TFE 731 Chap 76 (1)

Garret TFE 731 Turbofan Engine (CAT C)

CHAPTER 76

Page 1 of 42 FOR TRAINING PURPOSES ONLY © TFE 731 - ISSUE 2, 2010


CHAPTER 76


INTRODUCTION

0 TABLE OF CONTENTS

1 Introduction 4 2 Inputs and Outputs 5 3 Power Lever Potentiometer 6 4 Inlet Pressure and Temperature Sensor 7 5 N2 Monopole 8 6 N1 Monopole 9 7 TFE731-5 N1 Compensator 10 8 ITT Thermocouples 11 9 Torque motor 12 10 Manual Mode and Over Speed Solenoids 13 11 Manual Mode Switch 14 12 The Electronic Engine Control 15 13 Schedule Selection 16 14 Start Schedule 17 15 Governor Schedule 18 16 Acceleration 19 17 Surge Schedule 20 18 Governor Schedule 21 19 Deceleration 22 20 The Electronic Control 23 21 Digital Electronic Engine Control Features 24 22 DEEC Start Schedule 25 23 Idle 26 24 -2/-4/-5B DEEC Climb/Cruise 27 25 -5/-5A DEEC Climb/Cruise 28 26 Restricted Performance Reserve 29 27 RPR Logic 30 28 EEC Adjustments 31 29 Acceleration Adjustment 32 30 EFR Adjustment 33 31 Take-off Power 34 31.1 N2 Governor 34 32 DEEC Adjustments 35


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0 TABLE OF CONTENTS (Continued)

33 Level 1 Adjustment 36 33.1 Acceleration 37 33.2 ITT Limiter 37 33.3 Engine Flat Rate 37 33.4 Takeoff N1 (FR/Mn) 37 33.5 Idle Adjustment 38 33.6 When Changing a DEEC 38 34 EEC External Fault Monitor System 39 35 DEEC Fault Monitor 40


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ELECTRONIC CONTROL SYSTEM

1 INTRODUCTION This section describes the TFE731 electronic engine control. Various terms including fuel computer, computer, engine computer and EEC are used in reference to the control. For the purpose of this section, the terms EEC and DEEC will be used to describe the electronic engine control and digital electronic engine control respectively. In general terms, the EEC functions to reduce the pilot workload with regard to engine operation. The EEC provides automatic start sequencing, spool speed and temperature limits, and surge-free acceleration and deceleration. The EEC utilises a nominal 28 VDC supplied through a cockpit switch from the aircraft buss. The EEC requires a number of inputs from the engine in order to perform its functions. Outputs from the EEC go to a fuel control torque motor to meter fuel and to surge bleed valve solenoid to control surge. The EEC can energise an over speed solenoid, affecting an engine shutdown in the unlikely event of engine over speed. Fault detection circuits are integral with the EEC, which will automatically transfer the EEC to manual mode in case of an electrical fault. There are two basic electronic controls for the engine. The analogue system, referred to as an "EEC" (Electronic Engine Control), is installed on Dash 2 and Dash 3 engines. The digital system, referred to as a "DEEC" (Digital Electronic Control), is installed on Dash 2 (limited), Dash 4 and Dash 5 engines. The DEEC utilises modern digital circuitries that inherently possesses less drift, fewer components and solder joints thus providing increased reliability over analogue-type designs. There are two modes of engine operation: Normal mode - (sometimes referred to as "automatic" or "computer mode") Manual mode - EEC inoperative In the normal mode of operation, the EEC controls fuel. The EEC provides limits for ITT, N1 and N2. Conversely, in the manual mode of operation, fuel is controlled by the hydro mechanical fuel control and the operator must observe the engine limits to prevent over temperature or over speed.

The operator has the capability of switching from normal mode to manual mode during troubleshooting to assist in isolating system malfunctions. The discussion of the EEC and DEEC will be divided into five major areas: a. Input and output components b. Schedules c. Operation d. Field adjustments e. Fault detection circuits In each area of discussion, the DEEC will be discussed separately since some significant differences exist.


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2 INPUTS AND OUTPUTS The electronic control receives a number of signals from the engine in order to control engine operation. Low pressure spool RPM is received from a speed sensor mounted at the aft end of the LP spool. A speed sensor in the transfer gearbox senses the speed of the HP spool. Probes located in the interstage turbine area between the HP and LP turbines sense turbine temperature. The combined Pt2/Tt2 sensor located in the inlet supplies inlet air temperature and pressure data. Power lever position is received from a variable potentiometer located within the fuel control. Output signals to control engine operation go to the torque motor, manual mode solenoid, over speed solenoid, and to surge valve solenoids "A" and "B". An examination of the input and output components, how they operate and the location will aid in understanding the function of the electronic control.


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3 POWER LEVER POTENTIOMETER During the fuel system review, power lever angle (PLA) was used in reference to degrees movement of the fuel control input shaft. A position indictor on the fuel control indicates rotation of the fuel control shaft in degrees. Cut-off position is indicated at 0°, idle detent at 20°, and maximum shaft rotation is indicated at 120°. In normal mode, power lever position (PLA) is sensed in the electronic control by varying voltage from a power lever potentiometer in the fuel control. The PLA potentiometer is a variable resistor geared to the power lever input shaft. Rotation of the PLA input shaft would also vary resistance in the PLA potentiometer. A conditioned signal from the electronic control is varied by position of the PLA potentiometer and sent back to the control for speed reference. Takeoff speed reference is established at the maximum PLA (117-120°) and idle speed reference is established at the minimum PLA (0-26°). Intermediate power is set by modulating the signal from the PLA potentiometer to the EEC. There is an area of the potentiometer where rotation of the input shaft does not change the resistance value. Called a "dead band", this area is from 0 to 26° PLA on some installations and from 0 to 40° on others. With the engine operating at idle power, movement of the power lever from 20 to 26(40)° would not change engine speed, however movement past 26(40)° would cause a change in voltage to the electronic control with a resultant change in engine speed.

The purpose of the dead band is to accommodate thrust reverse installation. The thrust reverser levers piggyback mounted on each power lever achieve normal deployment, stowing, and reverse thrust control. Movement of the associated linkages during thrust reverser operation could cause flexing of engine control mechanisms and without the dead band, would cause changes in engine RPM.


