auxilliary tank plumbing practices

[Hargraves]

Auxilliary tank plumbing practices. Just brought an SS4 storch with standard twin wing tanks and standard 38 ltr centerline auxillary tank. The fuel system is plumbed in what I belive is the standard method and requires actuation of the auxilliary transfer pump,to pump fuel to the main tanks when required. While I understand the advantages of an overhead gravity assisted primary delivery system particularly on an aircraft who,s auxillary tank's pick up is 5' lower than the mains, the fact that fuel can not be transfered without the use of the transfer pump in any cicumstance leads me to question why it would not be a better option to have a fuel delivery system that delivered direct to the engine from the auxilliary when required via isolating the mains fuel taps and the procedure decaled suitably. The current system allows fuel in excess of an individual wing tank,s capacity to be dumped outboard via the tank vent this is problermatic due to the inherent yinaccuracy of the wingtank gauges in flight and the larger capacity of the auxillary tank. Any assistance and advice is most wecomed

 

 

[Downunder]

Sounds sensible, mains draining into Aux under gravity and the aux feeding the engine.

 

If you want to isolate the aux (from both the main tanks), you will need to vent it somehow, allowing air in when fuel is used.

 

That also presents the potential problem of removing the air when filling from main tanks.

 

You might want to look at what height the Rotax (?) mechanical pump is drawing from to the bottom of the aux tank.

 

I imagine Rotax have a spec for it. Could it be too low?

 

Especially if a tail dragger with high mains.....

 

 

[Hargraves]

Not what i had in mind exactly sir. Two seperate ways of feeding fuel is what i have in mind one from the auxillary tank transfer pump to engine with mains isolated or current mains feed with auxillary isolated with no cross tank transfering required..

 

 

[diesel]

MMmmmmmm, gravity rules. I have always pumped aux to the mains. It works fine and if done at half main tanks if something did go wrong you still have a lot of time. KISS Chas.

 

 

[Hargraves]

Gravity assist would allways be an advantage i agree

 

but the same something going wrong is also applicable and possible when the only method of transfering auxillary fuel from the external low mounted tank is via its seperate transfer pump. Looked at from the position of systems complication their would be more oppertunity for something to go wrong in a single continuious fuel delivery system involving two more tanks and plumbing than the proposed solution of two seperate and, independant deliver systems providing double redundency