TFE 731 Chap 73 (1)

Garret TFE 731 Turbofan Engine (CAT C)

CHAPTER 73

Page 1 of 34 FOR TRAINING PURPOSES ONLY © TFE 731 - ISSUE 2, 2010


CHAPTER 73


INTRODUCTION

0 TABLE OF CONTENTS

1 Fuel System Function 3 2 Modes of Operation 4 3 TFE Fuel System 5 4 Fuel System Module 6 5 Fuel Filter/Bypass 7 6 Fuel Pump 8 7 Motive Flow System 9 8 Fuel Pump with Anti-Ice and Motive Flow 10 9 Fuel Control 11 10 Two Methods of Regulating Flow 12 11 Metering Valve 13 12 P3 vs. RPM 14 13 P3 Controls Valve Position 15

14 Bypass Valve Reacts to P 16

15 Bypass Valve Operation 17 16 Manual Mode 18 17 Manual Mode – Governor Control 19 18 Troubleshooting P3 Signal 20 19 Mechanical Governor 21 20 Mechanical Governor 22 20.1 Normal Mode 22 21 Fuel Control Schematic 23 22 Fuel Shutoff Valve 24 22.1 Cut-off 24 22.2 Above Cut-off 25 22.3 Ultimate Over Speed Solenoid Energised 26 23 Flow Divider 27 24 TFE731 Fuel System Schematic 28 24.1 Manual Mode at Idle 28 24.2 Normal Mode at Idle 29 25 Manifolds 30 26 Fuel Control Rigging 31 27 Adjustment Location 32 28 Fuel System Plumbing 33


CHAPTER 73


TFE731 FUEL SYSTEM

1 FUEL SYSTEM FUNCTION The aircraft fuel system must provide strained fuel at the correct pressure to the engine fuel system inlet. The engine fuel delivery system filters the fuel, heats it as necessary to prevent filter icing (when the optional fuel heater is installed), raises it to high pressure, and delivers it to the engine fuel control system. On some installations, the engine fuel delivery system also provides fuel flow to the airframe injector-motive flow system. The engine fuel control system meters the required amount of fuel to the engine combustor that corresponds to the power lever setting demand and to the atmospheric and engine operating conditions.


CHAPTER 73


2 MODES OF OPERATION There are two modes of engine operation, normal mode and manual mode. In normal mode of operation, the electronic engine control (EEC) provides surge protection, spool speed and temperature limits. The EEC precisely controls fuel to assure surge-free operation within the prescribed parameters. Conversely, in manual mode of operation, the surge valve remains partially open and fuel is controlled by the fuel control. The engine operator must realise that the EEC is not controlling engine parameters, and must observe the instrument indications to insure that speed and temperature limits are not exceeded. Engine acceleration time in manual mode is considerably slower than normal mode. The manual mode of operation can be selected during troubleshooting in order to isolate a problem within the fuel system. Comparing manual and normal modes of operation can often tell the operator whether the problem is in the electronic/electrical system or in the hydro mechanical control system. Dispatch of the aircraft with one engine in manual mode is allowed under some circumstances. Check your specific aircraft flight manual for your individual aircraft procedures. These actions concerning manual mode dispatch will be discussed in detail in the Chapter 76, Electronic Control section of this hand out.


CHAPTER 73


3 TFE FUEL SYSTEM Let us review the components involved in the fuel system and the path fuel takes through these components. Fuel enters the engine driven pump from the aircraft supply where it is filtered and the pressure is increased to provide good fuel atomisation at the fuel nozzles. From the fuel pump, fuel passes through a 200-micron screen into the fuel control. The fuel control will meter fuel to meet the requirements of the engine. When the temperature of the fuel leaving the pump is below 0°C, all fuel that is bypassed by the fuel control is routed through a fuel heater back to the fuel pump to prevent icing of fuel within the filter. From the discharge of the fuel control, fuel passes through a fuel/oil cooler that provides cooling of the oil. From the cooler, fuel flows through the flow divider and into the fuel manifold to supply 12 duplex fuel nozzles.


