Systems Final Exam Flashcards
what is the function of the electrical system on a large transport aircraft?
generate, regulate and distribute electrical power throughout the aircraft
how many electrical systems are there
there is usually more than one system and comprise of a mixture of AC and DC circuits
what is the normal config of a electrical system
will have a separate AC circuit powered by the alternator fitted to each engine and each circuit will feed a specific bus
why is AC more common in use than DC
AC alternators are more light than DC generators of similar capacity
diameter of AC wiring is smaller which makes it lighter
how is electricity supplied on an aircraft thru the battery
when connected to a complete electrical circuit, the battery converts chemical energy to electrical energy and a current will flow. the flow will stop if the circuit is switched off or the battery has exhausted it’s chemical energy.
explain the basic bus system
power supply is routed to a common bar, off which several components or services can be powered.
this splits the electrical system into channels so that some parts can be prioritized in the event of a supply breakdown
what are the different types of bus system
parallel type
split bus type
split parallel type
how is DC power provided on an aircraft
AC power is rectified using a full wave rectifier
(AC can be produced from DC using a static INVERTER in an emergency also)
DC circuits have their own bus system isolating them from the AC bus
explain how the CSD works
the AC alternator requires power at a constant 400hz. the frequency is determined by the rotation speed of the armature, which is driven by the engine. The engine however doesn’t output a constant speed so a constant frequency can’t be delivered unless a device (CSD) is installed so that no matter what the speed of the engine is, the alternator can still output 400hz.
what are the characteristics of AC generators
may be self excited, pilot excited or externally excited from the a/c DC bus bar
output is 400hz and power rating is 20-60 kilovolt amperes
what is the advantage of a 3 phase supply over a single phase supply
a 3 phase supply increases the amount of current available hence power to components.
what are the implications of connecting 3 phase motors to components
connecting 3 phase equipment correctly is important because if not done properly it will cause eg. motor to rotate in opposite direction.
connecting 3 phase motors in parallel with incorrect phase could cause burn out
what purposes do air conditioning and pressurisation have on an aircraft
- conditioned air for heating/cooling cabin/cockpit
- prevent condensation on a/c equipment leading to avionics damage to protect avionics
- provide conditioning for cargo as animals sometimes stored
what systems are the cabin air con and pressurisation systems broken down into
- pressurisation system
- air con system
- heating system
- oxygen system
- pressure cell
supplied inhabited areas of a/c with temp control and safe atmospheric pressure lvl - ensuring safe oxygen lvl at altitude.
what is PD
what is the standard PD
pressure differential, the difference between pressure inside the hull and the ambient pressure.
the system maintains a cabin alt of 8,000ft regardless of the altitude the aircraft is flying. if the a/c climbs beyond 8,000ft then PD will increase
how is PD controlled
controlling outflow of air from the fuselage. as air con flows in, the outflow is regulated at a lesser rate by a a rate controller. it also controls cabin VS.
when desired PD is achieved, outflow is increased again at a rate that will maintain the desired PD level
describe the cabin altitude profile as the a/c climbs
as it climbs:
- ambient pressure decreases and cabin pressure decreases BUT at a lesser rate than the a/c itself.
- cabin pressure change is controlled by cabin vertical speed in 1000’s fpm
- outflow valve, controlled by crew setting controls cabin pressure and cabin vertical speed to maintain safe PSID.
what is the function of the air con system
maintain a comfortable air temp within aircraft fuselage
achieved through use of a heating system and auto control system where crew can select temp it’ll maintain.
what are the 3 key requirements of a air con system
- a pressure cell designed to withstand pressure differential
- the ability to respond to rapid changes in ambient temp and pressure
- an air supply capable of providing sufficient air for cabin pressurisation and air conditioning.
how many cooling units are in the air con system
- there are 3
- 2 of them are primary and secondary heat exchangers, the 3rd being the refrigeration unit.
what is the purpose of the primary heat exchanger
- to cool the compressed air being bled directly from the engine compressor to a typical temp of approx. 300F.
what is the purpose of the secondary heat exchanger
- cooling the system air further so that the refrigeration unit can operate efficiently
what is the purpose of the refrigeration unit
- to provide the system with air at a temp appropriate for the provision of the necessary cabin comfort expected of air con systems.
what are the safety features of the environmental system
- pressure control valve
- pressure relief valve (used to ensure pressure does not build up beyond max operating pressure)
- negative pressure valve ( ensures pressure outside cabin never exceeds pressure inside the cabin and operates automatically
- emergency depressurisation valve (activated by WOW switch)
- emergency cabin altitude control
how is the environmental system controlled from the cockpit
simple systems have a OFF-NORMAL-RAM setting
most modern aircraft have a panel where individual temps can be set in the cabin and cockpit.
