Answers - Engine Electrical Systems | Aircraft Systems

Answers - Engine Electrical Systems

1. b
By replacing the slip rings of a basic AC generator with two half-cylinders, called a commutator, DC current is obtained. As the generator's armature rotates, the commutator elements act as a switch causing the current to flow in the same direction through the external circuit.

2. c
Because the windings in a series motor have such a low resistance, it is normal for these motors to draw a large amount when started. However, when this starting current passes through both the field and armature windings, a high starting torque is produced.

3. c

4. b
A series-wound motor produces the highest torque of any of the motors listed. Furthermore, a typical starter motor is operated by a 12- or 24-volt direct current battery. Therefore, the most common starter found on reciprocating engines is a direct current series-wound motor.


5. c

6. c
When an armature in a motor rotates in a magnetic field, a voltage is induced into the windings that opposes the applied voltage. This back, or counter electromotive force (EMF) increases with motor speed and, therefore, reduces the current flowing through the armature.

7. c

8. a
A rough or pitted commutator should be smoothed using very fine sandpaper, such as 000, and then cleaned and polished with a clean, dry cloth.

9. b
A generator master switch is provided in most aircraft so that a malfunctioning generator may be disconnected from the aircraft electrical system to prevent damage to the generator or to the rest of the system. Operation of this switch deactivates the voltage coil in the voltage regulator, resulting in generator output being reduced to residual voltage.

10. c
The opening and closing of the points in a vibrating type voltage regulator controls the generator output. When the points are open, generator output decreases and when the points are closed, generator output increases. Therefore, if the points stick closed, the generator's output will increase to its maximum.

11. c

12. a

13. c
According to FAR 25.1 351, aircraft equipped with more than one generator connected to a common electrical system must have individual generator switches which can be operated from the cockpit during flight.

14. a

15. b
Of the types of motors listed, the series-wound motor develops the most torque. Therefore, series motors are generally used as starter motors.

16. b
Generators are designed to operate at a specified voltage while generator ratings are usually given as the number of amperes a generator can supply at its rated voltage. Therefore, as the load on a generator increases, the amperage output increases up to the generator's limit, while the voltage remains constant.

17. c
The greater the flux density in the field of a generator the greater the resistance to rotation. The greater the resistance to rotation the greater the force required to turn the generator.

18. c
The purpose of the reverse-current cutout relay is to automatically disconnect the battery from the generator when generator output voltage is less than battery voltage. This prevents the battery from discharging through the generator and trying to drive it as a motor.


19. a
If voltage does not build when a generator's field is flashed, check for an open armature. To do this, remove the generator cover and inspect the commutator cover. If melted solder is found, then the armature is open.

20. c
A brushless alternator utilizes permanent magnet interpoles in the exciter stator to provide enough magnetic flux to start producing electricity. Therefore, there is no need to flash the field to put residual magnetism into the field frame.

21. a
The automatic ignition relight system is activated differently on different aircraft. One popular method of activating the system is to use pressure sensors installed at the compressor discharge. When used this way, a drop in discharge pressure automatically activates the ignition system.

22. c
In most brush-type alternators, the rotor winding is excited by direct current supplied by the battery and varied by a regulator. When the alternator load increases, the regulator supplies more current to the rotor windings and when the load decreases, less current is supplied.

23. c
As the specific gravity of the electrolyte solution within a battery rises, its freezing point drops. Therefore, in cold climates, the state of charge in a storage battery should be kept at a maximum so it does not freeze.

24. a
The capacity of a storage battery is rated in ampere-hours which is the amount of electricity that can be taken out of a battery when a current of one ampere flows for one hour. This rating indicates the minimum amount of amperes that a battery can put out in one hour. Therefore, a battery which can deliver 45 amps for 2.5 hours has a 112.5 ampere-hour capacity (45 amps x 2.5 hours = 11 2.5 amp-hours).

25. b
a battery that can supply 140 amperes in one hour can deliver 15 amps for 9.33 hours (140 amp-hour + 15 amps = 9.33 hours).

