Answers - Powerplant Ignition and Starting Systems | Aircraft Systems

Answers - Powerplant Ignition and Starting Systems

1. b
Almost all magnets, regardless of their retentivity, lose some of their magnetic strength when their lines of flux pass through the air. Therefore, during the overhaul of a magneto, when the rotating magnet is removed, it should be placed in a soft iron keeper to prevent loss of magnetism. A "keeper" is a piece of soft iron that is used to link the magnetic poles and provide a highly permeable path for the flux.

2. a
There are two common methods for checking the strength of a magneto magnet. If the magnet is removed from the magneto, it is checked with a magnetometer. However, if the magneto is assembled, the magnet's strength is checked by holding the points open and then checking the output of the primary with an AC ammeter as the magneto is rotated at a specified speed.

3. b
The number of degrees between the neutral position of the rotating magnet and the position where the contact points open is called the E-gap angle. When the rotor is at the E-gap angle the cam opens the breaker points and current stops flowing through the primary coil. This sudden stoppage of current flow causes the magnetic field in the coil core to collapse and induce a voltage into the secondary winding.

4. a
The greatest flux density within a rotating magnet-type magneto occurs when the poles of the magnet are fully aligned with the field shoe faces. This position is known as the fill1 register position.

5. b

6. c
The ignition harness on reciprocating engines serve several purposes. It supports the wires going from the magneto to each spark plug and protects them from engine heat, vibration, and weather. Ignition harnesses also serve as a conductor for the stray magnetic fields that surround the wires as they momentarily carry high-voltage current. Therefore, by routing the ignition leads through metallic conduits, the stray magnetic fields are easily conducted to ground, thereby reducing electrical interference with aircraft radio equipment.


7. b
Any time the gap between the breaker points in a magneto is increased, the points will open early. When the points open early, the spark within the cylinder occurs in advance of when it is supposed to. Furthermore, early opening points also cause a decrease in spark intensity.

8. a
Some magnetos contain a safety gap that protects the secondary coil. The safety gap is connected in series with the secondary circuit by two electrodes. One electrode is attached to the high tension brush holder and the other electrode is grounded to the ground plate. With this configuration, if excessive voltage builds in the secondary circuit due to an open that does not allow the spark to jump the spark plug electrodes, the excessive voltage jumps the safety gap to the ground connection. This helps ensure that the voltage cannot rise high enough to damage the insulation within the secondary coil.

9. c
When the timing marks within a magneto are aligned, the magnets are in the E-gap position and the breaker points are just beginning to open.

10. c
When internally timing a magneto, the breaker points should begin to open when the rotating magnet is a few degrees past the neutral position. The neutral position is defined as the point where the rotating magnet is 45 degrees past full alignment with the pole shoes.

11. a
The primary electrical circuit in a magneto consists of a set of breaker contact points, a capacitor, and an insulated coil. The capacitor is wired in parallel with the breaker points to prevent arcing between the points when they open, and to hasten the collapse of the magnetic field around the primary coil.

12. b
Current is induced into a magneto's secondary winding when the current within the primary winding is interrupted suddenly, causing a high rate of flux change. Current is interrupted by opening the primary breaker points when the rotating magnet is in the E-gap position.

13. c
With a "shower of sparks" ignition system, an electrically operated vibrator rapidly opens and closes both the retard and advance breaker points. This action causes the current flowing through the primary coil to be interrupted several times per second which, in turn, causes the magnetic field surrounding the primary coil to build and collapse at the same rate. The rapid successions of separate voltages that are induced into the secondary coil by the pulsating magnetic field around the primary coil produce a "shower of sparks" across the selected spark plug when both of the breaker points are open.

14. a

15. b
If a set of points is worn, the internal timing of the magneto is advanced. The next most common cause of timing drift is wear of the breaker cam which causes the timing to be retarded.

