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Question 45-1 : The very rapid magnetic field changes flux changes in the primary coil of a magneto are accomplished by the ? [ Success airport ]
Contact breaker points opening
.inside the primary circuit is a set of contact points that make the flux reversals more abrupt when they open and close at critical moments .when the points are closed current flows in the primary coil and a flux field builds up around it when a spark is needed they open which causes the field in the primary to collapse and induce a current in the secondary winding as the field moves across it which is fed to the spark plugs through the distributor .the more suddent the breaking of the primary circuit is the more current will be induced in the secondary coil
Question 45-2 : In a compressor stage of a jet engine the sequence is ?
Rotor stator
Comp r e s sor highlighted rs means correct order so rotor stator .turbine stage the opposite
Question 45-3 : The accessory units driven by the accessory gearbox of a turbo jet engine are the .1 thrust reverser pneumatic motors.2 ac generator through its constant speed drive csd .3 oil pumps.4 hydraulic pumps.5 high pressure fuel pumps.the combination regrouping all the correct statements is ?
2 3 4 5
Img908. 909.the accessory gearbox is driven by a radial shaft that meshes with a bevel gear driven by the n2 or gas producer rotor and is used to drive essential engine components such as the fuel pump oil pump fuel control unit generators and hydraulic pumps the accessory gear box is also used to drive engine rpm feedback tacho generators and is the means by which the starter drives the engine these are systems which must operate as soon as the engine starts .structure containing the gearbox can be utilised as a support as well .each component that attaches to the accesssory gearbox has its own mounting pad with its unique gear reduction for speed and rotational direction
Question 45-4 : Consider a jet engine whose control is based on the engine pressure ratio epr .1 with a constant epr the thrust decreases when the altitude increases.2 with a constant epr the thrust is independent of the mach number.3 at same environmental conditions a given epr setting maintains the thrust ?
1 3
Engine pressure ratio epr is a measure of the thrust of a jet engine obtained by dividing the pressure at the back of the engine by the pressure at the front .epr settings vary with altitude and temperature when the altitude increases the thrust decreases .answer 4 the epr is determined by the impact pressure difference between the turbine outlet and the compressor inlet is wrong this is a ratio not a difference
Question 45-5 : In the axial flow compressor of a turbo jet engine the flow duct is tapered its shape is calculated so as to ?
Maintain a constant axial speed in cruising flight
The duct in question is the convergent duct formed between the compressor and engine compressor casing .the airflow through a jet engine varies dependant on thrust selection since the engine runs for most of the time at cruise the engine is designed to give correct flow rates at this thrust setting
Question 45-6 : In the stator of a turbine the gas velocity v and static pressure ps vary as follows ?
V increases ps decreases
If you have multiple stage turbines there is a set of guide vanes between each turbine disc to redirect the flow again before the next turbine blade they are stationary so are sometimes called stators .with the turbine you want a high velocity so turbine blades are designed to extract power from the hot expanding gasses to turn the compressor by using the high velocity hitting the turbine blade the guide vanes straighten the flow before the next set of turbine blades but in effect it decreases the pressure and increases the velocity this also prevents excessive backpressure which could work its way forward and affect the combustion process .the compressor on the other hand is designed to increase the pressure through each of the stages so you can deliver a good mass flow to the combustion chambers however the increasing pressure will try to reverse the flow so the convergent duct is designed to keep the velocity constant through the whole compressor so that the momentum of the airflow prevents reverse flow in effect the velocity is therefore increased in each rotor section .velocity increases and decreases through each set of rotors and stators and by using a convergent duct the velocity therefore stays constant but the pressure is steadily increasing through each stage
Question 45-7 : Epr can be defined as the ratio of ?
The total turbine outlet pressure to the total compressor inlet pressure
Img911
Question 45-8 : A gas turbine engine turbine section may employ active clearance control ?
