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94 Cards in this Set
- Front
- Back
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What is dynamic pressure?
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The Kinetic Energy of fluid molecules in motion – VELOCITY
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What happens to Total Pressure as Pressure and Velocity vary in a closed system?
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Total pressure stays constant
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What does Bernoulli’s Theorem state?
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As any incompressible fluid passes through a convergent opening its velocity increases and pressure decreases
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What happens with air flow in a SUBSONIC nozzle and diffuser?
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Nozzle: convergent shape – velocity increases, pressure decreases
Diffuser: divergent shape – pressure increases, velocity decreases |
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What happens with air flow in a SUPERSONIC nozzle and diffuser?
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Nozzle: divergent shape – velocity increases, pressure decreases
Diffuser: convergent shape – pressure increases, velocity decreases |
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Describe what a nozzle and diffuser always do
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Nozzle increases velocity and decreases pressure
Diffuser increases pressure and decreases velocity |
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What happens during the Brayton cycle and when does each action occur?
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Intake, Compression, Combustion, Exhaust (suck, squeeze, bang, blow)—occur SIMULTANEOUSLY
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What are gross thrust and net thrust and when are they equal?
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Gross: measurement of thrust due solely to velocity of exhaust gases, must be stationary on the ground and requires standard day conditions.
Net: thrust that corrects for the effect of inlet airflow velocity They are equal on a standard day when vel = 0. |
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What are some factors that improve thrust?
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High density, high pressure, low altitude, low temperature, ram effect, high RPM
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What types of engines have Engine Pressure Ratio (EPR) gauges and what do they measure?
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Turbojets and Turbofans; it indicates pressure ratio between inlet airflow and exhaust airflow
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What is dynamic pressure?
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The Kinetic Energy of fluid molecules in motion – VELOCITY
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What happens to Total Pressure as Pressure and Velocity vary in a closed system?
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Total pressure stays constant
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What does Bernoulli’s Theorem state?
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As any incompressible fluid passes through a convergent opening its velocity increases and pressure decreases
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What happens with air flow in a SUBSONIC nozzle and diffuser?
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Nozzle: convergent shape – velocity increases, pressure decreases
Diffuser: divergent shape – pressure increases, velocity decreases |
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What happens with air flow in a SUPERSONIC nozzle and diffuser?
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Nozzle: divergent shape – velocity increases, pressure decreases
Diffuser: convergent shape – pressure increases, velocity decreases |
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Describe what a nozzle and diffuser always do
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Nozzle increases velocity and decreases pressure
Diffuser increases pressure and decreases velocity |
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What happens during the Brayton cycle and when does each action occur?
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Intake, Compression, Combustion, Exhaust (suck, squeeze, bang, blow)—occur SIMULTANEOUSLY
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What are gross thrust and net thrust and when are they equal?
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Gross: measurement of thrust due solely to velocity of exhaust gases, must be stationary on the ground and requires standard day conditions.
Net: thrust that corrects for the effect of inlet airflow velocity They are equal on a standard day when vel = 0. |
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What are some factors that improve thrust?
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High density, high pressure, low altitude, low temperature, ram effect, high RPM
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What types of engines have Engine Pressure Ratio (EPR) gauges and what do they measure?
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Turbojets and Turbofans; it indicates pressure ratio between inlet airflow and exhaust airflow
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What types of engines have a torquemeter in the cockpit and what does it measure?
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Turboprops and Turboshafts; it indicates shaft horsepower (aka power available)
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Which use a tachometer and what does it measure?
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All of them; it measures rpm
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What type of inlet is designed for aircraft operation in both subsonic and supersonic flight regimes? How does it work?
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The Variable geometry inlet duct uses mechanical devices such as wedges, ramps, or cones to automatically change inlet shape as aircraft speed varies between subsonic and supersonic.
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What is the centrifugal flow compressor Diffuser Passage shape?
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Divergent IOT increase pressure of air being fed to burner (all airflow within the engine is subsonic)
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Purpose of the compressor?
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Provide high pressure air free of turbulence to send to the burners for max efficiency. As air travels through the compressor, it decreases velocity and increases pressure and temperature.
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Describe a dual-spool axial flow compressor.
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The compressor is divided into two independent rotor spools, each driven by their own turbine and drive shaft. Order from fore to aft is Low Pressure Compressor, High Pressure Compressor, High Pressure Turbine, Low Pressure Turbine.
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Why is a dual-spool compressor design used?
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Allows for greater pressure buildup, is easier to start, and provides compressor stability and helps prevent compressor stalls
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In a turboprop engine with a dual-spool axial compressor design, which turbine drives the propeller?