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4 INLET PRESSURE AND TEMPERATURE SENSOR The inlet pressure and temperature sensor (Pt2 Tt2) senses engine inlet air pressure and temperature for use by the electronic control. The sensor incorporates an electrical heating element for the prevention of ice formation when the pilot selects anti-icing. The sensor is mounted at the 12 o'clock position in the inlet duct, forward of the fan on all engines except the -5, which has the sensor mounted between the 11 and 12 o'clock position in the airframe air inlet duct. The temperature sensing element is a resistor made of a platinum-beryllium-copper combination that changes resistance with temperature. As temperature increases, resistance increases, with the resultant change in voltage sent to the electronic control. A typical sensor would read 500 ohms resistance at 0°C and increase in resistance at the rate of 2 ohms per 1° temperature increase. Inlet total pressure (Pt2) is sensed through a plumbing connection from the sensor to the front face of the electronic control. The pneumatic pressure signal is converted to an electric value by a pressure sensing potentiometer. Note the auxiliary port in the upper left. If the air entry ports become restricted (ice, dirt, etc.), the auxiliary port would provide a static pressure signal to the electronic control.


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5 N2 MONOPOLE Motional pickup transducers called monopoles generate spool speed signals for both N1 and N2. Each monopole consists of a dual-element transducer that transmits an independent signal to the electronic control and the cockpit indicator. The N2 monopole is mounted in the transfer gearbox. The monopole utilises the motion of a special 50-tooth gear to generate an AC pulse signal. For a given N2 RPM, and AC HS signal is generated by the monopole. Peak-to-peak output voltage can be measured at the monopole connector. Shims are installed between the monopole mounting surface and the gearbox to provide the necessary clearance between the monopole and the gear teeth. Insufficient clearance may damage both monopole and gear teeth. Excessive clearance will generate low voltage and/or weak signals leading to operational problems. Shimming is performed by measuring the distance from the base to the tip of the transducer, the depth of the gear teeth from the gearbox housing and adding sufficient shims to attain a clearance of 0.012 to 0.015 inch. Following the formula and specific procedures in the maintenance manual will insure proper monopole installation. Since this monopole is mounted in an oil-wetted area, attention should be paid to the "0" ring seals to prevent oil leaks after installation.


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6 N1 MONOPOLE N1 speed is measured by the N1 monopole at the aft end of the low pressure rotor shaft. A speed gear is attached to the low pressure shaft and rotates around the monopole that senses the changes in the magnetic field as each gear tooth passes. Like the N2 monopole, it also has two elements fabricated into a common assembly, one to provide a speed signal to the electronic control, the other to provide the cockpit N1 signal. The monopole elements are positioned 180° from each other. Monopole dimension and internal speed gear diameter are set by the manufacturer to provide the proper clearance between the pickup and the gear teeth, thereby requiring no shimming or adjustment. Access to the N1 monopole is gained through the number 6 bearing sump. Maintenance manual procedures should be adhered to during maintenance in this area since improper installation of gaskets and back-up plate could result in complete loss of oil during engine operation.


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7 TFE731-5 N1 COMPENSATOR The TFE731 uses fan speed (N1) as the cockpit indication of thrust level, but the thrust-RPM relationship varies from engine to engine. For instance, engines with high core thrust, when set to flight manual N1 speeds, can be expected to produce more than rated thrust, and a higher operating temperature than necessary results. The purpose of TFE731-5 N1 compensation is to bias the N1 indicator to read rated RPM when specified thrust is achieved. Thus, the pilot can continue to set thrust by setting N1 to the existing published value for all engines and will be assured of obtaining the required thrust while subjecting the engine to lowest possible ITT. The N1 monopole and compensator is engine mounted. Signals from the N1 monopole are appropriately biased by the compensator and displayed on the cockpit indicator. The compensator resistance is selected during calibration of the engine and controls the compensation bias circuit dependent upon actual thrust-RPM relationship. The amount of compensation (bias) is indicated in the compensator part number. Always replace a faulty compensator with one of the same part number.


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8 ITT THERMOCOUPLES The turbine gas temperature is measured by 10 chromel-alumel thermocouples mounted on bosses in the turbine interstage transition duct and extending into the gas path between the high pressure and low pressure turbines. This area is engine station 5; consequently interstage turbine temperature (ITT) is commonly referred to as T5. Two thermocouples rakes of 5 probes each are wired in parallel to form an averaging circuit. The 10 probes in the assembly present an average sensed temperature. The harness is mounted aft through the turbine interstage transition duct and provides connections for the electronic control and for cockpit indication. The chromel-alumel junctions generate a voltage in proportion to the heat they sense. The voltage output of the thermocouples is directed from one of the parallel circuits to the electronic control, the second parallel circuit is used for cockpit indication. Later configurations of thermocouple harnesses incorporate a more flexible two-piece harness. Field experience has revealed that the one-piece harness is susceptible to damage caused by improper handling and installation. Continued product improvement is an on-going effort with all engine systems and components. Experience has shown that minor changes in gas path stratification in the interstage turbine duct can have significant effect in ITT indication with no real change in engine performance. This appears as

deterioration in indicated performance that sometimes leads to a premature engine maintenance action. Use of a harness containing alternating long and short probes provides a dual immersion into the gas path improving temperature indication. The alternating length probes reduce the sensitivity of the ITT indicating system changes in gas path temperature distribution and provide a more consistent measurement of average gas path temperature.


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9 TORQUE MOTOR The electronic control sets the engine fuel schedule by means of a torque motor in the fuel control. Differential pressure across the metering valve is sensed in the Delta P bellows and compared with the force of the torque motor. Torque motor force is proportional to the current output of the electronic control. Current to the torque motor will position the Delta P bellows and operate the fuel bypass valve to set the fuel schedule. The electronic control provides an appropriate output current to the torque motor based on input signals from the PLA, Pt2, Tt2, spool speeds, and ITT. The torque motor can be referred to as a DC motor with permanent magnets and an armature with two coils. The two coils are matched to within 10 ohms resistance in each coil. The electronic control monitor system continuously evaluates the condition of the coils. The monitor system functions will be discussed later in this section.