16. b
The shielding used on ignition leads to protect against radio interference can act as a capacitor and store electrical energy that is released when the spark jumps the spark plug gap. When this happens, the energy that is stored in the capacitance of the ignition harness is returned to the spark plug, and afterfiring occurs. To minimize this problem, spark plugs have a resistor installed inside the spark plug insulator that prevents the electrical energy stored in the harness from reaching the spark plug gap.

17. c

18. b
The purpose of the capacitor in the primary electrical circuit is to prevent arcing between the points when they are opened, and to hasten the collapse of the magnetic field surrounding the primary coil.

19. a
A high voltage is induced into the secondary winding when there is a rapid change in the magnetic field surrounding the primary coil. The amount of voltage within the secondary winding builds from its lowest value immediately before the breaker points close to a maximum value just prior to the spark plug firing.

20. b

21. b
The amount of voltage induced in the primary coil of a magneto varies with the rate at which the magnet's lines of flux are cut. Therefore, the faster an engine runs, the faster the flux lines are cut and the greater the induced voltage.

22. b
When internally timing a magneto, the breaker points should begin to open when the rotating magnet is a few degrees past the neutral position. The neutral position is defined as the point where the rotating magnet is 45 degrees past full alignment with the pole shoes.

23. b
Some magnetos contain a safety gap that protects the secondary coil. The safety gap is connected in series with the secondary circuit by two electrodes. One electrode is attached to the high tension brush holder while the other electrode is grounded to the ground plate. With this configuration, if excessive voltage builds in the secondary circuit due to an open that does not allow the spark to jump the spark plug electrodes, the excessive voltage jumps the safety gap to the ground connection. This helps ensure that the voltage cannot rise high enough to damage insulation within the secondary coil.

24. b
One of the purposes of a capacitor in a magneto is to prevent arcing across the breaker points once the points open. Therefore, a good indication of a defective capacitor is breaker points that are burned or pitted from arcing.

25. a
In a low-tension ignition system, a primary and secondary coil are located near each of the spark plugs. Therefore, a standard 18-cylinder engine with two spark plugs per cylinder utilizes 36 secondary coils.

26. b
The ignition switch in a magneto circuit is wired in parallel with the breaker points and the primary capacitor. In other words, from the ignition switch, there are two possible paths current can flow; either through the breaker points or through the primary of the coil.

27. c
The shielding used on spark plug and ignition wires serves to prevent or reduce interference with radio reception. Without this shielding, the stray magnetic fields that surround the wires when they momentarily carry the high-voltage current could make radio communication virtually impossible.

28. c
An impulse coupling is designed to induce a momentary high rotational speed which increases the rate at which the lines of flux in the primary coil are cut. This produces a more intense spark that aids in starting.

29. c
An impulse coupling is designed to induce a momentary high rotational speed which increases the rate at which the lines of flux in the primary coil are cut. This produces a more intense spark that aids in starting.

30. c
In a dual-ignition system, the spark plugs may be set to fire at the same instant (synchronized) or at slightly different intervals (staggered). When a staggered ignition is used, the spark plug in the exhaust side of the cylinder is always fired first to compensate for the slower burn rate of the diluted fuel/air mixture in this portion of the cylinder.

31. c
Magnetos require adequate drains and proper ventilation to provide cooling and prevent moisture from building inside a magneto and shorting across the internal components. In addition, good magneto ventilation helps ensure that the corrosive gases produced by normal arcing across the distributor air gap are carried away.

32. b

33. b

34. a
When using a timing light to time a magneto to an engine, the master switch must be turned on and the ignition switch placed in the BOTH position.

35. c

36. a
Several different types of test devices are used for determining the serviceability of a high-tension ignition harness. One common type of tester is capable of applying a direct current voltage up to 15,000 volts. Once voltage is applied to an ignition harness, a high-tension lead tester is used to measure any current that leaks through the insulation.

37. b

38. b
With the primary breaker points closed, the rotating magnet's flux generates an opposing magnetic field in the magneto coil. This opposing force is the source of magnetic field stress, and the point where this stress is highest is a few degrees beyond the neutral position or the E-gap position. It is here that the points open to induce the highest possible voltage into the secondary coil.