To provide enhanced clearances between the blade tips and the casing
.active clearance control acc is a method used in gas turbines to improve fuel efficiency this is achieved by controlling the tip clearance it is particularly effective on turbofans since they are required to respond quickly to changes of the thrust setting .the active clearance control consists of the acc valve which receives the air from the bypass duct and routes the air to flow over the pipes surrounding the turbine casing which in turn reduces the thermal expansion of the turbine case and maintains the accurate clearance between the turbine case and the blade tip this clearance should be maintained accurately which is essential for the engine efficiency and its performance the acc valve opening is adjusted automatically by the fadec system depending on the throttle position
Question 45-9 : In a gas turbine engine the maximum gas temperature is attained ?
Within the combustion chamber
Question 45-10 : A pressure relief valve that does not fit on its seat properly would result in ?
Low oil pressure
To prevent too high a pressure a pressure release valve is fitted it comprises a stainless steel ball bearing and a spring .the ball is forced by the spring against an oil port when the oil pressure is sufficiently high the ball if forced off its seat and oil is allowed to bypass this restriction thus making sure that the pressure never gets too high .if the pressure relief valve is not hermetic there will be a leak and the pressure decreases
Question 45-11 : In a gas turbine engine the power changes are normally made by controlling the amount of ?
Fuel supplied
Question 45-12 : In a free turbine engine ?
There is no mechanical connection between the compressor and the power output shaft
Question 45-13 : During take off the fan in a high by pass ratio turbofan engine produces ?
The greater part of the thrust
Question 45-14 : At constant fuel flow if engine compressor air is bled off for engine anti icing or a similar system the turbine temperature ?
Will rise
The ratio of the tertiary airflow around the combustion flame tube that cools the turbine decreases thus less cooling flow as the warm air is diverted for anti icing so the egt will increase as a result
Question 45-15 : A hung abortive start of a gas turbine engine may be caused by ?
A too low pneumatic starter pressure
.after the normal light up sequence until the starter 'cut out' a hung start is identified by light off followed by abnormally slow acceleration and rpm stabilization below idle a hung start may be result of fuel scheduling being either too lean or too rich a lean hung start is associated with low fuel flow and proportionally low egt a rich condition can be recognized by a high fuel flow and an egt rise which may tend to develop into an over temperature condition and possible compressor stall ..hung starts may be caused by . starter air pressure too low to accelerate engine to self sustaining speed. premature starter deactivation . fod foreign object debris to compressor . faulty pressurizing valves not opening at scheduled fuel pressure setting . incorrect scheduling of hp compressor igv and variable stators . turbine section damage
Question 45-16 : A reverse thrust alert on the flight deck occurs when .1 the reverser doors are unlocked with the reverse lever in the stowed position .2 the reverser doors are locked with the reverse lever in the stowed position .3 the reverser doors remain stowed with the reverse lever in the deployed ?
1 3
Boeing fcom .thrust reverser .each engine is equipped with a hydraulically operated thrust reverser consisting of left and right translating sleeves aft movement of the reverser sleeves causes blocker doors to deflect fan discharge air forward through fixed cascade vanes producing reverse thrust the thrust reverser is for ground operations only and is used after touchdown to slow the airplane reducing stopping distance and brake wear .when reverse thrust is selected an electro mechanical lock releases the isolation valve opens and the thrust reverser control valve moves to the deploy position allowing hydraulic pressure to unlock and deploy the reverser system an interlock mechanism restricts movement of the reverse thrust lever until the reverser sleeves have approached the deployed position when either reverser sleeve moves from the stowed position the amber rev indication located on the upper display unit illuminates as the thrust reverser reaches the deployed position the rev indication illuminates green and the reverse thrust lever can be raised to detent no 2 this position provides adequate reverse thrust for normal operations when necessary the reverse thrust lever can be pulled beyond detent no 2 providing maximum reverse thrust .downward motion of the reverse thrust lever past detent no 1 reverse idle thrust initiates the command to stow the reverser when the lever reaches the full down position the control valve moves to the stow position allowing hydraulic pressure to stow and lock the reverser sleeves after the thrust reverser is stowed the isolation valve closes and the electro mechanical lock engages .if the reverser doors are unlocked with the reverse lever in the stowed position or if the reverser doors remain stowed with the reverse lever in the deployed position the reverser light illuminates the master caution and eng system annunciator lights illuminate
Question 45-17 : In a single spool gas turbine engine the compressor rpm is ?
