Final Exam Flashcards
Difference between soft & hard magnets?*
Soft readily susceptible not good at retention (steel/iron)
Hard iron opposite.
What 6 ways can electricity be generated?
Friction Pressure (piezo electric) Heat Light Chemical Magnetism
What are the 3 requirements for electromagnetic induction?
A magnetic field
A conductor
Conductor move through magnetic field
Describe the basic principal of an alternator
Stator encloses a rotating bar (rotor) to generate AC, which slip rings or silicon diodes rectify to DC.
Magnetic field lies in rotor.
Current produced in armature (stator)
Describe the basic principal of a generator
Stationary magnetic field and rotating conductor (armature)
AC is rectified to DC using split rings commutator and carbon brushes.
Current produced in armature (rotor).
What is the purpose of the electrical system?
Generate, regulate and distribute electrical power.
How many electrical systems are on board a large a/c? Of what type?
At least 2 for redundancy of a mix of AC/DC.
AC more common, largely due to lightweight alternator attached to each engine and greater power demands. They also have smaller gauge wire. Will have an alternator attached to each engines to drive AC circuit.
What is a bus bar?
Power is supply is routed off a common bar, which several components are connected too. This breaks the electrical system into channels which can be prioritised.
What 3 types of bus bar are there?
Parallel, split bus, split parallel
What type of power is supplied from a battery?
Can this type be isolated from the other?
DC.
Yes it can be using it’s own bus.
How to convert AC - DC and DC - AC?
AC-DC: AC is rectified using full wave rectifier to DC.
DC-AC: static inverter
What purpose is a CSD?
Output of electrical generators must produce power at a constant frequency.
A constant speed drive is required to produce a constant frequency power supply of approx 400hz.
An alternator output frequency is determined by rotation of armature which is driven by the engine, which does not operate at a constant speed. Hence the need for a CSD (works of CFF flyweights balanced by springs).
What is the most common type of CSD
Hydro mechanical.
Mechanical governors maintain rotation speed for this type.
Is the oil in the CSD the same as the engine?
They are independent. Oil inlet and outlet temp, low oil px are monitored.
Describe the principal of operation of the CSD
Integral to generator/alternator.
Input from engine on a cog, attached to 2 cogs of same size, one attached to fixed displacement. The faster it spins the more charge oil is entered and enters the control cylinder which provides a braking action to variable displacement cog. This spins a similar sized cog attached to an output shaft maintaining RPM. Governor maintains a stable oil px.
Is there an option to disconnect the CSD? Why/why not?
Yes, in the event of a gearbox or generator failure (overheat/mechanical), as rotation must be stopped before serious damage would occur.
A guarded switch “GEN DISC” activated a solenoid switch which pulls a dog clutch breaking the connection between the engine gearbox and CSD. Once disconnected, it can’t be reset inflight.
What is a CSD a called on Boeing/Airbus?
Integrated drive generators (Boeing)
Integrated constant speed drive (Airbus)
What advantages are there to 3 phase AC over single phase?
Any disadvantages?
Increases the amount of current available.
Connection to phase sensitive devices (motors/transformers) is essential to be correct otherwise the incorrect phase will cause motor to spin in the opposite direction. Connecting these devices in parallel with incorrect phase can cause burn out.
What areas other than the cabin are air conditioned and/or pressurised? Why?
Air con to A/C equipment compartment (avionics) to prevent condensation due heat build up.
Air con/pressurised to cargo compartment as animals often carried.
What a/c systems make the PAX environment?
O2, pressurisation, air con, heating, pressure cell.
Main purpose is to control pressure and temp to ensure comfort and sufficient O2/safe px.
What is pressure differential? What the normal cabin altitude?
How is this cabin altitude controlled?
PD: difference between internal cabin px and ambient outside px.
8000ft.
Controls outflow rate to be less than the inflow from the aircon by a rate controller. Once desired PD met outflow rate increases. VS in cabin is also controlled by rate controller to ensure pax comfort.
Compare a flight profile of the internal cabin px compared with ambient outside px.
What control this?
Both are initially the same on the ground. As altitude increases, pressure decreases. Both the a/c and outside px will “inc in altitude/dec in px” but the a/c will at a slower rate that is comfortable to PAX.
This rate is controlled the outflow valve, which is controlled by crew settings (in 0000’s of ft/min) to ensure safe PSID.
What is the purpose of an air conditioning system?
Maintain comfortable cabin temp within the a/c fuselage. Will utilise an automatic control system so that the crew set a temperature and the system will automatically maintain this temp.
