Engine Indication Systems (15.14) PDF

Engine Indication Systems (15.14) Learning Objectives 15.14.1 Describe exhaust gas temperature and interstage turbine temperature indicating systems (Level 2). 15.14.2 Describe engine thrust indication: engine pressure ratio, engine turbine discharge pressure or jet pipe pressure (Lev...

Engine Indication Systems (15.14) Learning Objectives 15.14.1 Describe exhaust gas temperature and interstage turbine temperature indicating systems (Level 2). 15.14.2 Describe engine thrust indication: engine pressure ratio, engine turbine discharge pressure or jet pipe pressure (Level 2). 15.14.3 Describe oil pressure and temperature indication systems (Level 2). 15.14.4 Describe fuel pressure and ow indication systems (Level 2). 15.14.5 Describe engine speed and indication systems (Level 2). 15.14.6 Describe vibration measurement and indication systems (Level 2). 15.14.7 Describe torque indication systems (Level 2). 15.14.8 Describe power indication systems (Level 2). 2024-07-23 B1-15b Gas Turbine Engine Page 190 of 290 CASA Part 66 - Training Materials Only  Engine Instruments Introduction to Engine Instruments Although engine installations may differ, depending on the type of both aircraft and engine, gas turbine engine operation is usually controlled by observing some or all of the instruments. Engine indications are divided into three groups: Performance instruments Condition instruments Warning systems. 2024-07-23 B1-15b Gas Turbine Engine Page 191 of 290 CASA Part 66 - Training Materials Only  Engine Performance Instruments The power of the engine is indicated by a performance instrument, which allows the operator to monitor the output or performance of the engine at a glance. The three performance instruments are: Engine Pressure Ratio (EPR) N₁ fan speed Torque. Performance instruments 2024-07-23 B1-15b Gas Turbine Engine Page 192 of 290 CASA Part 66 - Training Materials Only  Engine Condition Instruments Engine condition monitoring instruments include: Exhaust Gas Temperature (EGT) Fuel ow N₂/N₃ compressor speed Oil pressure Oil temperature Vibration gauges. Condition instruments These allow the operator to monitor the health of the engine. 2024-07-23 B1-15b Gas Turbine Engine Page 193 of 290 CASA Part 66 - Training Materials Only  Engine Performance Instruments Compressor Speed Turbine engine fan and compressor speed are displayed on a tachometer that is calibrated in percent rpm. In addition, a separate tachometer is used for each compressor section in an engine. For example, an engine with a twin-spool compressor will have an N1 tachometer for the low-pressure compressor and an N2 tachometer for the high-pressure compressor. Compressor speed 2024-07-23 B1-15b Gas Turbine Engine Page 194 of 290 CASA Part 66 - Training Materials Only  Compressor speed analogue Tachometers Magnetic Pickup RPM System Two slightly different kinds of electronic tachometers are found on turbine engines. The rst type is often used as a fan speed sensor to measure the rpm of the fan and low-pressure compressor. It uses a sensor which contains a coil of wire that generates a magnetic eld. The sensor is mounted in the shroud around the fan so when each fan blade passes the sensor, the magnetic eld is interrupted. The frequency at which the fan blades cut across the eld is measured by an electronic circuit and then transmitted to an rpm gauge (N1) in the cockpit. Impulse tachometer 2024-07-23 B1-15b Gas Turbine Engine Page 195 of 290 CASA Part 66 - Training Materials Only  Three-Phase Generator RPM System (Tachometer - Generator) This type of system is common on older aircraft. It is independent of the aircraft electrical system. The sensor is an engine-driven, permanent-magnet, three-phase AC generator. The generator drives a synchronous motor in the indicator. The tachometer generator sends a low-voltage three phase signal to the eld windings of the synchronous motor in the indicator. The rotor in the indicator is designed to remain synchronous with the speed of the permanent magnet in the generator. The frequency of the generator signal is critical, NOT the voltage. Electronic tachometer Rotational motion of the synchronous motor in the indicator must be translated into an angular displacement of the indicator needle. This is achieved by using a ux coupling to turn rotational speed into a torque value that is proportional to speed. The