CHAPTER 73


4 FUEL SYSTEM MODULE The fuel pump and fuel control are mounted to the accessory drive gearbox at the aft left pad. The fuel pump is mounted to the gearbox and the fuel control is mounted to the fuel pump by means of a quick-disconnect V-band clamp. Notice that a coupler shaft joins the fuel control shaft to the fuel pump. These two components form the fuel system module.


CHAPTER 73


5 FUEL FILTER/BYPASS The TFE731 engines utilise a dual element fuel pump capable of providing fuel flows of 5000 pounds per hour (PPH) at 1000 PSIG. A filter bowl housing a 40-micron paper cellulose filter element is integral to the pump. Mounted to the filter housing is a bypass indicating system consisting of either a pin-pop mechanical indicator or an electrical switch. The electrical indicator switch provides a signal to illuminate a warning light in the cockpit. These bypass indicators will indicate an impending fuel filter bypass.


CHAPTER 73


6 FUEL PUMP All pump models contain an integral boost pump (centrifugal), internal relief valve (to limit the discharge pressure), vane-type high pressure pump element, differential pressure indicator (to indicate when the filter is clogged), filter, and filter bypass valve. Some models also contain an anti-ice valve and/or a motive flow lockout valve and pressurising valve. Fuel not required by the engine is returned from the fuel control to the vane pump inlet through internal passages. A boss is provided on the fuel pump for the installation of a temperature probe to measure fuel temperatures at the fuel inlet. Fuel from the aircraft supply will flow into the centrifugal-type boost element where pressures will be increased to 35-43 PSIG and flow to the 40-micron paper cellulose-type filter. The fuel filter has a bypass valve in parallel with the filter element. If the filter element becomes clogged, the valve opens and permits unfiltered fuel to flow to the high pressure pump element. If the filter becomes contaminated, or if ice crystals form causing a differential of 6-8 PSID across the element, the differential pressure indicator will show that condition, either mechanically (a plunger pops out) or by electrical signal to a warning indicator. When differential pressures exceed 9-14 PSID, fuel will be bypassed around the filter. Situated between the low and high pressure pump elements is an anti-ice valve. The fuel passes over the expansion thermostat en route to the vane-type high pressure pump element. If the fuel temperature is below 0°C, the anti-ice valve will direct some bypass return fuel to an optional engine mounted fuel heater. Return flow from the fuel heater mixes with the boost pump discharge, preventing ice formation at the filter, and flows to the high pressure pump inlet. Here fuel pressures are increased to the limit of the high pressure relief valve. System pressures in excess of 1500 ±25 PSIG will be bypassed back to the inlet side of the pump. The use of Jet B or JP-4 fuels with the optional fuel heater can cause fuel vapour formation at the heater when


CHAPTER 73


operating at low flow rates. Additional aircraft fuel pressure of 8 PSI above true vapour pressure is required when using this fuel. Some installations use a fuel heater check valve module that permits operation with JP-4 or Jet B fuels with a minimum inlet fuel pressure of 5 PSI above true vapour pressure. Notice on the schematic that the two check valves would contain any JP-4 or Jet B fuel vapour created by fuel heating at the fuel heater. The true vapour pressures of normal kerosene-type fuels such as Jet A, Jet A-l, JP-5, and JP-8 are low enough so the fuel heater will not produce vapour.

7 MOTIVE FLOW SYSTEM Some aircraft configurations utilise a motive flow system to operate aircraft fuel transfer systems. On models so equipped, a lockout valve is used to prevent fuel from flowing to the motive flow circuit during engine starting. The spring-loaded lockout valve senses pump discharge flow through an orifice. At approximately 40% N2, the lockout valve opens to allow fuel to the motive flow pressure regulator. This valve is referenced to interstage pressure and provides regulated motive flow to 250-350 PSIG to the aircraft injector boost pumps. Consult your specific aircraft maintenance manual to determine your specific application and pressures.


CHAPTER 73


8 FUEL PUMP WITH ANTI-ICE AND MOTIVE FLOW A complete fuel pump with motive flow and anti-icing is depicted here. The fuel enters the boost pump at the far left of this schematic, flows through the filter to the anti-ice valve, and to the high pressure pump. From the high pressure pump, fuel flows to the 1500 PSI relief valve. When the motive flow lockout valve opens, as previously mentioned, fuel flows to the motive flow pressure regulator to provide fuel to the aircraft injector boost pumps.