What are the requirements of an air conditioning system?
A pressure cell designed to withstand pressure differential.
Ability to respond to rapid changes to ambient temp and pressure.
Air supply capable of providing sufficient air for cabin pressurisation and air conditioning.
How many cooling units are there? Explain each of their purposes?
3: primary/secondary heat exchangers and refrigeration unit.
Primary heat exchanger is to cool compressed air from bleed air (1/mid stage compressor) to a temp of approx 300*F. Cooled by RAM air. The amount of air to be cooled is controlled by the primary heat exchanger bypass valve.
Secondary heat exchanger cools this air down so that the refrigeration unit can operate efficiently.
The refrigeration unit will provide the system with air at a temperature that is comfortable.
What valves distribute air in the air con system?
How many air con systems are present in a large a/c?
Ducts and diffusers.
Multiple systems, one will usually run off 1 engine. They may feed different areas such as the cockpit, avionics bay, one side of cabin fed by 1 and other side/baggage fed by the other.
When may an air con system need to be shut down?
May be required in high altitude RWY, icing conditions or fully laden. These are all high power setting situations.
If one is shut down, the remaining system should be able to fully operate adequately on the remaining systems.
How much air is recycled through the air con system? Why?
Any redundancy in the system?
50% recirculated through the air con system to reduce load on air delivery system, especially when air is being drawn from the compressors.
Yes, an alternate RAM air system allows fresh air to be provided should both engine driven systems fail.
How is the environmental system controlled?
In the cockpit by a control panel can can be as simple as being selected to OFF/NORMAL/RAM. Modern a/c have temperature control, which can be different in cockpit/cabin.
What additional features are on a modern cabin environment controller?
Provision for duplicated systems. Will include pressurisation controls and instrumentation, such as cabin pressure controller, rate selector, landing altitude selector, barometric selector, emergency controls and warning lights in conjunction with the CWP.
If the APU has the ability to operate the system, the selection will also be available at this panel.
What valves are in the pressurisation system?
What 3 valves make up the safety system?
Outflow valve (pressure control valve): maintains correct cabin px under normal conditions.located mid a/c.
Safety:
Pressure relief valve: ensures the pressure does not exceed max predetermined limit. Auto located hull.
Negative pressure relief: ensure px outside never exceeds px inside. Auto located hull.
Emergency depressurisation: operated by WOW/cockpit, dumps residual px on landing. Located from a/c
Describe the temperature control system components
Cabin temp controller, temp selector knob, 2-position temp control switch (auto/man), modulating bypass valve and a control network.
When in “auto”, bypass valve seeks a gate position resulting in a duct temp that is selected, accomplished by the control network which transmits signals from sensing element to cabin temp controller which electrically positions this valve to the temp selected.
If “man” set, the controller controls bypass the valve directly without reference to duct temp. Desired temp is maintained by monitoring air temp know as varying conditions alter cabin temp.
What is temperature? What is latent heat? How does this relate to a refrigerant cycle?
Temperature is the measure of effect of heat on a body. Latent heat is the heat added/removed to a body without changing its temp (during state change).
A refrigerant changes from liquid to vapour, requiring heat to be added which it gets from the air (cooling air return to cabin) and will turn back into liquid outside of cabin, discharging the heat outside the a/c, returning cold.
High px refrigerant in condenser, low ox in evaporator.
What is the most common refrigerant in aviation? Why?
Refrigerant 12 (R-12) as it is stable at high/low temp and does not react with materials in the air con system (rubber in hoses/seals), is practically odourless and colourless.
Also known as Freon-12, Genetron-12, Isotron-12, Ucon-12.
Different end number is a different refrigerant.
How much O2 is generally carried?
Enough to get from cruise altitude to 14000ft in less than 4 minutes, without exhausting supply.
Advantages/disadvantages of gaseous vis chemical O2 systems?
Chemicals:
A: Lightweight, less maintenance, less equipment (less space consumed)
D: Greater fire risk, not tamper proof, won’t turn off
Gaseous are the opposite.
Which state of O2 takes the most room/least room?
Solid is most space efficient, gaseous is least space efficient.
What is the common passenger O2 system?
Plug in supply sockets fitted to cabin walls to which O2 masks are connected, or drop down masks auto fall if O2 is required.
O2 is supplied automatically. Any auto control (barometric control valve) can be overridden by a crew member.
What are the 2 most common type of fire detection devices?
Spot type detector and continuous loop detector (live wire).
Live wire is more common.
Explain the continuous loop detector system and name the 2 most common types.
