6.2.4 Ignition Systems
The basic requirement of any ignition system is to deliver a high-tension spark to the spark plugs in each cylinder in the correct firing order at the correct time. This spark ignites the fuel/air mixture powering the pistons and producing work so to turn the propeller.
A magneto uses a permanent magnet to generate an electrical 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 the crankshaft is rotating. The magneto system is mounted on the accessory drive unit located at the rear of the engine.
2502299114631500Most standard certificated aircraft incorporate a dual ignition system with two individual magnetos, separate sets of wires, and spark plugs to increase reliability of the ignition system. Each magneto operates independently to fire its own spark plug in each cylinder. The firing of two spark plugs in each cylinder improves combustion of the fuel-air mixture and results in a slightly higher power output. If one of the magnetos fails, the other is unaffected. This redundancy allows the engine to continue normal operation, although a slight decrease in engine RPM can be expected. The same concept of redundancy applies to the spark plugs. Operation of the magnetos are controlled in the flight deck through the ignition switch. The switch has five positions:
BOTH / START
With RIGHT or LEFT selected, only the associated magneto is activated. The system operates on both magnetos when BOTH is selected.
A malfunctioning ignition system can be identified during the pre-takeoff check by observing the decrease in R.P.M that occurs when the ignition switch is first moved from BOTH to RIGHT and then from BOTH to LEFT. A small decrease in engine R.P.M is normal during this check. The permissible decrease is listed in the POH. If the engine stops running when switched to one magneto or if the rpm drop exceeds the allowable limit, 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. It should be noted that “no drop” in R.P.M is not normal, the aircraft should not be flown and sent in for immediate inspection.
“No drop” means one of the magnetos is not grounding and can result in unintended start-up by turning the propeller by hand. Following engine shutdown, turn the ignition switch to the OFF position. Even with the battery and master switches OFF, the engine can fire and turn over if the ignition switch is left ON and the propeller is moved because the magneto requires no outside source of electrical power. Be aware of the potential for serious injury in this situation.
Even with the ignition switch 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. If this occurs, the only way to stop the engine is to move the mixture lever to the Idle Cut-Off (I.C.O) position, then have the system checked by a qualified AMO.
In order to produce a spark in the plugs, the magneto spins a magnet within a soft iron coil core. This creates an alternating current within the coil and creates as much as 20 000 volts to fire the spark plugs. Effective sparks are only produced once the magnet is rotating at speeds of about 500 R.P.M. Any speed below this results in weaker sparks and prolongs engine start-up. Low engine R.P.M on start-up require a delayed spark in order to prevent kick-back. This is a premature power stroke caused by normal magneto timing set for higher R.P.M and can lead to the crankshaft being forced in the wrong direction.
Impulse coupling is incorporated into one of the magnetos to help overcome this problem on start-up. The benefits are two-fold. Firstly, it accelerates the rotation of the magnet producing a higher voltage, and therefore better spark, and secondly to retard the spark at lower R.P.M experienced during start-up.
The magneto employs spring weights and a spring-loaded coupling which initially prevents the magneto from turning. Once the spring is fully wound it releases the magnet which now, due to the increased rotational velocity, results in both a hot and late start during start. Once the engine is running, the centrifugal force of the flyweights ensures the impulse coupling is disconnected.
Ignition leads carry the high voltage generated by the magnetos to the spark plugs. The spark plug is designed in such a way as to allow this high voltage to jump between the central insulated electrode and the grounded electrode, creating a spark of sufficient intensity to ignite the fuel/air mixture under compression.
The spark plug is a useful indicator of engine condition. At each mandatory inspection the plugs are removed for inspection and testing. Normal engine operation is indicated by a light grey coating of the end of the plugs. Excessive wear may indicate detonation. In cases where the engine has been operated with too rich a mixture, black sooty-like deposits will appear. Whereas white powdery deposits will be found on plugs from engines operated with too lean a mixture. Black oily deposits indicate excessive oil consumption. If hard brittle deposits are found in the end cavity, lead in the fuel is not being removed during combustion, and if left, the deposits can build up sufficiently causing the high voltage to ground without a spark. This often results in a “mag drop” which can be identified by a rough running engine and an excessive loss in R.P.M. If this is detected on the ground during magneto checks, the flight must be abandoned. The gap between the central and ground electrode is adjustable and should be set to manufacturer requirements. Too small, or large a gap will affect the efficiency and size of the spark produced and can lead to incomplete combustion and a rough running engine.