Ok,
I often fly with the family and I often start with a MTOW of 3725 lb in a PA30 with tip tanks.
I like to have 84 USgallons on board, which gives me around 5.5 hours endurance.
It allows me a return trip somewhere without refueling.

The question is, where to put the fuel?
I have been leaving the aux tank empty, and putting it all in the mains and tips.
Hopefully this keeps me in the weight and balance envelope.

Two concerns :-
1. My aux tank cells will dry out and spring leaks when used on a rare long distance solo sortie.
2. If the solenoid fails to the tip tank, the empty aux tank will feed the engine and then stop, requiring an untidy switch to main tank and then a diversion.

Any thoughts?

Gus,
This is an age old question with no real correct answer. If you are going to use the extra weight afforded to you by the Tip tanks, it works out to about 10.2 gal/side in the tips for the extra 125# of gross weight. I would suggest keeping about 5 gal in the aux's if you are really concerned about them drying out, and then only fill the tips to about 10 each. If you don't want to do that, then just leave a gallon or so in the aux's all the time, and fill them all the way when your load permits. As long as the cell is not totally dry for extended periods, you should be OK. Even a tank with 1-2 gal in it will slosh quite a bit of the tank with fuel while in flight. One other recommendation. If you are going to just leave a gal or two in the Aux for a while, treat that fuel with Marvel Mystery Oil. It works great as a tank preservative oil!
-Zach

Thanks Zach,
Useful advice.

I understand solenoid failure is very rare.
If it does occur, does the tip tank light on the fuel selector necessarily go out ?
Presumably the fuel guage continues to read the tip tank's contents despite feeding from the aux tank?
I imagine it could be tricky to diagnose in the air.

Gus.

Gus,

From what I've read from other tip tank owners, the solenoid seems most likely to fail when it isn't used often. We haven't had a tip tank failure but did come out of an annual with them wired backwards so shortly after a tank change from mains to full auxes (but really feeding from 1 gallon in tip tank) the right engine loss/surging gives the bad yaw - IFR, of course. Fast fingers got the fuel pump on and tank changed back before the engine died. I flew the rest of the flight on the mains and then diagnosed the problem on the ground. Did not have to do any extra cadio work out that day. Ironically, there had been no work done on the selector at annual but had changed out the gear conduits and wires had been impacted due to that work.

You can hear the solenoid click in a quiet hangar if you are able to pre-flight it there.

Assuming you climb to cruise altitude, change to the tip tanks so you've left the max left in the mains - worst thing that happens if the solenoid fails is you get to stop and have more fuel put in the mains to complete your flight. That works fine unless you are doing a long over water trip.

Zach is right about the aux tanks - although in Texas, it is so much drier here - if the plane is not hangared that would be another drying factor - even if it is hangared, the heat is a factor. After 40 years, the twin's aux tanks weren't leaking but I'd lost a main so next annual changed the aux tanks at my convenience. They were original, made in 1969. If your twin's are also original, you may just choose to put a few dimes in the piggy bank for when you do have to replace.

... just looked up your location - never mind about the dry heat but do suggest you keep in mind you point of no return for either high terrain or water expanses. Also, I'm assuming you practice good fuel hygiene and drain the crossfeeds before the first flight of the day - I wasn't taught this but learned it later. I've also made it a point to use the crossfeeds every once in awhile - usually when I'm solo. The crossfeed can help you extend your range a bit if a solenoid does fail.

If a solenoid does fail and you have one full tip and have used 'all' of the other fuel, be prepared for an aircraft that flies strangely. It is not as bad as an engine out but does give you a heavy wing. I found it relatively easy to manage but I was solo, far short of gross with no passenger fears to manage.

Hope this helps.

Gus, You guessed it, the light on the fuel selector panel is only a light indicating the position of the Aux/Tip
switch position. The light will light up when you select tip, regardless of what position your solenoid is at, aux or tip, ie a broken
power wire or ground to the solenoid will cause the fuel to be drawn from the aux even though the light for the tip selection is on.

Gary