Provides a more complete coverage of fire detection of an area.
They are over heat systems, or heat sensitive devices that complete electrical circuits when a certain temperature is reached.
Kiddie and Fenwall systems.
Briefly explain the Kiddie and Fenwall fire detection systems
Draw diagrams
28V DC
Fenwall: metal bar running through casing of ceramic beads treated with enteric salt (semi conductor). As temp rises salt breaks down and conducts electricity so the EMF applied to the rod travels to the outer tube which is grounded, completing a circuit.
Kiddie: same principal, expect 2 rods. One of which is grounded and the other has 28V DC EMF applied (hot lead). Circuit is completed to ground once resistance of thermistor material breaks down so that relay coil is energised to activate alarm.
Both will go back to normal if heat disappears.
What does the typical fire extinguishing system on engines contain?
Cockpit control switch, fire extinguisher agent container, agent distribution system.
Engine can be protected by a single bottle or 2+ on cross feed.
How is the extinguisher agent released?
Relief valve is a fusible disk which ruptures if the bottle overheats. To discharge from cockpit, electrical current is applied to contractor which detonates an explosive cartridge and releases agent.
Green=good, yellow=intentional discharge, red=unintentional discharge.
What fire extinguishers are kept in cargo/cockpit/cabin?
Explain the cargo system
Hand held in cabin/cockpit and in cargo a fixed remote controlled system.
First an initial amount is sprayed then slowly more agent is released to maintain the amount in the compartment, which depends on the size.
Some cargo systems have auto fired system, but this is usually inside toilets.
Explain the type of fire extinguisher found inside toilets
Eutetic head (squib) which melts at 70-75*C and allows agent to be released.
What type of extinguishing agents are there? How do they work?
Water: cools fire/excludes O2
CO2: displaces O2
Dry chemical: smothers fire (excludes O2)
Halogenated hydrocarbons: chemically interferes with combustion. Aka Freon
What types of fires are the previous agents used for? Advantages or disadvantages?
Water: non electrical fires, risk of electrocution otherwise
CO2: electrical fires (leaves residue/difficult to breathe)
Dry powder: any type of fire (not used in cockpit due to visibility issues/residue)
Halogenated HC: used in aviation on all fires. Low toxicity, effective in lower concentrations (not deprive people O2), little reside.
How do smoke detection systems work?
Collects air in strategic locations where smoke is expected to accumulate before the heat from the fire activates the fire protection devices.
Can be type 1: CO detectors
Type 2: photoelectric devices (light transmissibility)
Type 3: visual detection of presence of smoke by viewing samples.
How does a single wire thermal switch work?
28V DC is applied to both paths and if an overheat alarm/fire occurs which closes a switch, the path is complete creating a circuit. This arrangement means one open circuit and system will still provide fire protection at all the fire surveillance points.
A test switch tests entire loop.
Short circuit will provide false alarm.
What factors control the effectiveness of the controls?
Force and arm from CoG.
Where force is determined by deflection angle and dynamic pressure.
What is an issue with flight controls in high speed flight?
Effectiveness may be too great so that flexibility in structure with a large force results in a twist about its torsional axis.
To fix this (and provide system redundancy) split controls are used. Used in tandem at low speeds and inboard sections at high speeds (or flaperons/elevons). Lockout system is activated by ADC speed input.
What is a problem with power boosted controls?
During transonic flight, shock waves form on control surfaces and cause buffeting and this causes feedback into the system.
To prevent these forces reaching the pilot, many a/c use power operated irreversible controls.
How does power boosted controls work? What is a disadvantages? How is this fixed?
Flight controls in cockpit actuate control valves which direct hydraulic fluid to control surface actuator known as power control units (PCU). Since the pilot has no actual feel, Q feel or artificial feel to control column must be added that is proportional to flight load on control surfaces.
What can be done if all hydraulic power control is lost?
Some a/c have a manual disconnect or manual reversion system which allows manual handling.
Modern a/c use electrical power.
In extreme cases, fuel transfer and power imbalance can control an a/c.
How does fly by wire work?
Position transducers convert flight control movement to analogue electrical signals, which are sent to actuator control electronics. They are converted to digital signals and sent to flight computers so pilot inputs can be analysed to be sure they are inside flight envelope and adjusted if they are not.
Can be operated by IFS providing auto flight control and feed flight control position to data recorder.
Notes) flight control-CPU-electric signal-actuator
What is the Brayton cycle?
Is the term used for the continuous thermodynamic combustion cycle for a gas turbine engine. Is also known as a constant pressure cycle (as pressure constant during combustion as volume increases so speed increases).
