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  #16  
Unread 11-22-02, 08:59 AM
jcthomas jcthomas is offline
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I agree with the concept that the excess fuel returns to the manifold and not the aux tanks. If you look closely at the diagram the arrows going back to the mains and aux tanks are white. The other diagrams have a different legend or color.

My question is if are you saying you are burning 18 gallons each side of your aux tanks? Are you saying that you can get an 1 1/2 hour burn?

When my fuel starvation occurred it happened aftet I ignored the first burp from the engine. I did have an enginesputtering to a stop the second time before I switched and used the high side pump.

I am interested in your theory if you used all 18 gallons of aux fuel.

John
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  #17  
Unread 11-24-02, 04:36 AM
Kevin McDole Kevin McDole is offline
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Bob & John,

I think you guys are right.

Up through the F models, the excess fuel just circulates in the manifold. The return lines allow the vapor to go back to the tanks.

Starting with the G models, the excess fuel and vapor is returned to the tanks.
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  #18  
Unread 11-25-02, 06:28 PM
bawb bawb is offline
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Okay, so why can't he use all the fuel? What would be the easy things to test for? It reminds me of a C-150 I had that would quit after you'd burned about half tanks. Turnded out the vent was plugged with a dreaded mud dauber. Vacuum on top of the fuel increased as fuel was used until gravity was no longer able to feed fuel to the motor.

If this is happening to just the front engine, check the vent line by blowing in the vent tube with just the aux tank cap off and listening at the tank. Or, check the length of time required to drain the aux tank via the strainer and compare it to the rear engine. Does all the fuel drain out of the tank?

If none of that helps, we can check more difficult items.

Bob
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  #19  
Unread 11-26-02, 08:15 AM
jcthomas jcthomas is offline
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BAWB

Good idea. I do not think this is my problem because there is only one vent tube. The main tanks work fine. So we have to dig a little deeper on any thoughts concerning the use of fuel in the aux tanks.

John
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  #20  
Unread 11-26-02, 01:44 PM
bawb bawb is offline
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John,

It's not very likely, but the vent line could be plugged between the aux tank and the T with the main vent. Since it is so easy to check for, it would be worth ruling out. It's always nice to know the vent system is unobstructed anyway.

Bob
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  #21  
Unread 11-26-02, 05:33 PM
Paul Sharp Paul Sharp is offline
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Well I said earlier that my excess fuel went into "the mains." Actually it probably goes back to the "manifold." But that doesn't mean it is all "used" again by the engines. If that were the case you'd never have to worry about burning some fuel out of the mains before switching to the aux. tanks. My '67 turbo (I'm at work and can't look at the diagrams) is probably the same, and if you have full mains and switch immediately to the aux. tanks you won't get full use of the fuel because the excess (that not burned by the engines) goes bye-bye. Once an hour's worth is gone from the mains you can safely switch because no matter how much goes back into the mains it can't overflow them since you'll always be using some out fo the aux. tanks. And it never goes back into the aux. tanks.

I customarily get aroun 1.5 hours on the aux. tanks, running around 72% power settings.
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  #22  
Unread 11-26-02, 08:53 PM
jcthomas jcthomas is offline
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Paul!

Looking at the diagrams, there is a difference between a 67 Turbo and 69 Turbo. But, I think the only defference is the type of fuel guages we have. However, when I look at the diagrams, (sheet 2 and 3 of figure 11-1) the legend does not show any fuel returning to the vent lines. If you look at the legend on the NA 337 on the same page, it is clear that the excess fuel can go to the vents.

Please see if you agree with my view on this. I am pleased that you are getting 1.5 from your aux tanks. I would like to be able to do the same.

I still haven't had the chance to try and see if any fuel will come out of the vent lines if I switch to aux tanks after start-up. This is a challenge which I would like to know the answer. It sure would make for safer flying. although, there is nothing wrong with the 1 hour rule and witch to mains for one hour and back to mains. It works all of the time.

John
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  #23  
Unread 11-26-02, 09:18 PM
jcthomas jcthomas is offline
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Paul and BAWB!

I started to read the manual---it helps to read!!! It indicates I need to check the vent lines at the wing tips for the turbo. Bawb may be right on the issue of not getting all of the fuel.

Even though the diagram indicates something different for excess fuel, the written comments would seem to contradict the diagram.

It gets more challenging every time I step forward.

JOhn
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  #24  
Unread 11-30-02, 03:01 PM
bawb bawb is offline
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Nowhere in the POH for my '67 Turbo is it stated that you need to burn an hour out of the mains before using aux fuel. Other models differ significantly.

Bawb
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  #25  
Unread 12-01-02, 08:42 AM
jcthomas jcthomas is offline
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Bawb!

I keep going in circles trying to figure this thing. I agree with you that you do not have to burn one hour in a turbo. Does that mean you are still going to pump the excess overboard or is the manifold going to get it back and burn it? One thing for sure is tha nowhere does the diagram or the written explanations demonstrate that excess fuel, if any, will return to the mains.