CHAPTER 73


9 FUEL CONTROL The fuel control contains the fuel-metering section, power-lever input, shutoff valve, outlet pressurising valve, and a mechanical governor. The governor section provides manual control when the electronic control is de-energised and functions as an over speed governor for the N2 spool. During normal operation, the electronic control is scheduling fuel to the engine by an electrical input to the fuel metering section of the fuel control unit. The inlet from the pump is shown in the upper left-hand corner of the control drawing as the "inlet filter". This filter is a removable and serviceable item. Also visible on the drawing is the fuel discharge port. From this fuel discharge, fuel passes through the fuel/oil cooler and on to the fuel nozzles. Near the bottom of the fuel control is a P3 pressure inlet. This pressure is used to aid in operation of the engine. There is also an overboard drain shown in the drawing. It is not identified as a fuel drain, but as a P3 pressure drain or discharge. Finally, at the extreme right-hand side of the drawing is the power lever input shaft. The power lever is connected to this shaft and it rotates in proportion to movement of the power lever.


CHAPTER 73


10 TWO METHODS OF REGULATING FLOW It might be helpful before going any further to review some basic information concerning fuel systems. It can be said that there are three requirements of a fuel system. First, a supply of fuel is needed, a pump to increase the fuel pressure enough to cause the fuel to atomise, (reduce to a fine spray), and a metering valve to regulate the amount of fuel flow to the engine. This allows the required thrust settings necessary for flight conditions. Next, an examination of the third requirement of the fuel system, the metering valve. Two methods of regulating flow can be employed. Looking at the illustration, it is evident that if the pressure is held constant across the metering valve, and the opening is varied, the flow of fuel to the engine is changed. The other method of regulating flow would be to hold a constant opening in the metering valve and change the pressure by use of the bypass valve. If more fuel is bypassed, the pressure is reduced which, in turn, reduces the flow. Both of these methods are utilised in the TFE731 fuel control. Before discussing the operation of the bypass valve, a more detailed description of the metering valve is necessary.


CHAPTER 73


11 METERING VALVE Located between the pump discharge and the atomisers, the metering valve meters all fuel going to the engine. A stop is located at the top of the metering valve, referred to as a minimum flow stop. The stop is adjusted at the factory to deliver a minimum 130 PPH fuel flow to the fuel nozzles. Some specifics of metering valve operation are described below. Regulated fuel pressure enters and provides a pressure on the shoulder area of the metering valve. The same regulated pressure is also fed through an orifice to a chamber on the bottom side of the metering valve. If the pressure on both sides of the valve were equal, the valve would move up, or to the closed position, due to the surface areas on the shoulder and the bottom of the valve. A valve inside the chamber can be opened and closed by moving a beam that pivots on the pointed area in the lower right-hand corner of the chamber, identified by shading in this picture. If this valve is opened, the pressure within the chamber will decrease, and the metering valve will move down, or into an open position. This will increase the flow of fuel to the atomisers. Conversely, if the valve inside the chamber is closed, the pressure will increase, forcing the metering valve toward the closed position, decreasing the amount of flow to the engine. Because of the way this metering valve is drawn, it is apparent that the beam that controls the valve in the chamber would have to be moved by some means in order to meter the proper amount of fuel to the engine for the desired RPM.


CHAPTER 73


12 P3 VS. RPM As this graph reveals, as RPM increases, the P3 pressure also increases, proportionally. Therefore, P3 pressure can be used within the fuel control to provide more uniformity in the addition of fuel during acceleration.


