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110 Cards in this Set
- Front
- Back
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What effect does airspeed have on thrust?
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As airspeed is increased, thrust is decreased. 'Ram effect' increases thrust by increasing density.
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What effect does air density have on thrust?
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As air density decreases (i.e. altitude is gained), thrust decreases. The decrease is more rapid above the tropopause.
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What effect does pressure have on thrust?
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As pressure is decreased, thrust decreases.
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What effect does temperature have on thrust?
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As temperature increases, thrust decreases.
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What is the thrust formula?
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Thrust of engine = (total mass flow out tailpipe x exhaust velocity) - (incoming mass flow of air x velocity of incoming mass flow)
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When is rich mixture flame out more likely?
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During rapid throttle advance or in turbulent conditions.
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When is lean mixture flame out more likely?
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When on descent at high airspeeds, low rpm and low fuel pressure.
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What are the advantages of annular combustion chamber types over can-annular types?
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More space efficient, suffer less pressure loss and can be shorter in length.
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What are the advantages of reverse flow annular combustion chambers?
What are the disadvantages? |
Engine length significantly reduced as turbines are within combustion chamber. Also lower weight.
Disadvantages: some loss of efficiency, not suited to large engines with high mass airflows. |
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What percentage of core gas stream energy is extracted by the turbine to drive the compressor?
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75%
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In a turbofan powerplant, where does the gas stream reach its highest velocity?
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Exhaust propelling nozzle
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What kind of duct do nozzle guide vanes associated with the 'impulse type' turbine blades form?
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Convergent duct.
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What is the purpose of stagger angle in a turbine blade?
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Straightens the gas flow path and ensures all parts of the blade extract equal energy.
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What is the purpose of honey comb shrouds and knife edge seals?
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Reduce turbine tip air leakage.
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What is the purpose of 'fir tree design' turbine blade attachments?
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Ensures centrifugal load is equally shared by all the fir tree serrations.
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What is the purpose of the turbine shroud ring?
Where does the shroud ring cooling supply come from? |
Stiffens the blade tip to reduce blade vibration and distortion under heavy gas loads.
Compressor bleed air is passed up through blade body to cool shroud ring. |
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What kind of duct does the jetpipe form? What effect does it have on airflow velocity?
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Convergent duct
Increases velocity |
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What are the three purposes of exhaust struts?
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1. Bracing the exhaust area
2. Supporting the turbine shafts 3. Straightening the gas flow, making it more axial. |
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What is the main difference between 'pressure relief valve' and 'full flow' lubrication systems?
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Pressure relief valve operates on a constant pressure basis.
Full flow types vary pressure depending on engine speed. |
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What happens if the oil filter becomes blocked by thick oil on very cold days when starting the engine?
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Filter bypass valve will open allowing unfiltered oil to engine components. Last chance filters at these components will filter fuel.
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What are the characteristics of synthetic oil regarding the following:
- Undersirable impurities - Likelihood of developing carbon deposits - Health hazard if exposed to skin |
Do not contain undesirable impurities
Less likely to produce carbon deposits Health hazard |
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Are oil tanks pressurised?
Why/why not? |
Pressurised.
Prevents cavitation and supply a positive and constant supply to the pumps. |
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List three types of aviation fuel in order of specific gravity (low to high)
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Avgas
Avcat Avtur |
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List five advantages of Avtur over Avgas.
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1. High calorific value (specific gravity)
2. High flashpoint and fire point (low fire hazard) 3. High boiling point (prevents vapour lock) 4. Low freeze point 5. Significantly cheaper |
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List four disadvantages of Avtur
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1. Contains wax and sulphur
2. Tends to retain water 3. Supports microbial infestations 4. Not as volatile as Avgas (slower flame front) |
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Can Avtur hold more water when warm or cold?
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Warm
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What is the purpose of supplying engine bleed air to air gap above fuel tanks?
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Prevent cavitation (vapour lock)
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What advantage do tail (stabiliser) tanks provide?