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10 MANUAL MODE AND OVER SPEED SOLENOIDS The manual mode solenoid and over speed solenoid, both components of the fuel control system, are housed within the fuel control. The manual mode solenoid is normally closed and energised open when the electronic control is in the normal mode of operation. With the electronic control operating in normal mode, the manual mode solenoid is energised, resetting the manual governor control to 105% N2 RPM. The manual mode governor then becomes a 105% over speed governor and the manual governor is set high enough to prevent its interfering with the electronic governor. The resultant function of the fuel control in normal mode was outlined in the Fuel System section. The over speed solenoid is a normally closed solenoid used to prevent over speed of the engine in the unlikely event of complete failure of the electronic governor schedules and failure of the hydro mechanical governor in the fuel control. The electronic control incorporates an over speed switch function that continuously monitors N1 and N2 spool speeds. The module which detects an over speed is executed every 10 milliseconds. Both spool speeds are input and the rate of change of speeds is calculated. If either spool speed is equal to or greater than the over speed threshold, the solenoid is energised. This action would interrupt fuel flow to the engine, causing it to shutdown. The over speed threshold is 109% N1 or 110% N2 for EEC's. For DEEC's the threshold is 107% N1 or 109% N2. For system redundancy, a secondary over speed module is incorporated that, except for monopole inputs, is effectively isolated from the primary system. In the event of electronic control "manual mode" operation, the secondary over speed module remains active. A functional check of the over speed protection circuit is performed during scheduled inspections. Note also the position of the power lever potentiometer and indicator dial in the

fuel control.


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11 MANUAL MODE SWITCH A two-position guarded switch located on the front of the electronic control may be used to provide over speed protection in the manual mode. The switch is labelled NORMAL and MANUAL. Over speed protection circuits of the EEC for both low and high pressure spools remain activated in the manual mode provided the cockpit switch for the EEC is on, and the NORMAL/MANUAL switch on the face of the EEC is set to MANUAL. Over speed circuits of the digital electronic engine control (DEEC) for both N1 and N2 remain activated during manual mode operation provided the cockpit auto/manual switch is positioned to manual or when the DEEC transfers to manual mode automatically.


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12 THE ELECTRONIC ENGINE CONTROL The electronic control in its automatic mode of operation controls engine fuel flow during starting, acceleration, deceleration and steady-state operation based on parameters that include PLA, atmosphere conditions, and preset engine parameters. Surge valve scheduling, ITT limiting and maximum power limits are automatic functions of the electronic control. Specifically, the electronic control provides the following:

Automatic fuel enrichment during start and acceleration of N2 to idle RPM.

Provides an acceleration schedule to maintain the preset compressor surge margin or prevent excessive turbine temperature.

Provides a deceleration schedule that prevents flameout or large HP-LP spool speed mismatch.

Establishes an idle RPM based on inlet temperature and pressure.

Sets a maximum N1 speed to achieve flat rated thrust or maximum thrust available based on ambient temperature and pressure altitude.

Provides turbine temperature limiting during all engine operations.

Provides a thrust level directly proportional to power lever settings.

Controls surge bleed valve opening and closing to prevent compressor stalls/surges.

Provides an N1 spool speed limiter and over speed shutdown capabilities.

Monitors input and output signals and transfers to manual mode if faults are detected.


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13 SCHEDULE SELECTION During engine operation, the electronic control constantly receives input signals from the engine and, based on five primary schedules within the control, will govern fuel flow and surge bleed valve position. The input signals are shown at the left of this diagram. During all steady state operation, the governor schedule is controlling engine speed. However, what schedule will be selected to start, accelerate and decelerate the engine to prevent surge, over speed or flameout? Notice that signals from the four schedules enter the Low Wins Circuit and of these four signals, one is passed on to the High Wins Circuit. As the name implies, the low wins circuit will select the lowest value of the four inputs and pass that value on to the high wins circuit. It can be said that the low wins circuit selects the lowest fuel schedule to control engine speed. Notice that the Minimum Fuel Schedule is also input to the High Wins Circuit. The electronic control will select the higher of the two schedules to operate the engine and prevent flameout. This value will then be sent to the fuel control torque motor. By selecting these most and least fuel schedules, the electronic control is providing optimum fuel flow for surge-free engine operation within the designed speed and temperature parameters. Understanding the five basic schedules shown here will enable you to recognise normal electronic control operation and to evaluate anomalies. The

next several pages will show a typical engine start, acceleration, deceleration and shutdown highlighting the individual schedule functions. The inputs to the schedule and schedule functions will be explained using a series of drawings indicating the parameters involved.


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14 START SCHEDULE During a start, the electronic control will cause the engine to accelerate to idle within the established time and temperature limits. The objective is to increase fuel flow until the gas generator develops sufficient energy to accelerate the engine to idle. In order to accomplish this, fuel flow will be increased based on N2 RPM. The graph shows that as N2 RPM increases, fuel flow will increase. At 10% N2 RPM the start fuel schedule is increased. This increase, or enrichment, improves fuel atomisation at light off and acceleration in cold ambient temperatures. As 200°C ITT is sensed, the control reduces the start fuel schedule. In addition to N2 RPM, ITT is an input to the start schedule. The ITT input signal will prevent the engine from exceeding the start temperature limit. If turbine temperatures exceed the ITT limiter setting on start, fuel flow would be reduced to keep the temperature within limits. As the engine continues to accelerate to idle, at approximately 50% N2 RPM the control will provide a circuit to disengage the starter and ignition. At approximately 60% N2 RPM, the governor schedule will be lower than the start schedule, consequently the control will select the governor schedule and engine speed will be maintained by the governor schedule. Some aircraft are equipped with a cockpit ENRICH/SPR switch. Use of this switch during start will provide increased fuel scheduling beyond the 200°C ITT limit of automatic fuel enrichment. Caution must be exercised, however, to not exceed allowable temperature limits when using this function. Consult your aircraft flight manual for specific procedures.


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15 GOVERNOR SCHEDULE When at a steady state operating condition at idle, the engine speed is being controlled by the governor schedule as shown here. The graph indicates a direct relationship between power lever angle and N2 RPM. Although N1 RPM is normally monitored at idle, the governor schedule controls N2 speed. Notice that, at idle, N2 RPM, PLA, ITT and PT2, TT2 inputs are sensed by the governor schedule. When idle is selected with the power lever, the governor establishes an N2 RPM that will deliver approximately 30% N1 to provide adequate idle thrust. The N1 RPM will vary with ambient conditions, while providing consistent thrust. The ITT input to the governor schedule will play an important role at maximum power.