26. a
Electrical power is the product of voltage and current. Since direct current systems produce low voltage, the corresponding current must be large enough to produce sufficient power to sustain heavy electrical loads. As a result, conductors carrying direct current must be large and heavy to carry the high current. Alternating current, on the other hand, utilizes much higher voltages and, therefore, lower currents. Since low current can be carried in smaller wire, the use of AC power on large aircraft produces substantial weight savings.

27. c

28. c

29. c
Most aircraft systems use 400-hertz alternating current. At this high frequency, inductive reactance is high and current is low. As a result, motors can be wound with smaller wire, and transformers can be made much smaller and lighter.

30. a
Generators use field coils wrapped around soft iron cores to produce the magnetic field required to generate current. Soft iron retains little or no residual magnetism when the magnetizing field is removed. As a result, generator fields must be flashed to restore residual magnetism.

31. c
An alternator uses solid-state diodes in its rectifier circuit. These diodes act as electrical check valves and allow current to only flow in one direction. This isolates battery current from the alternator.

32. b
A typical aircraft generating system produces a constant voltage that supplies power to the primary bus and charges the battery. This type of system utilizes a fixed voltage that is slightly higher than the battery voltage.

33. c
A ni-cad battery can be charged using either the constant current or constant voltage method. With the constant current method the charge takes longer but is more effective in maintaining cell balance.

34. b
First, locate the column on the left side of the chart representing a 28V system with a 1 volt drop. Move down this column to a wire length of 45 feet, which is between the 40 and 50 foot callouts. From this point, project a line to the right just beyond the 10 amp diagonal line. Since this point is above curve 1, installation in a bundle carrying continuous current is permitted. Now, project a line down vertically to the bottom of the chart. The line falls on the #12 wire size. Therefore, a #12 wire is required.

35. c
An aircraft starter circuit does not have a fuse or circuit breaker because the current draw is so great that a fuse or circuit breaker would continually be blowing.

36. c
According to AC 43.1 3-1B, no more than four terminals can be attached to a single terminal stud. If more than four terminals must be attached, use two adjacent studs connected by a small metal bus strap.

37. c
According to AC 43.1 3-1B, no more than four bonding jumper wires should be attached to one terminal that is grounded to a flat surface. If more than four jumper wires are used, a proper ground may not be obtained.

38. a
Manufacturers specify the exact wear limits for their parts. However, as a general rule, starter brushes should be replaced when they are worn to approximately one-half of their original length. Allowing the wear to progress much farther will affect the spring tension and the ability of the brush to stay in contact with the commutator.


39. c
Electrical switches should be derated from their nominal current rating when they are used to control direct current motors. The reason for this is that DC motors will draw several times their rated current during starting. Therefore, if the switch is not derated, it will not have enough of a safety factor to prevent overloading.

40. c

41. b
AN and MS electrical connectors consist of a female socket and a male connector with a set of pins. When installing an electrical connector, the socket should be installed on the voltage side while the pin section should be attached to the ground side. When assembled this way, the recessed sockets make it extremely difficult to short or ground a circuit.

42. b
In a solenoid-operated electrical switch, part of the core is movable and is spring loaded open. When the control circuit to the solenoid is completed, a magnetic field pulls the solenoid closed. This completes the primary circuit. When power is removed from the control circuit the magnetic field dissipates causing the movable core to return to its original position and open the circuit. Therefore, as long as the control circuit is complete, current will flow through the coil and the switch will close.

43. b
The electrical load check described in this question is an acceptable method for determining an aircraft's total electrical load. According to AC 43.13-1B, unless the aircraft is placarded or contains monitoring devices, the total continuous electrical load may be held to approximately 80% of the total rated generator output capacity. The total load on the system is only 76 percent of the generator's capacity. Therefore, the load is considered to be within the generator's load limit.

44. b
Prior to pulling electrical wires or cables through conduit, soapstone talc is dusted on the cables to act as a lubricant. The talc helps keep the wire from binding and chafing against the walls of the conduit

45. c

46. b
One of the purposes of bonding jumpers is to provide a ground for electrical circuits. If the bonding and grounding is properly done, there should be virtually no resistance in the return path. Some guidelines to follow when attaching bonding jumpers is to make them as short as possible and install them in such a manner that the resistance of each connection does not exceed 0.003 ohm.

