armed. Automatic protection circuits are installed to shut down the APU in case of re, low oil pressure or overspeed. In case of re, the re shut-off relay changes over, interrupting the power to the main relay. The result is the control circuit de-energises, the fuel valves close and the APU stops. The air intake door also closes, and automatic re extinguishing takes place. In case of low oil pressure above 45% speed, the oil pressure switch signals the electronic speed- sensing unit to change over and de-energises the turbine hold relay. The result is both fuel valves close and the APU stops. In case of an overspeed, the overspeed circuit closes the fuel valves and stops the APU. 2022-08-24 B1-15c Gas Turbine Engine Page 18 of 104 CASA Part 66 - Training Materials Only  Typical APU control Producing Bleed Air There are two basic methods of producing compressed (bleed) air from an APU: Bleed air is taken from the APU compressor (left image below). Bleed air is taken from a separate load compressor (right image below). The load compressor is a separate compressor used only for generating bleed air for the aircraft systems. It is typically directly coupled on a common shaft to the gas generator compressor. Bleed air 2022-08-24 B1-15c Gas Turbine Engine Page 19 of 104 CASA Part 66 - Training Materials Only  There are two bleed air valves: Load control valve Surge bleed valve. The load control valve is a Pressure-Regulating and Shut-Off Valve. It controls the passage and pressure of compressed air to the aircraft pneumatic system. The surge bleed valve (sometimes also called a surge control valve) protects the APU compressor from stalls and surges that may occur as a result of bleed air being taken off the compressor. With the APU running, these two bleed valve positions oppose one another. That is, when one valve is open, the other valve is closed. Load control valve - open (LHS) - closed (RHS) When the ight deck APU bleed air switch is in the OFF position, the load control valve is closed and the surge bleed valve is open. Switching within the load control valve signals the surge bleed valve. If the load control valve fails to open when bleed air is selected, the surge valve remains open. The surge bleed valve is open when bleed air is not selected ON to ‘relieve’ the compressor, reducing the chance of compressor surge and reducing EGT. The surge bleed valve also of oads the APU compressor during APU start. As soon as APU bleed air is selected ON, the surge bleed valve closes. Typically, the load control valve is designed to fail in the CLOSED position. This is termed ‘fail-safe closed’. Load Control Valve With the APU running at or near 100% (on-speed condition), the bleed air system is now armed and available. Most cockpits have a bleed air switch for all sources of pneumatic power, i.e. engines, APU and external ground pneumatic power. Placing the APU bleed air switch in the cockpit to ON opens the APU load control valve and closes the surge bleed valve, if tted. 2022-08-24 B1-15c Gas Turbine Engine Page 20 of 104 CASA Part 66 - Training Materials Only  When the APU bleed is selected ON, the solenoid valve energises and retracts the ball assembly and allows compressor discharge pressure (CDP; red) to act on the butter y actuator. This moves the butter y valve towards OPEN. Whenever the APU is running, air pressure is acting on the rate control diaphragm (purple), and when the valve starts to open, this pressure decreases as air ows through the aircraft ducting. This has the effect of allowing the poppet valve to raise and allow some CDP to escape. Now there is less pressure acting on top of the butter y actuator, slowing its opening speed. The speed with which the bleed air pressure dissipates from the rate control diaphragm is governed by the rate adjustment screw, and this controls the speed at which the butter y opens. When bleed air is selected ON, the pneumatic thermostat control valve closes, which switches the pneumatic thermostat connection to the butter y actuator valve (previously it was connected to the acceleration limiter for start). The pneumatic thermostat valve ensures that EGT is held within limits when air is bled off the compressor. If a high EGT is sensed, the pneumatic thermostat opens and relieves control pressure from the butter y actuator. This moves the valve towards CLOSED, reducing the output of bleed air, thus lowering EGT. Some systems have separate thermostats for fuel acceleration limiting and bleed air. The bleed air function perform by an APU is to supply compressed air for air conditioning and engine starting. But it also provides bleed air for other cabin services, hydraulic system reservoir pressurising and anti-icing. Load control valve operation 2022-08-24 B1-15c Gas Turbine Engine Page 21 of 104 CASA Part 66 - Training Materials Only  An APU operating in harsh conditions, with degraded performance or under high load demand, may experience dif culty supplying both electrical power and bleed air loads within a normal EGT range. An example of this may be supplying electrical power for the whole aircraft and also pneumatic power for engine start while on the ground with hot outside air temperatures. In these cases, APU electrical loads usually have priority. They are controlled electronically and sometimes load shedding is built into the system. For example, galley power (power to ovens and coffee/tea makers) are tripped off when ENGINE START is selected. 