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Keep centre of gravity closer to the centre of pressure thereby reducing drag and increasing range.
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What is the requirement for fuel quantity checks prior to flight?
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2 separate checks, differing by no more than 3% of the larger amount.
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Low pressure fuel boost pumps.
1. Located where? 2. Powered by? 3. Cooled and lubricated by? 4. How many? |
1. Submerged in fuel tank.
2. Electrical (both AC or one DC) 3. Fuel 4. 2 in each tank |
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High pressure fuel pumps.
1. Located where? 2. Powered by? 3. Cooled and lubricated by? |
1. Cold section of engine
2. High pressure compressor shaft 3. Fuel |
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How do gear type high pressure fuel pumps work?
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Driven directly from engine.
Any excess fuel returned by 'spill valve' which essentially controls delivery rate. |
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How do variable stroke high pressure fuel pumps work?
How much HP do they consume? |
Consists of a plunger controlled by FCU. Only delivers what is required.
Consumes 60hp |
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How do vaporising tubes work?
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Turns the fuel/air mixture through 180 degrees, pre-mixing and vaporising the fuel.
More stable combustion at low RPM |
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What is the purpose of an APU?
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Used to start engines, provide air conditioning during ground operations and can provide electrical power.
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Name four principles of convergence.
1. _________ process 2. Velocity __________ 3. Pressure __________ 4. Temperature __________ |
1. expansion
2. increases 3. decreases 4. decreases |
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What happens to air as it flows through an axial flow compressor in regards to pressure and velocity?
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Pressure continuously rises.
Velocity remains relatively constant. |
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In a turboprop aircraft, what percentage of thrust is developed by the exhaust stream?
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10%
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What is the purpose of a thrust sensing unit (TSS)?
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Provide auto-feather capability if a turboprop engine loses thrust during take off or initial climb.
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List the three main jet types in order of best TSFC.
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Turboprop
Turbofan Pure jet |
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What is the purpose of water (or water methanol) injection?
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Increases mass flow through the engine to increase thrust at takeoff
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Name four advantages of multi-spool axial compressors over the centrifugal types.
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Greater compression ratios
Greater fuel economy (lower SFC) Less chance of surging Reduced engine diameter Less spool up time (than single spool) |
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What is the primary purpose of 'variable inlet nozzle guide vanes'?
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Reduce chance of compressor surging, especially during starting.
They restrict the amount of air fed to N1 so that it doesn't choke N2 with more air than it can handle. |
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When is compressor surging most likely?
(8 situations) |
- low RPM (especially engine start/acceleration)
- thrust levers opened or closed too quickly - if AoA at inlet is too great (rotation off runway) - in turbulence or heavy rain or hail - at high altitude - if compressor blades are damaged - during reverse thrust (especially at low speeds) - if aircraft is side slipping. |
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What are the symptoms of compressor surging?
(4) |
- Rising EGT
- Steadying or reducing fuel flow - Steadying or reducing RPM - Engine popping, vibration or loud banging sounds |
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What should you do if you experience a compressor stall?
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Reduce throttle, lower nose and increase forward airspeed.
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What methods are used to prevent compressor stalling?
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- Multi-spool compressor in engine design.
- Fit stators that change their angle as engine speed changes (variable stator vanes) - Fit variable inlet guide vanes to restrict airflow to N1 - Avoid engine starts in strong tailwinds - Fit bleed valve near N2 to bleed out air if too much. |
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What is the purpose of the 'interconnector tube' between multi-can combustion chambers?
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Ensure all chambers operate at the same pressure and to pass the flame.
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Which turbine shaft drives the high speed accessory gearbox in a twin spool turbofan engine?
