Opinion on topping off tanks! Before Flight or After?

[wcb]

Just looking for opinions on topping off tanks!  I like to top off after flight so when I want to go I know that the plane should mostly be ready.  However,  I have some weeping and recently had D. Max patch (but doesn't look like we got them all, I have not been able to take it back to D. Max to finish).  If the plane sits a couple weeks as it has recently (weather has been bad to fly in Houston for TOOO LONG and especially when I am trying to fly) I loose up to 6 gallons depending on the length between flights. When do you prefer to top off?  If you top off after your flights do you top it all the way (I hear that FULL FULL can cause some issues?)

[RLCarter]

six of one, half a dozen of the other. Seems if I fill up after a flight my next one is always pushing gross, also being in S. Tx I loose some to  evaporation in the summer heat. When I do fill up before putting it away in the hangar I fill to about an inch below the neck for expansion (no since in letting it puke on floor when it heats up 

[1964-M20E]

I generally like to top off when I return to my home base for the reasons you suggested.  Depending on schedule, time of day and weather I adjust accordingly.

When away it will vary fill up when I get there or right before I leave, FS or SS???

 

In your case when returning to home base I would defer filling up until I was ready to go.  That is until you get the seep fixed.

[RobertGary1]

I have an F so topping off would be a ton of fuel. Flying a Mooney with full tanks is like pulling a boat with a vette. It may serve a purpose sometimes but not common. I think outside of Mexico in 20 years I’ve topped off a few times. 6 hours of fuel isn’t needed too often. 

-Robert 

[Jeff_S]

Losing six gallons between flights seems like a lot of leaking.  That's up to $30 lost into the air (or on the hangar floor). Having full tanks means there's more pressure on the leaks, so that would only compound the issue. Also, it would depend on the price of gas at your home-drome compared to elsewhere.

I don't have leaks to worry about, but gas at KPDK is a bit pricey. My approach is to fuel-plan to get home with at least 2 hours (30 gals in my case) so that gives me flexibility for general trips. If I know I'm headed out on a longer trip, I'll usually plan a fuel-flight a few days before to shake the rust off and get the gas I need for that trip.

[Boilermonkey]

Get those leaks fixed.  Wasted money and eventually a safety issue.

I prefer to top off when I put the airplane away.  It reduces the amount of air and possible water condensation that could enter the tanks when air cools.  More importantly, I always tend to have more time when I land than when I am leaving.  I usually want to get to the hangar, pre-flight and get going.

[Bob - S50]

I almost never top off.  Our operating procedures call for us to park the plane with 30 - 35 gallons of fuel.  That's enough fuel for us to fly  2+ hours without needing to buy fuel before we go.  By leaving out that last 30+ gallons of fuel, it gives us another 180+ pounds of useful payload.

When I fly long cross country flights, I usually just fill to 50 gallons anyway since that's enough to fly 600 NM with reserves in our J.

By parking with less than full tanks, it also gives me the opportunity to tanker fuel.  I can buy more fuel than I need at an out station that is cheaper than the fuel at home, and then skip stopping at the pumps when I get back.

A perfect example would be if I was doing an Angel Flight with a drop off at Eugene, OR (EUG).  I could stop at Independence (7S5) on the way home and buy Phillips 66 gas.  With the rebate from Phillips I would pay a net cost of $3.85/gallon vs $5.47 at home.  I could fill the plane to 45 gallons at 7S5, land with about 33 gallons, and go straight to the hangar.  The extra stop would only cost me about 1.5 or 2 gallons of extra fuel, but if I bought 35 gallons at 7S5, that would save me net about $45.

Another example, on our way home from Skypark (BTF) in September, we stopped at Mountain Home, ID (U76) to stretch and check the plumbing.  I filled to 59 gallons so I wouldn't need to buy fuel at home.  Saved about $1/gallon times a lot of gallons.

[Mooneymite]

My C has bladders.  If I top off after I fly, the next day, I find that I can add 1-1 1/2 gallons more.  Apparently the bladders expand a bit with full fuel making room for more.

So, if I know that I'm going to need "full fuel", I fuel the evening before, then top off again before takeoff.  However, I rarely plan my fuel such that 1 1/2 gallons is a significant factor.  The bladders supposedly give the C a 54 gallon capacity instead of the stock 52.

@Marauder and I have discussed this and have both experienced this oddity.

 

 

[AH-1 Cobra Pilot]

The conventional wisdom on this is to always top-off, because by leaving room in the tanks for water vapor to condense would result in water in the bottom of your tanks.  I am not sure how accurate the theory is and am open to arguments.

[bonal]

Thread drift about bladders, we also have a drop in level but was told that it's due to the slow passage of fuel between cells and takes several minutes to get maximum fuel. As to the OP I would not top off until the leaks are fixed. Many years ago I had an RD 350 with a cafe fuel tank that had a vent in the cap. Topped off at the end of the day on a hot night after a few minutes there was a huge explosion and when  I ran to the garage the bike was in flames. Long story short the fuel expanded out the vent and fumes hit the water heater. Leaking fuel is a major fire hazard 

[RobertGary1]

7 minutes ago, Ah-1 Cobra Pilot said:

The conventional wisdom on this is to always top-off, because by leaving room in the tanks for water vapor to condense would result in water in the bottom of your tanks.  I am not sure how accurate the theory is and am open to arguments.