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The Low Pressure Turbine
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What happens with air entering the combustion chamber/burner?
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25% of air is Primary air mixed with fuel for combustion.
75% is Secondary air, flows around chamber for cooling and flame control. |
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What are some advantages of a can combustion chamber? Disadvantages?
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+ Ease of maintenance
- Poor use of space in chamber and uneven heating of the turbine (failure of one can lead to turbine damage due to cold spots) |
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What are the advantages of an annular combustion chamber? Disadvantages?
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+ Uniform heat distribution on face of turbine, better use of space
– Replacement/maintenance requires complete engine overhaul |
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What does the turbine do and how is its energy distributed?
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75% of energy produced by turbine drives the compressor and accessories
25% of energy produced is used for thrust. |
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What is creep? What method is used to prevent it?
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Deformation due to overheating
Fir tree |
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What is the purpose of exhaust?
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To direct the flow of hot gases rearward to cause a high exit velocity to the gases while preventing turbulence.
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What engine types incorporate afterburners?
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Turbojets and turbofans
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How does afterburner operation affect thrust and fuel flow?
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Thrust increases 50%; fuel flow increases 300%
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What are the components of the afterburner and what do they do?
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Spray bars – inject fuel into afterburner
Flame holders – create turbulent eddies to mix fuel and air Screech liner – shock absorber, prevents vibrations and turbulence Variable exhaust nozzle – increases air’s exit velocity. |
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How do you get relative wind entering a gas generator?
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Components are compressor RPM and inlet airflow vectors
Decreasing inlet air component or increasing RPM increases AOA |
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What are indications of a compressor stall?
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Mild pulsations, loud noises/bangs, vibrations
RPM decay, ITT increase, torquemeter wavers |
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What are some mechanical malfunctions that can cause compressor stalls?
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Variable inlet guide vane failure, FCU failure, FOD, variable exhaust nozzle failure
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How should the pilot fly to avoid compressor stalls?
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Avoid abrupt PCL changes and rapid attitude changes. Fly the minimums at all times.
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What are some design features that help prevent compressor stalls?
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Variable inlet guide vanes, dual spool compressor design, interstage bleed air, variable exhaust nozzle
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How do you make a turbojet engine?
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Add an inlet and exhaust section to the basic gas generator
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What is Thrust Specific Fuel Consumption (TSFC)?
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The amount of fuel needed to produce one pound of thrust.
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How does a jet produce thrust?
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By highly accelerating a small mass of air through the engine
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Why are turbofans more fuel efficient?
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Bypass air means less air goes through the gas generator, thus less fuel is added to the air to burn
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Why are fans inefficient at high altitudes?
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Lower density at high altitudes means the fan is not accelerating as much airloss of thrust
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How much thrust does a turboprop produce?
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90% thrust from prop and 10% thrust from exhaust
Thrust is produced by a smaller amt of acceleration to a large mass of air |
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What are the three parts of the propeller assembly?
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Blades, hub, dome
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What controls prop pitch?
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DOME
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How does a gas generator turn a propeller?
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With a reduction gear box (RGB) that converts high RPM low torque from gas generator into low RPM high torque required for propeller operation
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Why is low RPM desirable for the prop?
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Prevent blade tips from going supersonic
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What is the torquemeter assembly?
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The shaft that connects the gas generator to the RGB. Two parts:
Torque shaft—inner, coupled to the compressor (or free power turbine), experiences torsional load Reference shaft—outer, rigidly connected only at one end (means it doesn’t carry a load). Difference in twisting between torque shaft and reference shaft is used to measure torque. |
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How much thrust does the turboshaft exhaust create?
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Negligible
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What is the purpose of hydraulics?
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To provide extra power and a mechanical advantage in various aircraft components and to multiply force
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What is Pascal’s Law?
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Pressure applied to a confined liquid is transmitted equally in all directions without loss of pressure and acts with equal force on equal surfaces.
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What are the types of hydraulic systems in an aircraft and what do they do?
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Power control system – operates flight controls
Utility system – operates everything else, and is a backup to flight controls |
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What does the hydraulic reservoir do?
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Storage for hydraulic fluid
Overflow basin for excess fluid forced out of system by thermal expansion Allows air bubbles to be purged Separates some foreign matter from the system |
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What do pumps do? Name 2 pump types and their subunits.
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Supply fluid under pressure.
Hand pumps – emergency systems Power pumps – mechanically or electrically powered, 3000psi Variable displacement: regulates volume delivery IAW system demands Constant displacement: delivers constant fluid output for any rpm. Requires regulator/unloader valve. |
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What is the purpose of an accumulator?
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Shock absorber for pressure surges in the system.
Can supply pressure for one-time emergency use |
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What does a check valve do?
