Final Exam Systems Flashcards
Describe an alternator compared to a generator?
Alternator:
=AC current
=Requires battery to energise windings before it can generate
=Rotating electro magnet, static wires
Generator: =Larger and heavier =Less current per RPM =Built in commutator =Brushes and slip rings =Rotating wire =Commutator rectifies to DC
Describe generally how electricity works? how does current vs electrons flow?
- Negative electrons around a nucleus can be dislodges and ‘flow’ creating current
- When wire or conductor connected to positive and negative terminal a potential difference occurs, creating electron flow
- Electrons ‘flow’ from negative to positive
- Current said to ‘flow’ from positive to negative
Describe a Volt and Amp?
Volt: measure of electrical pressure (force needed to make electrons flow)
Amp: rate of current flow
What are common types of aircraft battery?
Lead Acid
NiCad (Nickel-Cadium)
What types of electrical circuits will be present on an aircraft and how many? Why?
AC and DC circuits
Will often have more than one for the purpose of redundancy
What is the normal configuration of electrical systems on a large/twin aircraft?
- Separate AC circuit powered by alternators on each engine
- Each circuit feeds a specific BUS
- Split, Split-parallel or parallel BUS
What are the advantages of an AC circuit?
- Lighter weight for same power output
- Alternators produce more current per RPM
- Can be rectified to produce DC, or provides AC
- Can be transmitted long distances more readily
- Circuit breaker operate better under high loads at high altitudes
What is DC power required for? How is it obtained?
- Required for battery charging on ALL aircraft
- Required for some equipment
- Obtained by using full wave rectifier
How are the AC and DC circuits linked?
They are totally isolated on different busses
How is AC produced from DC?
By operating a static inverter from the DC battery
What is a CSD? Why is it required?
Constant speed drive is a hydro-mechanical drive similar to an automatic gearbox
Maintains constant output for electrical generators due to varying speed of engines
What RPM is typically required for alternators? What RPMs can be found on engines?
Gas turbine: 7000-10,000RPM
Piston: think DA40
Generator requires 400RPM (+/- 8Hz)
What is the principle of operation of the CSD?
- Input shaft rotates 2 identical gears to drive a differential unit and Fixed displacement hydraulic pump
- Fixed displacement pump will pressurise oil under high RPM and apply a brake to a variable displacement pump through the control cylinder
- Also maintains pressure to governor with maintains stable pressure to control cylinder
- Variable displacement pump drives a gear linked to differential unit and to output shaft
- Differential unit similar to car clutch and is constantly slipping under every situation except 400RPM engine operation
- Output shaft also drives oil pump for charge oil
How does the CSD oil system work? What instrumentation is provided for it?
It is independent of the engine oil system
Oil inlet and outlet temp
low oil pressure
What happens to the CSD if the Generator malfunctions?
- Guarded “GEN DISC” switch activates a solenoid
- This pulls a dog clutch and will shear inlet drive shaft
- Cannot be reset in flight
What is the Boeing and Airbus terminology for CSD?
A: Integrated constant speed drive (ICSD)
B: Integrated Drive generators (IDG)
What things will a modern constant frequency AC generating system have? Electrical Rating? How is it excited?
- Rotating rectifier, brushless AC generator
- May be excited by Pilot, external or self excited
- 115V, 400Hz, 20-60 kVA (kilovolt amperes)
What is the advantage of 3 phase AC supply compared to single phase?
Increase current available
What happens if you connect 3 phase AC to equipment when the 3 phases are not in sync?
Motors will rotate in opposite direction
Could cause burn out
What is the purpose of the cabin pressurisation system?
- pressurisation for safe/ comfortable cabin environment
- prevent condensation where there is heat build up
- create safe environment in cargo areas for live animals
How can the air conditioning and pressurisation system be broken down? Sub-systems?
- Pressurisation system
- Air conditioning system
- Heating system
- Oxygen system
- Pressure Cell
What is PD? how does it vary with altitude?
- Pressure differential - the difference in pressure between the inside and outside of the aircraft hull.
- System will try and maintain cabin alt of 8000ft and so any climb above 8000ft will increase PD
How is PD controlled? Describe system?
- By maintaining cabin outflow rate lesser than the air conditioning airflow in.
- Rate controller controls air outflow rate and when desired PD is reached it will maintain air outflow rate equal to the air inflow rate through the air conditioning system
- Cabin vertical speed also controlled through this means
How does the aircraft RoC relate to the cabin RoC? How is the Cabin RoC/RoD controlled?