torque is turned into angular displacement by the action of a hairspring as explained below. The ux coupling consists of a permanent magnet and drag cup. Relative motion between the magnet and the drag-cup induces eddy currents in the drag cup. These currents create a magnetic eld which reacts with the permanent magnetic eld, causing a torque reaction between the two. This torque tries to rotate the drag-cup in the same direction as the magnet rotation, but rotation is opposed by the calibrated hairspring. Drag-cup rotation stops at a position where drag cup torque is balanced by spring tension. The resulting movement of the drag-cup shaft and gear train thus positions the pointers over the dial to indicate fan, compressor or propeller speed. 2024-07-23 B1-15b Gas Turbine Engine Page 196 of 290 CASA Part 66 - Training Materials Only  Inductance Type Speed Sensor The speed sensors provide the ECU channels A and B with signals that are representative of the rotational speeds, N1 and N2. They are induction type tachometers, which provide electrical output signals. These outputs are Alternating Current (AC) signals, and the frequency is directly proportional to the rotational speed of the dedicated rotor. Speed wheel sensor The sensing element is an electrical winding with a core made up of a permanent magnet. Both sensors feature three independent sensing elements insulated from each other, thus there is one output signal per connector. The third connector allows a signal to be sent to the Engine Vibration Management Unit (EMVU) for vibration analysis, in conjunction with data from the vibration sensors. One of the teeth is thicker than all the others and is installed in the same angular position as fan blade No 1. The thicker tooth generates a stronger pulse as it passes the sensor, and this is used as a phase reference in engine vibration analysis. A spring in the speed sensor housing dampens speed sensor vibrations. In addition, two dampers are on the speed sensor shaft to prevent sensor vibration. The passage of a sensor tooth ring modi es the magnetic eld around the core of the winding and causes a magnetic ux variation in the coil. Each tooth induces a pulse onto the coil, and therefore, the number of pulses is proportional to the sensor ring speed. 2024-07-23 B1-15b Gas Turbine Engine Page 197 of 290 CASA Part 66 - Training Materials Only  Speed wheel sensor elements 2024-07-23 B1-15b Gas Turbine Engine Page 198 of 290 CASA Part 66 - Training Materials Only  Engine Pressure Ratio Introduction to Engine Pressure Ratio Engine pressure ratio (EPR) is a measure of the thrust being developed by the engine. The gauge compares the engine turbine discharge pressure to the pressure of the air at the inlet of the engine and is considered a measure of thrust being produced. The EPR transducer compares the two pressures (Pt₇ to Pt₂) and with adjustments for temperature, airspeed and altitude, sends an electrical signal to the indicator. Engine pressure ratio EPR is the ratio of turbine discharge pressure to compressor inlet pressure. When planning a ight, pilots use ambient temperature and a predetermined Take-Off Thrust Setting Curve to calculate the EPR required for take-off. 2024-07-23 B1-15b Gas Turbine Engine Page 199 of 290 CASA Part 66 - Training Materials Only  Turbine Discharge Pressure In some older aircraft, turbine discharge pressure is indicated on an individual Pt gauge. This instrument displays the total engine internal pressure immediately aft of the last turbine stage. When corrected for variations in inlet pressure caused by airspeed, altitude and ambient air temperature, turbine discharge pressure indicates thrust. Turbine discharge pressure Torque Turboprop and turbo-shaft engines are designed to produce torque for driving a propeller or rotor. Therefore, these engines often utilise a torquemeter to provide a pilot with engine power output information. Torquemeters can be calibrated in percentage units, foot-pounds, shaft horsepower and psi. Several techniques are used to measure the torque produced by an engine. The electrical signals from the sensors are processed and used to position the cockpit indicator. Cockpit torque indicators The engine torque, or turning moment, is transmitted through the reduction gearbox to the propeller. The torquemeter system is the primary performance instrument for turbo‑propeller engines. 