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Lycoming Aircraft Engine

Ignition System Checks

The fuel air mixture in the combustion chamber needs to be ignited at the correct moment to ensure efficient combustion and power generation by the engine. This is the job of the ignition system, be that the old fashion magneto of the good old days or a modern fully electronic microprocessor controlled FADEC system.

For safety reasons the ignition system does not rely on the the aircraft electrical system must be dual and each system operates one of the two spark plugs in each cylinder.

These ignition systems apply to avgas (spark ignited) engines and not to diesel, which are compression ignited engines.

Before flight and just after engine start and runup the pilot in command must ensure proper operation of the aircraft and its engine(s). The ignition system is one of them and here we explain what to look for.

Runup & Preflight

Unless you fly with an FADEC or mechanically controlled compression ignition engine you will need to do a check of the ignition to make sure they both are operational and are within limits set forth by aircraft and engine manufacturer.


Except for some old historic aircraft most have an electric starter system consisting of a battery (12/24 V), connected with wires and starter motor and with an ignition switch in the cockpit. During engine startup the current draw by the starter motor will be very high and therefore it needs thick cables (ie. low resistance, low power loss) to minimize the voltage drop. The starter switch is connected to a relay or solenoid in the engine compartment so that the wiring is as short as possible (again: low resistance).

Some models have a red starter warning light which indicates that power is applied to the starter motor, the light must go out the moment the ignition switch is released.

Ignition switch

Only one of the two magnetos is equipped with the impulse coupling (usually the left) therefore the other magneto is switched off during start, this is done electrically in the ignition switch. And it has the following positions: OFF-RIGHT-LEFT-BOTH-START. The START position is spring loaded, meaning the key won't remain in this position but its pushed back to BOTH.
Some older type aircraft have a start switch or button separate from the magneto switches.

Oil pressure

Make sure that after start the oil pressure rises within 30 seconds, and within 60 seconds maximum in cold weather. If not, shut the engine down to avoid damage due to improper lubrication. Cold oil is thicker (higher viscosity) and it needs some time to get pumped around.

Dead cut check

Ignition Switch

After the engine has started, perform a dead cut check between 800 and 1000 RPM. This is to ensure that both mags are operating before you do a runup mag check at high power. For example: PA-28's are runup at 2000 RPM and GA-8 Airvan at 2100 RPM. The Airvan has a 300 BHP engine and runup is not the time to discover that a magneto has failed and the engine suddenly stops at high power settings. Especially so with turbocharged engines.

Magneto check

After the dead cut check is satisfactorily completed a high power runup with mag check can be done. Cessna aircraft runups are done at a typical 1700 RPM and note the RPM drop on each mag and the difference between those readings. Typical values are max drop 125 RPM and max 50 RPM difference.

Rotax four stroke engines (912, 914) runup at 4000 engine RPM and may drop 300 with a max difference of 120 RPM. Those Rotax powered aircraft using a propeller RPM indicator use 1700 PROP RPM for runup (TECNAM P92-JS).

Running rough

If the engine starts to run rough on one magneto the spark plugs probably fouled up because of the rich mixture and high lead content of avgas. Perform the runup check again and lean the mixture for a minute or so to clean the plugs, this should resolve the problem. If not, consult an aircraft engineer to have the engine checked.

Shutdown check

Some pilots check the magnetos prior to engine shutdown. Do this as follows on 1000 RPM or lower: BOTH-LEFT-RIGHT-OFF -> BOTH. The engine should experience a sudden loss power indicating that both magnetos are really switching the ignition off. You can then be sure that the aircraft has no live magneto when the keys are taken out of the ignition switch. After this check proceed to pull the mixture to idle cut off (ICO) to stop the engine as usual, then remove the keys from the ignition switch.

Before you exit the aircraft, remember to ALWAYS remove the keys from the ignition switch and place them on the dash, in the side pocket or take them with you.

Written by EAI.

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