39. b
The retard points are timed so that they open later than the normal points. This retarded timing ensures that the engine will not kick-back when it fires. Both sets of points must be open for the Shower of Sparks system to operate. After the starter switch is released, the retard points have no function.


40. c

41. c
In a low-tension ignition system, the magneto produces a low voltage which is fed to an individual secondary coil located near each spark plug.

42. c
A distributor used with any four-cycle engine must rotate at one-half the engine's speed to fire each spark plug in every two crankshaft revolutions. However, the speed of a magneto with an uncompensated cam is calculated by dividing the number of cylinders by twice the number of poles on the magnet. In this case, magneto speed is 1.125 times the crankshaft speed (9 ÷ (4 x 2) = 1.125). Based on this, the distributor rotates at the slowest speed.

43. b
Spark plugs that are gapped too wide generally lead to hard starting. The reason for this is that the wider gap requires a higher voltage to produce a spark that will jump the gap. Since the production of a higher voltage requires the magneto to rotate faster, starting becomes more difficult.

44. c
Before a spark plug is removed, its ignition harness lead must be disconnected. If the lead is not pulled straight out of the plug barrel, damage to the core insulator and the ceramic lead terminal may result.

45. a
One end of a magneto secondary coil is grounded to the primary coil or the coil core while the other end is connected to the distributor rotor. Therefore, if the distributor rotor in a magneto is cracked, the current in the secondary coil has a less resistive path to ground through the crack to the metal shaft of the magneto.

46. c
Unlike the ignition system for a reciprocating engine that produces a high voltage, low amperage spark, turbine engine ignition systems deliver a high-energy spark with a substantially higher amperage. This high-energy spark is needed to ignite the fuel/air mixture in low temperatures and at high altitudes.

47. b
A high-voltage capacitor discharge ignition system in a turbine engine produces high voltage pulses at what is called a trigger transformer. Current is supplied to the trigger transformer through a contactor and large storage capacitor. When the storage capacitor becomes fully charged, the conductor is closed by the mechanical action of a single-lobe cam. With the contactor closed, the energy in the storage capacitor is allowed to flow to the trigger transformer where a high-voltage pulse is formed and sent to the igniter.

48. c
One purpose of the capacitor in a magneto ignition system is to prevent arcing between the breaker points. Therefore, a faulty capacitor can be suspected if the breaker points take on a coarse-grained or sooty appearance.

49. a

50. c

51. a
The shielding used on ignition leads to protect against radio interference can sometimes act as a capacitor and store residual electrical energy as the high voltage charge flows through the lead to the spark plug. When this happens, the energy stored in the ignition harness is returned to the spark plug after the primary spark has occurred.

52. a

53. c

54. a

55. c
In a constrained-gap igniter plug, the center electrode is recessed into the body of the plug. Therefore, in order for the high-intensity spark to get from the electrode to ground, it must jump out away from the plug's tip. Because of this, constrained-gap igniter plugs do not have to project into the combustion chamber liner which allows them to operate at a cooler temperature.

56. b
When inspecting a magneto, all accessible condensers should be cleaned with a lint-free cloth moistened with acetone. However, you should always observe the magneto manufacturer's recommendations since some cleaners can damage the protective coating on some components.

57. b

58. b
Great caution should be exercised in handling damaged transformer units from turbine engine ignition systems since some units may have radioactive material on the air gap points. This material is used to calibrate the discharge points to a preset voltage.

59. c
The high-energy current used to fire turbine engine igniters, if used continuously, would quickly cause electrode erosion. However, since combustion in a turbine engine is self-supporting, igniter plugs are only used for short periods and, therefore, they maintain a relatively long service life. The ignition system is used primarily to start an engine and ensure ignition during takeoff, icing conditions, landing, and moderate to severe turbulence.

