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Aircraft Systems 2 > Chapter 4 - Aircraft Electrical Systems 1 > Flashcards

Chapter 4 - Aircraft Electrical Systems 1 Flashcards

1
Q

What is the international standard voltage for aircraft DC systems?

A

24 V

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2
Q

At what voltage are aircraft DC generators usually rated at to ensure a positive charge state is maintained to the 24 V batteries?

A

28 V

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3
Q

What is the standard AC generator used in both military and civilian aircraft?

A

200 V

3 Phase

400 Hz

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4
Q

What two categories do modern aircraft tend to fall in to regarding power generation and power requirements?

A

Low electrical demand - Primarily DC based

High electrical demand - Primarily AC based

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5
Q

What are the 3 categories of power generation for aircraft?

A

Primary

Auxiliary

Emergency

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6
Q

What comprises Primary power generation?

A

Main batteries - 24V

AC/DC generators - AC provide 200V, 3 phase, 400Hz
- DC provide 28V

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7
Q

What comprises Auxiliary power generation?

A

Ground Power Unit (GPU)

Auxiliary Power Unit (APU)

Airborne Auxiliary Power Unit (AAPU)

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8
Q

What comprises Emergency power generation?

A

Aircraft batteries

Ram Air Turbine (RAT)

Emergency Power Unit (EPU)

Note - Less demanding requirements for emergency power are frequently met by the use of an AAPU

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9
Q

What are the two main types of aircraft battery?

A

Primary cell - Non-rechargeable

Secondary cell - Rechargeable

Provide electrical power for engine starting + for long term, low current requirements

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10
Q

What is battery amperage?

A

How long it can supply a set current.

E.g. a 28Ah battery will run a 28amp component for 1hr.

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11
Q

What is the role of generators in the aircraft’s electrical system?

A

Provide power to the aircraft electrical system whilst simultaneously re-charging the battery(s).

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12
Q

What drives the aircraft’s generators?

A

The engine ACCESSORY GEARBOX.

Convert mechanical energy into electrical energy by electromagnetic induction.

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13
Q

How does a generator create electrical power?

A

A magnet is rotated within an iron core which has a coil of copper wire on it.

A flow of electricity is produced across the coil due to the rotating magnetic field.

N and S pole magnetic flux travel in opposite directions, therefore an alternating flow of electricity is created, AC.

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14
Q

How can a generator be designed to produce more power?

A

Stronger magnet

Increasing the length of the coils

Increasing the magnet’s speed of rotation (the only thing we can control in the aircraft once generator is in service)

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15
Q

What does a simple, two stage generator comprise of?

A

Exciter section

Main generator section

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16
Q

What is the purpose of a Constant Speed Drive Unit (CSDU)?

A

To maintain a constant generator RPM, and therefore constant voltage output, even when the engine gearbox RPM is changing.

It is attached to the generator.

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17
Q

What frequency and RPM do standard aircraft AC generators operate at?

A

24000rpm

400Hz

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18
Q

What is the purpose of using a 3 phase generator?

A

Generate more electricity in a single generator.

Note - Any more than 3 magnets has a detrimental effect on electricity generation, therefore 3 phase is the max used.

In a 3 phase aircraft AC generator, EACH phase produces 200V at 400Hz.

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19
Q

How do DC generators turn an AC supply to DC?

A

Use a Transformer Rectifier Unit (TRU)

Only samples the positive part of the AC sine wave.

20
Q

What transformation does the TRU cause in standard aircraft electrical systems?

A
  1. 200V AC to 28V AC

2. 28V AC to 28V DC

21
Q

What types of power conversion equipment are found on aircraft?

A

Inverters - Primary DC to Secondary AC

Converters - Change the frequency of the primary AC supply to a different secondary frequency

Transformer/Rectifier Units (TRUs) - Convert AC input of one voltage into DC output of another voltage

22
Q

What does an Inverter do?

A

Primary DC supply to Secondary AC supply

E.g. 28V DC to 200V AC

23
Q

What does a Converter do?

A

Change the frequency of the Primary AC supply to a different Secondary frequency.

