Received a question this morning from Jim:
When flying a piston single (e.g. C172), I’m trained to do a run-up before every leg of every trip. I can understand the value of a runup before the first leg of the trip — you want to be sure the engine is running fine. I’m less clear about why we do a run-up after landing after a pee break on a long cross-country flight. What exactly is the run-up testing for? What problems in a second or subsequent leg of a day’s flying is it intended to catch?
You have asked a great question Dave that I am sure a lot of pilots have wondered at one point or another. Isn’t just one engine run up per day or per trip enough? Why do I have to do it every single leg?
For those who may be just getting into aviation let me quickly explain a engine run-up. A engine run-up is referring to a series of checks a pilot does before takeoff. On a single engine piston aircraft (like a Cessna 172) a engine run-up consists of usually of checking the aircraft’s carburetor heat and a quick check of the airplane’s magnetos as well as the basic engine instruments such as oil pressure, oil temperature and cylinder head temperature.
Why these checks are necesary:
Carburetor Heat: Carburetor heat is necessary when operating at lower power settings where you might experience carburetor icing. Applying carburetor heat is usually as easy as pulling a single knob control in the cockpit. When you pull that knob you redirect hot air taken from a shroud around the exhaust system into a duct and directly into the air induction system which would melt any carb ice which might have built up. The less dense air entering the engine causes it too lose approx. 75 -100 RPM which is what you should see on the engine RPM gauge during the check.
But what happens if you don’t see a RPM drop? I could mean a few things.
1) It could mean a broken cable between that knob and the valve in the airbox
2) It could be a stuck valve in the airbox. Maybe it is already open (which is why you didn’t see a drop) or it could be closed, just not moving.
3) Or it could be something else.
Maybe one of those things (like the broken cable) happened to occur on the last leg. It might have been loose during your trip and finally disconnected itself during your landing (I’ve had landings hard enough that I wondered if the wheels were still attached). Wouldn’t you want to find out exactly what was wrong before the next flight? Do you also know that running with partial carburetor heat can be worse than none at all? (under certain atmospheric conditions).
Magnetos: Magnetos are mounted (usually) on the back portion of a piston engine. This is where the electrical charge (for lack of better word) for our ignition system and spark plugs originate. A magneto is a rotating magnet (duh) and as the magnet turns it excite a fixed coil of wires that send out current to the correct spark plug at the correct time. The current is just great enough that it causes the electrons to “jump” across the gap in the spark plug which creates the spark that ignites the air / fuel mixture in the cylinder, producing power that turns the prop. Most airplanes are equipped with two magnetos. Each magneto fires the “upper” side of the spark plugs on one side of cylinders and the “lower” side of spark plugs on the other side of cylinders. That way if you lose one magneto you would still have ignition on all cylinders.
Why a magneto check is important before EVERY flight
Most general aviation airplanes can and do start on a single magneto. When you place the switch in the start position you are only opening the ground for one magneto, not both. During your engine run-up you are checking to make sure that yes, both magnetos are really working and falling within required tolerances. Maybe you accidentally taxied out with the magneto switch in L side only. You wouldn’t want to takeoff like that right?
P-lead – The P-lead is a way to ground the magneto (turn it off). If for some reason the P-lead would break during a flight the magneto…well, it could never be turned off. That means your prop is always “live”. Even turning the prop by hand on the ground could technically start your engine and seriously hurt or kill someone. We also check for this after the flight by turning to magneto / start switch to off momentarily to make sure we can ground the magneto and kill the engine. I have had this break on me before.
Carbon Deposits – When you cycle a magneto off during the check you are anticipating a certain RPM drop associated with running the cylinder on half ignition. If you notice a significantly higher drop in RPM it could be a sign you are experiencing some form of carbon (unburnt fuel) deposit in that spark plug gap that we referred to earlier. To correct this we would want to lean out the mixture and increase power to try and “burn off” that excess carbon. You want to make sure that before flight you have each and every cylinder firing and producing peak power.
I hope I’ve help to explan the importance of the system checks and why we want to do these checks before each and every flight.
And thinking like a professional pilot for a second here, you also have to remember an important aviation rule, CYA. If for some reason you didn’t do the required checks as specified by your POH or AFM it could open yourself for litigation and investigation if something would happen during the flight that resulted in injury to someone or something because you failed to do an adequate before takeoff checklist as prescribed by the aircraft manufacturer. This shouldn’t be your motivation for doing the engine run-up but we live in the age of litigation so you always have to be careful and thourough.
Good luck out there and remember to do your engine run-up before each leg and always…