2022-08-24 B1-15c Gas Turbine Engine Page 22 of 104 CASA Part 66 - Training Materials Only  Load Compressor In more complex APUs, the power section drives a separate single-stage compressor and the accessory gearbox. This additional compressor is dedicated to supplying bleed air and is called the load compressor. The load compressor is supplied with air from the same air inlet as the power section compressor (gas generator). The supply of compressed air to the aircraft pneumatic system is controlled by a bleed air load control valve. In contrast to the previous description of the load control valve as a pressure- regulating valve, this valve is simply an open/closed shut-off valve. The energy required to drive the load compressor is obtained from the turbine. The fuel ow and EGT increase as the bleed air load on the load compressor increases. When bleed air is not required, simply closing the bleed air load control valve does not unload the load compressor ef ciently, and the load compressor continues to extract power from the turbine. To alleviate the demand on the turbine, the load compressor is often equipped with variable inlet guide vanes (VIGVs). When aircraft bleed demand is low or when the bleed air load control valve is closed, the ECB signals the VIGV actuator to close the VIGVs. This of oads the load compressor. To prevent the load compressor from stalling at minimum air ows, the ECB controls the surge control valve to dump the unwanted bleed air overboard. Current versions of ECB-controlled APUs incorporate the load control valve and surge valve into one valve known as a Bleed Control Valve, which is fully controlled by the ECB. The ability to match bleed air output to aircraft demand reduces fuel ow, EGT, wear and tear on the APU hardware, and operating costs. 2022-08-24 B1-15c Gas Turbine Engine Page 23 of 104 CASA Part 66 - Training Materials Only  APU ECB engine bleed control valve APU control valves 2022-08-24 B1-15c Gas Turbine Engine Page 24 of 104 CASA Part 66 - Training Materials Only  The VIGVs are controlled by the ECB, which matches the load compressor output to the demand. The ECB modulates the VIGVs between open and closed, and also adjusts the fuel ow to maintain optimum EGT. The ECB also governs the APU protection modes, holds faults in memory, allows Built-In Test (BITE) testing and stores operating data. APU Cooling To cool the APU, ambient or ram air enters the APU intake plenum area, where a portion is tapped as cooling air for the APU compartment, oil cooler and generator. Most large APUs use a cooling fan to induce cooling air ow and increase the cooling air pressure. Typically, the cooling fan is driven by the APU gear train. The inlet to the cooling fan is ducted from the APU inlet plenum or a dedicated outside air intake via a motor-driven inlet door. APU cooling 2022-08-24 B1-15c Gas Turbine Engine Page 25 of 104 CASA Part 66 - Training Materials Only  APU Fuel System The APU fuel system provides the APU with the proper fuel scheduling for all phases of operation. A typical system consists of the aircraft fuel supply that includes the: APU boost pump APU fuel shut-off valve Fuel supply line. The APU fuel system supplies fuel from the aircraft to the APU fuel pump and lter. The FCU receives pressurised fuel and meters it to fuel nozzles through a ow divider and fuel manifolds. On FADEC APUs, the fuel is also used in the APU air control system as hydraulic servo power for the Inlet Guide Vane (IGV) actuator. Fuel is also used to cool the APU oil. This fuel, in turn, is heated and returned to the fuel pump and lter assembly to prevent ice from restricting ow through the lter. APU fuel system 2022-08-24 B1-15c Gas Turbine Engine Page 26 of 104 CASA Part 66 - Training Materials Only  Fuel Line Shrouds In the most common APU con guration, the APU fuel system receives fuel from the aircraft wing tanks through a shrouded line. Where the fuel supply lines to the engines and APU pass the pressurised aft cargo compartment and pressurised cabin, a shroud is installed to prevent fuel vapour from entering the cabin. In case of a leaking fuel supply line, the shrouds ensure the fuel is drained overboard via the overboard drain line and a drain port at the lower end of the