How much fuel do you typically keep in the tanks?

[BlackMammoth]

There has long been some belief that leaving a bunch of air in the tanks while the plane is parked produces water condensation in the tanks

Water has never been detected in the fuel when draining the sumps... even when the aircraft was parked outside next to the ocean (KPAO). In the life of the airplane, we have yet to see water contamination.

[Joey]

I'm in a two person partnership and we keep our da40 at 30 gal. The logic is that we can carry more when needed or fill up before a flight when desired. We are trying to avoid defueling and have never had to do it. We keep ours hangared and have never had a problem with water in the tank in 4 years of ownership.

[Colin]

My tanks are topped at all times, but I am an oddball.

I see one fuel-related incident per day in the FAA database. That seems like an easy one to eliminate: never take off without the tanks being full.

After a thousand hours I modified my rule: if the flight is planned for less than twenty minutes I can take off with two hours of fuel. From my home field that's just a handful of airports, so it doesn't happen often.

My kids are still small at 14 and 16 (we grow 'em small), and I don't have the extended tanks, so weight is less of an issue.

[Rich]

That seems like an easy one to eliminate: never take off without the tanks being full.

In some aircraft you can't get full utility from the plane if you always fill them all the way up. In our case, the decision to fill or not fill is made at takeoff time by the PIC at the time. You're not married to the condition based on a decision made a week ago by someone else.

And I used to have a Cherokee 140 that I routinely flow from fields at DA of 8000 ft. This condition, by the way is not even described in the POH for that aircraft. It was rare to fill those 50-gallon tanks in those conditions, because you needed to be substantially below gross weight to cop with the DA.

[Eric N]

I like the idea of putting the plane away with the tanks full, but if I do that--i.e. put it in the hangar--the right tanks weeps fuel out the vent and onto the floor (or into a can) for a week. I have sometimes been successful in getting the line guy to leave a 2 or 3" space at the top on the right but it still can drip a bit. We've had the fuel system, vent check valve, etc. checked several times but the path of least resistance seems to be just to fuel it with what the mission requires on departure. This is easier because my wife and I are the sole operators so we always know what it was put away with. The issue of condensation in a partially filled tank seems to be more theoretical than real for us as we've never found any. For those who tie down outside it is critical that the gas cap seal be properly set so it pulls up tight when the lever is folded down. We got a graphic demonstration of how important this is when we picked up our aircraft at the factory. There were three planes being delivered and when one of the factory guys was going through a proper preflight with us all they discovered about a gallon of water in one side (obviously the plane had been outside in the weather) and found that the fuel cap was so loose it could be lifted off even with the lever folded down.

[Nigel]

My 2005 DA40 (50 gal. tanks, EW 1731 lbs.) is rarely used with more than two persons + luggage so I usually top up at every leg of more than an 2 hours. I'm strictly VFR so I want maximum flexibility should my planned route fall apart due to weather or other unforeseen problems. The grass field I use in western NY has no fuel but is 3800' long so I try to top up before going into that area. I have not yet been west of the Mississippi so DA has not been an issue so far.

[Nigel]

P.S. Not a drop of water detected in tanks in the 7 years since I picked it up at the factory.

My 2005 DA40 (50 gal. tanks, EW 1731 lbs.) is rarely used with more than two persons + luggage so I usually top up at every leg of more than an 2 hours. I'm strictly VFR so I want maximum flexibility should my planned route fall apart due to weather or other unforeseen problems. The grass field I use in western NY has no fuel but is 3800' long so I try to top up before going into that area. I have not yet been west of the Mississippi so DA has not been an issue so far.

[Chris B]

Thanks for all the replies. This is very helpful!
And the good reports about lack of water in the tanks are very encouraging!!

The High Sierras are a popular destination out here. So we will almost certainly go with something like "leave the tanks at about 30 gallons" with a shared debit card so we can split all the variable costs by hour. Sharing a Google Docs spreadsheet seems like a very simple solution, so thanks for the tip!

Chris

[40flyer]

In response to Eric’s input, I usually top off my tanks whenever I refill. Today I noticed fuel was very slowly dripping from the Forward vent hose on the right wing. I have extended range tanks. The tank was probably 80% full so this was not likely due to fuel expansion but I have no other explanation. There are two vent hoses coming from the tank on each wing, one of which has an anti-return valve on it and the other has an in-line capillary tube.. i’m not sure which one has the slow drip. Anyone have any ideas?

[Scottsware]

What I have noticed is if you point the fuel nozzle toward the wing tip while fueling, fuel will come out the vents. I make sure to point the fuel nozzle inboard. That's the only explanation I have for fuel coming out the vents when the tank isn’t fuel.