2024-07-23 B1-15b Gas Turbine Engine Page 200 of 290 CASA Part 66 - Training Materials Only  An explanation of two different types of torquemeter systems is covered in the following paragraphs. These are: Hydraulic torque indication system Torque shaft indication system. 2024-07-23 B1-15b Gas Turbine Engine Page 201 of 290 CASA Part 66 - Training Materials Only  Hydraulic Torque Indication System The hydraulic torque indication system indicates torque by measuring hydraulic pressure created by a torquemeter system. The torquemeter system forms part of a reduction gear assembly between the engine drive shaft and the propeller shaft. The construction of the system depends on the type of engine, but all are based on the same principle of operation. The drive shaft from the engine supplies a torque to the reduction gear assembly. This drives the planet gears around in the same direction, but at a fraction of the engine speed. As the planet gears rotate, the propeller rotates. The propeller converts this rotation force into thrust. To do this, it resists the rotation of the propeller due to aerodynamic forces. This resistance causes the planet gears to transfer a portion of the torque to the stationary ring gear. The ring gear movement is resisted by pistons working in hydraulic cylinders secured to the gearbox casing. Oil is supplied to the cylinders from a special pump and can drain via a calibrated bleed line. The oil is subjected to a pressure which is proportional to the torque or load applied to the propeller shaft. An increase in torque causes the piston to reduce the bleed line ow, increasing pressure to the pressure transmitter. This oil pressure is sensed by a bourdon tube which is coupled to a synchro- transmitter. A simple synchro-indicator in the cockpit displays the torque information. Hydraulic torque indicator system 2024-07-23 B1-15b Gas Turbine Engine Page 202 of 290 CASA Part 66 - Training Materials Only  Torque Shaft Indication System Another method of obtaining an indication of torque is by measuring the amount of twist in a shaft called a torque shaft. The torque shaft connects the engine to the propeller reduction gearbox. A hollow shaft, called a reference shaft, is mounted so that it forms a sleeve around the torque shaft. Torque shaft indication system The torque shaft is connected to both the engine and the gearbox. The engine rotates and the propeller is dragged through the air. The propeller lags slightly, causing the torque shaft to twist slightly. The reference shaft is not subjected to any torque as it is connected only to the engine. On the end of both shafts is a gear, called an exciter wheel. A magnetic pick-up assembly is mounted directly above each exciter wheel. 2024-07-23 B1-15b Gas Turbine Engine Page 203 of 290 CASA Part 66 - Training Materials Only  Torque shaft and magnetic pick-ups As the shafts rotate, the teeth of the exciter wheels pass by the magnetic pick-up assemblies. Each tooth causes a pulse to be generated by its pick-up assembly. When the engine is not delivering any power to the gearbox, the teeth on the torque and reference shafts are aligned. Magnetic pick-up assembly 2024-07-23 B1-15b Gas Turbine Engine Page 204 of 290 CASA Part 66 - Training Materials Only  The magnetic pick-up assembly is mounted directly above each exciter wheel. When the engine is delivering power to the gearbox, the torque shaft is subjected to a torque that causes it to twist slightly. This results in the teeth on the torque shaft becoming misaligned with the teeth on the reference shaft (remember, the reference shaft is not connected to anything and therefore will not twist). Misalignment between the reference and torque shaft This system has two other components: a phase detector and a torque indicator. A complete system is shown below. The signals from both pick‑up assemblies are fed to the phase detector. The phase detector calculates the difference between the two signals and generates an output that represents the torque being measured. The output from the phase detector is used to drive a pointer in the torque indicator. 