Fuel systems

[Dale Campbell]

I see this same thing happening to many times. Not having enough fuel on board or not knowing what tank has the fuel. That is why I purchased the 337H latest model that has all tanks in each wing manifolded together to act as one tank in each wing.
I also keep way more fuel in each tank than I think I need. Just in case I have to devert to another airport or maybe that airport has no fuel and you have to go somewhere else. That has happened twice in the last few years. I keep no less than 60 gal. on board and most of the time it is 90-100 gal. I also installed a Shaden fuel monitor and check tanks often.

[Francisco]

[snip]See thread "337 Down In Utah " for the rest of the text of this message. -webmaster

PS can a skymaster be retrofited with the fuel system of the H model?

Francisco

[Ernie Martin]

I've owned Skymasters with both fuel tank designs, a '69 337D with separate auxiliary and main tanks, and my current 337G with long-range tanks but all tanks in each wing manifolded together to act as one tank. Without a doubt, the older design is far more difficult to operate. It's critical that you understand the design and know how to use it.

Because mistakes here could be life-threatening, I wrote a web page for owners of these aircraft (go to www.skymasterus.com and click on "Fuel Supply Management"). To give you an idea of potential problems, you can never switch to an auxiliary tank after running the main dry (as seemed to have happened in this accident), because the electric fuel pump doesn't work on the auxiliary tanks. And auxiliary tanks run dry twice as fast (so an engine using 9 gph will drain an 18 gal aux tank in one hour, not two).

To Francisco's question on whether a skymaster be retrofited with the newer system, my answer is two-fold. First, if it's doable it would be enormously costly, because there were lots of design changes in the entire wing tank system and would likely require a large engineering effort to STC the change to retain certification. Second, the older design, although more complex and harder to master, is actually more versatile and offers the pilot greater flexibility and, interestingly, potentially more range (or the same range with less risk).

Ernie

[kevin]

I'd like to second Ernie's comment on fuel systems. Both the old and new have advantages (having owned both). The new is simpler, and therefore potentially safer. However, the new has a disadvantage that the old system lessened. With the old system, it is easier to determine, with a dipstick, how much fuel is in the airplane, or rather, that there is enough. This is because you have a second fuel cap on each wing for the aux tank. On the new system, the tank can have 40+ gallons in it (for a total of 80 gallons in the aircraft), and it will look dry. On the old, you cold dip the aux tank and the main, and get a better idea of how much fuel is in the airplane.

Which would I prefer? I think the new, but the race is close. Simplicity probably wins out for me, but it was very nice to be able to manage the fuel load (on the ground) more easily with the old. Fill the mains, and not the aux, and you know you have a defined amount of fuel for a lower fuel load. And while flying, the amount of time you got out of the aux tanks before they ran dry provided a confirmation of your fuel burn rate. But those advantages are probably not worth the hassle of switching tanks on each flight, avoided entirely with the new system.

Kevin

[Francisco]

I have study Ernie's fuel flow chart and memorized it last year. it realy tough me about the fuel system.

Experience is what really counts.

Last Monday I decided to go to Diamondhead airport for fuel ($2,30 PG vs $3,18PG At Gulport.
On the ground at Gpt the gauges read over 1/4 fuel on the mains and the aux were full. Visually the main tanks showed about 1.5 inches (from the botom of the tank) when mesured with the stick. The aux were at the top.

As we level off at 2000 I noticed the needles were showing empty, or very closed to it. I decide to switch to aux and did so by turning the fuel pumps on and everything worked like the book says. As I started my decend to Diamonhead, I was concerned because I kept asking myself "will there be fuel on the mains or should I land with the aux"?
I remember my fuel burn since I last filled the tanks, were 3.2 Hours or "CLOSE TO IT" ( that is what kills pilots) the "close to it" Part.

Any way, I swiched back to the mains by the book and landed uneventfully. What realy surpice me was that it only took 25 galons each main, which tels me I had plenty of fuel considering the overflow from the aux.

