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Aircraft Engine Starting Techniques

Posted by on November 20, 2009 12 Comments Category : Flight Instructor Blog

Flying in a new or different airplane is always fun.  Such is the case with Jacob who had the joy of flying in his friend’s Mooney.   Unfortunately, the technique that his friend used to start the Mooney’s engine left him a little confused about starting techniques so he writes in asking:

I was flying in a friends plane the other day, a Mooney Ovation 2gx. When he started the airplane he had full mixture, with his hand on the throttle and the ignition. I didn’t ask him why the plane starts that way. I fly a 172. Whats the difference? Why do you start the 172 the way you do(prime, mixture lean, then upon starting full rich)? Would a “mixture full rich” start work on a 172?  Just curious.

Hi Jacob,

Aren’t Mooneys great? I’ve had the opportunity recently to fly a Ovation 2GX once as well. Man, was it a FAST airplane. Mooney’s really do seem like the “Porsche” of the general aviation fleet. Very slick.

There are obviously a lot of differences between your airplane, a Cessna 172, and your friend’s Mooney (as I am sure you noticed). However, I am a little curious about the starting technique that you use in the Cessna.  I’m going to assume for a second that you have a carburetor equipped Cessna and every carb equipped Cessna I’ve ever flown in you DO start with the mixture in full rich.  I am sure your instructor or FBO has a good reason why you start your particular airplane leaned out, but without further information about your particular model 172,  I can’t really tell you much more.  The only exception to a full rich start that I know of is a hot start or when attempting to start at an high altitude airport.   In both cases, you want the engine leaned out to prevent flooding due to the decreased density of the ambient air.

If in fact you are not a high altitude airport or conducting hot starts, then I’m a little worried about the condition of your carburetor.  It might be worth checking out the condition of your float.   The FAA just released a Special Airworthiness Information Bulletin on October 16, 2009 that addresses this exact issue.  You can check out the Airworthiness Bulletin for yourself at FAA.gov.  The service bulletin is NE-10-05 and the subject is “Control/Reciprocating Engine – Float-type Carburetors”.    This bulletin recommends that pilots “examine the engine area for evidence of fuel leakage.  During engine start be alert for carburetor flooding or the need for excessive leaning.  Hard starting might be an indicator of a deteriorating or damaged carburetor float”   The service bulletin also warns that “improper metering of fuel or fuel leaking from the carburetor…can lead to complete loss of power or engine fires.” I’m not saying that there is something wrong with your engine (especially if you are at higher altitudes or practice a lot of hot starts) but it might be worth checking out.

aircraft carburetor

Feel free to respond with any particularities that might help us figure out why your aircraft engine starting technique is a little different.

UPDATE: Shortly after posting this article I have received several comments about the starting procedures on the newer fuel injected Lycomming engines.  I haven’t flown one in a while and forgot that yes, you do indeed start with the mixture leaned and slowly increase the mixture during the start sequence.  Why Lycomming and Continentals have different starting techniques is inherent to the design of the engine.  I’m still looking up specifics and will post this information when I find out more.  Still, if you have a carb equipped C172 and are starting full lean, you might need to get something checked out.

Fly Safe.


  1. Mitch on Nov 20, 2009

    It could be that he flies the 172 R or S version like I instruct on at my school, which is not run on a carburetor system, but fuel is delivered via fuel injection. If that was the case, the proper method to start the engine is with the mixture full leaned, and as the engine fires you smoothly and slowly advance the mixture. This is the same for the hot and cold start. The only difference between these two starts that if your engine is warm then you do not prime the engine.

    If the Mooney has a carburetor and the 172 injected, then that’s a simple explanation there on why these two aircraft are started in different manners. (However I have no experience with Mooney’s and cant speak for them)

  2. Paul on Nov 20, 2009

    I would agree with you Mitch except that he mentioned the “primer” on his Cessna (which means it has a carburetor) and I know that the Mooney Ovation 2GX is a fuel injected system.

    Hopefully he will respond with some further clarification.

  3. Eric on Nov 20, 2009

    Paul, the fuel injected 172R and S models have a “prime” sequence but no primer: fuel pump on, advance mixture to 3-5 gph, mixture idle, fuel pump off. That’s very likely what Jacob is talking about.

    The main difference as I have seen is between fuel-injected Continental and Lycoming engines: Continental starts full rich (Cessna 350 & 400, T210) while Lycoming starts idle cutoff (Cessna 172R & S, 182T).

