Chapter 3 - Engine Indicating Systems Flashcards
What is N1, N2, NG and NF otherwise known as?
N1 - LP
N2 - HP
NG - Gas generator speed
NF - Free turbine (power turbine)
Which part of the tachometer generator is fitted to the engine?
Miniature 3 phase delta wound (squirrel cage) permanent magnet rotor alternator
Where a tachometer indicator uses two pointers, what will be the gearing between them?
10:1 always
What are required in the tachometer indicator to provide the torque force in the drag cup which will oppose the controlling hairspring force?
Eddy currents, will form magnetic fluxes which will be attracted to the rotating permanent magnet
What does a pulse probe (phonic wheel) consist of?
- Two permanent magnets
- Three pole pieces
- Coil assembly
What is the spacing of the pole pieces on a phonic wheel equal to?
The pitch of the teeth on the phonic wheel which is an integral part of the N1 and N2 shafts
Describe the operation of a pulse probe (phonic wheel type)?
Rotation of the shaft causes the teeth on the periphery of the phonic wheel to pass the appropriate pulse probe once per revolution
Magnetic flux experiences constant fluctuations because of the changing reluctance as the teeth pass the probes, which causes a voltage to be induced into a coil within the probe and an electrical signal to be produced
Frequency of voltage is directly proportional to the speed of rotation of the shaft phonic wheel
What is the result when the teeth of the phonic wheel are in line with the pole pieces of the pulse probe?
A path of relatively low reluctance is apparent
Why are two pulse probes used which are mounted 90° out of phase with each other?
To indicate the direction of rotation
In the electromagnetic method of measuring rotational speed, why are there two electromagnetic coils?
One dedicated to the standby indicator system, and a second dedicated to the primary N1 indication
What is the purpose of the higher tooth in the electromagnetic method of measuring rotational speed of the N1 rotor shaft?
Generate a stronger signal for a 1/rev reference for fan trim balance
Describe the operation of the eddy current sensor?
Electronic eddy current unit which consists of a permanent magnet and coil
Compressor speed is measured by the sensor unit which counts the number of fan blades passing a fixed point in a given unit of time
What does the N3 tachometer consist of?
Twelve pole permanent magnet rotor within a single phase stator
What does the propeller synchro work on the principle of?
Operating all engines at exactly the same RPM
Why is one engine always designated as the master?
The slaves will always copy the RPM of the master
What is the most common component used to measure the speed and phase difference between propellers?
Magnetic pulse probe
What happens if either the RPM or the phase of the slave engine needs to be adjusted?
Synch controller will signal the slave engine to either increase or decrease RPM as required through the governor assembly
How is EGT indication achieved?
By measuring an electrical signal produced by a system of thermocouples positioned in the hot gas stream and wired to form a parallel circuit
What are the two basic types of thermocouples?
- Contact
- Immersion
What are the materials normally used in the contact type of thermocouple?
Copper/constantan or iron/constantan
What are the materials normally used in the immersion type of thermocouple?
Chromel/alumel
What are the two types of immersion thermocouple?
- Stagnation
- Rapid response
Describe stagnation types of thermocouple?
Used in pure jet engines where gas velocities are high
Gas entry and exit holes in the sheath are staggered and of unequal size
Slows the gas flow down to allow the junction to respond to changes in gas temperature
Describe rapid response types of thermocouple?
Used in turbo prop engines where the gas velocities are low
Sampling holes are of equal size and diametrically opposite
Gas flow is directly over the junction to ensure rapid response to any change in temperature
Why are thermocouple probes normally connected in parallel?
So that the average temperature is measured
How many thermocouple probes are normally fitted?
Six or more
What is the purpose of the cold junction compensator?
To detect and correct any changes in ambient temperature at the cold junction
How else might indication of the thermocouple lead be ascertained where colour coding is impossible?
Alumel is slightly magnetic
How can you identify thermocouple leads by their colour coding?
Positive lead being chromel - red insulation
Negative lead being alumel - blue insulation
Which materials are used for the compensating cables in a thermocouple?
Constantan for alumel
Copper for chromel
Which resistor cannot be changed in a thermocouple EGT system?
Ballast resistor
Which wire material is used for trimming in the constantan cable?
Eureka
Which wire material is used for trimming in the copper cable?
Manganin
What are the two factors which affect the choice of wire material for trimming resistors?