- also will have individual pressure controls on the panel incl. emergency controls and warning lights
what other controls are on the cabin environment panel in the cockpit
- cabin pressure controller
- rate selector
- landing altitude selector
- barometric pressure selector
- emergency controls and warning lights
works in conjunction with central warning system
if APU has ability to operate system it will be available on panel
how is air distributed in the environmental system, is there more than one method?
- via ducts and diffusers
- some a/c may have duplicate systems feeding from each engine and feed to different areas of the a/c. (this has redundancy so if one fails the other will still work).
when might 1 environmental system have to be shut down
high power operating conditions eg
- high altitude runway ops
- heavy icing conditions
- fully laden a/c.
how much of the air in the environmental system is recycled
what’s used if both engine driven systems fail?
50% is recycled to reduce load on air delivery system
most systems will have an alternate ram air provision which allows for fresh air to be provided should both engine driven systems fail
What elements make up the temp control system
consists of:
- cabin temp controller
- temp selector knob
- two position temp control switch
- modulating bypass valve
- control network
How does the temp control system operate and what modes does it operate under
- when temp control switch is in AUTO, bypass valve will seek a valve gate position resulting in the duct temp corresponding to temp controller setting.
- accomplished though the control network which transmits signals from sensing element to cabin temp controller, which electrically positions valve in relation to settings of the temp control knob
- when temp control switch is in MAN, controller will control bypass valve directly without reference to duct temp. desired temps are maintained monitoring the air temp knob.
how does the primary heat exchanger work
- the unit reduces the temp of the engine bleed air by routing it through the veins in the core of the heat exchanger.
- during flight the core is cooled by ram air and the amount of air to be cooled is controlled by the primary heat exchanger bypass valve.
explain how vapor cycle air conditioning works
on the principle of latent heat
the refrigerant changes state from liquid to vapor, and in doing so absorbs heat from the cabin. this heat is sent outside the aircraft as the refrigerant returns to a liquid state.
what material is used as a refrigerant for commercial a/c
dichlorodifluoromethane AKA
- Refrigerant 12 OR
- R-12
what are the characteristics of dichlorodifluoromethane
- stable compound at high and low temps
- doesn’t react with any materials in the system
- doesn’t attack rubber sealings
- colorless and odorless
what else is termed as dichlorodifluoromethane
- Refrigerant 12
- R-12
- Freon 12
- Genetron 12
- Isotron 12
- Ucon 12
- Freon 22 is similar to Freon 12 but NOT the same thing as it has different vapor pressure.
what is the requirement for oxygen supply on large transport aircraft
enough for the aircraft to descend from cruising altitude to 14,000ft in less than 4 minutes without exhausting the oxygen supply
what are the advantages and disadvantages of chemical oxygen systems
advantages:
- lightweight
- less equipment
- less maintenance
disadvantages:
- fire hazard higher
- not tamperproof
- won’t turn off
How is the continuous flow oxygen system arranged in the passenger environment
- may consist of a series of plug in supply sockets fitted to the cabin walls adjacent to pax seats to which oxygen masks can be connected
- can be a drop out arrangement where masks are presented automatically is pressurization fails.
- in both cases oxygen is supplied from a manifold and any automatic control can be over ridden by a crew member.
Why are solid state oxygen generators the most efficient option
- the oxygen occupies much less volume as a gas so it is more efficient in space.
- less equipment and maintenance is required for solid state oxygen converters.
what fire and smoke detection systems are common in use
- spot type system
- continuous loop detector system (AKA live wire detection system - the most common one)
What are the characteristics of the continuous loop detector system
- permits more complete coverage of a fire hazard area than spot type.
- they are overheat, heat sensitive units that complete electrical circuits at a certain temp, activating the warning system
what are the 2 main types of continuous loop fire detection systems
- Kidde system
- Fenwal system
how does the smoke detector work in the engine nacelle
uses photoelectric cells with a light beam directed toward them. if enough smoke is present, the light beam refracts light to the cells causing the alarm to sound.
how does fire extinguishing work inside the engine nacelle
- bottle is powered at 500-600psi.
