Aux Fuel
 

I own a 1969 T337. While flying on the aux tanks, went a little longer than one hour and heard the front engine begin to suffer from fuel exhaustion. Switched tanks and everything went according to plans.

Question which is being debated is, Why in a Turbo with 18 gal aux tanks do you only get one hour of fuel? Keep in mind that the diagram of the fuel system in a Turbo is different from a NA 337. The diagram does not show the excess fuel returning to the mains. Further proof of that is when we filled the aux tank each only took 13 gallons, meaning there was still 6 gallons in each tank.

My partner and I debated this issue and his logic seems better than mine. I always thought the excess retuned to the mains, but it doesn't. The proof is that upon fill-up of the auxs' they only take 13 gallons each.

Anyone have any ideas or know more about it?

Thanks

John

Nice puzzle. I know a bit about the fuel system of normally aspirated 337's (see the Fuel Supply Management link in www.SkymasterUS.com), but have not seen the diagram for Turbos.

Does the diagram show the excess fuel and vapors going back to the auxiliary tank in use? It seems that way if you only consumed 12-13 gal in one hour. Two things occur to me. First, see in the POH if the USABLE amount is only 13 gal (I know it's far fetched to have 5-6 gal unusable per tank, but it's worth checking). Second, might there have been some debris which floated into the tubing shutting off the flow? It would be interesting to check flow out of this aux tank on next flight.

Ernie

Hi Ernie!

First, we have checked this twice and found there is 6 gallons left in each tank (aux) when we re-fill. The POH says "18 gallons useable in each tank." It also says,"...any excess fuel and vapor from the engine driven pumps is retruned to the fuel line manifolds." The returned vapor passes throught the fuel line manifolds to the vent lines and is routed overboard. The excess fuel passe4s into the fuel line namifold and is returned to the dengine driven pumps."

the POH also indicates that there is 19 gallons in each aux tank with 18 being useable.

I am still confused by it. It would be nice if you could squeeze the extra 6 gallons out of the aux tanks. I think the one hour rule is probably the controlling factor.

If you think of anything else, let me know. If you remember the last time we visited the aux issue, I lost a bet on the aux tanks because I told mypartner about the procedure for a NA 337. He then pointed out the difference in the diagrams between the Turbo and the NA 337.

Interesting problem here. I would like to know why.

JOhn

That does sound strange. My 67 Turbo does the same insofar as returning excess fuel into the system - which means the old caveat about not using the aux tanks until you've burned at least an hour or so off the mains (assuming the mains were topped off) holds. But having the aux not burn it's full amount does sound like a new mystery item.

Paul!

Are you saying your turbo returns the excess to your aux tanks? Or do you not have any excess left in the aux tanks when you refill?

John

Fuel return
 

If you look at the fuel line schematic for a 67 turbo you will see that the excess return line is Y'ed and returns to both aux & main tanks at the same. I am not sure how that works but it mite explain how the aux gets burned off and then after switching to mains, refilled with 6 gallons. I tried to attached a schematic but I couldn't get it to work. If you don't have a schematic (service manual) let me know and I will E-Mail you one.

Fred N358

Fred!

I do have the shematic of the fuel system. I can see how the fuel is transferred. I just do not understand the reason you have fuel left in the aux tanks which is not useable.

John

If I follow these last messages, and excess fuel is Y'ed and goes back both to the main and aux tanks, then the 6 gal always left in the aux makes sense: it's excess fuel that was put there AFTER you switched back to the mains. The problem now is this: what happens when you take off with full fuel tanks? Is the excess arriving at the aux tanks blown overboard? That can't be, so there must be more than meets the eye here.

Ernie

Let us assume the excess does go back to the aux tanks, why is it not useable? The POH says 18 gallons of the 19 in the aux tanks is useable.

From the diagram, you cannot tell if excess go to aux or main tanks.

If I recall, when this occured, I had switched from the aux to main after one hour. Later in the flight, My partner said switch back to aux and use the other fuel remaining in the aux. Shortly after I switched, that is when the fuel ran out. So, in theory, if I had use one hour of aux, and the excess went back to the mains back to the auxs, it would appear there should have been enough fuel to fly a half hour longer on the auxs.

This is a very interesting issue. I really would like to get the answer to the puzzle. I would like know what is going on when I am using the aux tanks.

I do really appreciate all of you offering your thoughts. Hopefully, we will all be able to come to a common conclusion.

Let me know anything else you can think of.

John

AUXFUEL
 

Hi,
The aux tanks on my 337B- N5476S - have gone to near empty on long flights, and can see the fuel flow start to flux at empty. also as others have said the overflow from the engnie driven fuel pump vents to the respective main tank. Your can see the main tank fuel qty increase as you burn the aux fuel - if the gauge is working. The POH says use fuel from the main for 60 minutes before using the aux fuel - if you started with full mains.
The 128 gals useable gives longer flight legs than my bladder can
handle! :-))
I've had our friendly Skymaster since 1985, and plan to keep it as long as I can fly! :-))))

What I was saying is that on my '67 turbo, when I burn from the AUX tanks, the excess retunrs to the MAINS. That's why I always burn at least an hour from the mains befor using the aux tanks; that way the excess doesn't just get dumped (and thus wasted) out of the overflow tubes into the air.

