SkyMaster Wing Mods

[Alfonso]

It has the original 46 gallons main fuel tanks and the 20 gallons auxiliary, for a total of 128 gallons, no reserve. I was planning to install the AE 20 gallons wing-tip tanks, but it is on hold until the NTSB clears those tanks from the C337 crash in NJ. I am considering instead internal aluminum tanks. Any suggestions?

FUEL QUESTION: Does any one has any experience in not getting fuel to the engines when low in fuel (7 gallons remaining on each main tank) and on level flight with flaps extended (low nose)? I understand some main tanks have front and rear tank-pipes to drain the fuel into the fuel selector, and others only have one pipe in the rear of the tank. Any suggestions or explanations are appreciated. Thanks.

Are there any further news from the NTSB or FAA on the C337 crash in NJ?

Alfonso.

[skymstr02]

The main tanks all have fwd and aft outlets on all of the 337's. Both feed a small header tank in the lower boom, aft of the lift strut attach point. That is where the sump drain valve is located.
The aux tanks have a 18 gal capacity, not 20 gal, and all is usable.
If you are having problems with fuel feed, check your vents to assure that they are free and clear.

[Alfonso]

After 3 hours of flight, I used the auxiliary tanks for 30 minutes, which I estimated it used 5 gallons per auxiliary and transferred about 7 gallons surplus fuel to each of the main tanks, leaving about 7 gallons on each auxiliary tank. Is there any procedure or requirement that states that when the auxiliary tanks are used, they have to be used all the way until they are emptied? I landed with a total of 26 gallons of fuel, but I do not like hearing the engine sputtering due to lack of fuel even though there was sufficient. The C336 generally flies with the nose always high about 2 to 3 degrees up, compared with the C337. So when the flaps are extended the wing flies flat relative to the earth, and the fuel in the main tanks spreads to the whole flat tank-wing rather than depositing in the rear of the tank when the nose is up, and where the fuel pick-up pipe is located to send the fuel to the fuel control valve. I remember reading about several years ago of a Skymaster, I do not remember if it was a C336 or a C337, that crashed short of the airport due to fuel starvation even though they apparently have plenty of fuel. Any comments, suggestions, articles, etc would be much appreciated. Thanks.

[Alfonso]

You are correct, the auxiliary tan has a capacity of 18 gallons, all usable. I was thinking in the 20 gallos tip tank. Thanks again. Any other ideas are welcome. Alfonso.

[Ernie Martin]

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

[hharney]

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.

[skymstr02]

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.

[Roger]

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.

[John Hoffman]

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

[Alfonso]

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.