  4. Paul on Nov 20, 2009

    Huh…ok, so maybe I need to do a little more research. Most of my experience has been with carb equipped 172s or fuel injected Continentals like the Baron and Bonanzas.

    Let me see if I can find out why the Continentals fuel injected engines start with mixture rich and the Lycomings start with mixture idle.

    ( I think I learn more than anybody on this site)


  5. Jon on Nov 20, 2009

    I agree a bit with what the others have stated here.

    I’m a flight instructor on C-172s, and our entire fleet uses fuel injected Lycomings.

    We do however use a “prime” procedure on our engine start checklist, like others have mentioned:
    – Mixture full
    – Aux. Fuel pump on for 3-5 seconds
    – Mixture out
    – Start engine, advance mixture as engine starts to fire

    I think this might be what the original asker was referring too

  6. Paul on Nov 20, 2009

    The Bonanza (and the Baron) have a similar “priming” technique
    – Mixture Full
    – Aux Fuel pump on for 3-5 secs while increasing the throttle to 17-18 GPH
    – Fuel Pump Off
    – Throttle 1/4 inch open
    – Start engine

    So the Cont. engines are with mixture RICH and the Lycommings are with mixture lean.

    After reading your comments and looking at the original question, I think you might all be right.

    The summary is this: Different engine manufacturers have different starting techniques. It’s best to read and follow the directions in the POH.

  7. Iliya Maximov on Nov 20, 2009

    TCM injection (Continental) and Bendix RSA (Lycoming) are generally different, the later more complex.
    One of the differences that I could remember is that the lycoming system fuel pump was of a constant debit and the Continental was of constant pressure. Thus there is the danger of flooding the engine on start-up, which is why you should cut-off after priming

  8. Paul on Nov 20, 2009


    Thanks for the added info. I’m not quite sure what you mean by constant “debit”? Maybe you mean a constant volume or amount?

    Either way, the theory makes sense.


  9. Jacob on Nov 20, 2009

    I am sorry for not being specific enough in my question. My plane is a 2008 Skyhawk 172SP. It is Fuel Injected. The Mooney was a 2006, I am assuming it was fuel injected as well. I have always wondered why the 172 starts like this… it seems as though it were backward. In my mind, the engine would want MORE fuel during the startup. I have done some further research and have found that, as previously commented, it is dependent on the engine setup. The IO- 360- L2A contains fuel lines that run across the top of the engine. The intial prime(and by prime… fuel pump on, mixture rich until peak fuel flow, fuel pump off, mixture lean) runs just enough fuel through these fuel lines to the engine. It puts just enough fuel in the engine to start. However, it is unfavorable to have fuel in these lines during the startup.When the engine starts, it produces a considerable amount of heat. The fuel in these lines above the engine will subsequently vaporate due to the heat. Since vaporized fuel is harder for the engine to burn, it will fail to start. To solve this problem, you start with the mixture lean. Once the engine does start, than you apply full rich.
    I will try to find some more links/ info on this. Feel free to correct me… I may be wrong on this.

    But this does bring to question… why does the engine run at all. During high temperatures in cruise, wouldn’t all of the fuel be nearly vaporized upon reaching the engine? Hmmmmmmm……..

  10. Eric on Nov 20, 2009

    Jacob, at high power settings the fuel flow & pressure may very well be high enough to prevent vaporization… that’s my best guess.

    Fuel flow into a 310 hp Continental TSIO-550, for example, is only 2gph or less at ground idle, while in flight at maximum power it can exceed 30 gph. While the Skyhawk’s 180 hp IO-360 doesn’t have quite the fuel flow at maximum power, it can still push 15 gph at high mixture settings; ground idle is probably less than 1 gph.

    So, the somewhat higher CHT of cruise is probably offset by the significantly higher fuel pressure.

  11. Best of the Web — Golf Hotel Whiskey on Dec 05, 2009

    […] If you are confused about aircraft engine starting techniques, Paul answered a query on the Ask a CFI blog from a reader whose friend used a technique to start a Mooney’s engine that had left him a little […]

  12. Raymond Zapata on Jan 21, 2013

    I have cesssna 182T 2004 , when I prim the engine for start up (3-5 gph) I notice fuel coming out of my fuel vent line at lower cowl driping fuel over nose wheel pant. Would like to hear comment.

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