Both start at departure airfield altitude
Cabin altitude increases at a lesser rate than the aircraft itself
Controlled by:
- Crew settings control outflow valve position (via rate controller)
- Cabin vertical speed in 1000’s of fpm
What capabilities/ components must the air con system have?
- Pressure cell to withstand PD
- Ability to rapidly respond to ambient temp changes
- Supply capable of sufficiently pressurising aircraft
- Sufficient supply for air conditioning
How will cabin temp be obtained?
Heating system or air con system where crew sets temp.
System will automatically maintain temp by engaging and disengaging different systems.
What are the requirements of a cabin air con system?
- Provide minimum stratification (hot/cold layers)
- Heating and cooling cockpit and cabin
- Humidity control (prevent condensation on windows)
- Keep wall and floor panels at comfortable temp
describe the process through which conditioned air reaches the cabin? Where does it come from?
- Compressor bleed air cooled by primary heat exchanger to 300˚F
- Secondary heat exchanger cools air further to allow for efficient use of refrigerator unit
- Refrigeration unit provides cabin air at appropriate temperature
How is cabin air distributed? How do redundant/ duplicated systems integrate into the system
- Through vents in the cockpit and cabin
- May have duplicated systems, with one engine feeding one half of the cabin
- System 1: cockpit, avionics bay, half of cabin
- System 2: half of cabin, cargo bay
Redundant ram air system can be used if both air con systems fail
Why might the crew opt to shut down one air con system? What conditions?
How are loads further reduced on the system?
When high power settings are required:
- High altitude runway
- heavy icing conditions
- Heavy load
Loads further reduced by recirculating 50% of of air
Describe a simple vs modern environmental control panel?
Simple: ‘Off’ - ‘Normal’ - ‘Ram’
Modern: zone controls with temp adjusting.
- Provision for duplicated systems
- Pressurisation controls and instrumentation
- Cabin pressure controller
- Rate selector
- LDG altitude selector
- BARO pressure selector
- Emergency controls and warning lights
- APU selection to operate if available
What safety features are present in the environmental control system?
- Pressure control valve (outflow valve in normal ops)
- Pressure relief valve
- Negative pressure relief valve
- Emergency depressurisation valve operated by WoW or cockpit
- Emergency cabin altitude control in cockpit
What components make up the temperature control system?
- cabin temp controller
- temp selector knob
- 2 position temp control switch
- bypass valve
- control network
How does the temp control switch work? What happens when it is set to different positions?
Auto: bypass valve will seek valve position to maintain duct temp the same as that on the temp selector. Electrically positions valve
Manual: temp controller will directly control bypass valve position. Temp maintained by adjusting temp knob as cabin conditions change
How does the primary heat exchanger work? Where can heated air come from? What controls how much air is cooled?
- Engine bleed or supercharger air is ducted through veins in the core
- Core is cooled by ram air
- Amount of air to be cooled is controlled by primary heat exchanger bypass valve
Describe the vapour cycle air conditioning system?
- Heat can be added or removed to refrigerant without changing its temp
- Heat absorbed from atmosphere by refrigerant as latent heat and changes its state from liquid to gas
- Heat is removed to external air and refrigerant returns to liquid state.
What refrigerant is most commonly used in vapour cycle systems? Why? What are its properties?
Dichlorodifluromethane (Refrigerant-12 or R-12)
Stable at high and low temps Non reactive with system materials Won't attack rubber hoses and seals Colourless Odourless
How much oxygen must a transport aircraft carry?
Enough for it to descend from cruise to 14,000ft in less than 4 minutes without exhausting the supply
What are the advantages of Chemical Oxygen compared to gaseous Oxygen systems?
Chemical system is:
- More efficient space wise
- Requires less equipment
- Requires less maintenance
What are the disadvantages of chemical O2 compares to gaseous O2 systems?
- Fire hazard
- Not tamper-proof
- Can’t be turned off
What 2 types of systems may a passenger O2 supply system be?
Describe how oxygen flow is actuated?
- Drop out: where masks are presented if pressurisation fails
- Plug in: Oxygen masks plugged in to supply sockets next to each passenger
In both systems oxygen flow will be automatically supplied from a manifold, or overridden manually by the crew
What maintenance is required for a solid state oxygen system?
Only requires integrity inspection
What types of fire detection systems could be present on an aircraft (not engines)?
Which is more common?
Spot Type
More common: Continuous loop (live wire)
Generally, what is a continuous loop type fire detection system? What condition does it sense? What benefits does it provide?