Define the operation of the Brayton cycle
X4 actions are the same (intake, compression, expansion (power), exhaust).
A: ambient air is compressed towards “b” so volume decreases and pressure increases.
B: fuel is added so heat is added as it is burnt, at a relatively constant pressure towards “c” as volume increases. Pressure slightly decreases due to drag in the combustion chamber.
C: pressure losses toward “d” as gases expand back into atmosphere and some of the pressure is turned into mechanical energy by the turbine.
What are the similarities between the Otto and Brayton cycle?
Combustion Otto constant volume, in Brayton constant px (vol inc)
Both have induction, compression, combustion (power) and exhaust.
Otto only power in combustion where as in Brayton is continuous so greater power output for a given size of engine as more fuel can be burnt in short time.
As no large peak/fluctuating px in Brayton, jet fuel are low octane and lightly fabricated combustion chambers used.
Advantages of centrifugal and axial compressors
C: more robust (less susceptible to FOD), easier to develop/manufacturer, better on smaller engines, simple.
A: consumes more air of same frontal area so higher pressure ratios, more fuel burn, therefore greater power. Increase thrust by adding more stages. More efficient/higher SFC for equal thrust, avoids energy losses due axial flow.
What are inlet guide vanes, rotor blades and stator blades in the compressor?
Inlet guide vanes are at the inlet of the first stage compressor. Are stationary and purpose is to guide airflow into the rotor at the most desirable angle.
Rotor blades rotate and accelerate airflow rearwards through each stage. Are the first stage of compression. Are of aerodynamic design with varying AoA.
Stator receive air at high velocity from the rotors and diffuse the air (slow it down) to create px rise. Are also of aerodynamic shape.
Can draw graph that shows px change through a compressor.
What is a bypass ratio?
Used on some TJ and all TF engines.
Compares mass of airflow through bypass duct to the mass through the primary path in the same timeframe. Ie)bypass flow to core engine flow.
Under 5:1 is low bypass above is high bypass.
Benefits and purpose of multi spool compressors?
Developed for operational flexibility (high compression ratios, better stall characteristics and faster acceleration). They are able to provide greater thrust at altitude as the N1 speeds up and recovers lost pressure due to declining density so that pressure in HP a compressor remains same.
What is a compressor stall? What are the signs?
Abrupt loss of efficiency of the axial flow compressor when the AoA of the blades becomes excessive.
There are typically no signs of a complete stall, however, if a surge is about to occur vibrations, temp/FF/EGT fluctuations.
Symptoms of compressor stall?
Disrupted/turbulent airflow into compressor
Lean mixture due to rapid engine deceleration (inc gas flow)
Rich mixture due to rapid engine acceleration (dec gas flow)
Damaged turbine (inc gas flow due red comp)
Damaged compress (“”)
Operating outside of RPM limitations
What is the function of the turbine?
To drive the compressor and also the accessories/shaft of TP/TS.
It converts pressure energy from exhaust gases into kinetic energy… which the kinetic/heat energy is converted into mechanical work.
It extracts kinetic energy from the pressure energy by reducing pressure in convergent nozzles at the end of stator/rotor blades.
Describe the impulse blade design
A stator vane and rotor blade arrangement where the vanes form convergent ducts and the blades form straight (nor C/D) ducts that are curved. This creates a turning moment on the rotor by impulse as the gases impinge on the rotor.
Describe the reaction blade design**REFSG
A stator vane and rotor blade arrangement where the vanes are straight ducts and blades are convergent ducts. The convergent rotor blade is turned by the acceleration and expansion of the gas.
Describe the impulse reaction blade design
Works based on part impulse/part reaction, 50% impulse/50%reaction. Reaction at the tip and impulse at the root.
Velocity is kept constant base to tip.
Explain how an exhaust system can be “choked”
Exhaust gases are expelled into the atmosphere through a propelling nozzle (convergent duct…inc gas velocity). In TJ a/c, the exhaust is only subsonic at low thrust conditions, at med/high thrust the exhaust will reach speed of sound for the EGT for which no further gas velocity increase can be achieved, unless EGT rises. The propelling nozzle is now said to be choked.
How can choking an engine provide thrust?
Total pressure rises upstream above the value at which the propelling nozzle becomes choked, static px at exit increases above atmospheric px. Due to balloon theory, the px difference across the propelling nozzle gives a pressure thrust, effective over the exit area. The additional thrust is achieved through the change in momentum of the gas flow.
How does a convergent/divergent duct affect speed/px subsonic?