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16 ACCELERATION Movement of the power lever above idle will accelerate the engine as shown here. The graph reveals that for a given power lever position, a given N2 RPM is required. The electronic control will select either the surge schedule or acceleration schedule to accelerate the engine. Recall from the surge system discussions that movement of the power lever above the idle dead band causes the surge bleed valve to close. If the power lever is moved slowly or moderately from idle to maximum, the acceleration schedule will become the lowest schedule; it will be selected by the low wins circuit to control the torque motor. The low wins circuit will select the governor schedule when the acceleration schedule exceeds governor schedule. The governor will control the torque motor and maintain N2 RPM selected by the power lever. At the maximum power setting, a 100% N1 set point is compared with the N2 governor and the N2 speed setting is reduced when the set point is exceeded. This set point is the N1 limiter and is preset. The N1 limiter will operate in such a manner as to reset the N2 governor when the N1 governor set point is reached, thus preventing over speed of N1 RPM. Note that the surge schedule receives speed signals from the N1 and N2 spools. A programmed N1-N2 spool speed relationship will prevent N2 speed

from exceeding the point at which surge conditions could occur. Referring back to the low wins circuit, if a surge condition exists, the low wins circuit would select the surge schedule to accelerate the engine.


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17 SURGE SCHEDULE A closer examination of the surge schedule may provide a better understanding of how the engine accelerates and decelerates surge free. As explained previously, during acceleration the low wins circuit will select between the acceleration schedule and the surge schedule. The result is compared with the minimum fuel schedule, shown here as minimum schedule and the higher of the two is passed on to the torque motor. The surge schedule compares the N1 and N2 reference with torque motor signals. When N2 is low with respect to N1, the surge schedule will de-activate solenoid "A" causing the surge bleed valve to open 1/3. If spool speed mismatch continues, solenoid "B" is energised, causing the surge bleed valve to open full. By comparing fuel flow to reference speeds, the electronic control will open or close the surge bleed valve, preventing high combustor back pressures that can cause surge. As the engine accelerates, the PLA input signal (power demand) and N2 speed signal will become closer in value. The acceleration schedule will become higher than the governor schedule and at this point, through the low wins circuit, the control will select the governor schedule. The engine speed will be controlled by the governor schedule. A normal characteristic of the 731 engines is for N1 and ITT to overshoot above the values required for a stabilised power setting. This phenomenon is most pronounced during rapid power lever advancement. These overshoots are not

cause for concern provided allowable transient limits are not exceeded.


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18 GOVERNOR SCHEDULE When at steady state RPM, engine speed is maintained by the governor schedule. Note that N2 speed will be as determined by PLA position. Other inputs to the governor schedule are ITT, PT2, TT2 and, if installed, synchroniser. These inputs will influence the N2 speed. The synchroniser input will be discussed in Chapter 76 of this study guide. From the theory of operation discussion, maximum power was influenced by air density (PT2 TT2) and the maximum speed was determined by the flat rate (FR) or maximum speed (Mn) limits. The governor schedule will select FR or Mn depending upon the PT2, TT2 input signal. ITT (T5) also influences the governor schedule. A T5 limit is established within the control, and once established, will influence the governor schedule. In summary, the engine maximum speed - target N1 - power for the day, depending upon the term used, is determined by the governor schedule and is based on PLA, PT2, TT2, and ITT. The maximum ITT (T5 limit) and spool speed for a given OAT and pressure altitude is determined by the aircraft manufacturer and will vary depending upon the specific installation. For this reason, the flat rate (FR) maximum speed (Mn) and maximum ITT (T5 limit) are adjustable. These adjustments will be discussed in more detail later in this section.


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19 DECELERATION From what we have learned to this point, it should be apparent that when the power lever is retarded, the governor schedule is reset to low. The low wins circuit will select the lowest value of the start, governor, surge and acceleration schedules. The high wins circuit will select the highest schedule between the minimum fuel schedule and that schedule passed through the low wins circuit (normally the governor schedule). The purpose of the minimum fuel schedule is to prevent lean burner blow out. Normal engine deceleration then is accomplished through the reset of the governor schedule because of a change in PLA. It should be realised, however, that the N1/N2 spool speed input to the surge schedule will also influence deceleration, for example, a rapid deceleration command from the PLA would reset the governor schedule and allow N2 to decelerate to the point where a spool speed mismatch could induce a surge potential. At this point, the surge schedule would command opening of the surge bleed valve. Opening of the surge bleed valve would then relieve the surge potential. Remember that the surge valve will be full open on most applications with the power lever in the idle range (20-26°/20-40°). Because of the potential for compressor surge in some installations, the valve will open at the 60° PLA to prevent surge/stall during slow power reduction coming out of altitude. At the point at which the governor schedule becomes lower than the surge schedule, the governor will control and maintain N2 RPM. In summary, all engine operating parameters are controlled by five schedules and, through the low wins and high wins circuits, the electronic control will select the schedule best suited to provide rapid, surge-free response while maintaining the desired temperature and speed limits. The schedule selection

process is dependent upon inputs from N1 and N2, ITT, PT2, TT2 and PLA. It is apparent that erroneous input signals or spurious inputs will cause abnormal engine operation. A significant part of engine anomalies can be traced to failure of the electronic control input and output components and/or the electrical connections. A basic understanding of the schedules will provide a solid foundation for you to build upon in troubleshooting the engine operation in normal mode. While both the EEC and DEEC share common input/output components, and are generally the same in operation, some differences do exist between the part number 2101142 and 2101144 EEC's and part number 2118002 DEEC's. These differences should be examined prior to reviewing the adjustment and troubleshooting procedures.


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20 THE ELECTRONIC CONTROL The part number 2101142 series electronic control was designed to replace the part number 949572-S/-8 control used on TFE731-2 engines and added significant improvements such as manual mode dispatch, combined FR/Mn adjustments and improved thrust matching. Numerous product improvement efforts have been made to date to obtain optimum engine performance with the 2101142 series EEC. Our discussion of the EEC to the point has centred on the 2101142 EEC. The part number 2101144 series electronic control was designed for the TFE731-3 engines. While functionally the same as the 2101142 series, it utilises an N1 limiter (preset) that allows higher fan speeds on some aircraft applications, and an improved flight descent feature that will reduce N1 speed as a function of pressure altitude. Referred to as flight idle, the feature was incorporated to improve power management during climb and descent. From takeoff to cruise altitude, approximately three power lever adjustments are required as compared to seven with previous EEC's. During descent with the power lever above 40° PLA, a linear decrease in N1 RPM is provided which prevents a sudden rollback as 18,000 feet is approached. The part number 2118002 series digital electronic controls utilise modern digital circuitry which inherently possesses less drift, fewer components and solder joints, and increased reliability when compared to the analogue-type EEC's.

The input and output components are the same as previously discussed, however the DEEC offers additional features not available in the EEC.