60. a
In a typical magneto-type ignition system, the magneto produces a high voltage charge that flows through the distributor to the ignition leads and spark plugs. To complete the circuit, the electrical charge jumps the gap in the spark plug which is grounded through the engine structure.

61. a

62. b
When gapping spark plugs, you typically decrease the clearance between the center electrode and the ground electrode. However, it is not recommended that a gap be widened after it has been inadvertently decreased too much since damage to the center electrode insulator typically results.

63. a
Although newer ignition harnesses are flexible and can be installed with smaller bend radii, older ignition cables have a low tolerance for sharp bends. Over time, the stress imposed by sharp bends on an ignition lead's insulation can cause the insulation to break down and allow current to leak.

64. c

65. b

66. a
The ignition switch for a magneto ignition system shuts an engine down by supplying a path to ground through the ignition switch. If the engine does not shut down when the ignition switch is turned to the OFF position, there is either an open ground lead in the magneto or a faulty ignition switch.

67. a

68. a

69. b
The heat range of a spark plug refers to the ability of the insulator and center electrode to conduct heat away from the plug tip and transfer it to the cylinder head. "Hot" plugs have a long insulator that creates a long heat transfer path while "cold" plugs have a short insulator that rapidly transfers heat to the cylinder head.

70. a
When staggered ignition timing is used, the spark plug nearest the exhaust valve fires first. This is done because the fuel/air mixture nearest the exhaust valve is diluted and burns more slowly.

71. a

72. a

73. a

In addition to being able to check ignition leads, a high-tension harness tester can also be used to indicate the condition of the distributor block. If the majority of ignition leads being tested show excessive leakage, there is a good possibility that the distributor block is at fault.

74. a

75. b
The heat range of a spark plug refers to the ability of the insulator and the center electrode to conduct heat away from the plug tip. Hot spark plugs, or plugs that have a long insulator, slowly transfer heat and are typically used in engines whose cylinder temperatures are relatively low. Cold spark plugs, on the other hand, have a short insulator that rapidly transfers heat which makes them appropriate for hot running, high-compression engines.


76. a
In a normally operating ignition system, when a spark plug fires, current from the magneto passes to ground by jumping the spark plug's air gap. Therefore, if a spark plug lead becomes grounded, current would simply flow through the lead to ground with no effect on the magneto.

77. a

78. c
The heat range of a spark plug is a measure of a spark plug's ability to transfer heat to the cylinder head. The primary factor in determining the heat range of a plug is the length of the nose core. For example, cold plugs have a relatively short insulator to provide a rapid transfer of heat to the cylinder head while hot plugs have a long insulator nose that creates a long heat transfer path.

79. c

80. a

81. c
When performing a magneto ground check, the engine is operated at a specified rpm and the ignition switch is moved from BOTH to LEFT and back to BOTH; then from BOTH to RIGHT and back to BOTH. When this check is performed there should be a small drop in rpm at the LEFT and the RIGHT positions. This occurs because with only one magneto selected, only one spark plug fires in each cylinder thereby causing a slight decrease in power output.

82. c

83. c
A spark plug is fouled when it becomes contaminated with foreign matter to the point that the spark flows through the foreign matter to ground rather than jumping the air gap at the electrode.

84. b
A spark plug whose insulator tip is cracked should be replaced. This condition can affect the firing of the plug as well as its ability to transfer heat.

85. b
The heat range of a spark plug refers to the ability of the insulator and the center electrode to conduct heat away from the plug tip. Hot spark plugs, or plugs that have a long insulator, slowly transfer heat and are typically used in an engine whose cylinder temperatures are relatively low. Cold spark plugs, on the other hand, have a short insulator that rapidly transfers heat which makes them appropriate for hot running, high-compression engines. An engine which runs hot requires a relatively cold spark plug, whereas an engine which runs cool requires a relatively hot spark plug. If a hot spark plug is installed in an engine which runs hot, the tip of the plug could overheat and cause pre-ignition.