C: speed inc, px dec
D: speed dec, px inc
How does a C/D nozzle help increase thrust?
A convergent nozzle wastes energy as gases do not expand quick enough to achieve atmospheric pressure. Some high pressure ratio engines use a C/D nozzle to recover some of the wasted energy. The nozzle reached Vs at the C/D duct so the divergent part becomes SS a and a further increase in gas velocity can be achieved so an increase in thrust.
What are the requirements of oils?
Low volatility: minimise evaporation
Anti foaming: more positive lubrication
Low lacquer/coke deposit: min solid particle formation
High flash point: temp at which flammable vapours are released
Low pour point: lowest temp at which will gravity flow
Film strength: stick under compression, stick surfaces centrifugal
Wide temp range: -60F to +400F
High viscosity index: retains viscosity at operating temp
What is the purpose of magnetic plug and chip detectors?
Fitted to the return side of the oil system to collect any ferrous debris.
Magnetic plug is a permanent magnet inserted in the oil flow to collect debris. It can be readily removed and inspected to give early warning of impending failures. Chip detectors are magnetic plus incorporated into an electronic device which can transit a signal to the cockpit in the event of a significant collection/size of debris, either by completing a circuit or changing impedance of coil.
What is a FMU?
What other devices help prevent the engine exceeding parameters?
Fuel metering unit is an engine driven accessory which can operate by mechanical, pneumatic, hydraulic and electric forces in combination of hydro pneumatic, hydro mechanical and electro hydro mechanical to meter fuel.
Others include pressure control, flow control, pressure ratio control, acceleration and speed control achieved through the use of a governor.
None of which is controlled directly by the pilot, but by a CPU.
Purpose of fuel system?
Pulley precise amount of fuel to engine in all operating conditions.
Free of any dangerous operational characteristics.
Inc/dec thrust on demand in any operating conditions.
What are fuel nozzles?
Aka injectors/distributors.
They atomise or vapourish fuel to ensure rapid burning.
Can be simplex type or duplex type.
Explain simplex fuel nozzle with diagram
Is a chamber which introduces a swirl into the fuel and a fixed area atomising device.
Explain duplex with a diagram
Has a primary and main tube with 2 independent offices, one smaller than the other. Smaller office deals with smaller flow and the larger with larger flows
Purpose of after burning? How is it achieved?
Improving T/O and climb performance.
Introduces and burns fuel in the exhaust gases as they pass between the turbine/propelling nozzle. The increased thrust comes from the higher EGT therefore increased gas velocity.
60% of air remains unburnt from combustion chamber.
Purpose of spray bars?
Used to help concentrate flame around centre axis of the pipe to help cooling as only portion of unburnt air is burnt and remainder is used to flow around edges to help cooling.
How does water injection help increase thrust?
Power output is proportional to mass airflow (density) which can be achieved if cooler air is used. Introducing water into engine will cool the air and increase its density.
If all water is used on T/O pure water is used. If not, water/methanol combo is used to ensure water doesn’t freeze,
Where is water injected?
What is preferred for axial flow?
Compressor inlet or combustor inlet.
For axial flow engines the combustion chamber inlet is more favourable as a greater quantity can be used with a more even distribution.
Explain injection into the 2 different areas
When just water is sprayed into the compressor inlet, it cools the air so density and thrust increased, but reduced TIT. Adding methanol means the TIT is restored back due to methanol burning so power restored with no fuel flow adjustment required.
Adding coolant through the combustion chamber increase mass flow through the turbine relative to that through the compressor. The temp/px drop across the turbine is reduced resulting in increase jet pipe px which give additional thrust. The decreased TIT allows for increased fuel flow.
Benefits of AC?
More economically transmitted over long distances.
Lighter alternator… therefore simple, small and easy to maintain.
Less arcing so need to replace CB less.
Greater power for unit size.
Smaller gauge wire.
What are the methods of magnetism?
Repetitive stroking
Electromagnetism (device emits MF)
Magnetic induction (adopt magnetism overtime by left in single position/move through MF)
Methods to demagnetise?
Percussion (hitting)
Induction (2 opposite polarity magnets next to each other)
Heating (beyond Currie point dipoles disalign)
Degaussing (stepping down AC or moving object away from source… aviation)
What is the inverse cube law?
Strength of magnetic field decreases with the cube of the distance from the pole. Only applies in empty space with a single pole so in real life actually close to power of 4.
What are the magnetic axis?
Longitudinal axis P
Lateral axis Q
Normal axis R (can’t be compensated for affects compass in pitch/roll)