- Temperature coefficient must be small so that resistance value is unaffected by temperature
- Wire material must have similar thermoelectric properties as the cable in which they are placed
Describe an averaging dual element thermocouple?
Thermocouples placed into the same point of the gas stream but of different lengths
What will cause a solenoid warning flag to appear?
- In the event of 115V AC supply to the indicator falling below 100V
- Servo loop monitor detecting a failure of the servo loop to back off the error signal voltage
What is the pyrometer sensor and how does it work?
A lens that gets infrared light from the LPT first stage blades
Infrared light goes through the fibre optic cable to the electronics module, which turns the infrared light into the electrical signals for the EEC
What is the speed of the synchronous motor in the tachometer indicator entirely dependent on?
The frequency of the current fed to it
Where is the N3 (HP) tachometer mounted?
On the high speed external gearbox
What is the name given to the noise reduction systems often fitted on multi-engine propeller aircraft?
Synchronisation or synchro-phasing
What is synchrophasing based upon?
Measuring and adjusting the phase angle difference between the same numbered blade on the propellers of each engine
What are thermocouples used for on aircraft?
Cylinder head temperature indication on piston engines
Exhaust gas temperatures on turbine engines
What is the contact type of thermocouple used to measure?
Measures the temperature of a solid component
How can the accuracy of the temperature measurements of thermocouples be ensured, with regards to their connection to the indicator?
Connected by:
Extension leads - Same materials as the thermocouple junction
Compensating leads - Similar thermoelectric properties as the wire they are replacing
What else is incorporated on a servo operated thermocouple system, which is controlled by a comparator solid state switching circuit?
Overtemperature warning light
In a servo operated thermocouple system, what compensates for changes in ambient temperature of the indicator?
Cold junction reference circuit
What are the two air connectors for a pyrometer adapter?
Cooling air for the sensor & fibre optic cable and electronics module
Cleaning air for the sensor from the T3 sensor
What type of pressure transmitter is mounted in the engine for pressure indicating?
Inductor type
What type of indicators are used for pressure indicating?
AC ratiometer
What will always be shown on an oil pressure gauge?
A minimum pressure the engine is allowed to run at
What is a yellow band called used in an oil pressure indicator?
Precautionary band
What is the purpose of the vent tubes to atmosphere in an oil pressure gauge system?
Prevents build up of pressure within the transmitter that may interfere with the operation of the bellows at high altitudes
Describe the operation of a oil low pressure switch?
Oil system pressure acts against a switch diaphragm
Any pressure above the low pressure limit actuates a shunt disc closed, which opens the electrical switch
In the typical system, when will the low pressure fuel feed switches illuminate and extinguish?
Tanks 1 & 2 - illuminates at 5 psi boost pump output pressure, extinguishes at 7 psi
Centre tank - illuminate at 14 psi boost pump output pressure, extinguishes at 16 psi
What happens if both boost pumps fail in one fuel tank?
Applicable low pressure lights illuminate and in addition the fuel annunciator light and the master caution
Where is the oil temperature bulb mounted?
In a boss on the oil scavenge filter
What does an oil temperature indicator consist of?
- Stationary coil
- Moving coil
- Pointer attached to the moving coil
Describe the fuel temperature bulb arrangement?
Temperature bulb is an armoured, bayonet locking-type bulb
Temperature range of -50 to +300 °C
Contained in a housing which projects into tank No 1 through the rear spar
Housing is integral with the tank wall, and allows the bulb to be removed without draining the tank
What type of indicator is the fuel temperature indicator?
Resistance ratiometer
When is the greatest ratio that will be achievable for EPR?
Aircraft is static and the engine is operating at the maximum fuel flow available in the ambient conditions
What is engine pressure ratio?
Comparison between intake and exhaust pressure
What is EPR a useful parameter for?
Setting throttle levers for economical engine running and minimum take-off settings
What would the nominal throttle setting be on the ground with regards to EPR?
1.6:1
What would the nominal throttle setting be at idle with regards to EPR?
1.07:1
What is integrated EPR (IEPR)?
Includes by-pass pressure (PF) in the comparison of intake pressure to exhaust pressure
What is the formula for EPR?
Exhaust pressure / Intake pressure
What is the formula for IEPR?
(By-pass pressure + exhaust pressure)/intake pressure
What does adding by-pass airflow have the effect of?
Increasing the overall thrust ratio
What is the value of the ballast resistor in the EPR indicator system?