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21 DIGITAL ELECTRONIC ENGINE CONTROL FEATURES The DEEC was first developed for the TFE731-5 engine and after considerable flight experience was introduced for the Dash 2 engine. The DEEC features improved start schedules, reduced idle and climb/cruise schedules. Additionally, some installations feature a restricted performance reserve (RPR) designed to enhance hot day/high altitude takeoff characteristics. The system provides an increase over normal takeoff thrust and is controlled by pressure altitude and inlet temperature. A built-in test (BITE) feature will hold fault codes in memory and display malfunctions on a four-digit LED display located on the front face of the control. An 11-position rotary function switch, in conjunction with a calibration switch, provides total DEEC trim capability and fault interrogation. The latest improvement to the DEEC schedules has been to change the spool controlled by the unit from N2 to N1. This change provides consistent Flight Manual N1 speed throughout the operational range of the DEEC.


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22 DEEC START SCHEDULE The DEEC has an improved start schedule. As with the EEC, both automatic and manual enrichment is provided. During start, fuel flow is a function of N2, T5 and Tt2. If N2 RPM is less than idle, the start temperature limit is 732°C for the Dash 4/5 engines, and 663°C for the Dash 2 engine, as compared to the higher T5 limiter setting of the EEC. The improved starting characteristics provided by the new schedule results in cooler starts.


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23 IDLE There are two idle RPM schedules within the DEEC that control minimum RPM when the power lever is positioned at the idle stop (fuel control shaft at 18-22°). The higher idle is automatically selected when the squat switch or weight-on-wheels switch is activated and the aircraft is in flight. This schedule is termed normal idle. A lower idle setting, referred to as reduced idle, it utilised when on the ground. Specified idle RPM is not one distinct value but varies with changes in altitude and air temperature. The idle RPM charts are located in the adjustment/test section of the maintenance manual. The reduced idle setting results in approximately 220 pounds thrust while the normal idle setting produces approximately 350 pounds thrust. The graph shown here is for reference only. Consult the applicable maintenance manual for your specific idle speeds. As discussed earlier, a normal characteristic of the 731 engines is for N1 and ITT to overshoot above the values required for a stabilised power setting. This phenomenon is most pronounced during rapid power lever advancement. These overshoots are not cause for concern provided allowable transient limits are not exceeded. The DEEC incorporates an ITT feature which will reduce fuel flow should the ITT exceed an established trim schedule. This feature has limited authority and primarily serves to prevent excessive ITT overshoot during rapid power lever advancement.


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24 -2/-4/-5B DEEC CLIMB/CRUISE Normally during climb with a fixed power lever position, both N1 and ITT tend to increase. This characteristic necessitates periodic throttle adjustments during climb to prevent exceeding engine speed and temperature limits. The DEEC incorporates climb and cruise schedules that automatically controls N2 speed, (consequently N1 speed) during changes in pressure altitude and temperature. The climb/cruise schedule maintains a scheduled speed, which is programmed to produce N1 speeds specified in the aircraft flight manual. With the PLA in the climb range (117-105 °), the speed reference (either N1 or N2 depending on configuration) is scheduled as a function of physical speed, TT2 and PT2. With the PLA in the cruise range (105-95 °), the speed reference is scheduled to produce approximate Flight Manual N1 speeds for cruise. Fan thrust is maintained as a function of Tt2-Pt2, while maintaining climb/cruise ITT limits. The ITT limits are automatically calculated by the DEEC during initial trim checks. This feature reduces pilot workload and aids in prolonging engine life.


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25 -5/-5A DEEC CLIMB/CRUISE The climb/cruise schedule incorporated in the DEEC for the Dash 5 and Dash 5A engines accomplish the same result as described earlier, but in a slightly different manner. These DEEC's climb/cruise schedule maintains a constant ITT for a given power lever position. The N1-ITT relationship is established during the DEEC adjustment procedures, consequently as ITT changes because of altitude change (air density change) the DEEC adjusts fan speed. This feature is sometimes referred to as "locked-throttle climb/cruise". Generally, this feature is active when operating on the Mn schedule. However, there are certain ambient conditions when climbing or cruising at low altitudes (i.e.

less than 10,000 feet) and cooler air temperatures (less than EFR temperature) when this feature may not be available. From our discussions concerning FR/Mn schedules, it could be said that the climb/cruise feature is normally functioning on the Mn schedule and not on the FR schedule. Because of the low wins circuit, the N2 governor schedule will be selected as the lower of the N2 speed/ITT limit when operating on the FR schedule.


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26 RESTRICTED PERFORMANCE RESERVE Restricted performance reserve (RPR) is a feature available on some Dash 5 engine DEEC's that is designed to enhance hot day/ high altitude takeoff characteristics. The system is designed to provide an increase over normal takeoff power when operating at pressure altitudes of 3000 to 7000 feet with total inlet (TT2) temperatures above 18.5°C (65°F). This graph shows the approximate performance characteristics with and without RPR power at 5000 feet. In accordance with the takeoff procedures defined in the flight manual, the pilot arms RPR prior to takeoff by depressing a cockpit switch. Activation of RPR results only when the PT2 and TT2; as sensed by the DEEC, is within the envelope of 3000-7000 feet and above 18.5°C. RPR operates on a single engine basis since the inlet pressure and temperature sensed by the DEEC may vary slightly from engine to engine. Because of this, it is possible to have RPR activated on only one engine although all engines are armed. Advancement of the throttle from idle to maximum will provide a proportional increase in RPR. RPR is a schedule that is programmed into the DEEC. The program utilises the engines Pt2 Tt2 inputs to activate the system. The RPR schedule will allow partial system activation during certain operational conditions that in turn minimise thrust variants between engines. Once RPR is armed, it is automatically activated when needed. There is no provision to manually override this restricted feature. However, there is an overriding feature in the RPR control system that eliminates the possibility of a rapid decrease in the N2 control speed and thrust resulting from an inadvertent de-activation of the cockpit switch. A time delay circuit of 10 RPM per second will transition the RPR N2 set point to the normal N2 set point. For example, it

would take a total of 21 seconds if the system were providing full RPR to reach normal N2. An immediate increase in thrust would be available if the switch was reactivated.


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27 RPR LOGIC Once armed, the DEEC's N2 and ITT schedules are automatically reset when in the appropriate pressure and temperature envelope. RPR will activate when Tt2 is above 18.5°C and when Pt2 is within the region bounded by pressure altitudes of 3000 to 7000 feet as shown here. Notice that a maximum of 210 N2 RPM (0.7% N2) is allowed for Tt2 temperatures above 23°C and in pressure altitude range of 4000 to 6000 feet. The lowest N2 is selected between the pressure and temperature schedules to bias the maximum N2 and ITT (T5) takeoff schedule. RPR cycle logging has been incorporated into the DEEC. The system will log a cycle if the cockpit switch has been armed and actual N2 speed is greater than normal N2 takeoff spool speed. The number of RPR cycles logged will be displayed on the DEEC front panel. Use of RPR will affect engine cycle count, and as such, the maintenance manual should be consulted for actions to take in making the appropriate engine logbook entries. It is possible to test the RPR system functionally on some installations at idle. Check the maintenance manual for specific procedures.