CHAPTER 73


13 P3 CONTROLS VALVE POSITION A set of bellows has been added to the metering valve. If P3 pressure is supplied into the bellows, as the pressure increases, the bellows will expand. Expansion of the bellows will cause the metering valve to open and flow more fuel to the atomisers. This more detailed drawing of the metering valve reveals that the pump discharge flows through the metering valve to the atomisers. As the P3 pressure within the bellows is changed, the position of the metering valve is changed. As the acceleration bellows expands, it applies a downward force on the beam. The beam pivots at the point of the shaded area in this picture. This pivot action would open the seat within the chamber and allow fuel to flow back to the pump return. This would, in turn, lower the pressure within the chamber and allow the metering valve to open, supplying more fuel to the atomisers. The evacuated bellows applies the same force as the acceleration bellows to the beam. As fuel pressure increases and decreases within this chamber, both the acceleration bellows and the evacuated bellows will expand and contract with equal force. This will eliminate the possibility of pressure within the chamber affecting the opening of the main metering valve. The P3 pressure limiter shown here can be considered a safety valve. In the event of high plenum pressure (very cold day, high engine speeds) the limiter would relieve excessive plenum pressures. Compressor discharge pressure is sensed in the bellows of the limiter that is held closed by a spring. The spring rating is determined by engine model. When P3 pressure exceeds the spring ratings, the bellows will force the poppet open, reducing the pressure in the acceleration bellows. As this pressure drops, the metering valve will move toward the closed position, reducing engine speed.


CHAPTER 73


14 BYPASS VALVE REACTS TO P The purpose of the bypass valve is to bypass all fuel that is not required for engine operation back to the inlet of the pump. There is a definite reason for this fuel to be bypassed. The fuel pump on the engine is a positive displacement type pump. Therefore, all fuel not required for the engine must be bypassed back to the inlet of the pump in order to achieve ideal metering characteristics. As fuel is forced across the metering valve, the metering valve acts as an orifice, or restriction, in the line. This restriction causes a pressure drop and a differential pressure will occur across the metering valve. The differential pressure will be referenced at the bypass control shown here. Consequently, if the metering valve is opened, the pressure differential would be less and more fuel to the engine would be required. The bypass valve would close, due to the lesser amount of differential pressure applied at the bypass control.


CHAPTER 73


15 BYPASS VALVE OPERATION Understanding the basic function of the bypass valve permits a more in-depth look at its operation. The bypass valve is shown in the lower left of the diagram. Fuel from the pump discharge can enter the bypass valve and, as the bypass valve is opened and closed, the flow of fuel being bypassed back to the return of the pump can be regulated. The differential pressure across the two pistons of the valve, control the valve. Note that fuel pump discharge also flows through an orifice to the top of the piston in the bypass valve and into the chamber of the Delta "P" valve. Notice the flapper valve in the chamber. Opening and closing this valve change the pressure changed on the bottom side of the piston in the bypass valve. Moving the bypass valve to bypass more or less fuel is accomplished by means of the pressure within the chamber. The valve in the chamber is being moved by a set of bellows. Differential pressure across the metering valve is referenced inside the bellows. High pressure pump discharge is applied to the outside of the bellows. The difference in pressures will position the plate valve, allowing fuel to flow from the chamber. This causes the bypass valve to be positioned in response to the relative position of the metering valve, thus providing control of fuel flow to the engine.


CHAPTER 73


16 MANUAL MODE The next item in the fuel system discussion is the "governor control". The governor control - shown in the upper right-hand corner of this drawing - is merely a flyweight-type governor being operated from N2 RPM. As speed is selected with the power lever, the governor will react and regulate the proper amount of fuel to maintain the selected speed. The governor controls P3 pressure to the bellows and as the pressure within the bellows changes, so does fuel flow and engine RPM. In manual mode, an N1 RPM is selected, however engine speed is controlled by N2 RPM.


CHAPTER 73


17 MANUAL MODE – GOVERNOR CONTROL Added to the drawing is the governor control, shown in the lower part of the drawing. Note the input shaft from N2 RPM going to the flyweight-type governor, which operates a valve in the P3 line. This valve controls the P3 pressure inside the acceleration bellows. As the valve opens, P3 pressure is drained overboard. By decreasing P3 pressure within the acceleration bellows, the plate valve moves to the closed position. This increased pressure within the chamber positions the metering valve, reducing fuel flow to the engine. Also visible in the bottom centre of the drawing, is the location of the only adjustment provided on the fuel control. This manual mode adjustment involves setting the governor for maximum RPM, or maximum temperature. Making this adjustment affects a cam within the governor and adjusts the maximum setting of the fuel control.


