TFE 731 Maintenance (1)

Garret TFE 731 Turbofan Engine (CAT C) MAINTENANCE

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FOR TRAINING PURPOSES ONLY

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INTRODUCTION 0

TABLE OF CONTENTS 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 18.1 18.2 19 20 21 22 23 24 25 26 27 28 29

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Control System Operation Thrust Setting, Normal Mode Engine Control System Engine Control Indicators Engine Start Panel Operating Limits Pre-Start Procedure Abort Start Initiate Start Cut-off to Idle Light Off Starter Dropout Idle Acceleration from Idle Take-Off N1 Graph Take-Off Power (-10C, Sea Level) Take-Off Power (35C) Operating Characteristics Normal Mode Manual Mode Power Lever, Manual Mode Normal Mode vs. Manual Mode (35C) Normal Mode vs. Manual Mode (0C, Sea Level) Engine Shutdown Procedures Adjustment Problems EFR Adjustment Take-Off Power DEEC Climb/Cruise T5 Limiter Adjust Items Affecting Throttle Matching Power Lever Splits


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INTRODUCTION 0

TABLE OF CONTENTS 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 18.1 18.2 19 20 21 22 23 24 25 26 27 28 29

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Control System Operation Thrust Setting, Normal Mode Engine Control System Engine Control Indicators Engine Start Panel Operating Limits Pre-Start Procedure Abort Start Initiate Start Cut-off to Idle Light Off Starter Dropout Idle Acceleration from Idle Take-Off N1 Graph Take-Off Power (-10C, Sea Level) Take-Off Power (35C) Operating Characteristics Normal Mode Manual Mode Power Lever, Manual Mode Normal Mode vs. Manual Mode (35C) Normal Mode vs. Manual Mode (0C, Sea Level) Engine Shutdown Procedures Adjustment Problems EFR Adjustment Take-Off Power DEEC Climb/Cruise T5 Limiter Adjust Items Affecting Throttle Matching Power Lever Splits


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GROUND /FLIGHT CHECK-OUT 1

CONTROL S YSTEM OPERATION To this point, discussion of the engine was centred on specific isolated systems. The engine operating characteristics characteristics are a result of the the interaction of all systems. The engine discussion discussion in this section section will examine the operating characteristics and review normal and abnormal engine operating conditions. conditions. The engine operation will consist consist of a routine engine start and and operation with emphasis on normal parameters. parameters. To accomplish this, a "generic" engine installation with "typical" cockpit switches and indicators will will be used. While not intended to to reflect any specific aircraft installations, the configuration is applicable to most. The TFE731 engines utilise two two control systems. The primary system consists of an electronic engine control that schedules fuel through an electro/hydro-mechanical fuel control. This is the normal mode of operation. The back-up system consists of a hydro-mechanical fuel control incorporating a centrifugal centrifugal speed governor. This is termed manual mode and is used in the event that the EEC/DEEC becomes inoperative. Dispatch of the aircraft in manual mode is governed by specific aircraft flight manual/pilot operating handbook procedures, and as such will not be discussed in this section.

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Garret TFE 731 Turbofan Engine (CAT C) MAINTENANCE 2

THRUST SETTING, NORMAL MODE During the fuel system review, power lever angle (PLA) was used in reference to degrees movement of the fuel control input shaft. In normal mode, PLA is sensed in the electronic c ontrol by varying voltage from the PLA potentiometer. Cut off is referenced at 0° and idle stop is 20°. With the engine operating at idle power, movement of the power lever from 20 to 26° (40° on some models) does not change engine speed. This "dead band" area of the PLA is the idle range. The thrust setting range is from 26(40) to 120°. Movement of the power lever to the maximum position (120°) is the normal takeoff power position.

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ENGINE CONTROL S YSTEM

3.1

Related Components The power levers are mechanically linked by teleforce and Teleflex to the engine fuel control. The PLA is sent to the electronic control in the form of a voltage by the power lever potentiometer in the fuel control. The EEC/DEEC then sends a signal to the fuel control, adjusting fuel to correspond to power lever position. Initial travel of the linkage opens the fuel valve during both normal and manual modes of operation. Placing the power lever in the cut off position manually closes the fuel valve. This provides a positive mechanical means to control fuel to the engine. The ignition system is energised by microswitches located in the power lever pedestal. The annunciator panel contains engine related advisory lights for oil pressure, fuel pressure and electronic control. When the electronic control is off, or in the manual mode and when the oil or fuel pressure is below a determined value, the lights will illuminate.

