1. AIRCRAFT ELECTRICAL SYSTEMS

2. Objectives Students will be able to:
Describe the basic components of aircraft electrical system
Explain operation of electrical system
Interpret aircraft electrical diagram
Select proper size of wire for installation
Describe basic causes of electrical system malfunctions

3. Elements of aircraft electrical systems
An aircraft electrical system is mainly composed of :
Power sources
Components
- Control devices
- Conversion devices
- Protection devices.
Power distribution systems
Electrical loads

4. Electrical Power Sources
Electricity power sources on an aircraft may be classified into two groups :
Batteries
- Lead-acid
- Nickel-cadmium
Generators
- DC generators
- AC generators

5. Battery
A battery is a device that converts chemical energy into electrical energy.
It is a power reservoir that stores energy in chemical form.It does not produce energy.
Its functions are:
- To provide power when no other power source is available
- To assist in damping transient loads in the dc system

6. - To provide a short term source during emergency condition
The capacity of battery is measured in ampere-hours.
Its normal rate is a little over 24vdc in a 28vdc system.
It is automatically recharged when the engine-driven generator is operational.
Two types of batteries are used in aircraft
- Lead-acid batteries
- Nickel cadmium batteries

7. Lead-acid battery It is usually found in piston aircraft.
It is made up of cells which have positive/negative plates of lead and filled with electrolyte of sulfuric acid and water.
Each cell has app. 2.2v, but is simply rated as 2 v.
It has corrosive effects.
Frequent total discharge and remaining battery in discharged condition for a long time will shorten the life of the battery.

8. Battery Cell

9. Nickel cadmium battery
They are the most common type of battery used in turboprop and turbojet aircraft.
They provide electrical discharge at a high rate without voltage drop and accept high charge rates that shorten recharge time
They may be subject to thermal runaway condition caused by overheating, in which the battery destructs itself.

10. Nickel-cadmium battery
Cell
Terminals
Vent
pipe
Container
Main
connector

11. GENERATORS
A generator is a machine that converts mechanical energy into electrical energy by the process of electromagnetic induction
They are driven by the engine(s), and sometimes by APU.
A generator must be rated at adequate amperage to drive all the operating components on its circuit(s). The current required to operate each electrical component is known as its load.

12. All generators produce alternating current naturally
All generators produce alternating current naturally.The method which is used to take it from the coil will determine if the generator provides ac or dc to the circuit.
If a commutator is used for this purpose it will be a dc generator.
If a slip ring is used it will be an ac generator.

13. DC GENERATORS:
Commutator in the generator converts the ac current to dc current.
DC generator is designed to supply nearly constant voltage. Usually the voltage is 28vdc, but there are 270vdc systems developed recently.
They supply current up to 400 A.
Most aircraft do not have a separate dc generator, but ac produced by ac generator is converted to dc to power dc systems.

14. Basic dc generator

15. Operation of a basic dc generator

16. Producing steady dc

17. AC generator (Alternator)
On most large aircraft high-load electrical devices are usually AC powered.
AC is produced by ac generator which is usually called alternator.
AC generators use slip rings instead of commutators to provide the current to the circuits.
Alternator generates three-phase current, and standard aircraft voltage is 115 vac with 400 Hz.

18. Basic ac generator

19. Modern electrical power generation types
Mainly two types of electrical power generation currently in use on aircraft.
- Constant frequency Integrated drive
generators (IDG)
- Variable speed constant frequency
(VSCF) generators

20. Constant Speed IDG
It is essential that the alternator output have a frequency of 400 Hz with a very small tolerance. Thus rotational speed of alternator must remain absolutely constant.
This is accomplished by using a Constant Speed Drive (CSD) unit between the engine and alternator.
CSD provides a constant alternator rpm within a specified engine rpm range.

21. An alternator and constant speed drive unit combination is called integrated drive generator (IDG).
CSD is hydromechanically operated and needs to be correctly maintained in terms of oil level and oil cleanliness.
It is the most commonly used power generation method on today turbine-powered aircraft.

22. Integrated Drive Generator

23. Constant speed drive

24. Constant Speed Drive

25. VSCF
In this method, the variable frequency power produced by the alternator is converted to constant frequency 400 Hz, 115 vac by solid-state devices electronically.
Thus there is no need for CSD unit.
It is a new technique and limited in use.

26. Variable speed constant frequency generator

27. Power Generation Control
Power produced by generators is usually controlled by generator control unit (GCU).
The main functions of this device are:
- Act as a voltage regulator
- Direct current to battery for recharging
- Provide circuit and generator protection
by disconnecting the generator from the
system when electrical abnormalities
occur.

28. Control Devices
These devices are use to initiate and control the operation of the circuits.Control devices include
Switches
Rheostats
Relays
Solenoids

29. Switches
A switch is used to start, to stop, or to change the direction of the current flow in the circuit.
Toggle switches :They are on/off switches and extensively used in aircraft electrical system.
Push Switches : They are used primarily for operations of short durations
Rotary switches :When it is necessary to select several condition for a circuit, a rotary switch may be used.

30. Rotary Switch

31. Toggle switch

32. Rheostats : control the amount of current that flows throughout the circuit and used as dimmer devices for instrument and cockpit lights.
Relays :Electromagnetic switching devices which are used to remotely control electric circuits carrying large amount of current.
Solenoids : Remote control devices quite similar to relays; but they are designed to move a shaft over a short distance. Thus, solenoids are used as mechanical control devices to operate hydraulic or pneumatic valves, locking pins, etc.

