Skymaster Fuel Management

[rwenner]

I just finished reading the fuel management piece by E. R. Martin, great article, now I'm wondering what the differences are for the T337B. I have just purchased it and it appears to be a very different system (according to the POH)
Can anyone tell me from experience exactly how mine works?

The POH does not mention that fuel is pumped at twice the usage rate and returned to the mains, nor does it warn that the aux pumps work only on the mains

[Ernie Martin]

I don't know exactly how your T337B fuel system is configured, but with your help we can probably decipher it.

The fuel management piece you read was not the result of extensive research by me, but rather the result of a careful reading of my airplane's POH, along with some consideration as to how it applies to various situations.

First, in your POH, look carefully at the diagram of the fuel system and also at the markings on the airplane next to the auxiliary pump switches. This will solve your question of whether the pumps work on the auxiliary tanks or only on the mains. Moreover, it will tell you if the pump is associated with an engine or with a wing.

Now, read in the POH anything related to fuel, including crossfeed operation. This will tell you if indeed fuel goes into an engine at twice the rate needed by the engine (with the excess sent back to a tank via the vapor/fuel line). To see what tank the excess fuel goes to, look again at the POH diagram for the fuel system and follow the return vapor/fuel line. I believe that you will find that indeed roughly twice as much fuel goes into the engine and that the excess from the front engine (regarless of what tank it is feeding from) goes into the left main tank, and excess from the rear engine goes into the right main tank.

If you have trouble figuring all this from the POH, you can scan all relevant pages and send them by e-mail at the address posted at www.SkymasterUS.com or send them by fax to me at 305-668-7863.

Ernie

[FRED-E]

This may help if I can get it to work
FredN358

Right click on the picture and select "Save Picture As" to a file so you can see it better.

[Frank Benvin]

Correct me if I am wrong. Can't you put the fuel selector's on aux push the mixture forward and try hitting the electric pumps If your gauges show fuel flow you can pick up from the aux tanks. If there is no flow you can't. Ours is a Non-turbo 337b Electric fuel pumps only work on main tank and all excess fuel is pumped back in to the mains whether on main or aux. Does your POH say to burn 1 hr fuel from the mains before selecting aux. The reason is so there is room for the extra fuel that will be pumped back into the mains from the aux.

Frank

[Ernie Martin]

Thanks to Fred for taking the time to post the diagram and to Frank for his suggestion.

Looking at the diagram my conclusions are as follows:

1. The auxiliary fuel pumps will draw fuel from whichever tank you have selected. In short, each pump is associated with an engine, not with a tank. This is the case for my current 337G, but was not the case for my previous 337D (which is covered in the paper cited*).

2. There is return fuel going back to the tanks, as you can see from the code legend at the bottom left of the diagram. So, an engine receives more fuel than it needs and the excess gets sent back to the tanks. From the diagram alone I cannot determine whether the excess fuel is roughly the same as the burned fuel (i.e., the engine receives about twice as much fuel as it needs), as is the case for the non-turbo 337D.

3. It appears from the diagram that excess fuel from the front engine goes to both the main and auxiliary tanks of the left side, and excess fuel from the rear engine goes to the tanks on the right side. So this is different from the 337D, where the excess fuel goes only to the main tank. Frank's suggestion to see if your POH says to burn 1 hr fuel from the mains before selecting aux would confirm this. From the diagram, I'm concluding that your POH will not say that, because fuel returns to both. One unanswered question is how much to the main and how much to the aux.

4. If you are cross-feeding (which can only be from the opposite main, never from the opposite aux), excess fuel does not return to the tank from which you are feeding. If the rear engine is being fed from the left main, excess fuel goes to the right main and right aux.

From the above, you can infer operational scenarios. For instance, if you use all the fuel from both left tanks, cross-feed the front engine to draw from the right main and the right main eventually runs out of fuel, where will you find fuel? In both left tanks (the excess that came back from the front engine while it was feeding from the right main has partially refilled these tanks), and possibly in the right aux. You also recognize that the fuel in the aux left tank is not available to the rear engine and the right aux fuel is not available to the front engine; in a very long flight over inhospitable terrain (like Brian von Herzen's Skymaster flight to Europe via Greenland and Iceland) it would be advantageous to use the aux fuel as early as possible in the flight, so that if an engine fails later, the remaining engine can have access to all of the remaining fuel.