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N2
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Low Energy Ignition Systems
1. When are they used? 2. High or low output? 3. Operated continuously? 4. Spark rate? 5. Voltage? |
1. During takeoff, heavy rain, turbulence or in icing conditions.
2. Low joule output (3-6 joules) 3. Yes. 4. 4-8 sparks per second 5. 2,000 volts |
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High energy ignition system
1. When are they used? 2. High or low output? 3. Operated continuously? 4. Spark rate? 5. Voltage? |
1. Engine starting, in flight re-lighting
2. High joule output (12 joules) 3. No. 4. 1-5 sparks per second 5. 20,000 volts |
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Bucket type reverse thrust
1. How are they actuated? 2. Exhaust gas, fan gas or both? 3. How do they work? |
1. Hydraulic
2. Hot exhaust gas 3. Use buckets to divert hot exhaust gas forward. |
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Clamshell thrust reverse
1. How are they actuated? 2. Exhaust gas, fan gas or both? 3. How do they work? |
1. Pneumatic
2. Hot exhaust gas 3. Clamshell deflectors divert the core exhaust gas forwad through cascade vanes. |
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Cold stream reverse thrust
1. How are they actuated? 2. Exhaust gas, fan gas or both? 3. How do they work? |
1. Pneumatic
2. Fan gas only 3. Use blocker doors to deflect cold air through cascade vanes which are exposed by a translating sleeve. |
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What effect on engine RPM (N1 and N2) does altitude gain have and why?
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N1 and N2 RPM will rise as the less dense air creates less drag.
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What effect on RPM (N1 and N2) does a warmer than ISA day have and why?
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RPM will be higher due less drag of less dense air (same effect as higher altitudes).
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To maintaina fixed Mach number in a climb through the troposhere you need to do what?
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Increase EPR or N1 value by advancing the throttle levers. Will eventually exceed turbine limit temperature.
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For a constant altitude/speed combination, what effect does a warmer day have on fuel flow?
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Fuel flow will increase as RPM will increase due to more air being drawn into engine to account for reduced density.
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For a constant altitude/speed/temperature combination, what effect does a lighter aircraft have on fuel flow?
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Fuel flow will reduce as EPR setting will need to be reduced to maintain the same speed.
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What are the differences between high frequency noise and low frequency noise?
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High frequency attenuates more rapidly.
Higher frequency lowers perceived noise levels. |
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What are the two greatest sources of noise of a high bypass ratio turbofan powerplant?
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1. The fan
2. The exhaust |
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What do corrugated internal mixer chutes and multi-lobe mixers accomplish and how?
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Promote rapid mixing of the atmosphere and exhaust gas stream (reducing velocity) which increase the noise frequency and lowers perceived noise levels.
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Where are the EPR sensors found?
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First stage compression inlet and last stage turbine outlet.
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Blockage of the EPR inlet probe by ice would cause what effect of EPR gauge accuracy?
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EPR gauge will over-read.
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What instrument does a crew use to set power on a twin spool turbofan?
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EPR or N1
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To lower oil temperature in a turbine engine, you should?
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Increase engine power.
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Discharging a fire extinguisher bottle in an engine will cause what to happen before discharging the extinguisher?
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Fuel and ignition to affected engine will be shut off.
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Where is the greatest increase in pressure in a gas turbine engine?
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Compressor.
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What happens to gas volume, pressure and temperature in the combustion chamber?
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Volume increases.
Pressure remains relatively constant. Temperature increases. |
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Why is bleed air anti-ice normally switched OFF during take off?
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Bleed air reduces amount of thrust developed.
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Increasing speed causes what effect on:
1. Thrust 2. Propulsive efficiency 3. TSFC |
1. Decrease
2. Increase 3. Increase |
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What are the advantages of a free power turbine?
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1. Increases individual efficiency of the turbines.
2. Less power is required for start 3. Can use propeller brake |
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What is the primary function of the combustion chamber?
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Supply the heat energy for transformation into mechanical energy.
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Complete: the combustion chamber converts ________ energy into ________ energy.
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1. Chemical
2. Pressure |
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Propeller efficiency is about ____%
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80%
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What is involved in flat-rating an engine?
Name one alternative. |
1. Involves operating an over-size engine to a lesser power output to provide sea-level take off power to some high altitude.