Oviously my gauges are not calibrated properly, but most scary is my ignorance over the amount of fuel I carry, burn, think I have, etc

The more I fly the more I will learn. I sure would like to have a fuel totalizer in the plane. (and about 200 more Hours as PIC)

Just today I have learned something new. Ernie states that a main tank should never be run drie and then switch to the aux, and that makes sence since the aux can't use the buster pumps. but I would like to know if any one has ever switched to aux with out turning aux pump on. THEN WHAT HAPPENED? Does the engine quits? doe it get vapor locked?
have any of you landed on the aux tanks?
And what happens if you cross fed the tanks?

Food for learning!


Francisco


It's Geting complicated

[Ernie Martin]

Francisco:

I'm not sure I understand the questions, but let me try to answer them.

If you run a main dry and go to the aux, you can't get flow. Turning the electric pump on doesn't help, because it's sucking from the dry main tank which anyway is no longer connected to the engine (the aux tank is). So the engine will windmill but produce no power.

If you're running good on the main and switch to the aux, uninterrupted flow occurs and the engine keeps running without even burping.

If you run a main dry and switch to the opposite main, you can get flow by turning on the electric pump for that side. (Careful: the pump will be forcing fuel from that main into both engines, so I prefer to put that side on its aux tank first before turning on the pump, to avoid any chance of flooding my working engine!); so if F engine sputters because L main is dry, my procedure is R engine to aux tank, wait several seconds to confirm good flow, F engine to cross-fed R main, turn R main electric pump on for a few seconds until the F engine re-starts, and then you can switch the F engine to the L aux without burping).

You should never land on aux tanks. First, because the book says so, second because there's no accurate way of knowing how much fuel is on that tank (remember it's draining at roughly twice the normal rate), and thirdly, I believe that the supply lines are smaller diameter and will not support the flow needed for full-power if you need to do a go-around (although I have not confirmed this last point, but I've heard it). Having said this, in an emergency -- like a leaking main tank that can't hold fuel -- I think you could do it safely. Generally, however, you should land on the main, even if its near empty: just run on the aux circling over the airport for an extra 5-10 minutes, and then when you're ready to get on the pattern switch to the main which will now have in it fuel that will last roughly however long you were on the aux tank -- incidentally, this is part of question #3 in the quiz at the end of my fuel management web page.

Ernie

[Francisco]

Ernie, Does't the mechanical fuel Pump draws fuel from the aux tank?

Francisco

[Mitch Taylor]

The mechanical pumb will draw from the aux tank, but if there's air in the lines (because you ran the main dry) it will take quite a while to get fuel flowing again. The system I use when flying longer trips it to take off and fly the first hour on the mains. After that hour, switch to the aux tanks for an hour. At that point, switch back to the mains. The return flow from the aux tanks will have topped the mains back up, and you will have a small reserve remaining in the aux tanks. The Air Force dash one says to run the aux tanks dry, then switch back to the mains, but I don't like things getting quiet suddenly. As far as landing on the aux tanks, there are two reasons not to. First, if the engine stutters, no boost pump. Second, there isn't a sump on the aux tanks, so you can unport the fuel pickup in a turn or bouncing around.

[Ernie Martin]

Mitch suggests that the mechanical pump will eventually draw fuel from an auxiliary tank after you've run the lines dry, with air bubbles in them. It's the first time I've heard of it. I'm under the impression that the mechanical pump will not draw fuel, which is why the POH makes it clear that you should draw from the auxiliaries in the middle of your cruise (not at the start, when the mains are full, and not at the end, when the mains are dry). Even if Mitch is right, who knows how long it takes. I think the safest approach is just to assume that it won't work.

Ernie

[Francisco]

THe possibilities are growing. For starters my 336 has sumps on aux tanks. I wonder if amy one (At Cessna During the Development of the design) has tested the possibility of starting the engines with the mechanical pump? and how long would it have taken? I certainly are not traying it, but would be nice to know. Say it takes 4 min or 15 min. that will tell you how high you must be.