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ENGINE CONTROL INDICATORS Five indicators mounted on the main instrument panel can monitor engine operation. Shown are the N1 and N2 spool speed indicators, ITT, fuel flow, oil pressure and oil temperature indicators. Note that the "typical" N1, ITT and fuel flow indicators shown throughout this section have a digital indication in addition to the analogue. Range marks are commonly used to provide instantaneous visual reference.

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ENGINE START PANEL A typical engine start panel, normally located on the aircraft overhead panel provides the necessary switches for engine starting. A two-position switch provides for starting from the battery buss (normal) or from external power. The switches labelled "CMPTR 1" and "CMPTR 2" controls the electronic engine control. Those engines equipped with a DEEC utilise a three-position switch as shown here, while EEC's utilise a two-position on/off switch. The "IGN" lamps will illuminate when the ignition system is energised. The three-position switches labelled "IGNITION-START-MOTOR START STOP" is used to select the conditions for an engine start. The up position provides continuous ignition. The centre position is used for normal starts. Moving the switch down disengages the starter and is used for engine motoring. Pushing the “PRESS TO START” push buttons energises the starter. During subsequent operating procedures, the operation of switches and controls with reference to the engine function does not reflect a specific installation. Some aircraft installations may combine switches with each other and with other controls. Refer to your specific aircraft document for cockpit control arrangement and operation.

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OPERATING LIMITS It is important to be aware of engine operating limits prior to operating the engine. The primary document is the flight manual. A table of limits is located in the engine maintenance manual and contains the necessary corrective actions if limits are exceeded. The table includes engine start times, oil pressure and temperature limits, ITT limits and rotor speed limits. The table and the approved aircraft flight manual should be reviewed prior to operating the engine.

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PRE-START PROCEDURE Prior to starting the engine, perform a pre-flight inspection or "prior to run" inspection as outlined in the aircraft flight manual. Ensure that the aircraft covers for engine intake and exhaust are removed. Check the run-up area for any loose objects near the engine air inlet and exhaust. Remove any objects that could interfere with the air intake. Check all engine controls for freedom of movement. Note indications on engine control indicators. If available, use an external power source for engine starts. If used, check to ensure proper connection and that the unit is capable of 1000 amp minimum capacity. Do not run the engine without the proper aircraft primary (core) exhaust nozzle and aircraft inlet duct installed. Engine damage will result if ducts are not used as specified. Review the applicable emergency procedures prior to attempting a start. Consult the aircraft flight manual for specific engine start/abort procedures and/or engine fire-on-ground procedures.

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ABORT START Before continuing a typical start sequence, a review of the conditions requiring an aborted start is necessary. If an indication of ITT has not occurred within 10 seconds after power lever movement, or ITT is near the limit and climbing rapidly, the start should be aborted. As N2 RPM increases to 20%, an N1 indication should be displayed. Abort the start attempt if N1 RPM is not indicated when N2 accelerates to 20% RPM. No indication of oil pressure within 10 seconds of light off is cause for abort. A start should be aborted if the allowable acceleration time is exceeded or if the engine continues to accelerate above idle. To abort a start attempt, first place the power lever in cut off and move the start selector switch to the "motor/start/ stop" position. It should be emphasised to review the specific start procedures applicable to your installation prior to attempting a start.

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INITIATE START With the electronic control on, aircraft "CMPTR" and "FUEL" lamps off, ignition switch in the normal start position, the engine is ready to start. Prior to initiating a start, the residual ITT should be below 300°C. Start can now be initiated by pressing the start switch. An increase in N2 RPM should be observed. N2 should accelerate smoothly.

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CUT-OFF TO IDLE When N2 RPM increases to 10-15%, note cockpit N1 indication. If positive N1 speed is indicated, move power lever from cut-off to the idle position. Movement of the power lever opens the high pressure fuel shutoff valve in the fuel control and through a micro switch operation in the throttle linkage, energises the ignition unit. An indication of fuel flow and ignition should also be observed at this time. Note in the illustration an indication of N1 when 10-15% N2 is attained. This is a normal indication for initial start and depicts a free-turning LP spool. The prudent operator will insure that the LP spool is free prior to moving the power lever out of the cut-off position. Failure to do so could result in turbine distress or damage.

