1w11 engine insruments

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Unit level: 3Issue Date31/05/2006

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AIRCRAFT GAS TURBINE ENGINESCore code: Group A

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ENGINE INDICATING AND MONITORING SYSTEM

CONTENTS

11.1 INTRODUCTION

11.2 GROUPING OF POWERPLANT INSTRUMENTS

11.3 TYPICAL ENGINE INDICATING SYSTEM

11.4 DIFFERENT PARAMETERS INDICATED/MONITORED

11.5 ENGINE GAS TEMPERATURE INDICATION SYSTEM

11.6 ENGINE PRESSURE RATIO MEASUREMENT AND INDICATION

11.7 FLUID PRESSURE MONITORING

11.8 FLUID TEMPERATURE MEASUREMENT AND INDICATING

11.9 SPEED MEASUREMENT AND INDICATING

11.10 FUEL FLOW MEASUREMENT AND INDICATING

11.11 VIBRATION MEASUREMENT AND INDICATION

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11.1INTRODUCTION

The engine indicating parameters are displayed in the flight compartment onindicators, and, where installed, on ECAM displays or alarms, thus enabling the crewto monitor the engine.

11.2 GROUPING OF POWERPLANT INSTRUMENTS

The specific grouping of instruments required for the operation of power plants isgoverned primarily by the type of power plant, the size of the aircraft and thereforethe space available. In a single engined aircraft, this does not present much of aproblem since the small number of instruments may flank the pilot's flight

instruments thus keeping them within a small `scanning range'.

The problem is more acute in multi-engined aircraft; duplication of power plantsmeans duplication of their essential instruments. For twin-engined aircraft, and forcertain medium-size four-engined aircraft, the practice is to group the instruments atthe centre of the main instrument panel and between the two groups of flight instru-ments.

In some large types of public transport aircraft, a flight engineer's station is providedin the crew compartment and all the power plant instruments are grouped on thecontrol panels at this station. Those instruments measuring parameters required tobe known by a pilot during take-off, cruising and landing, e.g. rev./min. and turbinetemperature, are duplicated on the main instrument panel.

The positions of the instruments in the power plant group are arranged so that thoserelating to each power plant correspond to the power plant positions as seen in planview illustrated in Figure 11.1. It will be apparent from the layout of Figure 11.1 that by scanning a row of instruments a pilot or engineer can easily compare thereadings of a given parameter, and by scanning a column of instruments can assessthe overall performance pattern of a particular power plant.

Another advantage of this grouping method is that all the instruments for one powerplant are more easily associated with the controls for that power plant.



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Figure 11.1 : Powerplant Instrument Grouping


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11.3 TYPICAL ENGINE INDICATING SYSTEM

Engine indicating and status monitoring is done in modern aircraft through:

• Engine Indicators/Gauges• Electronic Central Aircraft Monitoring (ECAM) system through:

- CRTs,- Master Warning and Master Caution Lights- Central Aural Warning system: Continuous Repetitive

Chime/Single Chime

(See Figure 11.2)

Figure 11.2: Engine indicating and status monitoring arrangement on atypical aircraft



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11.4 DIFFERENT PARAMETERS INDICATED/MONITORED

(i) Power management parameters: EPR, EGT, N1, N2, FF/FU. These areindicated in indicators, ECAM pages and TRP (Thrust Rating Panel). See Figure11.3 showing sensing of these parameters and also see Figure 11.4 illustratingtheir indicators in the main instrument panel as engine main parameters in atypical engine.

(ii) System status monitoring parameters: The engine system statusmonitoring parameters are: Oil pressure, Oil temperature, Oil quantity, Oiltemperature, Fuel pressure, Nacelle temperature, Vibration measuring of the LP (N1)

and HP (N2) rotor etc . These are mostly displayed on ECAM system display units inglass cockpit. They are also displayed in gages and status indication lights. (Figure11.5)



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Figure 11.3: Engine indicating parameter sensing andindicating/monitoring



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Figure 11.4: Engine Main Parameters Indicators



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Figure 11.5: Engine system parameters on ECAM

11.5 ENGINE GAS TEMPERATURE INDICATION SYSTEM

Engine gas temperature is required to be instrumented for:

(i) Monitoring at engine start as positive indication of engine light off.(ii) Indicating Level of power(iii) Shoot up maximum temperature level at take off(iv)

Assessment of EGT margin(v) Assessing maintenance in case of exceeding red line value.

