I know that we went through this recently, but I, too, would appreciate a
well-presented explanation of why or why not the sections of 91.205 apply.

Marv,

 

Here is a poor presentation just trying to define "tank."

 

The C337G that I flew for many years concealed three interconnected tanks in each wing (extended range option).  A "brain" attempted to compute a wing total by processing information from each tank.  It met the FAA requirements by displaying empty when all three tanks were actually empty.  It also frequently displayed empty when the tanks were not.  It sometimes displayed full when the tanks were topped up.  The other wing was similar but sometimes with different results.

 

Each "tank" did not have a filler, I.E. there was only one gas cap per wing.  Nor could each component tank individually feed the engine.  Only by "selecting" the wing would the collective fuel be available. Perhaps a tank is defined as a collection of vessels filled thru one cap.  My friend built his Lancair 360 with greater capacity by providing an additional tank in the outer wing bay (the area outboard of the aileron bell crank).  He has only one filler port in the outboard tank and only measures fuel in the wing tank formed inboard of the aileron bell crank.  That is, he knows that when his display says empty, the wing is empty (both connected tanks on that side). 

 

On the other hand, there are STC aircraft with auxiliary tanks that have their own filler ports and no gauge for those tanks at all.  Of course, one cannot "select" fuel from the auxiliary tank to directly feed the engine.  One must "pump" the fuel into a selectable, gauged tank that also has a filler port.  Hmmmm, then perhaps the definition of a "tank" should be one or more connected vessels filled thru one port that feeds the engine directly or directly thru a selector valve. Any other tankage would be defined as "auxiliary" and need not have a gauge. 

 

Since many Lancair 200/300 series aircraft follow the plans, wing fuel cannot reach the engine directly and must be pumped into the header tank and is then directly available to the engine.  One could label the wing tanks "auxiliary" and, as such, would seem to not need gauges for them.  The header tank would require a gauge.

 

Hmmmmm.  Back to my friend with the 360.  He actually has a 3-way selector and can feed fuel to the engine directly from either wing or the header.  Of course, he has each tank gauged.  Whew!

 

I am covered because my aux tanks are accurately gauged along with the header.

 

This information was provided free of charge and is worth every penny.