Ernie

_______________
* In early Skymasters, including the 337D, there is a pump for the left main and one for the right main. If the rear engine is cross-connected and being fed from the left main, you would turn on the left pump, and if you were connected to an auxiliary tank, neither of the pumps would work.

[rwenner]

Thanks to Ernie, Fred and Frank for your response. I posted my question and promply got called away to a family emergency, sorry for not responding sooner.

I have attached the text portion of my POH and as you will see it does mention excess fuel return to the fuel line manifolds for recirculation to the engine driven pumps.
I take that to mean it is not sent to the tanks. Since I have not flown the airplane yet I can't confirm much. Hoping to take delivery this weekend, will report findings

Again thanks to all for the help, its great to be associated with an active, interested group of owners. I'm looking forward to meeting all at an event very soon.

Ron

[Ernie Martin]

Ron:

I think the language of the POH still leaves unclear the question of fuel return. As you can see in the diagram, there is a return line from the manifold back to the tanks, with an arrow pointing to the tanks. Is the vent only for vapors or both vapors and fuel?My guess is that some excess fuel goes back to the tanks, but it might be a minimal amount, with most "recirculated to the engine-driven pumps".

Ernie

[SkyKing]

Ron,

You might want to take whatever 'time' is necessary to completely figure out the fuel system BEFORE you take delivery of the airplane or attempt to fly it... I sense you're on the right path in this regard. However, don't jump the gun and end up like the recent purchaser of a P210: the airplane crashed, was destroyed, and the new owner-pilot killed... simply because he didn't understand the fuel system and mismanaged it.

Actually sitting in the airplane and developing a flow system in your mind from looking at the placards with the POH in-hand would be an excellent method to navigate the fuel system.

SkyKing

[bawb]

The T337B is unique. Excess fuel returned from the engine goes to a fuel manifold, which looks like an aluminum whiskey flask mounted on the side of the fuselage. The engine is taking more fuel from the manifold than it is returning. The shortfall is made up for by fuel from the selected tank. The vent line comes from the high point of the manifold, and is connected to both the main and aux tank.

Consider what happens in that manifold: Fuel is drawn out by the pump and a lesser amount of fuel is returned by the pump. There is nothing forcing the fuel up the vent line except head pressure from the selected tank. Since the vent line is plumbed into the top of the tank, under normal circumstances, no fuel returns to either tank.

With this system, there is no need to burn an hour out of the mains before switching to the aux tanks. On a maximum range trip, the aux tanks should be selected early as aux fuel from the affected engine is not available to the surviving engine if one quits.

The POH for the T337B does not mention burning fuel from the mains for any length of time before selecting aux tanks. The manual does state on 2-1: "Vapor and excess fuel from the engine-driven fuel pumps are returned to the fuel line manifolds, and the returned fuel is recirculated to the engine-driven pumps." On page 7-1, regarding the auxiliary fuel system, the POH states: "Fuel vapor and excess fuel from the engine-driven fuel pumps are returned to the fuel line manifolds, and the returned fuel is re-circulated to the engine-driven pumps." Excess fuel does not return to the tank.

This system is distinctly different than that found on most other Skymasters including non-turbo models of the same year.

[Paul Sharp]

I have the same model, and find it as described herein; easier to use with less quirks than some of the other models.

[rwenner]

Paul,

Thanks for the reply I would like to talk to you more about our airplanes specifics. If you have the time email rwenner@charter.net

[Paul Sharp]

To rwenner: I just noticed your last msg, and sent you an email with me email address.

Sorry, but I dont' get to the site regularly like I once did...

[rwenner]

Paul Sharp,

The email address you sent me does not work. return says fatal error in address
Ron Wenner