86. b

87. b
The heat range of a spark plug is a measure of a plug's ability to transfer heat to the cylinder head. The primary factor in determining the heat range of a plug is the length of the nose core. For example, cold plugs have a relatively short insulator to provide a rapid transfer of heat to the cylinder head while hot plugs have a long insulator nose that creates a long heat transfer path.

88. a

89. a
When installing new breaker points in a magneto, the internal timing of the magneto must be checked to ensure that the point opening coincides with the E-gap position of the rotor. The timing of the magneto to the engine must also be checked to ensure that the firing of the spark plugs occurs at the proper time in relation to piston position.

90. a

91. b
Lead fouling can occur at any power setting, however, it is most frequently associated with cruising power settings with lean mixtures. At these power settings, the cylinder head temperature is relatively low and there is an excess of oxygen above that needed to consume all the fuel in the fuel/air mixture. The excess oxygen ends up combining with lead and builds up in layers on the cool cylinder walls and the spark plugs.

92. a

93. a

94. c
In both a battery and magneto ignition system, the spark occurs at the spark plug when voltage is induced into the secondary winding. Voltage is induced when the primary circuit is opened at the breaker points.

95. c

96. c

97. c
In a magneto ignition system, the purpose of the ignition switch is to ground out the primary circuit when the switch is OFF. By grounding out the primary circuit continuously, insufficient voltage is induced into the secondary coil to produce a spark. Therefore, if the ground wire to the ignition switch is disconnected, the engine will continue running when the ignition is turned to the OFF position.

98. c
The principal advantages of a dual magneto-ignition system are that if any part of one magneto should fail to operate, the other magneto will continue to furnish ignition. Furthermore, a dual ignition system firing two spark plugs ignites the fuel/air mixture in each cylinder simultaneously at two different places, resulting in more complete and quick combustion and increased engine power.

99. c
The shielding used on an ignition harness serves as a conductor to receive and ground stray magnetic fields that are produced when high-voltage current passes through the leads. By conducting these magnetic lines of force to ground, the ignition harness cuts down electrical interference with the aircraft radio and other electrically sensitive equipment.

100. a

101. a

102. c
Flashover in a distributor can lead to carbon tracking, which appears as a fine pencil-like carbon trail where the flashover occurred. This carbon trail typically collects on the distributor and forms a conductive path to ground which increases the potential for additional flashover to occur.

103. a
In order for a magneto to provide a spark at the appropriate time in the four-stroke process, the distributor must turn at one-half the crankshaft speed. Another way to look at this is that it takes two revolutions of the crankshaft to fire all the cylinders and, therefore, the distributor only needs to rotate at half the crankshaft speed.

104. c

105. c

106. b
An ignition switch check requires that the ignition be momentarily turned to the OFF position and then back to BOTH while the engine is running to see if all magneto ground leads are electrically grounded. To prevent backfiring when the ignition switch is returned to the BOTH position, an ignition switch check is usually made at the slowest possible rpm setting, typically between 500 and 700 rpm.

107. a
The primary breaker points in a magneto close at approximately the full register position. In this position, the poles of the magnet are perfectly aligned with the pole shoes and the maximum number of flux lines flow through the magnetic circuit.

108. b
A dual magneto incorporates two magnetos in one housing. This configuration allows somewhat of a weight savings because one rotating magnet and one cam can be used for both magnetos

109. c
If a breaker point mainspring does not apply sufficient tension to the breaker points, the points could bounce or float at higher speeds. A bouncing or floating point would prevent the normal induction buildup within the magneto thereby reducing the magneto output.

110. b
The secondary coil of a magneto is made up of a winding containing approximately 13,000 turns of fine, insulated wire. One end of the wire is electrically grounded to the primary coil or coil core, while the other end is connected to the distributor rotor.

111. a
The magneto ground lead, or P-lead, grounds the primary side of the magneto coil when the magneto switch is in the OFF position. This effectively shorts the breaker points, rendering the magneto inoperable. Therefore, if the P-lead should become disconnected or broken, the magneto will be on all the time regardless of the ignition switch position.

