Ignition Systems

Question 1

Explain the reasons for fitting independent dual ignition systems to engines.

Answer 1

• Each magneto separately supplies one of two spark plugs in each cylinder
• Safer in the event of failure of one system
• Smoother and more efficient combustion

Question 2

What are the components of an ignition system

Answer 2

•A source for the high voltage spark (magneto), Totally independent of the aircraft electrical system
•High tension leads (screened cables)
•Distribution system
•Spark plugs
- ignition switch

Question 3

Describe the operation of the following: aircraft magneto including the build.

Answer 3

Primary coil provides an amount of electricity (not strong enough to power the spark plugs, and as the permanent magnet rotates past the coils a magnetic field is induced.

Contact point opens which causes the magnetic field in the primary coil to collapse causing a voltage spike in the secondary coil.

Question 4

What’s in a magneto?

Answer 4

Rotating magnet
Primary coil
Secondary
Contact breakers

Question 5

Describe the operation of a distributor

Answer 5

Distributes the spark in sequence to the spark plug at the correct moment, Each cylinder fires once every 2 revolutions of the crankshaft.

High tension current supplied by the magnetos

Question 6

Describe the operation of high tension leads

Answer 6

Separate HT leads from distributor contact to individual spark plugs
•Proper insulation is important because of the high voltages (>20,000 volts)
•Radio interference – screened
•Also known as an “ignition harness”

Question 7

Describe the operation of the spark plug.

Answer 7

Work by providing an air gap in the secondary circuit which the high tension current jumps to create a spark plugs.

Question 8

What are the three main components of a spark plug and how do they work?

Answer 8

A central electrode connected to the ignition harness, through which the high tension current is led down through the plug.

A metal body screwed into the cylinder head. The body holds the side electrodes at the other side of the air gap.

A ceramic insulation to insulate the central electrode from the metal body.

Question 9

Explain the causes of spark plug fouling what spark plug fouling does and the methods of clearing such fouling.

Answer 9

Causes - Plugs often foul due to the mix of low engine temps and high led content during warm up at full rich settings
•Taxiing at low RPM for prolonged periods can also lead to plug fouling

Does - electrodes become covered with oil or carbon affecting its ability to ignite the charge correctly, also means a drop in RPM outside of the limits of magneto check

To clear spark plug fouling, engine power can be increased to 2000 RPM for 60 seconds or some people recommend leaning mixture. They have the effect of burning any contaminants on the spark plug.

Question 10

Explain the purpose of fitting an impulse coupling to a magneto.

Answer 10

Provides a short and sudden acceleration of the rotating magnet within the magneto generating a high voltage. (Fat retarded spark)

effectively retards ignition timing at low cranking RPM, immediately after startup allowing the timing of the spark to return to its normal position.

Question 11

Explain the reason ignition is timed to occur before top dead centre (TDC).

Answer 11

• Engine rotates about 120 rpm on the starter motor
• Magneto rotates at ½ engine speed
• Insufficient speed to supply healthy spark to light off successfully
• Spark advanced approximately 25° before TDC for normal operation
• Low RPM too much pressure builds before TDC causing crankshaft to rotate the wrong way
• Spark position must be delayed to ensure max pressure is after TDC

Question 12

Describe the operation of the ignition switch(es) in the cockpit. (4)

Answer 12

Left - left magneto is active and right magneto is earthed
Right - right is active left is earthed
Both - both are live
Start - pushing the ignition switch in and on will allow the starter motor to energise, releasing the switch will deactivate the starter. Impulse coupling is then activated

Question 13

Describe the function of the starter motor.

Answer 13

An electric starter motor draws a lot of current

Question 14

Describe the operational limitations of the starter motor.

Answer 14

Do not operate it for more than 10 seconds at a time as it risks flattening the battery or causing damage to components of the ignition system

Question 15

Explain the correct operation of a typical starter switch

Answer 15

Oil pressure must rise within 30 seconds or shut engine down.

RH - magneto de energised and the impulse coupling on the LH magneto used to supply high tension spark.

Key return to both, and both magnetos are on.

Question 16

Outline the correct procedures to be followed during magneto checks.

Answer 16

After start and at or below 1000RPM - live cut check
Prior to run ups - dead cut check
Set 1800RPM - magneto check to see it functioning correctly
Mag check
175rpm drop max, 50 rpm max difference
Prior to shutdown - complete another LIVE CUT check

Question 17

What is the live cut check procedure

Answer 17

After start, you are checking to see if the magnetos can be earthed
•Do this by briefly turning the key switch to ‘off’
•The engine should start to fail (as both magnetos are earthed and so no power is getting to the spark plugs)
•If this does not happen, stop the flight and take caution around the propeller
•In any case, always treat a propeller as live!

Question 18

What is the dead cut check procedure

Answer 18

You are checking to see if the magnetos can be earthed individually
•Done prior to conducting a magneto check at a higher RPM
•This is done to prevent damage to the engine if one magento is dead
•Slight drop in rpm on one magneto
•If no drop, do not continue

Question 19

What to do when hang swinging An aircraft

Answer 19

• Brakes on
• Chock the wheels
• Have a second person, they should be guarding the brakes in the cockpit
• Be on level, firm ground, The surface should not be slippery (due to loose gravel, oil, or being damp, etc)

Question 20

What are the limitations of magnetos? (6)

Answer 20

• Mechanical wear
• Oxidation/burning
• Need adjustment at regular intervals
• Mechanical variation
• Air in the distributor is insulator between rotor tip and plug lead terminals
• Insulation value of air reduced at altitude

Question 21

What are the components of a typical solid state ignition system? (7)

Answer 21

• input outputs
• power control module
• knock sensor
• cam sensor
• coil pack
• dual crank sensor
• spark plugs

Question 22

Describe the operation of a typical solid state ignition system.

Answer 22

Contact breakers, distributor and impulse coupling replaced by a capacitor.

Sensors for engine speed and compression, it is controlled via a microprocessor, which gets inputs about Oxygen levels, pressure, and crankshaft position, and adjusts the timing of the spark to ensure the best performance from the engine

•The high voltage needed to power the spark plugs is delivered by means of capacitors

Question 23

Explain the purpose for conducting typical solid state ignition integrity checks.

Answer 23

Same as magnetos, check the state and condition of these things

Question 24

Explain the procedure for conducting typical solid state ignition integrity checks.

Answer 24

Check Right and Left systems, check backup battery. Without power from the alternator or a battery the ignition system wont work

Question 25

With reference to a solid state ignition system:

(a) explain the significance of maintaining the ignition power supply;

Answer 25

Without any power from alternator or backup battery ignition system will not work and engine will fail.

Question 26

With reference to a solid state ignition system:

describe a typical power supply backup system.

Answer 26

Alternator is main power supply and if fails, Second battery solely for use of the ignition is used.