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Allows one-way flow
On engine shutdown, works with accumulator to maintain system pressure |
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What is the purpose of the pressure relief valve?
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Prevents pressure from building to a point where seals might burst or damage may occur
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What do hydraulic fuses do?
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Safety devices that detect ruptures, failed fittings, or other leak producing failures/damage.
Prevents loss of fluid while allowing rest of system to remain in operation |
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What does a selector control valve do?
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Directs the flow of fluid into an actuating cylinder
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What is an actuating cylinder?
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Converts fluid under pressure into linear or reciprocating mechanical motion
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What kinds of power do we have on aircraft? Which can be produced by a generator?
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AC and DC, both.
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What is a constant speed drive?
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A hydro-mechanical linkage between the engine and the generator
Ensures a constant RPM input to generator regardless of engine RPM |
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What is an inverter? A transformer-rectifier?
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Inverter-electromechanical device that transforms DC to AC
Transformer-Rectifier – electrical device that transforms AC to DC |
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What are sources of DC power?
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Battery, Transformer-rectifier, DC generator
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What is an electrical distribution network? What are the types?
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Essential – safety of flight
Primary – mission related Secondary/monitor – convenience circuits Starter – start the engine |
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What is a flash point? What are they for JP-5 and JP-8?
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The lowest temperature that allows for fuel-air mixture to form that ignites with momentary flame application.
JP-8 FP: 100 F JP-5 FP: 140 F – used aboard ship b/c of higher flash point |
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What must fuel system designers consider when designing a fuel system?
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High rates of fuel flow, low atmospheric pressure, piping system complexity, weight and size constraints, cold weather starts
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What is a boost pump and where is it located?
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An electric pump submerged in the fuel tank that ensures an adequate supply of vapor free fuel to the engine-driven fuel pump
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What is a fuel pressure gauge and where is it located?
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A pressure sensor at the outlet of the boost pump.
A drop in pressure may indicate a failed boost pump or an absence of fuel which could lead to cavitation |
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What does the fuel flow gauge do? (aka fuel flow meter)
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Located between FCU and fuel-oil heat exchanger, it measures fuel flow rate from FCU (in pounds per hour)
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What is the purpose of the fuel-oil heat exchanger?
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Preheats metered fuel and removes ice crystals before it enters the manifold, increases fuel’s volatility
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What is the basic purpose of the pressurizing and dump valve?
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To drain the combustion manifold upon engine shutdown and to adjust fuel flow during engine starts.
It is closed when the engine starts |
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What supplies fuel to the afterburner?
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Afterburner FCU meters fuel received from the engine-driven fuel pump via the Transfer Valve in the (main) FCU
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What are the time limits for rated thrust?
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Normal – no limit
Military – 30 minutes Combat (afterburner) – no limit |
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Define viscosity.
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A measure of a fluid’s resistance to flow. (For liquids) it is inversely related to temperature.
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What is a squeeze film?
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A very thin layer of lubricant that prevents metal on metal contact.
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What do oil tanks provide? What is inside the oil tank to feed oil regardless of attitude?
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Tanks store system oil supply, provide space for foaming/expansion, vent excess pressure
Swivel assembly uses gravity to pivot and supply oil regardless of maneuvers |
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What is the purpose of an oil pump?
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Supplies oil under pressure to the parts of the engine that require lubrication
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What diverts oil back to the oil pump inlet if safe operating pressures are exceeded?
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The oil pressure relief valve.
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What does a magnetic chip detector do?
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Detects metal fragments in the scavenge oil path downstream of AGB
Presence of metal chips indicates that gears in AGB are coming apart |
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What does the Oil Temperature Regulating Valve do?
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Directs the flow of return oil into the fuel-oil heat exchanger to allow heating of fuel to occur; when no heat needed, it stays closed and redirects the oil directly to the oil tank.
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What has an aneroid operated spring and bellows valve?
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The breather pressurizing system; sets system to sea level pressure
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What position is breather pressurizing vale in if you’re sitting on the deck?
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Open
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What is the purpose of a starter?
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To accelerate the engine until the turbine makes enough power to accelerate the engine on its own
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What are the indications of the four types of abnormal starts?
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False start – ITT normal, RPM stabilizes below normal
Wet start – fuel flows but no light off Hot start – ITT above normal Hung start – ITT above normal, RPM below normal |
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Which type of abnormal start is most dangerous?
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Wet start
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Name two types of starters
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DC electrical
Air Turbine |
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Describe two types of igniters
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Annular gap – protrudes into burner can
Constrained gap – outside can |
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When using an air turbine starter, do you still need electricity?
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Yes, for ignition.
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