- has a relief valve that will rupture if the bottle were to overheat
- the bottle is discharged from the cockpit by an electrical current applied to contactor which detonates an explosive cartridge and shatters a disk at the bottle outlet letting the agent flow
how does an x feed extinguishing system work
- allows a eg. number one engine fire to be taken out with either the number 1 or number 2 bottle using x feeding
how does the cabin and cargo area fire extinguishing system work
- has a fixed remote controlled system designed to extinguish fires in 2 steps:
1. - an initial amount of agent is sprayed into the compartment
2. - more agent is then slowly added in order to maintain the amount of agent in the compartment for 30 minutes or more depending on compartment size.
where does the cabin and cargo fire extinguishing container discharge
it discharges into a line which sprays the agent out into the compartment when the extinguisher switch is activated
what are the 4 types of extinguishing agents
- Water (cools fire and excludes oxygen)
- carbon dioxide (displaces surrounding oxygen)
- dry chemical (smothers the fire excluding oxygen)
- halogenated hydrocarbons (chemically interferes with combustion)
what are the characteristics of automatically fired extinguishing systems? where are they found?
usually found in toilets
- the fire bottle is fitted with a squib which melts at 70-75 degrees, allowing the extinguishing agent to exit the bottle
what are the different classifications of smoke detection instruments
Type 1 - measurement of carbon monoxide gas (CO detectors)
Type 2 - measurement of light transmissibility in air (photoelectric devices)
Type 3 - visual detection of the presence of smoke by directly viewing air samples (visual devices)
How can smoke detectors always remain reliable
they must be maintained so that smoke in a compartment will be indicated as soon as it starts to accumulate.
Smoke detector louvers, vents and ducts must not be obstructed.
how does the single wire thermal switch work
- 28V DC is applied to both paths of thermal switch loop
- if heating to alarm temp occurs which closes any switches, a ground is formed through closed switch.
- with that arrangement, one open circuit can occur and system will provide protection at all surveillance points.
REFER DIAGRAM
what are the test and circuit short functions in the single wire thermal switch
test switch tests entire loop and shows if an open circuit is present
a short circuit will cause false warning indications
dimming relay dims the light at night for night ops.
how does the continuous loop detection system work
- 28V DC applied to hot lead thru alarm relay coil.
- when heated, insulator material loses resistance and forms a path to ground
- relay coil is energized by the current flow and alarm light is illuminated
REFER DIAGRAM
how are buffeting forces dampened
power operated irreversible control system
cockpit controls actuate control valves which direct hydraulic fluid to control surface actuators (PCU)
- artificial load is built into the system so pilot has more control feel
what is the problem of the power boosted control system
during transonic flight, shock waves form on control surfaces causing buffeting
how does the fly by wire system work
crew command inputs are translated into electrical signals
signals go to actuator control electronics
signals converted to digital format and sent to primary flight computer
inputs are analyzed, and if outside flight control envelope the signals are modified so they stay inside
can be operated with IFS providing auto flight control and feed flight control data to the FDC
What do aircraft have for then there’s hydraulic power loss
- they have a manual disconnect or manual reversion system which allows crew to fly the aircraft manually (more recent backup systems are electrically powered)
- fuel transfer can be used to roll the a/c and engine power imbalance to turn in extreme cases
what does the effectiveness of a primary flight control depend on
- aerodynamic force generated by control deflection
- moment arm from CoG
what does aerodynamic force depend on
- amount of deflection
- dynamic pressure exerted by airflow
what can happen to the wing at high speed
- control effectiveness be so great that resultant force may twist wing about it’s torsional axis
- split flight controls minimise this happening
how do split flight controls work
- operated in tandem at low speeds and lockout system used to utilize to provide split operation at high speeds
- lockout system works on data fed from ADC so inboard ailerons are only used over a specified speed or flaperons/elevons may be employed.
How does the Fenwal continuous loop detection system work
an Inconel tube fitted with thermally sensitive salt and a center conductor
they are fitted to a control unit which impresses a small voltage on the sensing elements
when overheat occurs the resisitance of the salt drops causing current to flow to the control unit and sounding the alarm
when temp drops back down the resisitance increases again and it goes back on standby
how does the kidde continuous loop detection system work
2 conductors go thru tube with one going to ground and the other to the fire detection control unit
as temp increases resistance to ground decreases and fire detection control unit monitors this change
there is an overheat point and a fire point (overheat will have 10 sec delay)
rate of change of resistance indicates a short or fire, if to goes down real quick then its probably a short