47. c
When the master switch is on and the start switch is placed in the start position, power is supplied to the starter generator. However, if the starter solenoid is defective, the starter will not rotate.

48. a
Weak or worn brush springs allow the brushes to bounce, resulting in arcing and burned or pitted commutator surfaces.

49. b
According to AC 43.13-1A, the voltage drop in the main power wires from the generator or the battery to the bus should not exceed 2 percent of the regulated voltage when the generator is carrying rated current or the battery is being discharged at the 5-minute rate.

50. c
Hazardous errors in switch operation can be avoided by logical and consistent installation. For example, when two-position on-off switches are installed, they should always be mounted so that the on position is reached by a forward or upward movement. Furthermore, when a switch controls movable aircraft elements such as landing gear or flaps, the switch should move in the same direction as the desired motion.

51. b
Switches should be derated from their nominal current rating when they are used to control direct current motors, inductive circuits, and high in-rush circuits. The reason for this is that these types of circuits can draw several times their rated current when closed, so the switch must be capable of handling this without the contacts burning or welding together.

52. c
An arcing fault between an electric wire and a metallic fluid line can puncture the line and result in a serious fire. Consequently, every effort should be made to physically separate electrical wire from lines or equipment containing oil, fuel, hydraulic fluid, or alcohol. When separation is impractical, locate the electric wire above the flammable fluid line and securely clamp it to the structure. When installed in this manner, a leaking fluid line is less likely to drip onto the electrical wire and ignite.

53. b
Begin by locating the 28-volt column on the left side of the chart. Move down this column until you hit the number 25 which represents the wire length. From here, move horizontally left until you intersect the vertical line representing No. 10 copper wire. Since this intersection is above curve 2, the wire can carry a continuous current in free air. To determine the maximum continuous current that can be carried, interpolate between the 30 amp and 40 amp diagonal lines. The answer is 35 amperes.

54. c
The frequency of AC produced by an AC generator is determined by the number of poles and the speed of the rotor and can be calculated using the formula:
F = (P ÷ 2) x (N ÷ 60)
Where:
F = frequency of the AC in Hertz
P = number of poles in the rotating field
N = rotational speed of the generator in rpm.
To solve for N, the formula becomes:
N = (F x 60) ÷ (P ÷ 2)
Therefore:
N =400 hz x 60 seconds ÷ 8 ÷ 2
N = 24,000 hz/second ÷ 4
N = 6,000 rpm

55. b
The three basic types of circuit breakers used in aircraft electrical systems are the push-to-reset type, the push/pull type, and the toggle type.

56. a
14 CFR Part 23, Airworthiness Standards: Normal, Utility, Acrobatic, and Commuter Category Airplanes, specifies the airworthiness standards for the issue of type certificates. Within this part,l4 CFR 23.1357 states that each resettable circuit protection device requires a manual operation to restore service after the device has interrupted the circuit.

57. b
14 CFR 23.1357 states that each resettable circuit protection device (trip free device in which the tripping mechanism cannot be overridden by the operating control) must be designed so that:
1. A manual operation is required to restore service after tripping; and
2. If an overload or circuit fault exists, the device will open the circuit regardless of the position of the operating control.

58. c
A circuit breaker or fuse should open a circuit before the associated conductor reaches its maximum capacity. To accomplish this, the time/current characteristics of the protective device must fall below that of the associated conductor.

59. a

60. a
Switches are rated according to both the voltage and current they can control. A typical aircraft switch may be rated for 5 amps at 125 volts, or 35 amps at 24 volts.

61. c

62. c

63. c
Within Part 91, 14 CFR 91.205 specifically requires that one spare set of fuses, or three spare fuses of each type be carried in the aircraft during flight.

64. c

65. a

66. c
When attaching the terminal end of wires to a terminal strip, fan the wires out from the bundles so they will align with the terminal studs. If two or more electrical terminals must be installed on a single lug you must "stack” the terminals as necessary.

67. a

68. b

69. c
Copper is a very corrosive metal. Therefore, aircraft electrical wire is coated with tin, silver, or nickel plating to help prevent oxidation.


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