112. a
Closing the battery switch energizes the battery solenoid coil, which closes the battery solenoid and allows current to flow to the bus and one side of the start switch. Closing the engine master switch allows current to flow to the fuel valve and to the power lever switch. Once the power lever is advanced, the power lever switch moves to the Advance position and current flows to the power lever relay coil, causing the power lever relay to close and allow current to flow to the fuel pumps and one side of the ignition relay. Current is not made available to the starter relay or the ignition relay coil until the start switch is placed in the on position.

113. a
Only the engine master switch is necessary to be changed in order to allow the ignition exciters test switch to function. With start switch on, the test switch would be inoperative since exciters would already be activated. The test switch would be functional with the master switch and power lever switch on. The answer with master and power lever is chosen over the answer with master, battery and power lever since the battery switch would already be on to provide power to the bus, or if powered by external power, would be inoperative due to the external power receptacle switch.

114. b
The system illustrated may be operated with either battery power or external power, but not both at the same time. To prevent both an external power source and the battery from operating simultaneously, an external power receptacle switch is placed just after the battery solenoid coil that removes power from the battery solenoid coil and, ultimately, battery power from the bus when external power is plugged in.

115. a
Wire 8 supplies current to the starter solenoid coil which makes the solenoid close. With the starter solenoid closed, power flows from the bus, through the undercurrent solenoid coil and on to the starter generator. Since current is flowing through the undercurrent solenoid coil, the solenoid will close and provide a secondary power source to keep the starter solenoid engaged and, as long as the start switch is closed, the start sequence progressing normally if wire 8 should break or become disconnected.

116. a
The system illustrated may be operated with either battery power or external power, but not both at the same time. To prevent both an external power source and the battery from supplying power to the system simultaneously, an external power receptacle switch is placed just after the battery solenoid coil that removes power from the battery solenoid coil and, ultimately, battery power from the bus when external power is plugged in.

117. b
In a system that uses an undercurrent relay, an engine start is initiated by placing the start switch in the start position for a few seconds to allow current to close both the starter and undercurrent solenoids. Once these solenoids are closed, the start sequence will support itself and the start switch can be turned off. Then, once the engine speed increases and starter current drops, the undercurrent relay opens to disconnect power from the starter-generator and ignition circuits and enable the generator circuit.

118. a
When a hung start occurs, the engine fails to reach a self-sustaining speed and the starter continues to crank the engine. To abort a hung start, the star/stop switch must be pressed or toggled manually to de-energize the starter generator.

119. a

120. b

121. a
Series-wound motors are typically used for starter motors because they are able to produce a high starting torque. However, this high starting torque requires a great deal of current. On the other hand, as engine and starter speed increase, counter electromotive forces build and limit the amount of current the starter can draw.

122. b
Although the current provided to a starter is high at the onset of engine rotation and decreases as engine speed builds, the voltage provided to a starter remains relatively constant throughout an engine start.

123. c
The primary advantage of pneumatic starters over electric starters is that pneumatic starters have a much higher power-to-weight ratio. For example, a typical air turbine starter weighs from one-fourth to one-half as much as an electric starter capable of starting the same engine.

124. c
In a pneumatic starter that incorporates a sprag clutch ratchet assembly, the pawls are forced inward by small leaf springs to engage the sprag clutch ratchet when the engine is at rest. However, once the engine reaches a given rpm during a start, centrifugal force pulls the pawls outward, disengaging them from the sprag clutch ratchet. On coastdown the springs force the pawls to ride the ratchet gear until the engine comes to a stop.

125. a

126. a

127. c

128. b

129. c

130. b

131. b
If the pneumatic starter on a turbine engine is inoperative, the engine can be started by manually actuating the start valve. This is typically done by pulling a T-handle on the start valve, turning the T-handle to the OPEN position, and holding it in this position until the start is self-sustaining. If the T-handle is not returned to the CLOSED position after a successful start, the starter will overspeed and fail.


YOU MAY ALSO LIKE