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Is a engine run up really necessary?

Posted by on July 29, 2008 22 Comments Category : Flight Instructor Blog Tags :

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…

Fly Safe.

22 Comments



  1. Richard on Jul 30, 2008

    Good answer. I would just add two things:

    1. The carb heat check should also show any carb ice that may be building up while you taxi around. The conditions for carb ice formation could be very different at a rest/fuel stop than at the point of departure.

    2. The small piston engines in light singles are very reliable and will actually start and run smoothly in a condition you wouldn’t want to fly it in. A case in point, two years ago I was taking my niece for a flight. The engine started as per normal and ran smoothly during taxi to the run up area. However during the runup it was obvious that one cylinder wasn’t firing on the right mag. To keep the story short it turned out we had broken a ring and oil passing the piston was fouling the lower (in this case right) plug. Needless to say, no flying till the jug was replaced, but without a runup the problem may not have been noticed until we were in a much worse position than disappointed on the ramp.



  2. instructor on Jul 30, 2008

    Richard, thanks for the notes. Both good points about the importance of engine run-ups. I was looking for a good story to illustrate my answers but was coming up empty. Your story really brings it home. Great. Thanks.



  3. Jetwhine on Jul 30, 2008

    BlogFest @ AirVenture: The Ideas Flowed…

    Five minutes before the blog session began at AirVenture yesterday, I was wondering if the whole thing…



  4. Josh on Jul 30, 2008

    To illustrate why it is necessary on each leg of a flight not just the first flight of the day: Back when I was in college I decided to take a cross country trip to build some time, I flew the aircraft (a Cessna 172M) to a destination about an hour and a half away. I spent about a half hour on the ground just passing some time then went to leave, as I was doing the engine run-up before heading home I pulled the carburetor heat knob and it came right out of the fire wall into my hand. I was now stranded at an airport an hour and a half away from home. The flight school sent another aircraft to pick me up and left that one there till they could fix it. That is why it is important to always do a run-up, not only did not having carburetor heat make for a possibly dangerous situation, it was also illegal to fly the aircraft in that condition as it was technically no longer airworthy.



  5. PlasticPilot on Jul 30, 2008

    Thank you Paul for one more good post.

    My two cents: check idle power. Some of the checklists I use include that one as part of the run-up checks. I know that big (300HP) engines don’t like that much, but I think it’s important.

    As we don’t taxi or wait at idle power, there is no check of that before… the flare. Don’t we want to be sure that the engine won’t simply stop when power is back to idle before the flight ? Such a surprise on short final is not exactly pleasant.



  6. instructor on Jul 31, 2008

    Thanks Guys. I appreciate the good stories and additional run-up tips.



  7. AJ on Sep 23, 2008

    I’m not saying that it’s not a good idea – I definitely agree that it IS a good idea to do one…

    … but where is it WRITTEN that it is REQUIRED?

    Is it an FAA requirement?

    An aircraft manufacturer’s requirement?

    An aircraft operator’s requirement?

    Or just one of those Great Habits of Good Aviators?



  8. instructor on Sep 23, 2008

    Hi AJ.

    The FAA only specifically requires part 91 operators to comply with the operating limitations of the AFM (FAR 91.9) and does not specifically state that you have to follow the normal operating section where the engine run-up procedure is usually described however…

    If push came to shove and an accident or incident resulted because you failed to comply with the recommenced normal operating procedures, I would imagine that this fact would be stated pretty clearly in the NTSB report and on the report given to the insurance company. You can also be sure that it would void any kind of manufacturer warranty.



  9. AJ on Sep 24, 2008

    Instructor – Thanks very much for the informed response!

    To confirm:

    – For pilots, engine run-ups are part of the (highly) recommended manufacturer’s pre-flight checklist.

    – Not REQUIRED by the FAA, although we all fully understand the consequences + associated risks of failing to perform a run-up check.

    I am asking as I have a tenant at the airport where I work who is fully convinced that run-ups are not required, and is making an issue of aircraft performing run-up checks in close proximity to his location (even though they are in fact on a movement area when performing these checks).

    Thanks for the assist.



  10. instructor on Sep 24, 2008

    I would argue that it would be negligent of pilots NOT to perform a run-up. Although there is nothing specifically stated that they are required (unless you are on a checkride), the FAA could easily argue that failure to adequately check the aircraft and engine which resulted in an accident could be prosecuted under careless and reckless operations under FAR 91.13



  11. Richard Wyeroski on Jan 11, 2009

    I always teach do a full run-up on every start-up. Especially since there are some aircraft that are “VERY” sensitive to carb ice while taxing under certain conditions like the cessna 172 with the 0-300 continental 6 cylinder engine.



  12. Arthur Hill on Feb 15, 2009

    Thank you guys for your comments and inputs, it was really very informative.
    It’s better to be safe than to be sorry..



  13. chephy on Apr 13, 2009

    Interesting. At my school, I’ve only been taught to do a run-up before the first leg of a x-country trip. I’m in Canada, so I don’t know if that’s because Canadians do things differently, or whether it’s just my flight school or even just that one instructor who introduced me to cross-countries during my PPL training…



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  15. MICHELLE SY on Oct 12, 2010

    is there any certain parameter for GPU if u are conducting an engine run up?



  16. Brucew on Nov 21, 2010

    When checking magnetos, why is the procedure to turn from both to right, then turn back to both, then turn from both to left, then turn back to both? Is this to clear the cylinders of any possible unburned fuel between checks?



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  18. Camden on Nov 14, 2012

    Brucew, this is about 2 years late, but….

    By starting with the Right mag instead of the Left, you ensure that you don’t accidentally leave it on just the Left. For example, if you go from Both to Left (one click), back to Both (one click), then to Right (two clicks), back to Both (one click……or is it two??). You might not notice that you left it on the Left mag.

    I was always taught to check the Right mag first just for that very reason. Two full clicks to Right, two full clicks back to Both, one click to Left, one click back to Both.



  19. namgon lee on Feb 27, 2013

    HELLO, ALL MY FRIEND.
    I’M CURIOUS WHY WE HAVE TO KEEP THE R.P.M WITHIN 175 DURING MAGNETO CHECK?
    WHY BOTH MAGNETOS DIFFERENTIAL HAVE TO BE WITHIN 50?

    ANYONE?
    THANK YOU.



  20. gordon on Oct 10, 2013

    I took off once without runup, but a military pilot saw me and told me something new. He said even if the engine is fine, which it was on that flight, I should do the runup to burn and clean the spark plugs so that the engine does not cough on takeoff.



  21. Evan on Mar 28, 2015

    Guys,

    What you are checking on your mag run-up is to make sure the engine can run smoothly on both magnetos. If one magnetos were to stop working for whatever reason, you need to be sure that the other magneto can provide the spark to the spark plugs to run the engine. More than 175 drop? The magneto is providing inadequate electrical current to run the engine. Switching to L/R magneto and no drop? That magneto is not “grounding” meaning its not turning off when you ask it to so it could provide spark while you are on the ground! Too much drop? Carbon deposit due to unburnt fuel, ask you instructor how we deal with this.



  22. Amit on Sep 23, 2016

    can anyone answer me why do we do a right magnetos check before the left one ?

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