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28 EEC ADJUSTMENTS Provisions are made on the electronic control for the adjustments shown here. Part numbers 2101142 and 2101144 EEC's are calibrated to meet expected requirements for a nominal engine and will require trimming (adjustment) to match the individual engine performance characteristics. EEC adjustments are required when a new or replacement EEC is installed, or when an engine is changed. Since each engine application may be different in areas of performance (i.e. flat rate, T/0 N1 speed), specific adjustments for your particular installation are outlined in the Adjustment/Test section of the maintenance manual. Our discussion of EEC adjustments is intended to gain an understanding of the EEC function, the expected engine response and how maladjustments can cause undesirable engine performance.


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29 ACCELERATION ADJUSTMENT An 11-position guarded adjustment switch is located on the front face of the EEC. This adjustment, often referred to as specific gravity (S.G.), is used to adjust the engine acceleration rate. Note that with the switch full counter clockwise, the pointer is at position one. The position numbers shown here do not appear on the control. The adjustment positions are established by counting detent positions from the CCW stop on the control. Clockwise adjustment will result in faster engine acceleration and slower deceleration. Counter clockwise adjustment will result in slower engine acceleration and faster deceleration. Excessive clockwise adjustment is undesirable since it may result in engine compressor instability (surge) during rapid power lever advance. Excessive adjustment in the counter clockwise direction is also undesirable in that it may reduce engine acceleration capability to an undesirable extent and may result in lean blow out. To compensate for individual engine characteristics or installation effects a typical variation in acceleration adjustment setting (S.G.) may be to decrease by one value (setting number) or increase by two values (setting numbers). A secondary use of this adjustment is to compensate for different heating values (BTU/LB) of authorised fuels (Jet A/Jet B) by increasing or decreasing fuel flow to obtain the same operating characteristics. This setting has no effect on other EEC adjustments. Always refer to the aircraft flight manual and/or appropriate aircraft document for authorised fuels, and the maintenance manual for specific adjustment procedures.


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30 EFR ADJUSTMENT During the theory of operation discussion in Chapter 77 of this study guide, it was stated that the engine flat rate (EFR) was the temperature at which the FR and Mn schedules intersect. This graph depicts an engine flat rated at 22°C. That is to say, that the engine will produce rated thrust at sea level at outside air temperatures up to 22°C. Since the part numbers 2101142 and 2101144 EEC's are applicable to a wide range of aircraft installations, adjustment of EFR for the specific installation is necessary. Turning the EFR adjustment screw on the rear panel of the EEC facilitates the adjustment. The adjustment is required when a new or replacement EEC is installed. Referring again to the graph, if a value were assigned to the diagonal FR line and to the curved Mn line, the point at which the two lines intersect would be equal in value. Using the example, at 22°C the value of FR and Mn would be the same, however, at 27°C the value of FR would be greater than the value of Mn. Conversely, at 10°C the value of Mn would exceed that of FR. Reading the value of FR and Mn and recognising the required difference (Delta) between the two at any OAT would then provide the basis for EFR adjustment. For example, at 27°C OAT and at idle, the value of FR was 4.875 VDC and the value of Mn was 4.850 VDC. The voltage difference (delta) is 0.025 VDC, and FR exceeds Mn. Using our example, a chart in the manual indicates that the delta voltage at 27°C should be 0.035 VDC. EFR must be adjusted until the required delta of 0.035 VDC is obtained. This value indicates that EFR is adjusted to 22°C. Precise adjustment can be accomplished using the EFR tester. The procedures for EFR adjustment are detailed and specific for each installation. Improper adjustment can cause power lever splits at some ambient conditions. In the event a tester is not available, an alternate adjustment procedure can be used. Referring to data on a decal adjacent to the EFR adjustment screw and using the procedures outlined in the maintenance manual facilitates the alternate adjustment procedure.

To ensure that climb/cruise N1 can be attained and prevent power lever splits, the adjustment should be verified when the tester is available.


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31 TAKE-OFF POWER

31.1 N2 Governor

Adjustment of takeoff N1 (FR/Mn) and idle speed is accomplished by adjusting the N2 governor schedule. Although N1 speed is monitored at idle and at takeoff power, the gas generator speed is adjusted to attain the desired N1. Idle RPM will vary with ambient conditions to provide 200-300 pounds idle thrust depending upon the aircraft type. Takeoff N1 will also vary with ambient conditions, and as discussed earlier, will provide takeoff thrust for existing conditions. Referring to the graph, notice that the power lever angle (PLA) determines N1 speed. Idle adjustment will not affect the maximum RPM (FR/Mn) but will affect idle and intermediate positions. At idle, a range of plus or minus 10% N1 adjustment is available. At maximum PLA, the adjustment range is plus or minus 10% N1. This adjustment will affect both the FR schedule and Mn schedule but will not affect idle. Adjusting idle speed is accomplished by turning the idle adjustment on the front face of the EEC. The adjustment procedures are outlined in the maintenance manual. A table is provided to determine N1 speed requirements based on OAT and pressure altitude. When adjusting idle speed, engine bleed air demand should be zero and engine accessory drive loading should be at minimum. Takeoff N1 speed (FR/Mn) adjustment is accomplished by turning the FR/Mn adjustment screw on the front face of the P/N 2101144 EEC and on the back face of the P/N 2101142 EEC. The FR/Mn adjustment is determined by obtaining the prevailing OAT and pressure altitude and, with this information,

referring to the aircraft flight manual and/or appropriate flight document to determine the required N1 speed. Obtain the OAT from a shaded area immediately outside the aircraft. Reflected heat from the parking ramp or shaded temperature probes of aircraft temperature indicating systems may provide erroneous temperature indications. To obtain the pressure altitude, set the aircraft altimeter to 29.92 inches of mercury (1013 HPa) and read the pressure altitude directly from the altimeter.