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LIGHT OFF An increase in ITT indication should be observed within 10 seconds of power lever movement. The increase in ITT shown here indicates a good start. Remember that the electronic control will provide automatic fuel enrichment to approximately 200°C. As N2 increases, fuel flow will increase. ITT should be monitored at this point to ensure that the established limit is not exceeded.

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STARTER DROPOUT An indication of oil pressure should be observed within 10 seconds of light off. As N2 RPM reaches 50%, check for the ignition light out. This indicates that the ignition circuit has been de-energised and the starter has disengaged. Generator function can be confirmed at this point by observing the aircraft DC load meter. The ITT indication should have peaked at this point and begin to stabilise. Observe that N2 and N1 are accelerating to idle.

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IDLE Observe that N1 and N2 stabilise at idle speed. The indications shown here reflect approximately what the operator will see at idle. The specific value will be determined by engine type, aircraft installation and OAT. The important thing to emphasise at this point is that the spool speeds have stabilised, oil pressure is above 25 PSIG and the temperature is within the operating range. A normal characteristic of the 731 engine that may be observed at idle speed (28.5% N1) 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 cycling by increasing the power lever position to provide 250 to 300 pounds per hour fuel flow, which stops cycling. The cycling characteristic is normal at low flow rates and has no effect on operation or flight safety.

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ACCELERATION FROM IDLE Movement of the power lever above the idle dead band causes the surge bleed valve to close. This is indicated by a slight reduction in ITT as the engine starts to accelerate, followed by the normal expected increase in ITT. The engine should accelerate to the selected setting surge free and stabilise at that setting. The normal characteristic of the N2 controlled 731 engines is for N1 and ITT to overshoot the values required for a stabilised power setting. This is due in part to the fact the engine speed is controlled by the N2 governor. The N1/ITT overshoot is most pronounced during rapid power lever advancement. These overshoots are not cause for concern provided allowable transient limits are not exceeded. The N1 controlled 731 produces required N1 with minimal overshoot. This is due to the way the digital control is programmed to produce speed.

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TAKE-OFF N1 GRAPH Takeoff power is determined based on ambient conditions. The graph shown here is typical of the chart used by the pilot to determine takeoff performance. Located in the aircraft flight manual performance section, takeoff power is determined by entering the chart at the bottom with the OAT and moving upward until either the flat rate (FR) line or the maximum speed (Mn) line is intersected. The percent N1 displayed on the left of the chart depicts takeoff power. Note that the pressure altitude is a determining factor in N1 speed when operating on the flat rate sc hedule.

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TAKE-OFF POWER (-10 C, SEA LEVEL) This depicts typical indications for TFE731-5 engine at takeoff power for -10°C OAT and sea level pressure altitude. Remember that N1 RPM does not remain at 100% for all takeoff power settings. N1 RPM will be controlled by the electronic control and is determined by OAT and pressure altitude. Using the conditions depicted here note that with matched N1 speeds, ITT and fuel flows vary. This is typical and is due to individual differences in the engines.

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TAKE-OFF POWER (35 C) Notice the changes that take place as the OAT increases to 35°C. N1 speed is higher, ITT's are approximately 20° higher. Though difficult to discern, N2 RPM is 98.4% as compared to 91.6% on the previous example. Oil pressure is essentially the same, however oil temperature is higher, caused by the higher ambient at the oil coolers. Again, using the conditions depicted, note that N1's are matched. The engines are operating on the Mn schedule. The indications shown here and on the preceding page are not intended to reflect the expected or normal indications for any specific installation. They are intended to show the differences in operating parameters. While the specific values will vary due to engine type and installation, matched power levers should produce matched N1's.

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OPERATING CHARACTERISTICS

18.1

Normal Mode When operating the engine in normal mode, the electronic engine control provides automatic start sequencing and engine parameter control. Some of the operating characteristics are outlined above. The power lever operates the high pressure fuel shutoff valve located in the fuel control. During start, the first 8-12° travel of the power lever opens the shutoff valve. The power lever sets N1 RPM between idle and maximum power setting. During start, the EEC provides automatic enrichment to provide a smooth acceleration to idle speed, and automatically deactivates the ignition and starter after start. The electronic control will set the minimum power setting for idle thrust based on OAT and pressure altitude. Maximum thrust will be set for takeoff power by the electronic control when the power lever is in the maximum position. Engine speed and temperature limits are provided by the electronic control.

