Question 49-1 : A twin spool engine with an inlet air mass flow of 500 kg/s and a hp compressor mass flow of 250 kg/s has a bypass ratio of ? [ Quiz landing ]
1
Img939.bypass ratio = bypass mass flow / hp compressor mass flow.500 250 = 250.250 / 250 = 1
Question 49-2 : A 'fan' stage of a ducted fan turbine engine is driven by ?
The low pressure turbine
Img948
Question 49-3 : The oil supply to gas turbine engine bearings is maintained in the event of the main oil filter becoming clogged by ?
Differential pressure opening the bypass valve of the main oil filter
Question 49-4 : One of the purposes of compressor bleed valves is to ?
Decrease the airflow across the rear compressor stages whilst that across the early stages is increased
Bleed valves are fitted to prevent compressors stalling .vdo1055.like the stall of a wing compressor stall is caused by a breakdown of airflow .the bleed valve removes excesss volume of air at low compressor speeds to increase the flow over the front compressor blades and reduce the flow over the latter stages .as speed of the compressor increases the bleed valve closes
Question 49-5 : Turbine blade stages may be classed as either 'impulse' or 'reaction' in an impulse blade section the pressure i across the nozzle guide vanes and ii across the rotor blades ?
I drops ii remains constant
Img940.nozzle guide vanes = stator .the pressure drops across the stator blades and remains constant across the rotor blades
Question 49-6 : One advantage of having three instead of two spools in some high bypass turbofan engine is that ?
Fewer variable stator vanes and variable inlet guide vanes are required to prevent compressor stall at low rotor speeds
Variable inlet guide vanes v i g v s are fitted to engines which have a particular problem with inherent compressor stall at low rpm or during engine acceleration or deceleration the vanes are fitted just in front of the first rotor stage they can be automatically pivoted around their own axis to vary the path of the airflow going into the compressor so maintaining the proper relationship between compressor rotational speed and airflow in the front compressor stages at low compressor speeds the v i g v s are angled to impart the greatest amount of swirl to the air thereby correcting the relative airflow to obtain the optimum angle of attack over the rotor blades this optimum angle of attack allows a smooth and rapid engine acceleration .on a three spools high bypass turbofan engine an additional intermediate pressure compressor stage is added and reduces the risk of compressor stall at low rotor speeds
Question 49-7 : The compressor surge effect during acceleration is prevented by the ?
Fuel control unit f c u
With modern engines there are various ways to control surge one of them is the use of blow off valves also called compressor bleed valves .on all modern turbine engines with electronic fuel control systems the valves are controlled by the fuel control unit .the end result will be that the blow off valve will reduce the pressure in the compressor to prevent the surge
Question 49-8 : Creep in a gas turbine engine is ?
The permanent increase in length of metal parts due to a combination of tensile stress and high temperatures over a prolonged period
Question 49-9 : About a twin spool compressor ?
The low pressure spool runs at a lower rpm than the high pressure spool
Question 49-10 : In addition to providing 'customer' bleed for aeroplane systems air may be bled from compressors of a turbofan engine to increase stall margin ?
At low rotor speed
During engine starts most turbojet/turbofan engines use engine bleed air to reduce the possibility of compressor stalls .compressor stall occurs when the compressor sucks too much air for the turbine wheels to handle excessive pressure then builds aft of the compressor and the compressor blades stall also happens with centrifugal type compressor .bleeding air from an engine air is bled just aft of the compressor relieves that excessive pressure and the compressor can more easily 'push' the sucked air behind him
Question 49-11 : One reason to fit a tip shroud to a turbine blade is to ?
Increase turbine efficiency
The shrouded blades form a band around the perimeter of the turbine which serves to reduce blade vibrations the shrouds improve the airflow characteristics and increase the efficiency of the turbine the shrouds also serve to cut down gas leakage around the tips of the turbine blades
Question 49-12 : The most important oil system instrument indication during gas turbine engine start is ?
Oil pressure
Question 49-13 : Turbine blade stages may be classed as either 'impulse' or 'reaction' in an impulse turbine stage ?
The pressure drops across the stator blades and remains constant across the rotor blades
Img940.nozzle guide vanes = stator .the pressure drops across the stator blades and remains constant across the rotor blades
Question 49-14 : When a gas turbine engine is fitted with can type combustion chambers ?
A total of only two igniters are usually fitted since the chambers are inter connected
Most gas turbine engines only have two igniters typically located diagonally opposed in fact the engine would probably start quite readily with only one operating however because there are only two another means of passing the starting flame between the combustion chambers has to be found this is the interconnector
Question 49-15 : The purpose of gas turbine engine trending is ?