Francisco

[Paul Sharp]

Just remember, for purposes of this thread, that all of the models aren't the same, beyond just the main/aux. and all-in-one difference.

My model will draw fuel just fine with the fuel pumps even if you run a tank dry. Doesn't make any diff. if you run it dry or switch when things start sputtering except for the extra seconds to start if you let the line run dry before turning the switches.

And far as I can tell there is no such thing in my particular model that requires the mains to be run down an hour before you can switch to the aux. and not lose fuel overboard.

So they're not all the same.

[kevin]

What model do you have Paul?

Kevin

[Ernie Martin]

Yes, Paul, what model and also do you have separate auxiliary tanks -- meaning that there is a separate cap to fill them and that the fuel selector valve has a separate position for the auxiliary (i.e., main, auxiliary, cross-feed, off)?

Ernie

[hharney]

1968 337C (IT'S NICE TO KNOW THE MODEL)
TWO MAIN TANKS
TWO AUX TANKS

THE FIRST THING I LEARNED FROM MY DAD ABOUT FLYING WAS VERY SIMPLE.

FILL THE TANKS BEFORE YOU FLY
WHEN YOU COME BACK, FILL THE TANKS
THEN THEY ARE READY WHEN YOU FLY NEXT

MY SKYMASTER WILL FLY WITH FULL TANKS AND 4 – 5 PEOPLE WITH NO PROBLEM. WHY EVEN QUESTION THE LEVEL. WHY BUY EXPENSIVE GADGETS IF THE TANKS ARE ALWAYS FULL. IF YOU CAN’T AFFORD TO FILL THE TANKS, YOU BETTER SELL THE PLANE.

OK, NOW I AM OFF OF MY SOAP BOX
ON LONG TRIPS, MORE THAN 2.5 HOURS, I WILL FILL ALL 4 TANKS
START WITH THE MAINS FOR 1 HOUR (TO ALLOW ROOM FOR THE RETURN FUEL FROM THE AUX’S WHEN THE AUX’S ARE SELECTED), SWITCH TO AUX, WITHOUT USING THE BOOST PUMPS (NO NEED TO)
SET THE TIMER ON ONE HOUR AND WHEN THE HOUR IS UP WAIT FOR THE FIRST ENGINE, ALWAYS THE REAR ON MINE, TO SPUTTER AND SWITCH BACK TO THE MAIN. SOMETIMES I WILL GIVE IT JUST A QUICK BOOST TO HELP THE FUEL TO THE ENGINE. THE FRONT IS USUALLY ABOUT 3 MINUTES AFTER THE REAR. SAME PROCEDURE, I HAVE OPERATED LIKE THIS FOR 25 OR SO YEARS. NOW THAT I AM FLYING MORE IFR I LIKE TO KEEP 10 GALS IN EACH AUX. IF PERHAPS I NEED TO HOLD OR FIND THE ALTERNATE I HAVE 25 – 30 MINUTES ON THE AUX’S TO USE AND THE REMAINDER IS FED BACK TO THE MAINS. I NORMALLY WILL USE THIS 20 GALS ON SHORTER INTERMEDIATE VFR FLIGHTS JUST TO ROTATE THE STOCK.

GENTLEMEN, WITH THE SKYMASTER THERE IS NO REASON TO TAKE OFF AND FLY ON THE BOTTOM HALF OF THE TANKS. I JUST WON’T DO IT.
FILL EM’ UP!

hi hharney, i agree with you. fill the tanks. the only time you have to much fuel is if you are on fire. i always take off with full tanks. most of the time there is 1 to 3 people with me. i also make
2 to 3 hr trips,and my tanks are full when i take off. i think the plane flys better when its heavy. when i switch to the aux tanks i have my finger on the pump switch just in case it coughs (never has).i don't like it when thay cough.

thanks
rick