Excess Fuel?
 

Well, I am glad I put this question out for us figure out the problem. I have examined the turbo fuel diagram in the Cessna Service manual. I do not believe there is any excess fuel being dumped back to the mains. Examining the diagram, it appears the excess returns to the manifold and is used. This is my humble opinion.

I have already lost one dinner on a bet with my partner on this subject. I am going to try a test tomorrow. I am going to fill all of the tanks and start the engines. I will switch to the aux tanks and see if any excess fuel will be dumped over. I need to see if it is true that it the excess goes back to the manifold or does it go back to the mains.

Maybe I should wait to hear from some of you before I try ths. Of course, I will do this on the ground.

Let me know your thoughts.

John

Retract Comment
 

I would like to retract my earlyer comment about the Y'ed return line. It is the vent line that is Y'ed not the return line, sorry about that, so I still don't know why you can't burn that other 6 gal.
Fred N358

Fred,

From my novice reading of the diagrams, I think you were right the first time.

It looks like the Turbo 337 up through the D model share the vent line with the return line. And it is "Y'ed" to both the aux and main. That means that the return fuel may go into either the aux or main tanks (with no actual control over it).

All other models (throughout the years) pipe the return fuel into the main tank (regardless of whether the aux or main is in use).

I have been pondering this problem since it has been posted and my best guess is that the 6 gallons present in the aux tank at fill-up time was simply the returned fuel making its way into the aux tank when John was feeding from the mains. I realize his data doesn't really support this theory.

John,

Some questions that might help to get to the bottom of this are:

How much fuel was actually present in the aux tank when the engine because starved for fuel? Doing a little bit of flight planning, would you predict that only 12 gallons were used when the engine faltered? Of course, something like a Shadin would make this really easy to determine.

When you filled the mains, did they take the amount that you would have predicted (or did they take 6 gallons less?).

If I'm following this correctly, this was a single event where the pilots had been flying on the aux tank for about an hour and "heard the front engine begin to suffer from fuel exhaustion." Now we are trying to figure out why the engine quit but six gallons are still in the aux tank. I'd like to suggest the possibility that the engine was not about to quit, but the pilots were anxiously expecting it to. When I run a tank dry on a long flight, I wait patiently until there is absolutely no doubt that there is no more fuel coming out of that tank and there is 0% power from that engine for at least 15 seconds. Usually I can use up most of the "unusable" fuel. I've put 19.2 in my aux tanks. I also select aux tanks at least 30 minutes apart so both engine don't quit at once.

My '67 T returns unused fuel to the respective manifold and then back to the engine. It would take an unlikely malfunction for that fuel to make it back to a tank.

I believe the D model is plumbed like my B model, but can't say for certain.

I'd suggest running the test again. If the engine does really, really quit, go to the main and land and report back.

If you are reluctant to let the engine quit completely, just think back to your multi training when you routinely shut down, feathered and secured critical engines in those lopsided conventional twins. Skymasters excel at single engine flight. Switch to a good tank and they light right up.

But, I run lean of peak, with 6 quarts of W-65 in temperate weather, so what do I know?

Bob

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

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.

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

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

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

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.

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

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

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

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

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.

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.

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

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

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.

fuel system education
 

Hi Bunch!
After all the chat back and forth I started reading more manuels. My 65-73 maintenance manuel shows that the nonturboed models - thru 337D - return excess fuel to respective mains, the turbo diagram - as several have stated - goes thru a "y"and could go to either main or aux tanks.
The O2 setup is same as 67/68 models.
I don't fly a turbo model, so have no practical experience with the related fuel management proceedures.

WHOAAA!!
 

After 3 hours, my bladder says, when you gonna land!!
Reminds me of a joke from AVWEB

Pilot: How long can we expect to hold
ATC: Is there a problem?
Pilot: Do the words "Daddy I have to go to the bathroom" mean anything to you?
ATC: You're next for the approach.

Larry!

The question is not how long one can fly, but it goes to the issue of turbo aux tanks. It would be good to know how much fuel one has especially if you get caught upstairs in a sudden weather change. Of course if you don't get caught in any stuff like that, then the qestion is moot.

I guess I am puzzled by the way the Cessna service manual confuses everything. I, as I am sure you are as well, would like some clear material from them. I look at the diagrams and the the discussions in 11-6, etc and the manual seems to contradict itself.

I just keep flying mine for the time being like it is a NA 337. It doesn't hurt and works every time. But if I knew the correct answer to our puzzle, I would be a little more sure about how much fuel I have.

But your right about the bladder and in addition to that, how about a little fatigue honing in on a long flight.

John