wing fairing. This port is also used for the drain valve in the engine fuel supply line. APU fuel shroud 2022-08-24 B1-15c Gas Turbine Engine Page 27 of 104 CASA Part 66 - Training Materials Only  Starting the APU Before starting any APU, check the integrity of the re-detection system by performing an APU re test. Once the APU start sequence has been initiated, acceleration and on-speed control are fully automatic in APUs with FADEC/ECB. However, it is still good practice to monitor APU EGT during the start. Depending on the aircraft, sometimes this is not possible if the aircraft is on battery power. A typical start procedure follows: Turn battery switch to ON. Turn APU master switch to ON. Wait for the APU inlet door to open. Turn APU start switch to START or press button. The starter motor is engaged and the APU cranks. The ignition circuit is energised at about 10% rpm and fuel is introduced. Check for light off and acceleration. The starter cuts out at 45%–50%. Check for acceleration and watch EGT rise. The ignition circuit disengages at approximately 95%. Continue to monitor acceleration and EGT. EGT should stabilise. Depending on the model, APU is on-speed at 100% ±1%. When it is on-speed, the bleed air system is armed. Some models quote on-speed rpm according to the load, e.g. 100% for generator only, 102% for generator and bleed air on. Typical overspeed protection might occur at 105%. 2022-08-24 B1-15c Gas Turbine Engine Page 28 of 104 CASA Part 66 - Training Materials Only  Stopping the APU Manual Shutdown Stopping the APU involves shutting the fuel shut-off valve. Depending on aircraft type, the normal shutdown procedure is pressing the STOP button or selecting the APU push-button or toggle switch to OFF. In many aircraft, the normal shutdown uses the overspeed protection function to stop the APU. The electronic speed-sensing unit or ECB signals the fuel solenoid to close. This proves the integrity of the overspeed protection system. If the normal shutdown does not stop the APU, the cause must be investigated. Stopping the APU 2022-08-24 B1-15c Gas Turbine Engine Page 29 of 104 CASA Part 66 - Training Materials Only  Depending on the aircraft, an APU can also be shut down by any of the following switching procedures: Battery switch – OFF APU master switch – OFF APU control circuit breaker – trip APU re handle – CLOSED External APU stop switch – STOP. B737 APU ground shutdown and warning 2022-08-24 B1-15c Gas Turbine Engine Page 30 of 104 CASA Part 66 - Training Materials Only  Automatic Shutdown To protect the aircraft and APU from damage, the APU automatically shuts down if any of the following occur: Overspeed – actual or test Fire warning – actual or test Low oil pressure High oil temperature Excessive EGT (ECB controlled) Vibrations (ECB controlled). Different aircraft and APU types and models behave differently for an automatic shutdown during an actual re warning and an APU re test process. For example, in later model Boeing and Airbus aircraft, the APU automatically shuts down during an actual re warning only when the aircraft is on the ground. When the aircraft is in the air, the pilot is required to shut the APU down manually during an actual re warning. Automatic shutdown 2022-08-24 B1-15c Gas Turbine Engine Page 31 of 104 CASA Part 66 - Training Materials Only  Ground-Running Precautions The APU is tted in its own reproof compartment with re-detection circuitry and a dedicated re extinguisher. The operator must be aware that running the APU with the cowlings open compromises the re- sensing and extinguishing capabilities. The compartment is constructed to minimise the possibility of re. If during ground tests it is necessary to run the APU with the cowlings open, it must be monitored by trained personnel with appropriate re ghting equipment. Prior to starting any APU, the operator should always conduct an APU re test to check the integrity of the APU re-detecting circuitry and extinguisher. Boeing 737 APU compartment 2022-08-24 B1-15c Gas Turbine Engine Page 32 of 104 CASA Part 66 - Training Materials Only  APU Ground-Run Test An APU ground run is used to verify the normal operation and condition of the APU. It usually involves starting and stabilising the APU, then progressively applying pneumatic and electrical loads. This checks the automatic load control and EGT-limiting functions. In some aircraft, a record of EGT is made with every progressive load applied. These records form part of ongoing trend analysis for the APU to gauge the internal condition of the engine. Some aircraft have no cockpit indication other than an indicator light showing that the APU is on- speed and supplying AC electrical power. Start parameters are automatically monitored by the APU protective circuitry. 2022-08-24 B1-15c Gas Turbine Engine Page 33 of 104 CASA Part 66 - Training Materials Only