CHAPTER 73


18 TROUBLESHOOTING P3 SIGNAL How do you troubleshoot the P3 signal? If there were a broken line, or a "B" nut not installed for the P3 pressure line, the following events would occur. With no P3 pressure in the acceleration bellows, the seat in the chamber would not open. If this seat did not open, then the main metering valve would not open, and consequently, the maximum flow to the engine would be limited to approximately 130 pounds per hour (PPH). This would be enough to achieve a light off, however, it would not be enough to permit acceleration to a normal idle RPM.


CHAPTER 73


19 MECHANICAL GOVERNOR Now let us look at the function of the manual mode governor in normal mode operation. The power lever controls the EEC and the internal electronic governor will, in turn, control the bypass valve. With the EEC "on", a 28-volt signal to the manual control resets the manual governor control to 105%. The manual mode governor then becomes a 105% over speed governor and is set high enough to prevent its interfering with the electronic governor. It is necessary at this point to remember the discussion of the two methods of regulating fuel flow. These two methods involved either holding the pressure constant and varying the opening of the metering valve, or holding the metering valve opening constant and change the pressure with the bypass valve. In manual mode, we adjust the main metering valve opening. In normal mode, the bypass valve is adjusted and, therefore, the change in pressure.


CHAPTER 73


20 MECHANICAL GOVERNOR

20.1 Normal Mode

What happens when the EEC is on? Look at the manual mode solenoid valve near the bottom of the drawing. When the EEC is on, the manual mode solenoid valve is energised. This allows two routes of regulated fuel pressure into the governor chamber. The two openings or orifices are larger than the opening in the centre of the governor manual piston. Therefore, a pressure builds up and moves the piston all the way to the left, against the stops. Through this action, the speeder spring of the flyweight governor is reset to 105% and it then serves as an N2 over speed governor in normal mode.


CHAPTER 73


21 FUEL CONTROL SCHEMATIC The source of regulated pressure to the metering valve is depicted in this schematic. The pressure regulator shown at the top of the schematic regulates the pressure to 205 ±5 PSI, bypassing excess pressure. Also shown downstream of the metering valve is an outlet pressurising valve. Fuel leaving the metering valve, en route to the fuel atomisers, must pass by the outlet pressurising valve. The valve is spring-loaded closed and requires a fuel pressure of about 185 PSI to open it. The outlet pressurising valve assures that sufficient fuel pressure is available for operation of the various pressure sensitive devices within the fuel control.


CHAPTER 73


22 FUEL SHUTOFF VALVE

22.1 Cut-off

The fuel shutoff valve allows fuel to flow to the fuel atomisers when the power lever is moved from cut-off to idle. This action causes high pressure fuel to position the valve, allowing metered fuel to flow through the valve to the atomisers. When the power lever is placed in cut-off, the mechanical action turns the rotary valve, bypassing high pressure fuel to the pump return. The spring in the fuel shutoff valve positions the valve shuttle to the left causing metered fuel to bypass back to the pump. The fuel shutoff valve is located in the fuel control and is operated by mechanical linkage from the power lever. Notice also the electrical solenoid valve in the upper right of the schematic. This is a normally closed, energised open, solenoid valve that will be used to shutdown the engine in case of over speed.


CHAPTER 73



22.2 Above Cut-off

Shown here is the rotary valve position when the power lever is out of the cut-off position. Notice that high pressure pump output fuel passes through the valve, through an orifice and to the back side of the spool valve, forcing the spool to the right. This action allows metered fuel from the fuel control to pass through the valve and on to the fuel manifolds.


CHAPTER 73



22.3 Ultimate Over Speed Solenoid Energised

In the event of an over speed, the EEC would energise the ultimate over speed solenoid which would close the valve, stopping flow of fuel to the engine. Let us look at the sequence of events. If the EEC senses a speed signal above 109% N1/110% N2 RPM for analogue electronic control systems or 107% N1/109% N2 RPM for digital electronic control systems, the ultimate over speed solenoid would be energised open. This action would allow two paths of fuel to flow to the spool valve. Pressure on the right, with the aid of the spring would force the spool valve to the left closing the path of fuel to the engine and opening a path of fuel to return to the pump inlet. Specific operation of the ultimate over speed solenoid will be addressed in Chapter 76 Electronic Engine Control section of this study guide.


