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.