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Garret TFE 731 Turbofan Engine (CAT C) MAINTENANCE 18.2

Manual Mode The engine operating characteristics change when operating in manual mode. As with normal mode, the power lever provides positive control of fuel. With the electronic control inoperative, engine RPM is controlled by the power lever. Movement of the power lever controls N2 RPM through a mechanical governor in the fuel control. Maximum ITT is limited by the fuel control. The maximum available thrust is generally less than normal mode due to the adjustment of the mechanical governor. Engine response to power lever movement is slower due to the surge bleed valve in the one-third open position. This results in reduced engine efficiency while maintaining an optimum surge margin. The effect that these changes have on engine operation will be examined later in this section. Since all the normal mode temperature and speed protection schedules are inoperative during manual mode operation, the operator must closely monitor engine spool speeds and temperature to prevent exceeding established limits. For this reason, manual mode operation is not approved for starting or takeoff. Dispatch of the aircraft with one electronic control in the manual mode can, under some circumstances, be approved. Specific procedures and requirements are maintained within your specific aircraft flight manual and local flight regulatory authority, and as such, are not discussed in this study guide.

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POWER LEVER, MANUAL MODE The power lever functions the same in manual mode as normal mode. Initial travel of the power lever opens the fuel shutoff valve. Cut-off is referenced at 0° PLA, idle at 20° PLA and 120° PLA is maximum thrust setting. Note that in manual mode, the thrust setting range is from 20° to 120° with no dead band. Movement of the power lever resets the governor in the fuel control for selected N2 speed.

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NORMAL MODE VS. MANUAL MODE (35 C) An examination of normal mode and manual mode to compare parameters is appropriate prior to completing our operational sequence review. The parameters illustrated are with the left engine in normal mode and the right engine operating in manual mode. At 35°C OAT, the normal mode engine parameters are the same as depicted earlier. Notice that at this OAT, N1 is only slightly lower than normal mode, but less than "target N1". ITT is near the 952° limit imposed on this engine type, and N2 is near the limit at 99.3 percent. The significant indication of engine efficiency as compared to the normal mode is in fuel flow. The manual mode engine with the bleed valve in onethird open position requires a higher N2 RPM, fuel flow and consequently higher ITT to maintain almost the same N1. Notice that oil pressure and temperature are not appreciably affected by manual mode operation.

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NORMAL MODE VS. MANUAL MODE (0 C, SEA LEVEL ) When making the same comparative check on cold day conditions, the parameters are quite different. Again, the left engine parameters reflect normal mode operation while the right engine indicates manual mode operation. With the left engine power lever at maximum, the takeoff power is set at 94.0% N1 by the electronic control. At this ambient temperature, the manual mode engine power lever is retarded to prevent exceeding normal mode takeoff thrust. Again, N2 RPM, fuel flow and ITT are high, reflecting the loss of efficiency when compared to the normal mode engine.

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ENGINE SHUTDOWN PROCEDURES The applicable flight manual procedures should be followed for engine shutdown. Although differing slightly dependent upon configuration, shutdown is accomplished by firmly moving the power lever to the cut-off position. Prior to shutting the engine down it should be operated at least two minutes at 38% N1 or below to cool down the turbine components. During flying operations, taxi time can be included in the two minute period. The LP spool can be checked for internal binding or rubbing by rotating the spool at the fan or turbine. This check, if made, should be accomplished immediately (within one minute) after shutdown. It should be understood that thermal binding of the low pressure spool might occur within 20 to 45 minutes after shutdown due to the differential cooling of spool components. This condition is normal and should not be misinterpreted as a malfunctioning low pressure spool. If this condition is encountered, wait until the engine returns to ambient temperature prior to further troubleshooting. Refer to the Troubleshooting section of the maintenance manual for specific procedures. Always allow the engine to cool down at least 15 minutes before installing aircraft covers for engine intake and exhaust. Check the oil level in the oil tank within one hour after engine shutdown. Check the indicator pin on the oil and fuel filter for bypass. If the indicator indicates bypass, refer to the LMM for corrective actions.