To enhance maintenance planning and detect anomalies
Engine trending performance analysis monitoring and diagnostics permit to enhance maintenance planning and to detect anomalies
Question 49-16 : The flexible take off mode .1 can be used only if the engines are recent.2 reduces engine wear.3 can be used in situations where take off can be executed without the need for full engine power.4 can only be used with an auto throttle.the combination that regroups all of the correct statements is ?
2 3
Question 49-17 : Concerning the flexible take off mode the temperature selected in the fms is ?
Higher than the ambiant airfield temperature in order to achieve a reduced power setting
. extract from boeing fcom flight crew operations manual .assumed temperature reduced thrust .regulations permit the use of up to 25% takeoff thrust reduction for operation with assumed temperature reduced thrust use of assumed temperature reduced thrust is not allowed with anti skid inoperative or on runways contaminated with standing water ice slush or snow use of assumed temperature reduced thrust is not recommended if potential windshear conditions exist .a takeoff thrust less than the full rated thrust may be achieved by using an.assumed temperature that is higher than the actual temperature
Question 49-18 : One reason to fit a tip shroud to a turbine blade is to ?
Reduce blade vibrations and limit marginal leaks
The shrouded blades form a band around the perimeter of the turbine which serves to reduce blade vibrations the shrouds improve the airflow characteristics and increase the efficiency of the turbine the shrouds also serve to cut down gas leakage around the tips of the turbine blades
Question 49-19 : On a high by pass ratio fan engine reverse thrust is achieved by reversing ?
Only the direction of the fan airflow
On high bypass turbofan engines reverse thrust is achieved by reversing only the direction of the fan airflow in order to save costs and mass .it is not impossible to reverse the hot exhaust gas but it is complex the gain in reverse thrust will be less than 20% for an increase in maintenance costs and a higher inflight consumption due to weight of the system
Question 49-20 : If an abortive start hung start occurs when starting a gas turbine engine the correct action is to ?
Shut down the engine
Question 49-21 : Compressor surge may be controlled by means of ?
Bleed valves
Question 49-22 : The use of igniters in a gas turbine engine is required .1 throughout the operating range of the engine.2 during engine acceleration.3 for on ground starts.4 for in flight relights.5 during in flight turbulence.6 under heavy rains or icing conditions.the combination that regroups all of the ?
3 only
Normally in flight no igniters are in use as the combustion is self sustaining during engine start and for take off and landing igniters are in use .you have to use igniters in case of moderate or severe precipitation turbulence or icing or for an in flight relight
Question 49-23 : Vibration sensors are used in a gas turbine engine to indicate ?
Imbalance of one or both of the spools
Question 49-24 : The fan of a turbo fan engine is driven by ?
The lp turbine
Question 49-25 : Regarding a jet engine .i the maximum thrust increases as the pressure altitude decreases .ii the specific fuel consumption increases slightly as the pressure altitude decreases at constant tas ?
I is correct ii is correct
.jet engines are more efficient at higher altitudes because less fuel is needed for a given thrust due to reduction in lift and drag and lower temperatures through the inlet cause a significant reduction in specific fuel consumption up to the tropopause .specific fuel consumption is the amount of fuel burnt per unit of thrust for jets it reduces with altitude up to the tropopause
Question 49-26 : One of the causes of high oil temperature in a gas turbine engine at constant power setting is ?
Malfunction of the heat exchanger
.the fuel/oil heat exchanger is located in the tanks it cools the oil and heats the fuel so if at constant power setting you have a high oil temperature it may come from a malfunction of the heat exchanger
Question 49-27 : Given the following information about an aeroplane with a turbojet engine . mass air flow 50 kg/s. tas 90 m/s. exhaust nozzle gas velocity 150 m/s . exhaust nozzle static pressure1050 hpa. ambient static pressure 1000 hpa. cross sectional area of the nozzle 0 10 m .net engine thrust equals ?
3500 n
Jaredleto .the formula required to answer this question is .total thrust = mass thrust + pressure thrust.or t = m vj vv + a pj p0 .where m = mass air flow vj = exhaust velocity vv = tas .a = exhaust cross section area pj = exhaust static pressure and.p0 = static ambient pressure .in this instance the pressures are given in hpa and need to be converted to n/sq m .therefore t = 50 150 90 + 0 1 105000 100000 which gives the answer of 3500 n
Question 49-28 : Exhaust gas temperature sensing is usually taken ?