Over-heat systems that will activate warnings at a certain temperature
- Provide a more complete coverage than spot type
- Activate at a certain temperature
What are the two types of continuous loop fire detection systems? Briefly describe each
- Kidde: two wires inside a thermistor insulator
- Fenwal: one wire in a thermistor insulator with return circuit provided by conductive coating
Describe how the power plant fire extinguishing system works?
- Bottle contains pressurised extinguishing agent
- To discharge the bottle from cockpit, an electrical current is applied to the contactor, which detonates explosive cartridge
- This shatters disc in bottle outlet
- Agent then flows into engine
- Relief valve is fusible disc that will rupture if the bottle were to overheat
What can be done if the engine fire bottle has been used, and the fire is still not out? What pressure are they held at?
Held at 500-600PSI
Can be crossfed between engines.. eg. No.2 bottle contents to No.1 engine if No.1 bottle does not extinguish fire
What are the common extinguishing agents for turbine engines?
CO2
Methyl Bromide
What systems are usually used for cabin and cockpit fire extinguishing systems?
hand held fire extinguishers
How does the cargo compartment fire extinguishing system work? How long can it last?
- Fixed remote controlled system
- Container discharged into a line that is fed into the cargo compartment
- Initial discharge sprayed into compartment
- More agent slowly added to maintain amount of agent in compartment for up to 30min +
Describe an automatically fired extinguishing system and where they can be found?
Can be in cargo compartments, but mainly in toilets
- Fire bottle fitted with eutectic head (squib) which will melt at 70-75˚C
- Bottle will release contents to extinguish fire
What types of extinguishing agents are there and how do they work?
- Water: Cools fire, excludes O2
- CO2: Displaces O2
- Dry chemical: Smothers fire, excludes O2
- Halogenated hydrocarbons: Chemically interferes with combustion
What types of fires can each extinguishing agent be used for?
- Water: Non-electrical
- CO2: Electrical
- Dry chemical: Any type
- Halogenated hydrocarbon: Pax compartments/ low toxicity
What are the three types of smoke detection systems? Where would they normally be located?
Type 1: measurement of CO gas
Type2: Measurement of light transmissibility
Type 3: Visual detection by directly viewing air samples
Normally located where the anticipated type of fire will give off lots of smoke before an overheat is sensed
What is the principle of operation of the thermal switch loop? How is it tested?
- 28V DC applied to both paths of switch
- If an overheat has closed any of the switches, a path to ground will be completed and the alarm sounded
- If one open circuit occurs, fire protection is still provided at other surveillance points
-Test switch will show pilot if an open circuit is present in the power input lead of the loop
What is the principle of operation of the continuous loop fire detection system? What electricity does it use?
- 28V DC supplied to hot lead through alarm relay coil
- When cool insulator material doesn’t;t allow current to flow
- When heated, insulator loses resistance and a path to ground is completed
- When current flows, relay coil is energised and cockpit alarm is activated
What determines the effectiveness primary flight controls on large aircraft?
Aerodynamic force generated due to amount of deflection and moment arm from CoG.
What can happen when controls are actuated on large aircraft at different speeds? What is sometimes done for some controls?
High speed: Controls so effective that the flexible structure may cause torsional rotation of the wing
Primary flight controls are sometimes split to removing twisting and provide redundancy
How do the split primary flight controls work? Give an example
- Operated in tandem at low speed
- Lockout system actuated at speeds above threshold, when ADC sends speed signal
- At high speeds, inboard sections called flaperons or elevons will be used, at low speeds outboard and inboard sections will be used together
Describe a power boosted control system and the issues it faces?
- Control stick directs hydraulic fluid to actuator
- Problems arise in transonic range due to buffeting being fed-back into the system
How does the power operated flight control system stop mach buffet from reaching the pilot?
- Uses power operated irreversible control system
- control stick actuates control valves near cockpit
- control valves direct hydraulic fluid to actuators called power control units (PCU)
- Feedback is done artificially to make stick force proportional to flight loads on control surfaces
What alternative are available if total hydraulic power is lost?
- Manual reversion system
- Modern a/c use electric backup systems
- Extreme cases use alternate controls (thrust imbalance or fuel transferring)
How is a pilot control input made through the fly-by-wire controls to the control surface?
- Position transducers convergent crew inputs to electrical signals
- Signals sent to actuator control electronics to be converted to digital format
- Digital signals sent to primary flight computer for envelope analysis and modification (if required)
- Signals sent to actuators
What are the additional advantages of the fly-by-wire system?
- System can be operated by IFS
- Can provide auto-flight control
- Can feed flight control position data to the Flight data recorder
