When a pilot thinks of an engine runup in a single engine piston airplane, typically it is let’s find out if the engine is running rough or not, meaning a spark plug is fouled. Believe it or not, there is a little more to learn from a runup than just checking the spark plugs.
A distinct advantage to having an engine monitor that monitors all EGTs and CHTs is you get a better idea of engine health. When switching to one magneto or the other during the runup, all the EGTs should rise, showing that the temperature of the exhaust gas is going up from each cylinder. This occurs because when running on only one magneto and one set of spark plugs in each cylinder, the mixture takes longer to burn, so, when the exhaust valve in the cylinder opens, the burn is still on going and this hits the EGT probe, causing a temperature rise. An indication of a magneto failure in flight would be a rise in all the cylinders EGT without a change in mixture setting.
Now that we know what to look for when checking each magneto, what would one look for to show that there is a problem? The basic that each student pilot is taught still applies. The engine manufacturer puts a limit on RPM drop during the runup. When checking one magneto or the other, watch for an excessive RPM drop. If the RPM drops past the limit, this usually will be accompanied by engine roughness.
Why is that? Let’s look at the cause for the excessive RPM drop. For this, we need to go back to our EGT indicators. Let’s say we have a 6 cylinder engine. When the key is turned to the right magneto, the RPM drops 250 RPM and the engine gets very rough. Look at the EGT gauge. The cylinders where the spark plugs are firing normally will all show a temperature. The cylinder (or cylinders) where there is no combustion, meaning a bad plug, will show no temperature. This is because the mixture is just sitting in the cylinder and not igniting, therefore, no exhaust gases will be pushed out so there will be no EGT indication for that cylinder. This is also how you tell a mechanic which cylinder to check for the bad plug.
It could also be a bad magneto. If multiple cylinders EGT all drop, or the engine wants to quit entirely, you have a bad magneto or bad ignition harness.
What else can you learn during the runup? Electrical system health is key, especially in all electric airplanes (meaning no vacuum system). It’s a very good idea to turn on all the lights and pitot heat to ensure that a rise is indicated on the ammeter. This means the alternator is carrying the load and working properly.
The next time you do your runup, keep an eye on your EGT to see the rise during a magneto check.