Aux Fuel

[jcthomas]

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

[Ernie Martin]

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

[jcthomas]

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

[Paul Sharp]

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.

[jcthomas]

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

[FRED-E]

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

[jcthomas]

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

[Ernie Martin]

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

[jcthomas]

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

[EZCOPE]

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! :-))))

[Paul Sharp]

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.

[jcthomas]

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

[FRED-E]

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

[Kevin McDole]

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?).

[bawb]

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