The same as turbine rpm
The turbine is turning the compressor so one turbine and one compressor . 912. single spool gas turbine engine
Question 45-18 : The primary reason for a limitation being imposed on the temperature of the gas flow is to ?
Ensure that the maximum acceptable temperature at the turbine blades is not exceeded
The temperatures are highest in the combustion chamber but the engine is most sensitive to high temperatures just behind the combustion chamber at the turbine high temperatures and centrifugal loads can cause turbine blade creep . 913. jt9d engine
Question 45-19 : One of the purposes of the bleed valves fitted to axial flow compressors is to ?
Reduce the tendency to compressor stall
Question 45-20 : A modern auxiliary power unit apu is designed to provide power for ground starting of an engine it also supplies both in the air subject to certification limitations and on the ground ?
Air conditioning and electrical services
Question 45-21 : The disadvantages of axial flow compressors compared to centrifugal flow compressors are .1 more expensive to manufacture.2 lower airflow.3 greater vulnerability to foreign object damage.4 lower pressure ratio.the combination which regroups all of the correct answers is ?
1 and 3
Img914.compared to centrifugal flow compressors axial flow compressors are more expensive to manufacture .due to the design and function of a compressor engine's air inlet the possibility of ingestion of debris always exists this causes significant damage particularly to the compressor and turbine sections when this occurs it is called foreign object damage fod typical fod consists of small nicks and dents caused by ingestion of small objects from the ramp taxiway or runway .however fod damage caused by bird strikes or ice ingestion can also occur and may result in total destruction of an engine .example of a centrifugal compressor . 915
Question 45-22 : A stage in an axial compressor ?
Consists of a rotor disc followed by a ring of stator blades
Question 45-23 : In a turbo jet the purpose of the turbine is to ?
Drive the compressor by using part of the energy from the exhaust gases
Question 45-24 : The gas turbine illustrated is of the following type . 916 ?
Free turbine and centrifugal compressor
For a free power turbine engine the turbine driving the main rotor transmission is mechanically independent from the gas generator . 915. radial centrifugal compressor located at the front of the engine
Question 45-25 : The control of free turbine engines on turboprops is accomplished by . a propeller control lever used to select .1 propeller rpm.2 turbine temperature.3 turbine rpm. a fuel control lever used to select .4 propeller rpm.5 torque.6 turbine temperature.the combination which regroups all of the ?
1 5 and 6
Propeller control lever controls rpm so out of 1 2 and 3 only 1 is correct .fuel control lever controls torque and turbine temp so out of 4 5 and 6 both 5 and 6 are correct .the correct answer option is then 1 5 6
Question 45-26 : The static thrust of a turbo jet at the selection of full power .1 equals the product of the exhaust gas mass flow and the exhaust gas velocity.2 is obtained by pressure of the exhaust gas on the ambient air.3 is equivalent to zero mechanical power since the aeroplane is not moving.4 is ?
1 and 3
Static thrust is the measured thrust produced when movement is prevented .thrust in its simplest form is .f = m x a.thrust f = mass flow rate x jet velocity tas .statement 1 it is true .2 it is a nonsense .statement 3 it is true no movement = zero mechanical power .statement 4 wrong static thrust is dependant of the outside air temperature since mass flow will vary with temperature/density
Question 45-27 : The purpose of a compressor bleed valve is to prevent surging ?
At low compressor rpm
.the compressor bleed valve removes excesss volume of air at low compressor rpm to increase the flow over the front compressor blades and reduce the flow over the latter stages
Question 45-28 : Severe compressor stall can be indicated amongst others by ?
A rise in turbine gas temperature tgt and a change in vibration level
A compressor stall means less cooling flow for the turbine so the egt will increase as a result as for a wing vibrations will occur during the stall
Question 45-29 : In a gas turbine engine compressor blades which are not rigidly fixed in position when the engine is stationary take up a rigid position when the engine is running due to ?