Between the gas generator turbine and the free power turbine
Question 49-29 : Flat rated jet engines are designed to provide constant maximum take off thrust up to a certain level of ?
Ambient temperature
.the term flat rating is so called because of the flat nature of thrust versus the ambient temperature curve the curve shows that from the lowest operating temperature possible to the specified temperature the engine produces full thrust above this temperature full thrust will not be permitted .a 'flat rated' turbofan engine is controlled in such a way that at constant ambient air pressure there is no variation of take off thrust with air temperature up to some specified temperature limit .above that limit the engine operates in such a way that either . the fan speed or the compressor speed is constant or . the turbine inlet temperature is constant.and in either case the thrust then decreases with increasing temperature
Question 49-30 : Engine temperature limitations of a gas turbine engine are usually imposed due to temperature limitations on the ?
Turbine section
Question 49-31 : For a fan jet engine the by pass ratio is the ?
External airflow mass divided by the internal airflow mass
Img939.bypass ratio = bypass mass flow / hp compressor mass flow .by pass mass flow is the external airflow .hp compressor mass flow is the internal airflow mass
Question 49-32 : If the main oil filter of a gas turbine engine becomes clogged oil to the main bearings is still assured ?
Because the oil filter bypass valve opens
Question 49-33 : In a free turbine engine the energy delivered by the free turbine stages ?
Is dependent on the rotational speed of the gas generator
Julien35 .from bristol book 'free power turbine engine the mechanical link between the gas generator and the propeller turbine is removed so each element is free to rotate at its optimum speed' ..yes but read the question carefully it talk about the energy delivered by the free turbine stages .if the gas generator does not rotate the free turbine stages will delivered 0 energy if the free turbine stages deliver full thrust it is possible only with a high rotational speed of the gas generator .so the energy delivered by the free turbine stages is dependent on the rotational speed of the gas generator
Question 49-34 : Which statement is correct for a gas turbine engine with a constant speed propeller and free power turbine when the power setting is increased .i the hp spool speed increases .ii the egt increases ?
I is correct ii is correct
Hp spool speed egt and gas generator speed all increase for a gas turbine engine with a constant speed propeller and free power turbine when the power setting is increased
Question 49-35 : Which statement is correct for a gas turbine engine with a constant speed propeller and free power turbine when the power setting is increased .i the gas generator speed increases .ii the egt increases ?
I is correct ii is correct
Hp spool speed egt and gas generator speed all increase for a gas turbine engine with a constant speed propeller and free power turbine when the power setting is increased
Question 49-36 : The bypass ratio .i is the ratio of bypass air mass flow to hp compressor mass flow .ii can be determined from the inlet air mass flow and the hp compressor mass flow ?
I is correct ii is correct
Img939.bypass ratio = bypass mass flow / hp compressor mass flow .the bypass ratio can be determined from the inlet air mass flow and the bypass mass flow
Question 49-37 : The bypass ratio .i is the ratio of inlet air mass flow to hp compressor mass flow .ii can be determined from the inlet air volume flow and the hp compressor volume flow ?
I is incorrect ii is incorrect
Img939.bypass ratio = bypass mass flow / hp compressor mass flow .the bypass ratio can be determined from the inlet air mass flow and the bypass mass flow and not by volume
Question 49-38 : Compressor stall in a gas turbine engine ?
Can be caused by an axial airflow velocity that is low relative to the rotor speed
.like the stall of a wing compressor stall is caused by a breakdown of airflow .if your wing is suitable for high speed and you fly at low speed and large angle of attack the stall is most likely to occur .a compressor stall is most likely to occur when the air flow stagnates in the rear stages of the compressor a compressor blade is hitting the oncoming airflow at an angle of attack that is determined by a combination of the rotational speed and the incoming airflow .when the incoming speed is low the angle of attack will be high when the angle of attack gets too high the blade stalls so when the axial flow in a compressor backs up the blades stall pressure increase in the rear stages this slows down the flow .you can prevent this by dumping some late stage air to atmosphere via bleed valves
Question 49-39 : The air flow passages between adjacent axial compressor rotor blades are divergent but the absolute exit velocity is higher than the absolute entry velocity of the rotor this is because ?
Part of the compressor work delivered to the air is converted into kinetic energy
.compression of hot gas in the rotor is followed by an expansion before entering the stator
Question 49-40 : A twin spool gas turbine engine is one in which the ?
Lp turbine drives the lp compressor and the hp turbine drives the hp compressor
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