On your question on whether the auxiliary tanks have to be used all the way until they are empty, I know of no such requirement. If the plumbing of a 336 is similar to the early 337s, it is, however, a prudent move because if a main goes dry you can't get to whatever fuel remains in the auxiliary (for more see the Fuel Management page at www.skymasterus.com).

Ernie

In the C336 Service Manual it states that the auxiliary fuel tanks capacity is 19 gallons of which 18 gallons are usable. See the first attachment.

The main tank system (two interconnected tanks) have a capacity of 46.4 gallons each. If the C336 has 128 usable fuel then this would allow the mains to have 46 gallons usable each. Leaving 0.8 gallons unusable. See second attachment

Third attachment is the C336 fuel schematic.

The main tanks have an aft connection and a forward connection on the inboard main tank. The outboard main tank is interconnected to the inboard main by an upper and lower front connector and a lower aft connector. These connections are depicted on the schematic attached. Here is a photo of the tank http://www.337skymaster.com/messages...ead.php?t=2309


As pointed out on Ernie's site, when on extended flights the auxiliary fuel tanks should be use as soon as possible (60 minutes from departure) after mains have been used at least 1 hour. This is because the auxiliary tanks can not be used for landing. The electric fuel pumps will not provide fuel from the aux's if a engine driven pump fails and the aux's cannot crossfeed to opposite sides for front / rear engines. Therefore, I always use my aux tanks as soon as possible on long trips. Keep all the fuel you can in the mains for emergency issues that may arise.

It would be best to use all of the aux fuel because you cannot use the fuel boost pumps with the aux tanks for landings or takeoffs, if needed. Should an engine driven pump crap out, you'll loose the engine if the electric pumps cannot push the gas thru.

I select one aux tank, wait about 10 minutes, and switch the other tank over to aux. After about 55 minutes later, wait for the first engine to surge like its running out of gas, and switch back to the mains on that one. About 10 minutes later, the same with the opposite engine. Running an aux tank dry is acceptable and the only way to know that all the useable gas is out of it.

Gas that is in an obtainable tank serves no purpose.

One of the major mishap causes on the 336/337 models is fuel mismanagement.

I am surprised that no one has developed an STC to just hook all of the tanks together, like in my 1980H model. Back when I had my 1970 model it was always time this, wait for that then turn this one, then hope the 5 way ball-cock worked when you ran the aux dry (i.e. hope for no vapor lock). It is so much safer to fly with the interconnected tanks.

Linked tanks
 

Hooking up all 3 tanks must have started with the -G models, my 1973G is that way - no switching.

Tanks Interconected
 

ROGER AND JOHN HOFFMAN:

Your news is a great news. Please could either of you email me or post here all the drawings and information on how the tanks are interconnected. I would like to see it, and I will consider installing or implementing it on my C336... it definitely is a safer operation. Please, you may email it to: fiestair@erols.com. Thanks a million and best regards, Alfonso.

Forget it, Alfonso. There is no STC and trying to adapt the design first implemented in 1973 on a different aircraft -- because the 336 and the 337 ARE different -- would be hugely expensive to be legal (i.e., after figuring out how to do it, it would require, as a minimum, engineering certification that the design is safe).

Your current design, on the other hand, is safe and effective (my '69 337D had it). Yes, it requires adhering to procedure, but it works fine if you understand the system and how to use it. That's what my Fuel Management page does at www.skymasterus.com.

Ernie

Could someone summarize to a neophyte like myself which Skymaster versions had the simplest fuel management? Were the tanks interconncted from the -G model and simplified their use?

It is my understanding that from 1973 and later (the G and H models) have the simplest fuel systems. Others please jump in here and either confirm or correct me as I am not positive.

My 1973 G model has the tanks in each wing all interconnected and under normal operations there is no fuel management. The right wing feeds the rear engine and the left wing feeds the front engine.

It is my personal procedure before engine start up to "exercise" the fuel selectors by rotating them throughout their full range of positions because otherwise they would never move. I also periodically cross-feed on the ground (right wing to front engine and left wing to rear engine) just to make sure the system works as designed and the engines continue to run in the cross-feed configuration.

The only other consideration with this fuel system is that because the tanks are interconnected with relatively small (though adequate) hoses, these models must be fueled SLOWLY!!! Most fuel pumps pump fuel much faster then the tanks can accept it. As a result the outboard most tank, which has the fuel port, fills quickly and gives the appearance that the system is full. IT WON'T BE! If you sit and watch for a few minutes, you can watch the level in the outer most tank dwindle down as the fuel gradually makes its way to the inboard tanks.

Ed

Good summary, Ed. I exercise the fuel selectors on the ground less often, maybe every 3 months, and have never found them frozen, provided you grease the mechanism at the wing root at each annual. And sometimes, on solo flights, I excercise them in flight, always near my destination airport, in case there is a problem.

Ernie

Jim Stack once commented that a new fuel selector was $1200.