Ignition and Starting Systems

Question 1

What is the main advantage of a magneto ignition system over a battery ignition system for an aircraft reciprocating engine?

Answer 1

A magneto has its own source of electrical energy, and it is not dependent upon the battery.

Question 2

What is the function of the capacitor in a magneto?

Answer 2

The capacitor minimizes arcing at the breaker points, and it speeds up the collapse of the primary current as the breaker points open.

Question 3

What is a compensated magneto cam, and on what kind of engine is one used?

Answer 3

A compensated cam is a special cam used in magnetos mounted on high-performance radial engines. The cam has one lobe for each cylinder, and the lobes are ground in such a way that the breaker points open when the pistons in the different cylinders are the same linear distance from top center.

Question 4

What is the significance of the numbers on the distributor of an aircraft magneto?

Answer 4

These numbers are the sparking order of the magneto, not the firing order of the engine.

Question 5

What happens in a magneto ignition system when the ignition switch is placed in the OFF position?

Answer 5

The primary circuit is connected to ground.

Question 6

What is the reason for having a low-tension ignition system on some aircraft?

Answer 6

Low-tension magnetos are used on aircraft that fly at high altitudes where there is a problem with flashover in the high-tension magneto distributor.

Question 7

What is the basic difference between a low-tension magneto and a high-tension magneto?

Answer 7

The low-tension ignition system has only one coil in the magneto, and it uses a carbon-brush-type dis-tributor. Low voltage is distributed to high-tension transformers that are located on the heads of each cylinder.
A high-tension magneto produces high voltage in the magneto coil, and it is sent to the correct spark plug by the built-in high-voltage distributor.

Question 8

What is meant by an all weather spark plug?

Answer 8

A shielded spark plug that has a recess in the shielding in which a resilient grommet on the ignition lead forms a watertight seal.

Question 9

What is meant by the reach of a spark plug?

Answer 9

The length of the threads on the spark plug that screw into the cylinder head.

Question 10

What is the difference between a hot spark plug and a cold spark plug?

Answer 10

A hot spark plug has a long path for the heat to travel between the nose core insulator and the spark plug shell. In a cold spark plug, the heat has a shorter distance to travel, and the spark plug operates cooler than a hot spark plug.

Question 11

What is checked when a magneto is internally timed?

Answer 11

Internally timing a magneto consists of adjusting the breaker points so they will open at the instant the rotating magnet is in its E-gap position, and the distributor rotor is in the position to direct the high voltage to cylinder number one.

Question 12

What is the advantage of fine-wire spark plugs over massive electrode spark plugs?

Answer 12

Fine-wire spark plugs have a firing end that is more open than that of a massive electrode spark plug.
The open firing end allows the gasses that contain lead to be purged from the spark plug so they will not form solid lead contamination.

Question 13

Why is it important that the spark plugs be kept in numbered holes in a tray when they are removed from an engine?

Answer 13

Spark plugs tell a good deal about the internal condition of the cylinders from which they were taken.
By knowing the cylinder from which each spark plug came, the mechanic can take the proper action when a spark plug indicates such conditions as detonation or overheating.

Question 14

What is the purpose of staggered timing between the two magnetos on an aircraft engine?

Answer 14

Engines in which the exhaust gas scavenging from the cylinders is uneven use staggered ignition tim-ing. The spark plug nearest the exhaust valve, where the air-fuel mixture is diluted, fires before the spark plug on the intake side. By using staggered timing, the flame front caused by the two spark plugs will meet in the center of the piston.

Question 15

In what position should the ignition switch be placed when using a timing light on the magnetos?

Answer 15

In the BOTH position.

Question 16

Why is it important that a torque wrench always be used when installing spark plugs in an aircraft engine?

Answer 16

If the spark plugs are not put in tight enough, there is the possibility of a poor seal, and if they are put in too tight, there is danger of cracking the insulation.

Question 17

Why are pressurized magnetos used on most reciprocating-engine-powered aircraft that fly at high altitudes?

Answer 17

Pressurized air is a better insulator than less dense air. By pressurizing the distributors, the high voltage is kept from arcing across to the wrong electrode and causing vibration and loss of engine power.

Question 18

What type of ignition system is used on most turbine engines?

Answer 18

High-intensity, intermittent-duty, capacitor discharge ignition systems.

Question 19

What is meant by the E-gap in magneto timing?

Answer 19

The E-gap angle is the position of the rotating magnet when the primary current flowing in the magneto coil is the greatest. The breaker points open when the rotating magnet is in its E-gap position.

Question 20

What is the function of an impulse coupling?

Answer 20

An impulse coupling is a spring-driven coupling between the magneto and the engine. When the engine is being started, the impulse coupling holds the rotating magnet until the piston passes over its top center position and starts down. The impulse coupling releases the magnet and the spring spins it fast so that it produces a hot and late spark.

Question 21

What kind of gauge should be used to measure the electrode gap in aircraft spark plugs?

Answer 21

A round wire gauge.

Question 22

How many igniters are used in most turbine engines?

Answer 22

Two.

Question 23

What are two types of ignition systems used in turbine engines?

Answer 23

High-voltage systems and low-voltage systems.

Question 24

With which type of ignition system is a glow plug igniter used?

Answer 24

A low-voltage system.

Question 25

How is the strength of the magnet in a magneto checked?

Answer 25

The magneto is put on a test stand and rotated at a specified speed. The breaker points are held open and the primary current is measured. The strength of the magnet determines the amount of primary current.

Question 26

In what position is the magnet in a magneto when the greatest change in flux density in the coil core takes place?

Answer 26

It is a few degrees beyond its neutral position. When it is in this position, the breaker points open and the primary current is interrupted. The flux change in the coil core is the greatest.

Question 27

In what position is the magnet in a magneto when the breaker points begin to open?

Answer 27

In its E-gap position, just a few degrees beyond its neutral position.

Question 28

What malfunction in the ignition system would cause an aircraft reciprocating engine to continue to run after the ignition switch is placed in the Off position?

Answer 28

The ignition switch is not grounding the magneto primary circuit.

Question 29

What turns on the autoignition system in a turbo-prop engine?

Answer 29

A torque pressure switch energizes the system when the engine stops producing torque.