E.g. 400Hz to 50Hz

24
Q

What does a Transformer/Rectifier Unit (TRU) do?

A

Converts AC input of one voltage to DC output of another.

E.g. 200V AC to 28V DC

25
Q

What is the purpose of contactors?

A

Used to remotely operate components.

Allow the use of very small switches in the cockpit.

26
Q

What two parts is a contactor divided into?

A

An electromagnet (low tension) which is operated by a switch

A pair of contacts (high tension) which are held open by a spring

27
Q

How does a contactor work?

A

Switch is activated - contact creates a circuit which energises the electromagnet.

Electromagnetic relay only draws a SMALL amount of current.
Known as the LOW TENSION circuit.

The electromagnet affect pulls together a more substantial pair of contacts which were help open with a spring.

These contacts create a second circuit to earth which allows the battery to power a component that demands a significantly higher current flow.
This is called the HIGH TENSION circuit.

28
Q

How does an Emergency or Fail Safe contactor differ from a standard contactor?

A

Reversed

Switch OFF = Switch contacts are closed and electromagnet is energised.

Main contacts are HELD OPEN by the electromagnet, which overcomes spring pressure.

Switch ON = Electromagnet de-energised and the contacts are closed by spring pressure (FAILSAFE)

Means the system remains ON in the event of an electrical failure.

29
Q

What is used to distribute electrical power to systems and components?

A

Busbars

30
Q

What is the VITAL DC Busbar?

A

Permanently connected and powered by the batteries.

Used in the event of a crash.

31
Q

What Busbar does No.1 generator feed?

A

ESSENTIAL DC Busbar

32
Q

What does No.2 DC generator supply power to?

A

Non-essential system services.

Through a busbar and an inverter to deed non-essential AC services.

33
Q

What is the electrical power source for the VITAL DC Busbar?

A

Batteries

34
Q

What is the electrical power source for the ESSENTIAL DC Busbar?

A

No.1 Generator

35
Q

Into what 3 categories of importance are consumer services split in a split-busbar system?

A

Vital

Essential

Non-essential

36
Q

What are Essential services?

A

Required to ensure safe flight during in-flight emergency situations, e.g. radio and instruments.

Connected to busbars in such a way that they can always be powered by either a generator or batteries.

37
Q

What are Vital services?

A

Services needed after an emergency landing or crash.

E.g. fire extinguishers, emergency lighting.

Fed directly from the Main and Emergency batteries.

38
Q

What are Non-essential services?

A

Not essential to flight and may be isolated during an in-flight emergency, either by manual or automatic action.

I.e. services connected to CBs, which will trip if the circuit connected to that component/system gets too hot due to excess current.

39
Q

How is the battery connected to the Essential and Vital busbars?

A

Essential busbar via the battery contactor.

Vital busbar is always connected

40
Q

What busbar is used to charge the battery?

A

Essential busbar

41
Q

What busbar is fed by No.1 AC generator?

A

Dedicated AC Busbar

A TRU then converts AC to DC which is fed to the Non-essential (DC) and Essential busbars (DC).

Note - Look at diagram in course manual and LEARN + be able to label

42
Q

What busbar is fed by No.1 AC generator?

A

Dedicated AC Busbar (No.2)

A TRU then converts AC to DC which is fed to the No. 2 Non-essential (DC) and No. 2 Essential busbars (DC).

43
Q

What is load shed?

A

In the event of an emergency/malfunction, contactors are used to cut power (load shed) to Non-essential busbars, thereby prioritising Essential services.

44
Q

When might a GPU be used?

A

To provide power for starting the engines, instead of using the onboard batteries.

Can be used to charge the batteries.

45
Q

What connects busbars together?

A

Bus Tie Contactors

Increases flexibility and redundancy

No.1 and No.2 AC busbars are connected

No.1 and No.2 Non-essential DC busbars are connected

46
Q

What is the role of a crash switch?

A

In the event of a crash, it is automatically triggered and removes the earth from the battery contactor.

This isolates the battery from all busbars other than the VITAL busbar, preserving its power for these Vital systems only.

Aircraft Systems 2 (6 decks)

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