John
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  #26  
Unread 12-02-02, 09:10 AM
bawb bawb is offline
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Just as a curious sidenote: Have you ever looked at the manifolds? They look like aluminum whiskey flasks made in high school welding shop. The front engine's is located inside the cabin on the port side about in the middle. The rear's is in the main gear well.
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  #27  
Unread 12-04-02, 11:36 PM
Paul Sharp Paul Sharp is offline
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Well, I got curious so I did some research in my Maintenance Manual. I have a 1967 T337B model - turbo with aux. tanks, 128 gals. total.

My POH says basically what others have said here, BUT the maintenance manual gives a clear diagram of the system. And the description, in paragraph 11-6 (Turbos) of the maintenance manual, says:

"Fuel vapor return lines return vapor and unused fuel from the front engine-driven fuel pump into the front fuel line manifold, where the fuel is recirculated and the vapor is returned to the left fuel tanks."

Then is has a similar sentence, with "right" where that one says "left."

Now I don't know why it sort of separates the word "vapor" in that sentence (it doesn't do that in the description for the non-turbo A and B models - it simply says the fuel and vapor is returned to the main tanks and the manifold).

BUT if you look at the diagram for the turbo models (through B series), it CLEARLY shows that the return lines go to both the manifold AND main AND aux. TANKS. So that means what I've been told from the time I bought my aircraft may be incorrect: "Burn about an hour of fuel from the mains (if they are full) before switching to the aux. tanks because otherwise the excess will be vented overboard." It could be that advice came because of what the diagram shows for the non-turbo models (through B series), in which case it shows the return vapor line going ONLY to the main tanks and not the aux. tanks also; in that case I can see where that old piece of wisdom applies.

In my case if it returns to both mains and aux. on each side I can't see why it would make any difference whether you burned fuel from the aux. first or mains first (assuming both are full to begin with).

But I beg to differ from those who claim that they don't see how it could get back into the tanks. On their models that may be so; models differ over the years and their diagrams and models may be totally diff. from mine. But on the A and B turbo models it shows venting back to the tanks as well as the manifold.

There is a discrepancy between the POH and the service manuals. The POH says nothing about feeding back into the tanks but only the manifold.

None of this yet answers the question for the 1969 turbo, though. If the jcthomas' 1969 model is a "D" then the only diff. I can see in the diagrams for that year turbo model is a microswitch, evidently fow switching a single gage from main tank to aux. tanks.
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  #28  
Unread 12-05-02, 08:22 PM
jcthomas jcthomas is offline
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Hi Paul!

My 11-26-02 response agrees with you about the only difference between a 67 and 69 turbo.

Looking at the diagrams and the differences between the non-turbo and turbo is clear as far as the legends that are used. What I mean by that is the NA 337 shows in the diagram, clearly, a speckled tube mark, which indicates fuel returning back to the mains. The turbo diagram shows a clear tube legend which indicates only vapor.

Then you have the confusion o 11-6. I believe we can fly our auxs without burning one hour. HOwever, I feel why not burn the hour off and then you have now worries.

I would like to know the answer to the puzzle. It would be nice if someone could give an exact explanation of what Cessna means.

John
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  #29  
Unread 12-06-02, 02:51 AM
bawb bawb is offline
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Hello all!

Regarding the '67 T and I assume John's '69 T: Excess fuel from the engine driven pump (fuel not consumed by the engine) returns to the manifold from which the pump is simultaneously drawing more fuel than it is returning. The difference is made up from the selected tank. Return fuel does not go up the vapor line and into the tank. The vapor line comes off the very top of the manifold allowing any vapor to work it's way into the vent system. The vapor line off the manifold has no check valve nor any means of separating fuel from vapor. As the manifold is located well below the level of the tanks, head pressure from the selected tank will force fuel up the manifold vapor line to equal the level of the top of the fuel of the selected tank. However, the vent lines are above the highest fuel level, so when everything is right, no fuel should return to a tank. Since the main is slightly higher than the aux, head pressure from a full main would send liquid fuel a little higher up the vapor line but not enough to reach the tee and get into the aux.

One good reason to burn the aux early in the flight with this system is on a long flight over hostile or wet terrain, like Galveston to Belize City, should an engine failure occur, the fuel in the aux tank is not available for crossfeeding.

None of this answers the question of why John's engine quit and why there was 6 gallons in the aux tank. John, have you had a chance to go out to the plane and do any investigating?

Bawb
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  #30  
Unread 12-06-02, 05:13 PM
EZCOPE EZCOPE is offline
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aux fuel burn

Somewhere I read or heard about the fuel return from the engine driven pump returned fuel to the respective main whether using main or aux fuel. So far the only picture I found showing that is from an O2 manuel.

I plan aux fuel to use for one hour, and get itchey after 50 minutes to return to mains, but have ran the auxes dry and switch back to mains as soon as I see a fuel flow twitch. This also depends on the power setting you are using. I usually burn 8-9 gph per eng. My longest bladder test has been 5+50.
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