Start-Up and Run-Up checklists -- we all know them and use them. However, many students -- and even licensed pilots -- often get confused about what and how they're actually checking during the run-up.
Below is a list of the most misunderstood items on the ground checklists -- with explanations on how to do those checks properly, what to look for, and how to remedy problems that may arise.
All too often pilots would turn carb heat ON, then OFF, as part of this check, without asking themselves what it is they're actually looking for during the check. What are you checking for when you pull the carb heat knob out? Two things: one, that carb heat is actually working, and two, that you have no carb ice (which is likely to form on the ground when the conditions are conducive to ice formation, since you're operating at low rpm and carb heat off on the ground).
So, how do you check that carb heat is working? Well, if there is no carb ice, then turning carb heat ON should produce a drop in rpm (for your homework, reread your Flight Training Manual or the presentation on this site to figure out why). Run the engine with carb heat ON for at least half a minute to ensure that there is no ice, and then return it back to OFF (engine rpm should rise as a result).
And what if you do have ice? Well, after you turn carb heat ON, the ice will start melting, causing water to pass through the engine, so you might notice some engine roughness. However, you should also see an rpm increase as the carburetor ice clears and the fuel-air mixture can flow unobstructed. Leave carb heat ON until ice is melted, and then turn it back OFF (the rpm should rise further after that).
Finally, remember that whenever you turn on carb heat, you're using unfiltered air for the air-fuel mixture, so it's really important to do this check over a clear area so that your propeller is not picking up all manner of gunk and debris and feeding it right into your engine!
This is where you smoothly and slowly pull the mixture control out until the engine starts to run rough, and then return it back to rich, to make sure the mixture control is working as required. You should see a small rise in rpm as the mixture becomes more efficient and then a rather sudden drop. If you see no rise -- the carburetor is set too lean! Something for an AME to investigate. If you see no drop, the mixture control is not working. Definitely something for an AME to investigate! A little tip: when doing the check, not ram the mixture back full rich... ease it back in. Be gentle on that engine -- you don't want it to backfire!
New pilots can be fuzzy on this item at first -- because these are actually two items! The first is done at run-up power to check that you have no "dead mags" (i.e., magnetos that are not firing) ; the second is done at idle to check just the opposite: that you have no "live mags" (i.e., magnetos that are firing even when you turn them off).
The check at run-up power is performed by switching the mags from BOTH to LEFT to BOTH, then BOTH to RIGHT to BOTH. You should see a slight rpm drop when switching from BOTH to one mag, and a corresponding rise when switching back to BOTH. So far so good. But what if you see something different? Well, that could be a few things:
No drop: if you see no drop when switching from BOTH to, say, LEFT, chances are your RIGHT mag is dead. Do not switch to RIGHT to confirm; of if you do, do so at low rpm. The danger here is that if you switch the mags to RIGHT, and the engine begins to die, you'll be tempted to turn the key back to BOTH, which at high rpm could lead to a potential disaster. Instead, keep your hands off the mag switch, close the throttle, let the fuel evaporate, and only then restart.
Too much drop: What if instead of a POH-allowed drop of, say, 125 rpm, you see a drop of, say, 200-300 rpm, and the engine runs rough? Chances are, you're dealing with spark plug fouling -- product of incomplete combustion. Turn the mags back to both, increase the rpm somewhat and lean out the mixture. That makes your engine run hotter, clearing out the carbon deposits. After a few minutes, the engine should run smoothly again.
Ok, so what about the other check, the one where you're checking for a "live mag"? It's called a "dead cut" check: during this check, you turn the mags to OFF momentarily, and then immediately back on. What you should see is the engine begins to sputter in the off position. If it keeps running smoothly -- you've got a live mag, which means turning the propeller on the ground (e.g., adjusting it to fit the tow bar) could get the engine starting -- with obviously disastrous results such as chopping your head off.
There are two very important thing about the dead cut check: 1) do it at idle rpm and 2) do it very quickly. When you turn mags off, the unburnt air/fuel mixture keeps flowing through the cylinders into the exhaust, so if you do the check at a high rpm, or hesitate in the OFF position, there will be a lot of that happening... so turning the mags back on could cause a spectacular bang due to what's called an "afterfire", in which that unburned mixture suddenly ignites. This is not good for your exhaust system! As in, it could be blown to pieces!
5. Alternator/Generator Check
First of all, do you know the difference? And which one your plane is equipped with?
The standard alternator/generator check at run-up involves turning on all electrical equipment and checking that alt/gen can still supply power to the battery to run the full menagerie of lights, switches, and avionics. If your plane is equipped with an ammeter, the needle should be just above zero; if it's equipped with a loadmeter, the load should increase (again, do you know the difference between the two instruments, and which one your plane is equipped with?)
If the ammeter/loadmeter is not producing the desired indication, that means your alternator might be faulty! Sometimes this is accompanied by an overvoltage light coming on (which essentially tells you the alternator is out of the electrical loop), but sometimes the alternator will sort of run, but not well enough to power the battery. Either way, you might be heading for a full electrical failure... so don't take off! Taxi back to the ramp and get an AME to check things out.