Normally, gas temperature is important at the turbine intake because it is theturbine material at the intake that is the constraint for the ceiling of the gastemperature. But, there is problem of sensing this value at this position as lifeand durability of the sensor is under question. The indicator in the cockpit maybe labeled as Exhaust Gas Temperature (EGT), Turbine Inlet Temperature (TIT),Turbine Gas Temperature (TGT), Intermediate Turbine Temperature ITT or Jet



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Pipe Temperature (JPT). These names indicate the location at which thetemperature is measured (Location of the probe). TIT is the temperature

measured between the turbine stages, TGT is the temperature measured justafter the turbine, EGT is the temperature measured at the exhaust cone and JPTis the temperature measured at the tail pipe. Indicated value of temperature areall proportional to the TIT and hence, they have the proportional figures in thered line value having the same significance. Positioning the sensors at a coolerplace than the intake of the turbine has the advantage of increased life of thesensing elements.

There are different methods and devices for sensing gas temperature. The mostwidely used method is the thermocouple system. One of the main advantages of

thermocouple-type temperature measuring system is that, it is completelyindependence of aircraft electrical system, their operation being performed throughthe electrical energy produced by the direct conversion of heat energy at themeasuring source. The principle of thermocouple is based on the Seeback effect asshown in Figure 11.6.

Figure 11.6: Engine system parameters on ECAM



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The arrangement of two dissimilar metal wires joining together in this manner iscalled a thermocouple. The junction at the higher temperature being

conventionally termed the hot or measuring junction, and that at the lowertemperature the cold or reference junction. (In practice, the hot junction isin the form of a separate unit of sensing the temperature and this is regardedgenerally as the thermocouple proper.)Keeping these two separate junctions attwo different temperatures in this manner as in Figure 11.6 causes a currentto flow in the circuit, developing a thermo-e.m.f. across the junctions.

The complete EGT system for a turbine engine consists of (a) the probes thatsense the temperature of the exhaust gas, (b) the harness that surrounds the

engine tail pipe and serves as a connection for all of the probes, (c) extensionwires that carry the current form the probes into the cockpit, (d) resistors toadjust the resistance of the thermocouple to the value required for the system,and (d) indicating instrument in the aircraft instrument panel. The probes areconnected in parallel so that their output is averaged.

There are different combinations of metals in use, the most widely used combinationis the crumel-alumel.

Figure 11.7 illustrates a system used in a modern engine to measure EGT. Thegage showing the EGT displays and different significant markings are explained inFigure 11.8.



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Figure 11.7: EGT measuring system in a typical engine.



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Figure 11.8: EGT gage showing displays and markings in detail



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11.6 ENGINE PRESSURE RATIO MEASUREMENT AND INDICATION

Centrifugal flow engines often use RPM as an indicator of the thrust being produced,but axial flow engines more effectively use the engine pressure ratio (EPR) as theprimary indicator of thrust.

This instrument, a differential pressure gauge, measures the ratio between theturbine discharge total pressure and the equivalent of the compressor inlet totalpressure. These pressures are designated suitably like PT2 and PT7 (Figure 11.9).

The compressor inlet total pressure must be corrected for inlet duct loss, and theinstrument must be calibrated for each individual type of installation. The pressure

pick up for the compressor inlet total pressure (PT) Is not usually at the face of thecompressor, but it is as near the engine inlet as is practical, and corrections basedon flight tests are automatically biased into the indication system so it will indicatethe correct pressure ratio.

Figure 11.9: Engine pressure ratio measurement



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11.7 FLUID PRESSURE MONITORING

We often need to inform the pilot or flight engineer of a condition in the engine thatis likely to indicate an impending problem. Almost all of the larger aircraft have anannunciator panel where warning lights are grouped so any problem, regardless ofthe system can be easily noted. In such fluid systems as the fuel, oil, hydraulic, orpneumatic systems, the warning lights are turned on by pressure switch.

Fuel pressure is sensed at port P1, and ambient air pressure is sensed at port P2. Youwill notice that these two preset differential pressures exists across the diaphragmassembly, and when a preset differential pressure exists across the diaphragm, theactuator arm will close the micro-switch and turn on the warning light on the

instrument panel. See Figure 11.10.

