Most standard certificated aircraft incorporate a dual ignition system with two individual magnetos, separate sets of wires, and two spark plugs per cylinder to increase reliability of the ignition system through redundancy
This is designed so if one magneto or spark plug fails, the other is unaffected and will continue to run normally, with a slight decrease in power
In a spark ignition engine the ignition system provides a spark that ignites the fuel/air mixture in the cylinders and is made up of magnetos, spark plugs, high-tension leads, and the ignition switch [Figure 1]
Magnetos:
Magnetos are self-contained engine-driven units to supply electrical current to the spark plugs
A permanent magnet is used to generate this current completely independent of the aircraft's electrical system
The magneto generates sufficiently high voltage to jump a spark across the spark plug gap in each cylinder
The system begins to fire when the starter is engaged and the crankshaft begins to turn
It continues to operate whenever (or however) the crankshaft is rotating
Each magneto operates independently to fire one of the two spark plugs in each cylinder
Magneto Check:
Magneto checks test the ignition system's operation and are a critical element to ground checklists
When checking magnetos there a few conditions must be satisfied to ensure proper functionality:
The magneto grounding wires are connected
If not connected, there will be no drop in RPM when that magneto is selected
The drop in RPM falls within the recommended limits as defined in the Pilot Operating Handbook
A drop, but continued operation of the engine, ensures the aircraft can fly on a single magneto, albiet at reduced performance
If equipped, pilots should also monitor EGT for a rise on all cylinders
Note that when checking EGT, a drop in a single cylinder's EGT is one way to identify the malfunctioning magneto
The differential drop between magnetos is within limits as defined in the Pilot Operating Handbook
Excessive differentials demonstrate a malfunction in one of the magnetos
If the drop is greater than the recommended limits as defined in the Pilot Operating Handbook
Consider running the engine to a cruise/near-cruise RPM and leaning to peak EGT
Run lean of peak at high RPM for about 30 seconds
If magneto checks after a second or even third time, have the engine looked at by a mechanic
Although magneto checks are normally performed on the ground, they may also be performed in flight
Note that aging magnetos can manifest themselves and provide a warning through hot starts, before ever performing the magneto check
Magneto checks are usually performed before takeoff to:
Allow the aircraft to warm up
Check condition prior to takeoff (taxi may foul)
Avoid propwash concerns in the ramp area
Failed Magneto Checks:
If an anomoly in the magneto check occurs, the ignition system is not operating correctly
Anomolies are not meeting one of the previous mentioned conditions during the check, or something unexpected excessive rough engine idling during the check
Another cause for rough idle could be poor ignition timing
Low EGTs and high CHTs are a likely corresponding indication
Electronic Ignition:
Electronic ignition systems are becoming more common, supplementing or all-out replacing magnetos
These systems maintain the same functionality, but change the interface from an ignition switch with magneto selection usually via a key, to push-start systems
These systems generally maintain aircraft performance as compared to those with magnetos, but reduce maintenance costs through the use of a more simple system
Spark Plugs:
Spark plugs provide the source of ignition
Each cylinder has two spark plugs which improves combustion of the fuel/air mixture, results in a slightly higher power output, and provides redundancy
Ignition Switch:
The operation of the magneto is controlled in the flight deck by the ignition switch [Figure 3]
The switch has five positions:
OFF
R (right)
L (left)
BOTH
START
With RIGHT or LEFT selected, only the associated magneto is activated while BOTH uses the two simultaneously
A malfunctioning ignition system can be identified during the pre-takeoff check by observing the decrease in rpm that occurs when the ignition switch is first moved from BOTH to RIGHT, and then from BOTH to LEFT
The permissible decrease is listed in the AFM or POH
If the engine stops running when switched to one magneto, the rpm drop exceeds the allowable limit, or no drop occurs, do not fly the aircraft until the problem is corrected
The cause could be fouled plugs, broken or shorted wires between the magneto and the plugs, or improperly timed firing of the plugs
Following engine shutdown, turn the ignition switch to the OFF position
Even with the ignition, battery, and master switches in the OFF position, if the ground wire between the magneto and the ignition switch becomes disconnected or broken, the engine could accidentally start if the propeller is moved with residual fuel in the cylinder because it requires no external power
If this occurs, the only way to stop the engine is to move the mixture lever to the idle cutoff position
The video below is an example of a magneto check Always follow the procedures and limits listed in the AFM/POH for your aircraft when performing this check (i.e., 125 RPM is this aircraft's maximum drop, yours may be different)
Starter/Starter Motor:
Starting An Aircraft:
Most aircraft have starters that automatically engage and disengage when operated, but some older aircraft have starters that are mechanically engaged by a lever actuated by the pilot
The starter engages the aircraft flywheel, rotating the engine at a speed that allows the engine to start and maintain operation
Electrical power for starting is usually supplied by an on-board battery, but can also be supplied by external power through an external power receptacle
When the battery switch is turned on, electricity is supplied to the main power bus bar through the battery solenoid
Both the starter and the starter switch draw current from the main bus bar, but the starter will not operate until the starting solenoid is energized by the starter switch being turned to the "start" position
When the starter switch is released from the "start" position, the solenoid removes power from the starter motor
The starter motor is protected from being driven by the engine through a clutch in the starter drive that allows the engine to run faster than the starter motor [Figure 4]
When starting an engine, the rules of safety and courtesy should be strictly observed
Clear the area visually and make a call, "Clear prop!"
In addition, the wheels should be chocked and the brakes set, to avoid hazards caused by unintentional movement
When using a starter, don't hold the started for more than a few seconds or it can burn out due to excessive heat
To avoid damage to the propeller and property, the aircraft should be in an area where the propeller will not stir up gravel or dust
Combustion:
During normal combustion, the fuel/air mixture burns in a very controlled and predictable manner
In a spark ignition engine the process occurs in a fraction of a second
The mixture begins to burn at the point where it is ignited by the spark plugs, then burns away from the plugs until it is all consumed
This type of combustion causes a smooth build-up of temperature and pressure and ensures that the expanding gases deliver the maximum force to the piston at exactly the right time in the power stroke [Figure 5]