Fixed value selected to suit the type of engine and the indicator system
Which resistor in the EPR indicator system has its value stamped on the engine data plate?
Trimming resistor
Why is it necessary to trim the transmitter output to suit individual engine characteristics in an IEPR system?
Ensure that the indicated IEPR is the same for both engines at the same thrust
How are the various degrees of trim achieved in IEPR trimming?
A receptacle on the transmitter front panel has five active and five grounded sockets
Different combinations of connection of the active to the grounded sockets give varying degrees of trim
Why is fuel rate during an engine start important?
Too much fuel will result in a hot start
Too little fuel will result in a condition as a hung start
What units are usually shown on a fuel flow indicator?
Pounds per hour
or
Kilograms per hour
Why do fuel flow indicators measure fuel flow in terms of mass rather than volume?
So that the figures are not affected by changes in either the density or temperature of the fuel
Where would a helical vane flow meter normally be used?
Where high flow rates are to be measured
Why is fuel passed through straightening vanes in a helical vane flow meter?
To prevent false reading (outputs)
What are the two ways a helical vane flow meter measures flow rate?
Small magnet in one of the vanes and a coil on the body
or
High permeability vanes pass under a magnet and a coil
Describe the operation of a pulse switch flowmeter?
Fuel enters a swirl generator which gives the fuel a rotary motion
Rotary motion causes a rotor to rotate at a velocity dependent on the rate of fuel flow
Fuel leaving the rotor impinges on a turbine and deflects it against spring tension, an amount proportional to the fuel flow rate
A magnet on the rotor, induces an electrical pulse in a start coil attached to the case, and a stop coil attached to the turbine
Elapsed time between the pulses is proportional to the fuel flow rate
Where is a manifold absolute pressure (MAP) indicator used?
All aircraft that have a constant speed propeller to indicate engine power output
What are the permitted errors in manifold pressure gauges?
Between 25” Hg - 35” Hg - not exceed 0.75” Hg
The rest of the scale - must not exceed 1.5” Hg
What are the two types of torque indicator systems?
- Hydro mechanical
- Electrical
What does the hydro mechanical torque indicator system operate on the principle of?
Any tendency of some part of the gear transmission to rotate will produce a reaction so that the ring gear will tend to rotate in the opposite direction
How does an electrical torque indicating system operate?
Measures the deflection (twist) in the torque shaft relative to a reference shaft, by using two phonic wheels and associated pick-up assemblies
What is the indication of high torque in an electrical torque indicating system?
The first probe switches a circuit on, while the second probe switches it off
The longer a circuit is ON, the larger the torque
Why is an ‘idler’ tube used as the reference shaft?
It is not connected at one end so there is no twist in it
Describe the base, bellows and spindle assembly of the inductor pressure transmitter?
Flexible brass bellows soft soldered to a base at one end and a centre spindle at the other end
Two armatures are located on the centre spindle with their poles at 90° to each other
Why are the stator windings in an inductor pressure transmitter positioned at 90° to each other?
Reduce electrical coupling between the windings
What is the purpose of the circular disc mounted on the rear end of the shaft in an AC ratiometer indicator?
Positioned to lie between the poles of a magnet assembly and thus act as an eddy current damping device
How is the indicator movement designed in the AC ratiometer pressure indicating system?
Torques produced on each of the cam shaped discs are in opposition
What is the purpose of the compensating capacitors in the AC ratiometer pressure indicator?
Phase displacement of the eddy currents in the discs will change unequally, resulting in a change in the current ratio in the bridge circuit
Compensating capacitors reduce this effect
How are temperature effects compensated for in an AC ratiometer pressure indicator?
A high temperature co-efficient resistor is placed across the coils of the indicator
Describe a rotor torque flowmeter?
Impellor and drum rotate, though are connected only by a calibrated spring
Magnetic pick-offs are fitted to the flowmeter case for both the impellor and the drum
Greater the flow, the greater the displacement between them
Time gap is measured and in direct proportion to mass flow
What is the relation between manifold pressure and normally aspirated engines or turbocharged engines?
N/A - will not normally exceed ambient pressure
Turbocharged - can exceed ambient pressure
What is the purpose of the solid distance piece in the manifold pressure sensor?
Ensures accuracy of indication at all altitudes due to the tendency of the aneroid sylphon to expand or contract in opposition to the tendency of the manifold sylphon to do the same