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When adjusting FR/Mn it is important to operate the engine at maximum power for three minutes to stabilise conditions. Failure to thermally stabilise the engine prior to making adjustments may result in power lever splits during flight. Referring back to the graph, notice that at maximum power, the EEC will select the lowest of the N2 or T5 schedule. If, for example, the T5 limit was established below the N2 governor schedule (FR/Mn) increase of the FR/Mn adjustment would not result in an increase of N1 speed because the EEC would select the T5 schedule. The T5 limiter is preset on the 2101142 and 210144-1/-2 controls and does not require adjustment. The T5 limiter on EEC P/N 2101144-5/-6 is adjustable to accommodate a wide range of aircraft installations with varying temperature limits. The limiter is adjusted by turning an adjustment screw on the back face of the EEC. The adjustment procedures are outlined in the Adjustment/Test section of the maintenance manual. A procedure to check the T5 limiter setting termed "Over temperature Limiting Check" is outlined in the maintenance manual. This check, if incorrectly performed, may cause excessive ITT. Consult the manual prior to performing any adjustment.

32 DEEC ADJUSTMENTS All connections and adjustments are located on the front face of the DEEC. Electrical inputs and outputs interfacing with the cockpit, engine, and synchroniser/APR controller (if installed) are made through the J1 connector. The J2 connector is utilised for test equipment. The PT2 port receives the pressure input from the PT2 sensor. The air filter protects the unit from atmospheric contamination. The four-digit LED display is utilised to communicate detected faults and display adjusted values. The calibration switch is used to alter adjusted values or initiate erasure. The 11-position function select switch is utilised to select modes of operation for various DEEC functions. Position 1 is the normal run position. If the switch is

not in position 1, the cockpit manual mode annunciator light will blink on and off. Returning the function select switch to position 1 following adjustment stores the adjusted values. If the DEEC is de-energised prior to returning the function select switch to position 1 following adjustment, the new adjusted values will not be retained in memory. The range of all adjustments is limited. When the maximum or minimum limit is reached, the digital display will blink on and off.


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The operations and functions of DEECs are tailored to specific engine applications and as such, the control mode for each function switch position is not standardised for all installations. The discussion of DEEC adjustments will concern the capabilities and features available. Consult your maintenance manual for instructions concerning your specific installation. Two levels of adjustment capabilities are provided by the DEEC. Level 1 is for those routine line maintenance functions related to normal engine performance. Level 2 is for specialised adjustments for specific applications and may require the use of test equipment. The level 2 adjustments and displays are outlined in the applicable maintenance manual and will not be discussed in this study guide.

33 LEVEL 1 ADJUSTMENT Before discussion of the actual adjustments, some notes concerning all models DEEC's are offered. Position 1 of the function select switch is the normal run position. If the function select switch is in any other position, the cockpit annunciator (manual mode) lamp will blink off and on. A blinking display on the face of the DEEC (LED) indicates that the minimum or maximum adjustment range has been obtained. Adjustments are stored in memory by returning the function select switch to position 1 after adjustment and allowing one minute for the DEEC to store the adjustment. If the DEEC is de-energised prior to returning the function select switch to position 1 following adjustments, the new adjusted values will not be retained in memory and must be re-accomplished. The DEEC LED will display uncompensated N1 speed. All speed adjustments should be made using the cockpit indicator. Position 2 is utilised to perform a self-test of the DEEC and certain external components that interface with the DEEC. This test can only be performed with the engine shut down. Detected faults, if any, will be displayed on the front panel readout. A decal on the DEEC lists the codes for the various

components. This information is also located in the troubleshooting section of the maintenance manual. An additional function of Position 2 is the engine identification providing a cross check between the ID code generated by the engine harness and the ID code displayed on the DEEC front panel. This is to ensure that the appropriate DEEC functions are available for each particular engine application. The DEEC will transfer to manual mode if the engine harness ID does not match the appropriate DEEC ID. The displayed ID code number is adjustable and must match the unique code assigned to the aircraft manufacturer and engine application. Moving the calibration switch as necessary to achieve the desired ID code alters adjustment. The appropriate code can be found in the Adjustment/Test section of the maintenance manual.


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33.1 Acceleration

The acceleration (SG) adjustment is performed by placing the function switch in the applicable position and moving the calibration switch in the appropriate direction to obtain the desired setting number in the display. This setting has an effect on engine transient operation (i.e. starting, accelerating, and decelerating) and surge valve scheduling. Steady-state operation is not affected (i.e. RPM setting). Decreasing the setting number results in slower acceleration and faster deceleration. Extreme maladjustment to a lower than recommended setting could result in flameout during rapid deceleration. Increasing the setting number results in faster acceleration and slower deceleration. Extreme maladjustment to a higher setting could possibly induce compressor stall during rapid acceleration. This adjustment is primarily used to adjust engine acceleration response.

33.2 ITT Limiter

The ITT limiter (T5) can be checked and reset by moving the function switch in the required position. The setting of the ITT limiter is displayed in degrees Centigrade on the DEEC LED display. Moving the calibration switch in the appropriate direction until the desired setting value is displayed performs adjustment.

33.3 Engine Flat Rate

The EFR adjustment procedure has been made significantly easier when compared to the EEC procedure. With the function select switch in the appropriate position, the sea level EFR temperature will be displayed on the DEEC LED in degrees Celsius. Move the calibration switch in the appropriate direction to obtain the required EFR temperature. This adjustment can be accomplished without operating the engine.

33.4 Takeoff N1 (FR/Mn)

With the function select switch in the appropriate position, the uncompensated N1 for the prevailing ambient conditions will be displayed on the DEEC LED. Adjustment is accomplished by moving the calibration switch as necessary to obtain the desired N1 as indicated by the cockpit indicator. The power lever must be at the maximum position during the adjustment. This adjustment alters both the FR and Mn schedule simultaneously, and automatically performs climb/cruise adjustment on the Dash 5/-5A engine DEEC. As with the EEC FR/Mn adjustment, first record takeoff power setting (N1) from the aircraft flight manual and/or appropriate aircraft document for current OAT and indicated pressure altitude. Operate the engine at maximum power lever position and move the calibration switch in the appropriate direction to obtain the required takeoff power setting (N1) on the cockpit N1 indicator. A three-minute stabilisation period at maximum power lever position is required to thermally stabilise the engine. During the three-minute stabilisation, the DEEC display will blink. The DEEC display will stop blinking after the stabilisation period and display a continuous N1 indication. The steady N1 display indicates that the engine is stabilised and the function select switch can be returned to position 1 to store the FR/Mn adjustment, and on Dash 5/-5A engine DEEC's, the climb/cruise adjustment. Failure to thermally stabilise the engine can result in power lever splits at takeoff, climb and cruise. During previous discussions of the Dash 5/-5A engine DEEC climb/cruise schedule, the relationship of the N2 governor schedule and the T5 schedule was graphically shown. A review of that schedule reveals that the engine is governed on N2 at low and high PLA, however in the climb/cruise power lever range the engine is governed on T5. During DEEC adjustment, a relationship of N1/ITT is established and automatically computed during the stabilisation period. This N1/ITT relationship will determine N1 speed in the climb/cruise range, consequently any deviation from established LMM adjustment procedures may cause power lever splits and other performance problems.


