Comments on: Aircraft Engine Starting Techniques

By: Best of the Web — Golf Hotel Whiskey

Best of the Web — Golf Hotel Whiskey — Sun, 06 Dec 2009 03:07:14 +0000

[...] 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 [...]

By: Eric

Eric — Sat, 21 Nov 2009 04:09:26 +0000

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.

By: Jacob

Jacob — Sat, 21 Nov 2009 00:20:27 +0000

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……..

By: Paul

Paul — Fri, 20 Nov 2009 22:38:27 +0000

lliya,

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.

Paul.

By: Iliya Maximov

Iliya Maximov — Fri, 20 Nov 2009 21:37:18 +0000

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

By: Paul

Paul — Fri, 20 Nov 2009 18:53:33 +0000

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.

By: Jon

Jon — Fri, 20 Nov 2009 18:43:18 +0000

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

By: Paul

Paul — Fri, 20 Nov 2009 17:27:54 +0000

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)

Thanks.

By: Eric

Eric — Fri, 20 Nov 2009 17:16:46 +0000

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).

By: Paul

Paul — Fri, 20 Nov 2009 16:56:30 +0000

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.