M20F Fuel Quantity

[nugs314]

Could someone explain, in technical terms what exactly is happening here?  When I top off my M20F the visible fuel quantity is always a few gallons lower after a few hours..  Especially in the right wing.  I know the fuel is settling but it takes it so long?  Any gouge is appreciated!

[sreid]

More than likely, the technical term to describe it is "leak". See any evidence of stains under the wings, or smell any fumes in the cockpit? Have your tanks ever been worked on?

 

Steve

[carusoam]

Expect a while for fuel to escape into furthest reaches...

It makes no sense to me why, but the last few gallons take some time to put in.

If things work well, the fuel stays in the tank.

Best regards,

-a-

[nugs314]

I knew there would be one smart A^^ leak reply but thanks for taking the time to read and reply.  The aircraft holds 64 gallons of fuel but you only put 30 in each tank so it is going to the 4 gallon "liquid gold reservoir".  i just do not understand why it takes hours to happen which makes it a logistical issue to truly "top-off" the tanks. I was wondering if there was an easy answer. thanks!

[nugs314]

More than likely, the technical term to describe it is "leak". See any evidence of stains under the wings, or smell any fumes in the cockpit? Have your tanks ever been worked on?

 

Steve

 

Hey Steve the tanks where sealed before I bought the plane a few years ago and there is evidence of a very small leak but not enough to really notice.  No smell of fumes either.  Can you top your 20F off and have have the fuel at the same level the next time you fly or does is slowly seep into the far reaches of the tank?  Thanks Steve! 

[sreid]

Hey Steve the tanks where sealed before I bought the plane a few years ago and there is evidence of a very small leak but not enough to really notice.  No smell of fumes either.  Can you top your 20F off and have have the fuel at the same level the next time you fly or does is slowly seep into the far reaches of the tank?  Thanks Steve! 

 

When I still had the original sealant, I would definitely lose a few gallons to seeps if the plane sat for a week or two. There were no drips, but it was visibly seeping and apparently evaporating before it would reach the floor. After resealing, the fuel level does change a little after sitting awhile post-fueling, but I've only seen that if there is temperature change. For example I had it topped a few days before the temperature dropped, and the level is lower afterward. I've also topped it outside then moved into a warm hangar overnight. The next day, fuel will spill out if you open the caps in that case.

That's all I know about that!

Steve

[Marauder]

Could someone explain, in technical terms what exactly is happening here?  When I top off my M20F the visible fuel quantity is always a few gallons lower after a few hours..  Especially in the right wing.  I know the fuel is settling but it takes it so long?  Any gouge is appreciated!

 

There is no such thing as a topped off F model. Whether bladders or wet wings, as Anthony points out, there are pockets that never quite get filled. I have owned the same F for 23 years and had wet wings for a bit and bladders the rest of the time.

 

There are two things I noticed. If you fill it all the way to the filler cap ring, you can shake the wing and get another 0.5 to 1.0 gallon in per side. Shake it some more and you can get a little more in. After you park it filled, the next morning, you open the cap and it will be down another 0.5 gallons (on each wing) -- if it cools off overnight. What I think is going on is a large bubble at the top that is off-centered with the filler hole. Sort of a like what you see with an orienteering compass. With the compass to get a correct reading you need to get the bubble to center over the middle of the compass. In order to get the "bubble" out of the fuel tank would require some lifting of the tail, jiggling of the wings and who knows what else...

 

My process for dealing with this is the same. I fill up after every flight (unless I know I need some useful load for my next flight), park the plane and will top it off the next morning if I know I need every last drop.

[nugs314]

There is no such thing as a topped off F model. Whether bladders or wet wings, as Anthony points out, there pockets that never quite get filled. I have owned the same F for 23 years and had wet wings for a bit and bladders the rest of the time.

 

There are two things I noticed. If you fill it all the way to the filler cap ring, you can shake the wing and get another 0.5 to 1.0 gallon in per side. Shake it some more and you can get a little more in. After you park it filled, the next morning, you open the cap and it will be down another 0.5 gallons (on each wing) -- if it cools off overhight. What I think is going on is a large bubble at the top that is off-centered with the filler hole. Sort of a like what you see with an orienteering compass. With the compass to get a correct reading you need to get the bubble to center over the middle of the compass. In order to get the "bubble" out of the fuel tank would require some lifting of the tail, jiggling of the wings and who knows what else...