33. Rheostat

34. Schematic of a relay

35. Relay

36. Conversion Devices
There are many occasions within an aircraft electrical system where it is required to convert power from one form to another.
Typical examples of power conversion are :
- Conversion from dc to ac power
- Conversion from 115vac to 28 vdc
- Conversion from one ac voltage level to another
- Battery charging (from 115vac to 28vdc)

37. Following devices are used for these purposes :
Transformers
Rectifiers
Transformer-rectifier units (TRUs)
Inverters
Transistors

38. Transformers : Used to change ac voltage level.
Rectifiers : Used to convert ac into high-amperage, low-voltage dc.
TRUs: Combination of transformers and rectifiers and used as main unit to convert alternator output to dc in an aircraft
Inverters : convert 28 vdc power to 26 vac power particularly for flight instruments or 115 vac.
Transistors : Electronic devices that control electron flow and convert ac to dc.

39. Transformer

40. Analogy to TRU (ac adapter)

41. TRUs

42. Inverter

43. Analogy to inverter (ups for computer)

44. Protection Devices
These devices are used to protect circuits, cables and system components from damage due to failures.
Fuses
Circuit breakers
Diodes
GCUs

45. Fuses : They are designed to protect the cables against the flow of short-circuit and excessive current. They break the circuit and stop the current flow when the current exceeds a predetermined value.
Circuit breakers :They have same function with the fuses.The difference from the fuse is that they are resettable, while a fuse must be replaced.
Diodes : They are the electronic equivalent to the check valves in hydraulic system. They allow electricity to flow in one way only.

46. Circuit Breakers

47. Circuit Breakers

48. Primary Causes of Electrical Circuit Failures
Open circuit :
It is a circuit that is not complete or continuous. This is an uncommanded interruption of electrical power tosome components or systems. When an open occurs the affected component stop to operate, but the other components still remain in operative condition.

49. Common causes of open circuit

50. Short Circuit
It occurs when electricity is allowed to take a shortcut through or around a component or system. This is the most serious problem. It has two effects :
- Affected components have no power
and fail to operate
- Since current will not flow through
affected components, the other
components will be subjected to higher
level of current causing them to burn
out.

51. Common causes of a short

52. Power Distribution
An electrical distribution system is required in order to convey the electrical power to the equipments and systems that need it.
Busbar systems
Wire and cables

53. Busbar System
In most types of aircraft, output from the generators is sent to one or more conductors before distribution throughout the system.
These conductors are called busbars and they act as distribution centers for electric power.
A busbar system is set up so that each power source supplies one or more specific buses.

54. Analogy to bus

55. A variety of electrical items are hooked up to each bus for power.
The buses are interconnected via circuit protection devices which are called bus ties.
Bus ties are switches or relays used to connect or disconnect buses from one another.
They serve to isolate failed buses from working ones and/or send electrical power to buses that lost their normal power sources.

56. Each engine-driven generator, for example, normally drives its own generator bus. High-current drawn items are connected to these buses.
Items like fixed fire extinguishers and emergency lights are usually powered off battery bus. That way they are powered for use even when no generator power is available.

57. Typical 28vdc system

58. WIRES AND CABLES
Wires and cables conduct electrical power in its various forms and quantities to and between equipments.
There are various types of wires used in aircraft electrical system. The conductor is made of copper or aluminum. The insulation material may be nylon, PVC, or fiberglass.

59. Aircraft wires

60. Wire size selection
The wires installed in an aircraft electrical system are chosen on the basis of ability to carry the required current
- without overheating and
- without producing an excessive voltage
drop.
Electrical wire charts may be used for this purposes.

61. Maximum voltage drop in load circuits

62. Wire chart

63. Electrical wire size measurement

64. Aircraft wiring diagrams
There are different types of electrical diagrams available to understand electrical systems. These diagrams may be in the following forms :
- Block diagrams
- Pictorial diagrams
- Schematic diagrams
Many symbols are used in these diagrams.

65. Pictorial electric diagram

66. ELECTRICAL
DIAGRAM

67. Aircraft electrical system diagram

68. Electrical loads
Once the aircraft electrical power has been generated and distributed then it is available to the aircraft services.
These electrical services are distributed throughout the aircraft and may be broadly subdivided into following categories.
Motors
Lighting services
Heating services
Avionics

69. Aircraft Lighting System
Lighting system represent an important element of the aircraft electrical services.
External lighting systems
Internal lighting systems

70. External lighting system
Provides illumination for such operation as landing at night, inspection of icing condition, and safety from midair collision.
Most common aircraft exterior lights :
Position lights (navigation lights)
Landing/taxi lights
Anti-collision lights
Inspection lights

71. Position lights
- Red at left wing tip
- Green at right wing tip
- White at vertical stabilizer
Anti-collision lights
- Rotating beam lights
- Usually at the top of fuselage or tail
Landing and taxi lights
- Landing lights at the leading edge of wings
- Taxi lights at nose landing gear

72. Exterior lights

73. Aircraft exterior lights

74. Exterior lights

75. Exterior lights

76. Exterior lights

77. Exterior lights