In 20 years of being outside and not tipping off I can say it’s a wives tail. The fuel suspends more moisture than gets into the tank from venting. 

-Robert 

[TTaylor]

We have bladders and I prefer to top off before flight to leave flexibility on fuel loading for the next flight.  If I know I have a trip coming up that requires full fuel and time is tight or very early morning departure I will stop by the airport and fuel the night before.  As for fueling bladders, I usually ask the line guys to fill one tank, then fill the second, and then go back and top each tank to allow a few minutes for the fuel to settle and flow between bladders. 

[Shadrach]

I agree with @RobertGary1 on both counts.  I have never seen condensation in my tanks, nor have I ever gotten water from a hangared aircraft. 

Full tanks means 384lbs (64Gal) for an F model. In long distance mode I try to limit my legs to ~600nm. Most of my trips are between 300 and 500NM. There is just no reason to tanker around 2.5 to 3hrs of reserves.  Plus, I have a family; 384lbs of fuel limits us to 675lbs of payload in the cabin.  I keep between 120lbs (20Gal) and 240lbs (40gal) on board and fill as needed before departure.  This gives me flexibility.  If I'm traveling 300nm or less I can depart with 180lbs (30Gal) and put almost 900lbs in the cabin.

Edited October 26, 2018 by Shadrach

[Marauder]

My C has bladders.  If I top off after I fly, the next day, I find that I can add 1-1 1/2 gallons more.  Apparently the bladders expand a bit with full fuel making room for more.
So, if I know that I'm going to need "full fuel", I fuel the evening before, then top off again before takeoff.  However, I rarely plan my fuel such that 1 1/2 gallons is a significant factor.  The bladders supposedly give the C a 54 gallon capacity instead of the stock 52.
[mention=9886]Marauder[/mention] and I have discussed this and have both experienced this oddity.
 
 

The bladders do have some sort of phenomenon going on with filling them to the max and/or STC indicated level. The 54 gallon system I have should have ~1.25 to 1.5 gallons of unusable per side. The useful portion should be 54.8 gallons according to the STC. That means I should be able to put 27.4 gallons in per side.

When Terry and I did the install of the CiES senders we were really careful to measure the actual amount we could load. It ended up being 26 gallons per side. The next morning I was able to stick in ~ 1 gallon more per side. Since I will never operate the plane this way (topping off in the morning of a flight), I always will leave the usable at 26 per side.


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[salty]

Fuel is more dense when colder (morning) could that account for some of the additional fuel you can add?

[Marauder]

3 minutes ago, salty said:

Fuel is more dense when colder (morning) could that account for some of the additional fuel you can add?

This is what is probably going on. I also know that if it heats up during the day, a full bladder tank will do a Jose urination on the ramp if I step on the wing walk. Did this at Brian's July 4th party.

[skydvrboy]

I remember an older 170 pilot chastising me for not taking off with full tanks every time for added safety.  I got a pen and paper out and we started comparing performance of the two airplanes.  His max range with full tanks and an hour reserve was 380 miles. We calculated that I had the same range and reserve with 28 gallons on board.  You could actually see the look on his face change when he realized why I didn't need to fill my tanks to 64 gallons for every flight.

[RobertGary1]

@skydvrboy I agree. I think the idea of filling the tanks started when we soloed in planes with 20 gal capacity. 

-Robert 

[carusoam]

Some summary...

Seeing the future would really help answer this question...

1) Filling up leaves the risk of being over gross when you want to take some friends flying...  our tanks are so large they give the option of four seats or long (flying) legs..

2) a large mass of moist air in an empty tank could condense a few drops of water.  That’s why we sump first and think about it later...

3) fuel does adsorb moisture in equilibrium... this becomes evident in really cold conditions and fuel icing becomes a concern...

4) keeping the materials wet isn’t as much an OWT as it is a simplified explanation... Drying out would cause change in flexibility, and a few other really important physical characterizations... keeping the temperature controlled is what really works...  

5) left outside the wing’s top surface can get scorching hot... having fuel in contact with the top surface keeps the range of temperature in control... left half empty, the scorching heat is terrible for most flexible polymer materials...

6) parked outside and finding fuel leaking out and water leaking in... know there are two o-rings of which only one is visible.  Find that other o-ring in the cap...

7) look to see what the fuel cap is locking into... shiny metal... good.  Not so shiny? Not SS... (mid 60s) prone to rusting, sooner or later rotting through... near invisibly. Little rust bits get stuck in places... starting with drain seals.  At annual you will see the bits collecting in the bottom of the water separator... before and after the screen...