The resultant of aerodynamic and centrifugal forces
The blades are not rigidly fixed one common mehtod of fitting them is with the 'fir tree root' rotor blades are strong enought to withstand centrifugal forces they take up a rigid position when the engine is running due to the resultant of aerodynamic and centrifugal forces .watch the videos below . >beghfv2y2ry. you can here at 6 00 the 'clung' noise of the blade falling from their casing . >phluddufakg. this is for the turbine stage but construction is similar for the compressor stage
Question 45-30 : The characteristics of a centrifugal compressor are .1 a high pressure ratio by stage.2 a large diameter.3 a low pressure ratio by stage.4 a small diameter.the combination containing all of the correct statements is ?
1 and 2
Img938. axial compressor . 915. radial compressor .the pressure ratio obtained from a centrifugal compressor by stage is lower than that which can be obtained from the axial flow compressor by stage .the diameter of a centrifugal compressor is usually larger compare to an axial compressor for the same flow
Question 45-31 : The diffuser in a centrifugal compressor is a device in which the ?
Pressure rises and velocity falls
Diffuser = stator for a compressor .in the case of a compressor the velocity falls and the pressure increases in the stator the diffuser
Question 45-32 : The thrust reversers of jet engines may use clamshell doors in order to ?
Change the direction of the exhaust gas stream
Question 45-33 : By pass turbine engines are mainly used in commercial aviation because ?
At high subsonic airspeeds they have a better propulsive efficiency than propeller or straight jet engines
Question 45-34 : Engine auxiliary gearboxes ?
Provide the drives for a wide variety of services and may even provide for the support of the engine in the airframe
Img908. 909.the accessory gearbox is driven by a radial shaft that meshes with a bevel gear driven by the n2 or gas producer rotor and is used to drive essential engine components such as the fuel pump oil pump fuel control unit generators and hydraulic pumps .the accessory gear box is also used to drive engine rpm feedback tacho generators and is the means by which the starter drives the engine these are systems which must operate as soon as the engine starts .structure containing the gearbox can be utilised as a support as well .each component that attaches to the accesssory gearbox has its own mounting pad with its unique gear reduction for speed and rotational direction
Question 45-35 : The greatest risk created by a free turbine overspeed is ?
Bursting of the free turbine disk
In case of turbine overspeed centrifugal force can be so great that the turbines rotor blades can detatch .. 942.during a high powered engine run this 767 had a engine failure . 943.piece of the turbine is seen above lodged in the outboard side of the engine . 944.the photo above shows the gash made on the lower part of the fuselage in the air conditioning bay when a piece of the turbine exited the engine and ended up embedding in the engine
Question 45-36 : The thermal efficiency of a gas turbine engine will increase with a ?
Decrease in ambient air temperature
Thermal efficiency is the ratio of useful work to the heat put in expressed as a percentage .the power output of a gas turbine engine depends on mass airflow and a cold air mass has higher density thermal efficiency increases with cold ambient air temperature
Question 45-37 : Creep of turbine blades is caused by ?
High blade temperature whilst under centrifugal loading
Blade creep is the increase in length of turbine rotor blades during their operating life due to heat and stress
Question 45-38 : By pass ratio in a turbine engine is the ratio of the ?
Cold air mass flow to the hot air mass flow
Img939.bypass ratio = bypass mass flow / hp compressor mass flow .by pass mass flow is cold air mass flow .hp compressor mass flow is the hot air mass flow
Question 45-39 : On landing and prior to shut down a gas turbine engine should be run at 'ground idle' for a short period this allows ?
The core temperature to diminish
Question 45-40 : Both gas turbine and piston engines utilise a cycle of induction compression combustion and exhaust however in the gas turbine these processes are i and combustion occurs at ii .respectively i and ii are ?
I continuous ii constant pressure
Piston engine the combustion process is intermittent governed by the rythm of the back and forth movements of the piston in a cylinder the combustion occurs at constant volume in the cylinder .turbine engine the combustion process is continuous and occurs at constant pressure
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