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ADJUSTMENT PROBLEMS Electronic control (EEC or DEEC) adjustments are an important element in satisfactory engine performance. Maladjustments can be misinterpreted to be performance-related problems. Misunderstandings concerning the governor schedules and how they are influenced by temperature or misunderstanding the adjustment procedure can cause unnecessary troubleshooting procedures. For example, the FR/Mn adjustment controls the N2 governor schedule. Adjustment will influence takeoff power. The adjustment a uthority is plus or minus 10% RPM. This adjustment will affect both the FR schedule and the Mn schedule. Common corrective action for low power on takeoff is proper readjustment of FR/Mn. The technician not considering density altitude and OAT when calculating takeoff power settings causes common maladjustments of FR/Mn. This miscalculation often results in mismatched N1's, and low power for takeoff. The engine flat rate (EFR) adjustment must be accomplished any time the EEC/DEEC is replaced. Power lever splits during climb and mismatched N1's on the ground at some ambient conditions are often caused by mismatched EFR adjustments. Aircraft utilising the DEEC incorporate a climb/cruise feature that offers reduced power lever adjustments during climb. Improper engine stabilisation procedures during FR/Mn adjustment can cause mismatched power levers/low performance during climb. Maladjustment of the T5 limiter can limit takeoff power and performance at altitude. Additionally, N1 overshoot can be affected by maladjustment of the T5 limiter. The N1 limiter on EEC's is not a field adjustment, and can cause performance problems if adjusted. Some DEEC installations do, however, facilitate the adjustment. Understanding the N1 compensation/N1 limiter system and their relationship to the aircraft indicator system will prevent maladjustment with the resultant performance problems.

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Garret TFE 731 Turbofan Engine (CAT C) MAINTENANCE In the next few pages, several adjustment problems and their results will be explored in an effort to provide a foundation for troubleshooting.

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EFR ADJUSTMENT During the examination of the electronic control schedules and the adjustment description, it was stated that the EFR adjustment was made using an EFR adjustment tester and procedures outlined in the LMM. An alternate procedure is provided when the P/N 298226-2 J2 tester is not available. This procedure provides a set number of turns of the EFR adjustment screw to provide the required setting for your installation. Looking at the above graph, it should be apparent that by adjusting one engine using the P/N 298226-2 and the other using the alternate method, power lever splits and/or failure to attain climb/cruise N1 could result. If, for example, the EFR adjustment was accomplished using the alternate procedures on a cold day, takeoff N1 may not be attained on a hot day. Further adjustments of the FR/Mn may be required to match N1 as ambient conditions change. For this reason, the more exacting adjustment procedures utilising the EFR tester provide more consistent results in performance.

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TAKE-OFF POWER This familiar graph reflects the 10% N1 adjustment range available with the FR/Mn adjustment screw. This adjustment controls the N2 governor and effects N1 speed both on the FR (cold day) and Mn (hot day) schedule. Some important considerations are made prior to adjusting the EEC (the DEEC will be discussed next). When determining takeoff power using the aircraft flight manual charts, accurate OAT and pressure altitude parameters must be used. Failure to do so could result in higher or lower N1 values for takeoff power. Accurate EFR adjustments are essential prior to performing the FR/Mn adjustment. Failure to do so could result in further trim adjustments as ambient conditions change. It is important to operate the engine for three minutes at maximum power to allow thermal stabilisation. Failure to thermally stabilise the engine prior to making the FR/Mn adjustments may result in power lever splits during flight. The FR/Mn adjustment is accomplished with the assumption that the EFR setting is correct and that the temperature signal (Tt2) sensed by the EEC is the same as that used for referencing the takeoff power chart. Since they may not be the same, N1 error may become evident when ambient conditions change, requiring further trim adjustments.

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DEEC CLIMB /CRUISE The DEEC FR/Mn adjustment differs from the EEC in that adjustment of FR/Mn automatically performs the climb/cruise adjustment on some applications. 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 in the case of Dash 5/5A engines and on TT2 PT2 in the case of the Dash 2, Dash 4 and Dash 5B (excluding N1 controller) engines. During DEEC adjustments, 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 on the Dash 5/5A engine. Again, some important considerations for correct DEEC adjustments are offered. The cockpit N1 indicators on Dash 5 power aircraft display compensated N1 speeds, while the DEEC display is uncompensated. The FR/Mn adjustment is made by moving the calibration switch as necessary to obtain the desired N1 as indicated in the cockpit - not the DEEC. The power lever must be at the maximum position during this adjustment. The three-minute thermal stabilisation period must be adhered to. The DEEC display will blink during this period. 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 and the climb/cruise adjustment.

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Any deviation from established LMM adjustment procedures may cause power lever splits and other performance problems.