2024-07-23 B1-15b Gas Turbine Engine Page 205 of 290 CASA Part 66 - Training Materials Only  Complete torque shaft system 2024-07-23 B1-15b Gas Turbine Engine Page 206 of 290 CASA Part 66 - Training Materials Only  Engine Condition Instruments Purpose of Engine Condition Instruments Condition instruments show the operator how hard the engine is working to produce the power seen on the performance indicators. Engine condition instruments include: Fuel ow Turbo fan compressor speed N₂/N₃, Turboprop compressor speed Ng/N₁ Oil temperature Exhaust gas temperature Oil pressure Inlet air temperature Engine vibration. The relationship between instrument indications is a very important guide to engine condition, ef ciency and performance. For instance, if Torque Oil Pressure or EPR is lower than normal for a particular combination of turbine temperature, fuel ow, rpm, air temperature, aircraft altitude and airspeed, then a loss of engine performance can be suspected. By analysing instrument indications, ight crews and maintenance personnel can forecast trouble and take preventative action before a major malfunction develops. This is known as Engine Condition Trend Monitoring (ECTM). 2024-07-23 B1-15b Gas Turbine Engine Page 207 of 290 CASA Part 66 - Training Materials Only  Fuel Flow Indication Fuel ow for gas turbine engines is indicated in kilograms or pounds of fuel burned per hour. You may recall that the volume of jet fuel changes with temperature, and therefore the fuel ow on several turbine engines is measured in terms of mass rather than volume. Fuel ow indicator Fuel ow measuring systems basically consists of two units: A transmitter or owmeter An indicator. Transmitters are connected in the delivery lines of an engine fuel system. They are essentially electromechanical devices producing output signals proportional to the fuel ow rate. The systems used to measure fuel ow on gas turbine engines are: Impeller type Rotating vane type Turbine type Motorless mass ow type. 2024-07-23 B1-15b Gas Turbine Engine Page 208 of 290 CASA Part 66 - Training Materials Only  Impeller Type The impeller type uses a small turbine wheel in the fuel line. As fuel ows through this line, it spins the turbine and a magnetic pulse pick-up in conjunction with a digital circuit reads the number of revolutions in a speci ed period of time and converts this into a fuel ow rate. An iron core sensing coil is mounted in the tube in line with the position of the rotating magnet. The contour of the blades and the inside of the tube are calculated and machined to allow the impeller to rotate a set number of times for a xed amount of passing fuel. As fuel ows through the transmitter, its movement causes the impeller to rotate in a set direction. As the magnet passes the sensing coil, an electrical pulse is generated. With an increasing fuel ow, the impeller rotates at a greater rate and the pulses occur at a higher frequency. This pulsing signal is fed to an ampli er/signal conditioner, where it is converted to an output that is used to drive an indicator pointer. Impeller type fuel ow transmitter 2024-07-23 B1-15b Gas Turbine Engine Page 209 of 290 CASA Part 66 - Training Materials Only  Rotating Vane Type The rotating vane type assembly consists of a pivoted vane which enables angular displacement when fuel passes through the chamber. A small gap is formed between the edge of the vane and the chamber wall, which increases in size as more fuel ows through it. A rotor assembly is also mounted on the pivot shaft. As the shaft rotates, the rotor position is magnetically coupled to a stator connected to a potentiometer or a synchro-rotor assembly. The magnetic coupling prevents any electrical transmitter friction from affecting the movement of the vane. The electrical output of the potentiometer/synchro is indicated on the cockpit indicator. With no fuel ow, the vane is held at rest by the hair springs, and the indicator shows no ow. As fuel ows, the vane moves off the stops. With an increased fuel ow, the vane pivots further in the fuel chamber. With the fuel able to pass through the increasing edge gap, the shaft rotates and the tension of the hair spring increases. The distance the vane moves is proportional to and is a measure of the fuel ow. The hair spring calibrates the relationship between vane movement and fuel ow. A relief valve is tted at the top of the fuel chamber and relieves and bypasses excess fuel ow when the ow is greater than the capacity of the transmitter or the vane movement becomes jammed, restricting the ow of fuel. Rotating vane fuel ow transmitter 2024-07-23 B1-15b Gas Turbine Engine Page 210 of 290 CASA Part 66 - Training Materials Only  Turbine Type The turbine type