 

My process for dealing with this is the same. I fill up after every flight (unless I know I need some useful load for my next flight), park the plane and will top it off the next morning if I know I need every last drop.

Thanks for the info Marauder, I never considered the temperature aspect.  i started noticing it getting worse when I moved the plane to the DC area from Southern Cali.   I will shake the wings next time I fill her up.  Appreciate the response.

[nugs314]

When I still had the original sealant, I would definitely lose a few gallons to seeps if the plane sat for a week or two. There were no drips, but it was visibly seeping and apparently evaporating before it would reach the floor. After resealing, the fuel level does change a little after sitting awhile post-fueling, but I've only seen that if there is temperature change. For example I had it topped a few days before the temperature dropped, and the level is lower afterward. I've also topped it outside then moved into a warm hangar overnight. The next day, fuel will spill out if you open the caps in that case.

That's all I know about that!

Steve

 

Thanks Steve! 

[Bob - S50]

Only making assumptions here (not an A&P).  Wing ribs are attached to the skin.  That leaves a 'ridge' that fuel/air has to go over or under to get around.  I assume the ribs and skin form an open box on the top of the fuel cell.  Just like putting an open box, open end down, into a tub of water, if air can't get out, water can't get in.  In this case, if air can't get out, fuel can't get in.

 

When we calibrated our wings for a fuel stick we noticed that with the wing empty, if we added a gallon it would initially appear to have fuel below the fill hole.  However, if we waited a minute or so, all that fuel (that was trapped up slope from the rib) would drain down leaving the wing dry.  Add another gallon, same thing until we finally got enough in to cover the rib.  That leads me to believe there are weep holes in the ribs to allow fuel to flow downhill to the sump.

 

I assume the same thing exists on the top of the wing with small weep holes or gaps that allow trapped air to move out and fuel to move in.  So my 'guess' would be that when you fill the wing to full there is air trapped in the top of the fuel cell.  By the next morning, that trapped air has found its way out, allowing fuel in, lowering the fuel level for the rest of the wing, resulting in a tank that then looks less than full.

 

Since you said the wings were sealed a few years ago, perhaps they inadvertently sealed up most of the weep holes.  If that was the case, I don't know if it can be corrected or not.

 

The good news?  Even with only 60 gallons useable, your plane will still probably fly longer than your bladder/butt want to.

 

Have fun,

 

Bob

[kerry]

Do you shut your fuel selector to off when plane is not in use.  If not you'll loose fuel over time.

[DonMuncy]

Where does the fuel go?

[Lood]

If I fill my F to 64 gals, the fuel level is at the bottom of the two holes in the triangular indicator, right next to the filler. As has been said, if filled right to the necks, you do get in some more Avgas, but in my F, this extra fuel also seems to either drain out through the overflows or whatever, but whenever I fill to the necks, the fuel level will be down to the 64 gal mark after a day or two.

My tanks were resealed during the previous two years and are 100% leak free. If filled to the 64 gal holes, the fuel level will stay there for weeks.

[nugs314]

All good info Gents and I feel I now have a grasp of the situation.  I'm a military pilot and have only been in the Mooney world the last few years.  I have been to "pridefull" to ask many of the questions I find answered for me here on this board.  It is nice to have all of your expertise handy.  Thanks again and rest assured there will be more questions coming.

 

S/F,

Tim

[N201MKTurbo]

There are holes in the ribs at the bottom to allow water to migrate back to the drain. I think there may be holes in the top to allow air to migrate as well. I have pictures on my home computer of the inside of my tanks from the last time I had them opened up. I will take a look and see if they are there.

[Marauder]

There are holes in the ribs at the bottom to allow water to migrate back to the drain. I think there may be holes in the top to allow air to migrate as well. I have pictures on my home computer of the inside of my tanks from the last time I had them opened up. I will take a look and see if they are there.

 

I think you are correct about the holes on the top ribs.

[SkyPilot]

I fill up right to the rims. Then I go away for a month and the airplane is parked in a nice heated hanger. When I got back last time I put about 2 gallons in to top it up again. This is normal. No fuel smell or leaks my mechanic fixed those up real nicely. I have wet wings..