2. Water/methanol injection |
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What happens to thrust as an aircraft climbs through the troposphere and then the stratosphere?
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Thrust reduces gradually during climb through the troposphere and then markedly during climb through the stratosphere.
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Altitude has what effect on fuel flow at a constant power lever position?
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Fuel flow decreases.
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What duct/s are located at a jet engine inlet, prior to the compressor?
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A convergent duct then a divergent duct.
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What effect does increased compression ratio have on TSFC?
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TSFC decreases
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What is another term for thermal efficiency?
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TSFC
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What three factors affect the angle of attack at the compressor blades (and hence axial flow velocity)?
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1. Inlet and exhaust air pressures.
2. Inlet air temperature 3. High altitudes |
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Which compressor draws most of the energy from the gas stream leaving the combustion chambers - N1 or N2?
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N2
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Name three advantages to triple spool compressors.
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1. Shorter in length and suffer less flexing of turbine shafts.
2. Less prone to surging 3. Spool up fast |
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Name two advantages of 'wide chord' fan blades.
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1. Do not require mid span shrouds
2. Withstand FOD better |
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Name four advantages of a turbofan over a pure jet.
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1. Better fuel economy
2. Reduced noise levels 3. Greater thrust attainable 4. Lighter core components |
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Name three disadvantages of a turbofan over a pure jet.
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1. Increased drag due increased frontal area
2. Increased diameter 3. Mid-span shrouds often required to support blades against bending forces. |
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What is a disadvantage of a turbine engine compared to a piston engine?
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Costs of overhaul are higher
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What happens to air flow pressure through the turbine and exhaust?
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Decreases.
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Divergent duct:
1. ________ process 2. velocity _________ 3. pressure _________ 4. temperature ________ |
1. Diffusion
2. Decrease 3. Increase 4. Increase |
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Under what conditions should engine anti-ice be turned on?
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Air temp at or below +10 degrees and in visible moisture.
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What are the advantages of a centrifugal compressor over an axial compressor?
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More robust structure better able to resist FOD.
Is less complex and costs less to manufacture. |
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What are the compression ratio peaks of centrifugal type engines and axial type engines?
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15:1 centrifugal type
25:1 axial type |
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What is meant by the term "one stage of compression"?
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One rotor and it's associated stator.
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What two things effect the angle of attack on the rotor blades and must be matched carefully to prevent surging?
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Axial flow velocity and rotational speed.
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What is bypass ratio?
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Actual amount of air that moves through the core versus that which by-passes the core.
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How is airflow apportioned into the combustion chamber?
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Swirl vanes 12%
Flare 8% Primary combustion zone 20% Cooling zone 40% Dilution zone 20% |
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What is the optimum primary mixture ratio of fuel/air?
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15:1
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What is the optimum overall mixture ratio of fuel/air?
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75:1
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Is a compressor stall more likely to produce a lean mixture flame out or a rich mixture flame out?
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Rich
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What is the main advantage of multi-can type combustion chambers?
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Individual cans can be replaced easily.
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Which size aircraft are suited to which types of combustion chambers?
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Multi-can type: small engines only
Annular type: any size. |
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How is a can-annular type combustion chamber arranged?
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The cans are surrounded by a cooling airflow.
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Complete: It is unnecessary to de-ice _______________________ because the inlet air temperature rise due to compression eliminates the ice.
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axial flow compressor rotor blades.
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When is the engine said to have stabilised after start?
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EGT peaks, reduces and stabilises with an N2 speed of about 55%.
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What should pilot action be when encountering severe turbulence?
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Maintain a constant altitude.
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What is the definition of RAM ratio?
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Ratio of air intake dynamic pressure to compressor inlet ambient pressure.
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Which part disconnects the propeller gearbox from the propeller drive turbine?
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Safety coupling.
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What are the three adverse effects caused by undissolved water in Avtur?
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Specific gravity reduces
Microbial infestation Sludge |
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Why are vents fitted in fuel tanks?
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Ensures tank walls do not rupture in turbulence or during manoeuvres
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