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T5 LIMITER ADJUST TFE731-3 EEC's incorporate an adjustable T5 limiter to accommodate a wide range of aircr aft installations with varying temperature limits. The T5 limiter is adjusted to each specific installation and is depicted as a solid line on this graph. The N2 governor schedule is shown here as a dashed line. The limiter will allow the dynamics of the EEC to operate in such a manner as to reset (lower) the N2 governor set point. The magnitude of the N2 reset is limited to 4% RPM. Applying the "Low Wins" theory discussed earlier, it becomes apparent that a low adjusted T5 limiter could reduce takeoff power. Low adjustments have also had the effect of not being able to reach the required N1 speed at altitude. Conversely, adjustment too high could allow the engine to overtemp. With a T5 limiter adjusted too high, rapid acceleration could allow gross N1 overshoots. The only way to correctly determine the T5 limiter set point is to perform an over temperature limiting check using P/N 2899002/-3 tester assembly. Using the ITT (T5) limiter adjustment procedures in the Adjustment/Test section of the LMM will prevent incorrect T5 adjustment.

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ITEMS AFFECTING THROTTLE MATCHING As previously discussed, maladjustments of the EEC/DEEC can cause engine performance problems. Adjustments also play an important role in throttle matching. N1 idle and FR/Mn adjustments are determined using ambient temperature and pressure altitude. The EFR adjustment establishes the breakpoint between the FR and Mn schedules. Any maladjustment of these will cause a mismatch of power levers. The mechanical rigging of the fuel control must be verified in troubleshooting throttle matching problems. Since the power lever potentiometer sends a signal to the EEC to establish N1 RPM, correct rigging is essential in power lever matching. An erroneous pressure signal from the PT2 and TT2 sensor caused by leaks or obstructions would cause an incorrect N1 speed. Additionally, electrical malfunctions in the temperature sensing element can cause incorrect N1 speed. Variances between left engine and right engine sensors can cause mismatched power levers. A feature of some DEEC installations is the ability to read the temperature signal being received by the DEEC and to bias the TT2 signals to match. This procedure, if authorised for your installation, is outlined in the maintenance manual.

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POWER LEVER SPLITS At a given "matched" thrust setting (N1), it is many times observed that the resulting turbine temperature (ITT) is different from engine to engine on the same aircraft. The differences are sometimes made obvious in the cockpit by the use of "digital" N1 and/or ITT indicators. The mismatch of N2 RPM can be observed using the cockpit indicators. The normal relationship between N2 and ITT may differ from one engine to another at a matched N1 without any maintenance problem existing within either engine. This is because each engine is manufactured with specific tolerances. Individually, the tolerance band of acceptable engines can account for one of the differences in indicted engine parameters between "normal" engines. Although each engine is manufactured and assembled to exacting tolerances, the combination of many small tolerances provides no two engines that will perform identically. Each engine must however demonstrate the ability to produce specification thrust (N1) with N2 RPM and ITT within their respective limits. The operator is assured under these conditions that each engine is meeting specification requirements with some engine exhibiting greater performance margin within its normal limits. Once the engines are installed on the aircraft, there are a number of additional factors that may contribute to the difference between normal engine indications. Among these are:    

Indicator accuracy tolerances Unequal bleed air extraction High and/or low pressure bleed air leaks Nozzle (or thrust reverser seal leaks)

When engines are disassembled for maintenance or modification work, the resulting minor changes in clearances or aerodynamic flow path has some effect on engine matching upon reinstallation. These differences may increase or decrease the spread in indications between engines.

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Garret TFE 731 Turbofan Engine (CAT C) MAINTENANCE The following conclusions can be made: 1. 2.

3.

4.

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Engines may exhibit differences in ITT or N2 indications at matched N1 as a normal c ondition, both ground and flight operation. A change in the usual relationship of N1 to N2 and ITT can be expected after major maintenance or as an engine progresses towards a maintenance interval limit. A rapid change in the previous relationship of N1 to N2 and ITT between engines, during an operating period when no maintenance was performed may indicate an error in instrument calibration, a problem with bleed air management or an engine problem developing and should be reported for maintenance investigation. Due to instrument tolerances, changes greater than 25°C in IT match at matched N1 (at or near the takeoff limit) should generally be investigated. An easy first step is to inspect for fan damage. With the engine installed in the aircraft, engine performance can be verified by performing an engine calibration and performance evaluation run as specified in the maintenance manual.


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TFE 731 Maintenance (1)

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