8) expansion is real... but depends on vent location before the Piloto event occurs...

a Cessna wing with no dihedral... the fuel is very close to the vent.  Any expansion leads to an overflow...

Older Mooney’s have a vent tube that reaches up into the air pocket pretty far up the dihedral... it is possible that some of these tubes aren’t SS either...

Newer Mooneys have the vent closer to the end of the tank... and hide the external connection in a reverse NACA duct...

9) Seals are always suspect.  Fuel senders are known for allowing fuel out...

10) the rubber connector from the tank and fuel lines is another unseen part that can swell with age... crack and leak...

11) Leaks are not normal...

12 Having too much fuel... is not always appreciated.

13) what takes so long for fuel to find its level... lots of small air pockets related to ribs... they have a few tiny air holes in each one... they also have a few fuel drains in a similar style at the bottom...

14) Fuel bladders have a few details for similar reasons. Foam pieces under the bladders, Connections between cells, venting, and fuel cap differences come to mind... I don’t have any first hand experience on these...

So... refueling always depends on your next mission... and financial limits... and exposure to theft... So many cameras, booming economy, make low theft environs...

Losing six gallons? That is a leak.  There will be a deep blue colored spot near where the evaporation of that six gallons is occurring... over time the color appears more brown than blue...

The leak usually stops at the level of the leak...

Don't fill above this level will stop the leak under static conditions...

Flying isn’t static... if raising a wing in the traffic pattern has the aroma of fuel entering the cabin... you will know where to look more or less... check under the back rug for blue stains...

Fuel stains inside the cabin are a fix immediately thing in a modern world...

Best regards,

-a-

[Bolter]

3 hours ago, Marauder said:

 

The bladders do have some sort of phenomenon going on with filling them to the max and/or STC indicated level. The 54 gallon system I have should have ~1.25 to 1.5 gallons of unusable per side. The useful portion should be 54.8 gallons according to the STC. That means I should be able to put 27.4 gallons in per side.

When Terry and I did the install of the CiES senders we were really careful to measure the actual amount we could load. It ended up being 26 gallons per side. The next morning I was able to stick in ~ 1 gallon more per side. Since I will never operate the plane this way (topping off in the morning of a flight), I always will leave the usable at 26 per side.


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Can the rubber-like material of the bladder creep so that after 12 hours of constant pressure from being full it becomes a little bit larger?  If the bladder is not right up against metal structure on every surface, it has the opportunity to move a little, including towards the top of wing surface and the little corners where it may not make full contact without some pressure from the gasoline inside.  

-dan

[J0nathan225]

9 hours ago, Mooneymite said:

My C has bladders.  If I top off after I fly, the next day, I find that I can add 1-1 1/2 gallons more.  Apparently the bladders expand a bit with full fuel making room for more.

So, if I know that I'm going to need "full fuel", I fuel the evening before, then top off again before takeoff.  However, I rarely plan my fuel such that 1 1/2 gallons is a significant factor.  The bladders supposedly give the C a 54 gallon capacity instead of the stock 52.

@Marauder and I have discussed this and have both experienced this oddity.

 

 

Ok so I'm not making this up? I also have bladders I felt like I could get more in the next day and always planned for less than 52 gals in the tank because of this. I think topping off before bedding down depends if your leaky and how much your home drone fuel is. I've got bladders and cheap fuel in lower Alabama so I usually top off so I can depart quicker the next time. I've had 3 people in my aircraft once so I never realky need to drop fuel for lbs, so keeping topped off in my case makes sense. If your mission varies alot leaving fuel till the day of gives you the flexibility for fuel vs cargo. 

[McMooney]

always top her off after a flight,  one less step next flight.

[larryb]

I generally fill after a flight. Before a flight I am anxious to go and don’t want to wait. My full fuel is 75 gallons. I only fill to that level on long trips. Normal trip fill is 60 gallons.

[Shadrach]

8 hours ago, Freemasm said:

Keeping the sealant wet is better for the longevity of your tank integrity. This is long time tribal knowledge backed up by trchnical expertise of the PRC-Desoto product rep. There's some very knowledgible Polymer Engineers here on MS (calling @carusoam) that I hope will chime in. Whatever he says regarding elastomer life (wet vs. dry), is the right answer regardless of what I've stated. 

There’s a lot of tribal knowledge out there, some of it is even backed up by science. I’ve heard it stated in the past that fuel “preserves” the sealent. Unfortunately, I’ve never seen any technical information explaining why a solvent like avgas would contribute to the health of the sealant.

[Mooney-Mark]

16 hours ago, Boilermonkey said:

Get those leaks fixed.  Wasted money and eventually a safety issue.

I prefer to top off when I put the airplane away.  It reduces the amount of air and possible water condensation that could enter the tanks when air cools.  More importantly, I always tend to have more time when I land than when I am leaving.  I usually want to get to the hangar, pre-flight and get going.

Ditto for me. I like to store the airplane with full tanks and to always take off with full fuel. You never know when you may need it. Unless, of course, there is a weight issue, in which case I will plan accordingly.