transmitter consists of an impeller driven by an electric motor, a turbine interconnected with a calibrated restraining spring, and a rotary or angular position sensor (torque synchro/Linear Variable Differential Transformer [LVDT]). The impeller motor drives at a constant speed. When fuel enters the transmitter, it passes through passages in the impeller, which, on account of its rotation, causes the fuel to swirl at a velocity governed by the ow rate. The turbine has a greater angular displacement when a larger amount of fuel is being passed by the constant-speed impeller. Indication occurs via the synchro (or similar) system, which senses the angular displacement of the turbine. Turbine type fuel ow transmitter 2024-07-23 B1-15b Gas Turbine Engine Page 211 of 290 CASA Part 66 - Training Materials Only  Motorless Mass Flow Flowmeter The owmeter is a cylindrical, through- ow, double-skinned case containing a ow director, swirl generator, rotor and turbine. Mounted on the side of the case is an electrical connection. Contained between the skins are a stator coil and a circumferential coil. The coils are positioned in the rotational path of the two magnets attached to the rotor outer periphery at a preset angle relative to each other. The fuel passes through the ow director onto a swirl generator, which gives the fuel a rotary motion. This motion causes the rotor to rotate at a velocity dependent on the rate of fuel ow. The fuel ow leaving the rotor is directed through the turbine, de ecting the turbine against the restraining spring tension by an amount proportional to the fuel ow rate. The magnet attached to the rotor passing over the starter coil induces an electrical pulse on each rotation. The second magnet attached to the rotor, rotating in the path of the stop coil, induces an electrical pulse each time it passes the pulse generator xed to the turbine. The elapsed time between the two pulses is proportional to the mass ow rate. The time interval between the start and stop signals is measured and converted to a mass ow rate for ECAM/EICAS displays. Motorless mass ow transmitter 2024-07-23 B1-15b Gas Turbine Engine Page 212 of 290 CASA Part 66 - Training Materials Only  Exhaust Gas Temperature The most limiting factor in running a gas turbine engine is the temperature of the turbine section. This temperature must be monitored closely to avoid overheating the turbine blades and other exhaust section components. One common way of monitoring it is with an Exhaust Gas Temperature (EGT) gauge. In other words, EGT is an engine operating limit used to monitor the mechanical integrity of a turbine and overall engine operating conditions. Different names used for EGT include: Turbine Outlet Temperature (TOT) Turbine Gas Temperature (TGT). Even though the temperature of the gases exiting a turbine is lower than at the turbine inlet, it provides some surveillance of the engine’s internal operating conditions. Variations of EGT systems bear different names based on the location of the temperature sensors within the turbine section. Other common turbine temperature-sensing gauges include: Turbine Inlet Temperature (TIT) Interstage Turbine Temperature (ITT). Exhaust gas temperature gauges 2024-07-23 B1-15b Gas Turbine Engine Page 213 of 290 CASA Part 66 - Training Materials Only  Exhaust gas temperature indications The complete EGT system for a turbine engine consists of: Probes that sense the temperature of the exhaust gas A harness that surrounds the engine tailpipe and connects all of the probes Extension wires that carry the current from the probes into the cockpit Resistors to adjust the resistance of the thermocouples to the value required for the system An indicator in the aircraft instrument panel. The probes are mounted in the tailpipe and connected in parallel so that their output is averaged. 