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33.5 Idle Adjustment

Idle adjustment is accomplished after the engine has stabilised. Using the prevailing OAT and pressure altitude, consult the applicable table in the maintenance manual to determine idle N1 RPM. As with the EEC, adjustment of idle should be done with minimum accessory loading and no bleed air extraction from the engine being adjusted. With the power lever set at idle, turn the function select switch to the appropriate position and the uncompensated reduced idle speed will be displayed on the DEEC in percent N1. Move the calibration switch in the required direction to obtain the desired idle speed displayed on the cockpit indicator. Turn the function select switch to position 1 for one minute to store the adjustment. Normal idle speed is automatically set during adjustment of reduced idle on those installations so equipped. Verification of the normal idle can be done by selecting normal idle in accordance with your aircraft maintenance manual.

33.6 When Changing a DEEC

After installation of a new or replacement DEEC, or after engine installation, DEEC adjustments are required. In addition to those adjustments outlined here, some installations require adjustments that necessitate access to level 2. Check the maintenance manual for procedures to accomplish this. On those installations with restricted performance reserve (RPR) if a new DEEC is installed but the engine remains on the aircraft, all stored RPR counts must be transferred from the old DEEC to the new DEEC. Counts are applied to life-limited components. The specific procedures for your application are listed in the maintenance manual. The digital circuitry and non-volatile memory features of the DEEC allow simplified fault interrogation of the DEEC and input/output components. During our discussion of the fault monitor system next, we will return our attention to the function select switch and examine some additional features.

The N1 control DEEC's do not require any operational adjustments only static adjustments. The basic adjustments are: 1 Acceleration, 2 Engine ID, 3 N1 compensator, and 4 RPR/APR count. Check the maintenance manual for correct adjustment values.


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34 EEC EXTERNAL FAULT MONITOR SYSTEM In addition to monitoring input signals and internal schedules, the EEC continuously monitors electrical components. The external fault monitor system surveys the interface circuits shown here by superimposing a low DC current through the circuit. Should one of the above listed faults occur, the EEC would automatically transfer from normal mode to manual mode. An electrical open in the N1 or N2 monopole circuit will cause the EEC to transfer to manual mode. After compliance with SB TFE731-76-3018 (P/N 2101144-5 EEC) loss of N2 signal (shorted) while the engine is rotating will also cause transfer to manual mode. Previous EEC's do not have this feature and if N2 signal is lost, the engine may decelerate to idle or flameout. An electrical open in the power lever potentiometer circuit, manual mode or over speed solenoids will cause transfer to manual mode. Electrical opens in the surge bleed valve solenoids will also cause a transfer to manual mode, however, power lever position has an effect. For example, if Solenoid "A" is open and the power lever is in cut-off or in the idle range, conditions would be normal. As the power lever is advanced and solenoid "A" is energised to close the bleed valve, an open in the circuit would cause the EEC to transfer to manual mode. An open in Solenoid "B" would cause the EEC to transfer to manual mode when power is applied to the EEC. An electrical open or shorted TT2 circuit will cause the EEC to transfer. When a listed fault has persisted for approximately 50 milliseconds, the EEC automatically transfers to manual mode. The fuel control torque motor contains two matched coils that are monitored for differential resistance. A differential greater than 10 ohms will cause the EEC to transfer to manual mode. If an open were to occur in one coil, the fault monitor circuit would sense 200-260 ohms resistance in one coil and infinite resistance in the other, causing a transfer to manual mode. Conversely, if the torque motor connector was disconnected, the fault monitor circuit would sense infinite resistance in both coils less than 10 ohms differential and the EEC would not transfer to manual mode.

With no torque motor signal, the engine would accelerate to the mechanical over speed governor setting (105% N2) of the fuel control.


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35 DEEC FAULT MONITOR The DEEC provides built-in test (BITE) capability that monitors itself as well as the input/outputs previously discussed. The non-volatile memory allows delineation of essential and non-essential system faults. Essential faults (those causing transfer to manual mode) and non-essential faults are displayed on the DEEC display when the function switch is in the appropriate position. With the engine shut down and with electrical power applied to the DEEC, placing the function selector switch to position 2 will cause the DEEC to perform a self-test. Any detected faults will be displayed on the front panel display in a two-digit code. The fault codes are listed on a decal on the front of the DEEC. A typical BITE check is as follows: 1 Turn on electrical power supply to digital electronic engine

control. 2 Turn function selector switch to position 2. 3 If the front panel display stays blank, check DEEC power

circuit. 4 The DEEC control front panel display will now show in

sequence the following displays: (a) 8888 (b) .... (c) software version (example 1.60)

5 Self test of DEEC starts, any faults that are detected during this test will be displayed on the front panel display. See LMM for LRU fault display codes.

6 If the code 00 is displayed, no faults are detected. Code 11 indicates a failure within the DEEC, requiring replacement of the DEEC. Display of code 12 indicates that an adjustment is out of range. This would cause the DEEC to transfer to manual mode during power-up. Readjustment of the DEEC would be required if code 12 is displayed.

In addition to those faults that cause the EEC to transfer to manual mode, the DEEC also monitors T5 for opens and shorts. An open or shorted thermocouple will cause the DEEC to set the T5 input at 260°C and fault code 06 would be logged. Although this condition will not cause a transfer to manual mode, the engine operation will be significantly different. On some applications when the engine is shut down, the manual mode annunciator will flash, indicating to the pilot that maintenance action is required.


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The effect that an open or shorted T5 thermocouple has on engine operation will be examined in the troubleshooting description. Placing the function selector switch in position 3 displays all faults that have occurred since the last erasure. This information is especially useful as an aid to diagnosing intermittent or in-flight problems that cannot be duplicated during ground run-up. If the fault does not currently exist, it identifies the circuit on which troubleshooting should be concentrated. Selecting position 10 and holding the calibration switch in the increase position until the display blinks 88 accomplish erasure of the fault memory. The display will blink 88 during the erasure then display 00 for a second indicating erasure is complete. It will then go blank. The DEEC is ready to store any future faults.


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