CHAPTER 73


23 FLOW DIVIDER Fuel flows from the fuel/oil cooler to the fuel flow divider. Mounted to the fuel control, this unit is a spring-loaded closed, hydraulically actuated valve that provides a path for fuel flow to both the primary and secondary fuel nozzles. The flow divider contains a differential pressure bellows, a viscosity compensated restrictor, and a surge dampener. During engine start, fuel pressure is applied to the inlet port and across the viscosity compensated restrictor, the surge dampener and flows to the primary side of the duplex fuel nozzles. Fuel, under pressure, is simultaneously routed to the outside of the flow divider bellows and through the surge dampener to the inside of the flow divider bellows. The unequal pressures will cause the poppet valve to remain closed. As the fuel flow increases, the differential pressure at the bellows increases. When the pressure difference reaches approximately 33-43 PSID, the bellows compresses, allowing the poppet to open. Opening of the poppet allows fuel to flow to the secondary port of the duplex fuel nozzles. This normally occurs at approximately 150 PPH fuel flow. As the fuel flow increases the differential pressure across the bellows increases, allowing increased fuel flow to the secondary nozzles. A normal characteristic of the engine that may be observed at idle speed is a cyclic increase and decrease of N1. The cycling is characterised by a sequence as follows: fuel flow increases with no increase in N1; N1 then increases; fuel flow decreases with no decrease in N1; N1 then decreases, cycle repeats. This characteristic is due to normal fuel pressure changes within the fuel manifold caused by low flow rates that do not maintain constant flows from all secondary nozzles of the fuel manifold. This fluctuation of flow rates causes the flow divider to open and close with the resulting cycling.

This characteristic may be confirmed as the cause of the cycling by advancing the power lever to provide 250 to 300 PPH fuel flow, which stops cycling. The cycling characteristic is normal at low flow rates and has no effect on operation or flight safety.


CHAPTER 73


24 TFE731

FUEL

SYSTEM

SCHEMATIC

24.1 Manual Mode at Idle

The figure illustrates the TFE731 fuel system in Manual Mode at Idle power setting.


CHAPTER 73


24 TFE731

FUEL

SYSTEM

SCHEMATIC

24.2 Normal Mode at Idle

The figure shows the schematic layout and operation in Normal Mode at Idle power setting.


CHAPTER 73


25 MANIFOLDS Fuel flows into the 12 duplex fuel nozzles contained within two manifold assemblies mounted around the turbine plenum. Each manifold assembly contains six duplex atomisers. Primary and secondary fuel is routed to each atomiser. The atomisers are designed to provide a cone-shaped spray of finely atomised fuel. Each atomiser head is fitted with an air shroud that has compressor discharge air flowing through it to centre the atomised spray within the combustion chamber. Field level maintenance is limited to replacement of the manifold assemblies and air shrouds. Since each duplex fuel nozzle flow pattern is matched to the remaining eleven, the fuel manifold must be replaced in matched sets. Review the latest maintenance manual procedures prior to performing any fuel system maintenance.


CHAPTER 73


26 FUEL CONTROL RIGGING Fuel control rigging is required when a fuel control is changed, when an engine is changed, or when components of the mechanical control system are changed. A position indicator on the fuel control indicates rotation of the fuel control shaft in degrees. The positions are shown here, however the rigging procedures for your specific aircraft installation are contained in the aircraft maintenance manual. Always consult the aircraft maintenance manual for fuel control and power lever rigging procedures. Correct power lever and fuel control adjustments are an essential element in correct engine operation.


CHAPTER 73


27 ADJUSTMENT LOCATION The fuel control manual mode adjustment location is shown here. The manual mode adjustment is made to limit engine speed in manual mode. This adjustment is made when the fuel control is replaced or when the engine is changed. The procedures for this adjustment are contained in the maintenance manual.


CHAPTER 73


28 FUEL SYSTEM

PLUMBING The figure presents a layout of the fuel system plumbing


CHAPTER 73


PAGE INTENTIONALLY LEFT BLANK

Documents

TFE 731 Chap 73 (1)