2024-07-23 B1-15b Gas Turbine Engine Page 214 of 290 CASA Part 66 - Training Materials Only  EGT Thermocouple Probes and Harness Thermocouples may be either single or dual, meaning they may have one or two hot junctions. The single type (below) powers only a temperature indicator. The dual type provides an additional identical hot junction which can signal an electronic fuel control for scheduling purposes, or can be used as a test connection for calibrating the EGT circuit. In both types, hot gases impact the outer surface of the thermocouples, with some gas entering the inlet holes. The gases then pass over the hot junction(s) on their way to the outlet and back to the gas path. In this way, a total temperature of the exhaust gases is measured. The indicator is connected directly to the thermocouples, which are in parallel, and averages the output of the probes via a calibrating resistor. The indicator forms the cold junction of the circuit, while the thermocouples form the hot junction. The probes are installed using a mounting ange attached to a mounting boss on the engine. EGT thermocouple probe and harness schematic 2024-07-23 B1-15b Gas Turbine Engine Page 215 of 290 CASA Part 66 - Training Materials Only  Oil Pressure Indicating System Oil supply to an engine is critical and must be monitored. An engine oil pressure indicating system provides reading of engine oil pump pressure to an oil pressure gauge calibrated in psi. It guards against engine failure resulting from inadequate lubrication and cooling of engine components. Typical engine oil pressure indication system The indication system may be: Direct reading Remote reading. 2024-07-23 B1-15b Gas Turbine Engine Page 216 of 290 CASA Part 66 - Training Materials Only  Direct Reading Many oil pressure gauges utilise a Bourdon tube because its design enables the gauge to measure relatively high uid pressures. The gauge is connected by a metal tube directly to a point downstream from the engine oil pump. This measures the oil pressure being delivered to the engine. Bourdon tube pressure gauge The gauge operates by using a lever system that magni es any motion of a curved tube of elliptical cross section. The tube is attached to the oil supply line and a baseplate at one end, and sealed at the other end. As pressure increases within the system the tube attempts to straighten. A linkage between the end of the tube and the pointer moves around a dial that indicates the pressure in psi. Remote Reading A basic remote reading system consists of a supply pressure transmitter electrically connected to an indicator. Two types are commonly used on gas turbine engines: A synchro system An AC inductor system. 2024-07-23 B1-15b Gas Turbine Engine Page 217 of 290 CASA Part 66 - Training Materials Only  Synchro System A synchro-transmitter system uses 26-V AC from the electrical system and is connected by a hose to the engine’s oil pump outlet line. The pressure in the system forces the diaphragm to expand with an increase in pressure. This then causes the attached rotor to move, inducing an error signal in the stator of the transmitter. This error signal is transmitted to the indicator stator. The rotor of the indicator moves to balance the circuit. This rotor is connected to a pointer which then displays the pressure of the engine oil. Synchro type oil pressure system 2024-07-23 B1-15b Gas Turbine Engine Page 218 of 290 CASA Part 66 - Training Materials Only  AC Inductor System With pressure applied, the length of the air gap associated with stator Coil 1 is decreased while that associated with Coil 2 is increased. As the reluctance of the magnetic circuit across each coil is proportional to the effective length of the air gap, the inductance of Coil 1 increases while that of Coil 2 is decreases. The current ows in the coils respectively decrease and increase. This inductance then induces a current in the coil windings. The current is transmitted to the indicator. AC inductor type oil pressure system 2024-07-23 B1-15b Gas Turbine Engine Page 219 of 290 CASA Part 66 - Training Materials Only  Oil Temperature Most modern oil temperature systems are electrically operated and one of the following: Wheatstone bridge circuit Ratiometer circuit. Wheatstone Bridge Circuit The Wheatstone bridge circuit consists of three xed resistors and one variable resistor. The variable resistor is the temperature probe, which contains a coil of ne nickel wire. As the coil of wire is heated, its resistance increases and alters the current ow in the bridge, which moves the needle in the gauge. Electromagnetic attraction and repulsion move the pointer whenever the current ow through the meter changes. Aviation Australia Wheatstone bridge temperature indicator A disadvantage of the Wheatstone bridge is that any added resistance due to bad connections or any uctuations in the system voltage can cause inaccurate readings. For this reason, it has been largely replaced by the ratiometer. 