Figure 11.10: Electrical pressure switch for sensing differential pressure



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11.8 FLUID TEMPERATURE MEASUREMENT AND INDICATING

As with almost every other type of measurement used in aircraft instrumentation,the measurement of temperature is now being done electrically, with much greaterease and accuracy than was ever possible with non-electrical means. This can bedone in two ways: by measuring the change in resistance of a metal caused by achange in its temperature (in a Wheatstone bridge or ratiometer) and by measuringthe amount of voltage generated when a thermocouple junction made up to twodissimilar metals is heated (thermocouple).

RESISTANCE TYPE THERMOMETERS

The electrical properties of metals, as well as their physical dimensions, alterwith temperature. This characteristic is used when measuring the temperatureof outside air, carburetor air, oil, and even the cylinder heads in our modernaircraft. A fine nickel wound on a mica core, Figure 11.11, is placed where themeasurement is to be taken.

Figure 11.11

Some bulbs are stem-sensitive, some tip-sensitive and some fit flush with theairplane skin to measure outside air temperature. There are two basiccalibrations of these bulbs, one having a resistance of 50 ohms at zero degreesC, and the other with a resistance of 90.38 ohms at zero degrees C.



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Working Principle: Resistance-type temperature measurements may be madewith either a Wheatstone bridge-type indicator or with a ratiometer.

A. The Wheatstone bridge: Figure 11.12, operates with the principle ofcontrolling the flow of current through the indicator by varying the resistanceone of the legs of the bridge. In Figure 11.12 if the ratio of R 1 /R 3 is the sameas R 2 /X, the bridge will be balanced and the voltage at point B will be the sameas that at point C. No current will flow through the indicator.

Figure 11.12

When the temperature measured by the bulb increases, the resistance of thebulb will also increase, as will the voltage drop across it. These makes thevoltage at point C higher than at point B, so current will flow through theindicator. If the resistance of the bulb drops below that required to balance thebridge, the voltage at point C will be low enough for current to flow through theindicator in the opposite direction.



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11.9 SPEED MEASUREMENT AND INDICATING

Speed may be measured through tachometers and also by electrical means.Illustrated in Figure 11.13 is a tachometer system in a twin spool engine havingtwo tachometers, one to measure the speed of the low-pressure compressor, andthis is called N1. The other tachometer measures the speed of the high-pressurecompressor, and this is called N2. These systems are similar in operation to the ACelectric tachometer used with reciprocating engines, except that the indicators arecalibrated in terms of percent of engine RPM, and they have a vernier scale. Theindicators shown in Fig. 4B-5 show that the low-pressure compressor (N1) is turningat 97% of its rated RPM and the high-pressure compressor (NO is turning at 94% ofits rated speed. (See Fig. 4B-5 on page 40.)

Figure 11.13: Method of measuring RPM in a twin spool axial flow turbineengine.



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11.10 FUEL FLOW MEASUREMENT AND INDICATING

For turbine engines, it is important that the mass of the fuel flowing into the engine beknown, and a mass-flow type of fuel flowmeter is used. This type of instrument isshown in Figure 11.14. The impeller is rotated at a constant speed by a three-phase

AC motor, and as the fuel passes through the impeller, its rotation imparts a rotary, orswirling motion to the fuel. As this swirling fuel passes through the turbine, it tries torotate it, but since the turbine is restrained by the calibrated restraining springs, it candeflect but not rotate. The amount the turbine deflects is determined by both thevolume of the flow and the density of the fuel.

A permanent magnet is mounted on the end of the transmitter shaft, and this serves as

the rotor of a remote indicating transmitter, which is connected electrically with anindicator on the flight engineer's panel. This shows the actual number of pounds of fuelthat passes through the transmitter in a given period of time.

Figure 11.14: Mass flow type flowmeter for turbine engines.



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11.11 VIBRATION MEASUREMENT AND INDICATION

Engine vibration indication is required for analyzing mechanical integrity of theengine continuously during operation.

In many engines, vibration monitoring system uses Vibration sensors. In twin spoolturbine engines, vibration is conventionally sensed by two acceleration type vibrationtransducers, one installed on the fan bearing, the other on the turbine bearing orframe. Vibration level is displayed on an indicator located in the flight compartment.

Engine mechanical vibration loads of N1 rotor and N2 rotor are transformed intoelectrical loads (coulombs) on the faces of the crystals by the pressure of theflyweight supported by the crystals. See Figure 11.15 and 11.16

Figure 11.15: Typical vibration accelerometer (piezoelectric type)



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Figure 11.16: Typical vibration pickups and indications

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