2024-07-23 B1-15b Gas Turbine Engine Page 220 of 290 CASA Part 66 - Training Materials Only  Ratiometer Circuit The ratiometer circuit consists of two parallel branches: A xed resistor and coil A variable resistor and coil. The two coils are wound and formed for a meter movement. Both are supplied with electrical power. The two coils, attached to a pointer, are free to move in a magnetic eld of non-uniform strength. When the coils are energised, the torque they develop is countered by the magnetic eld, causing the pointer assembly to rotate until a state of equilibrium is found. Temperature changes affect the resistance value of the temperature bulb and the current ow through one of the coils. The pointer de ects in response to the changed current ow, which represents the new temperature. Supply voltage variations affect both coils equally and no change of torque is created. This allows the indicator to remain accurate even if voltage changes occur. Ratiometer temperature indicator 2024-07-23 B1-15b Gas Turbine Engine Page 221 of 290 CASA Part 66 - Training Materials Only  Oil Pressure Warnings The oil pressure warning system indicates in the ight compartment when engine oil pressure is below a predetermined setting, or when scavenge lter differential pressure is above a predetermined setting. Oil Pressure Light The oil pressure warning light indicates when engine oil pressure drops below speci ed limits, or when the scavenge lter differential is high. The light, one for each engine, is located on the pilot's centre instrument panel. Filter Pressure Differential Switch The lter switch is mounted on the same assembly as the low oil pressure switch. Its purpose is to provide a warning light earth if the lter is blocked beyond acceptable limits. Its two pipelines are connected, one to the lter inlet and one to the lter outlet. If the pressure difference between these two points exceeds normal values, the switch closes and completes the circuit for the warning light. Oil pressure warning indication 2024-07-23 B1-15b Gas Turbine Engine Page 222 of 290 CASA Part 66 - Training Materials Only  Vibration Analysis The Purpose of Vibration Analysis The main function of an engine vibration monitoring system (EVM) is to give the pilot a continuous indication of the engines’ vibration level, enabling them to take appropriate measures if the vibration reaches a dangerous level. For this reason, every engine vibration monitoring unit conditions some combination of rotor out-of- balance vibration data for cockpit display. According to the aircraft and engine type, the data are selected and conditioned differently. To give an unmistakable warning to the pilot in case of problems, the EVM usually monitors the vibration levels for exceeding a certain alert threshold and activates a cockpit warning. The system incorporates two engine-mounted accelerometers and a signal conditioner. One accelerometer is mounted on the fan casing, and the other on the turbine casing. They are mounted at right angles to the compressor/turbine shafts. The airborne EVM system utilises piezo-electric transducers (accelerometers) to sense engine vibration. The crystal oscillates with a predetermined electrical input. This oscillation is monitored. When the engine vibrates, it alters the oscillation frequency produced by the crystal. This produces a change in the signal output from the crystal, and the change in frequency is detected by the signal conditioner and sent to the indicator. Engine vibration sensor 2024-07-23 B1-15b Gas Turbine Engine Page 223 of 290 CASA Part 66 - Training Materials Only  Apart from catastrophic events, the out-of-balance vibration level of an engine usually shows a more or less steady increase over time due to mechanical wear (bird strikes, friction, etc.). Since the tendency of the vibration evolution over time is steady, it is easy to predict when the vibration will reach a certain vibration level, e.g. the maintenance alert level. This allows maintenance personnel to anticipate maintenance actions and to plan them in advance. For this reason, the vibration data from the EVM are usually sent to the Aircraft Condition Monitoring System (ACMS) or similar equipment, where they are used along with other engine parameters as input to the Engine Condition Monitoring (ECM) system. 2024-07-23 B1-15b Gas Turbine Engine Page 224 of 290 CASA Part 66 - Training Materials Only