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Keep looking...

One tank or both tanks?

evaporation is incredibly slow... but if that is happening... the fuel starts getting darker... as the blue is getting more concentrated... takes months....

If it is leaking inside the plane, most often, it comes with aromas and blue stains...

The most common places to look for leaks... fuel level sensors, and a nearby rubber connector hose for the fuel lines... both sides...

Ever See what is behind the wall panels... now may be the time to look.

Pull up the back rug in front of the seat... see if there is a blue stain anywhere...

PP thoughts of my M20C experience...

-a-

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Is the aircraft in a hangar or outside? Also, fuel reading will vary with system voltage. Are you using battery voltage to check fuel level, or with engine running? 

Edited by PilotCoyote

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5 hours ago, Badmoonraising said:

My fuel level slowly but most definitely goes down as the airplane sits. I've looked for a leak but have found no evidence. Suggestions? 

Does it smell like fuel when you open the cabin?  The staining could be under the carpet or side panels.  I lose some over time, but I know my right wing needs a reseal.  I get a blue stain at the seam just outboard of the gear well.  I am in a hangar and never have any fuel on the floor.  It evaporates before dripping on the floor.

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Community hangars tend to have more "fuel evaporation" than most.  However it's more prevalent on planes that have a lock open sump like a beechcraft....  

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Expansion can push some out the vent.  Topping off with cool fuel and then leaving aircraft in the sun can push some fuel overboard.  Check it the next morning and it won’t be to tabs.  

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Thanks, all good things to look for. To answer a couple questions; the airplane is outside. It appears to have lost a gallon or more over a week. The fuel loss appears to be both tanks. 

Thing is I don't see any blue stains. On the airplane or ground. 

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1 hour ago, Badmoonraising said:

Thanks, all good things to look for. To answer a couple questions; the airplane is outside. It appears to have lost a gallon or more over a week. The fuel loss appears to be both tanks. 

Thing is I don't see any blue stains. On the airplane or ground. 

How in the world are you measuring one gallon of lost fuel...with the gauges?

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2 hours ago, Badmoonraising said:

Thanks, all good things to look for. To answer a couple questions; the airplane is outside. It appears to have lost a gallon or more over a week. The fuel loss appears to be both tanks. 

Thing is I don't see any blue stains. On the airplane or ground. 

A few things to think about if you're topping off and finding the level down later. The temperature of the effects the volume.  We should all be buying fuel by the pound and at a precise temp (the temp is remarkably consistent for underground tanks). Air.  Truly topping off a Mooney requires a bit of patience. Vents notwithstanding, if I top off, replace the caps and then shake the wings, I can add more fuel after the sloshing stops. You may have a few air pockets that are taking a little while to get to the vent.

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2 minutes ago, Shadrach said:

if I top off, replace the caps and then shake the wings, I can add more fuel after the sloshing stops.

If I ever need that last little bit of fuel that is gained by bouncing the bubbles out...I will know that my flight planning sucked and I deserve to run out.

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14 hours ago, kpaul said:

If I ever need that last little bit of fuel that is gained by bouncing the bubbles out...I will know that my flight planning sucked and I deserve to run out.

If you ever did a tank reseal and wanted to know your true usable fuel capacity, you’d understand why I know. 

I'll also say that if you have the payload available on a long XC there’s really no reason not to do a max fill up. Planning minimizes risk, it doesn't eliminate it.

Edited by Shadrach

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Thanks, all good things to look for. To answer a couple questions; the airplane is outside. It appears to have lost a gallon or more over a week. The fuel loss appears to be both tanks. 
Thing is I don't see any blue stains. On the airplane or ground. 


Have you opened the under wing access panel front spar leading edge, forward of the fuel tank. You can be leaking fuel inside the wing and if slow enough will not see a stain outside.


Sent from my iPad using Tapatalk

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This doesn’t happen to everyone?? Last thing I need is another maintenance worry. I also generally see the fuel level down about a half gallon to a gallon when looking in the tank a week after topoff. Plane lives in a non- temp controlled hangar.  Honestly, I never thought anything of it until now. I assumed  it was some combination of evaporation, temp- volume changes,  venting overboard by expansion, and/or redistributing among the bladders (though I top off slowly at the end and watch for level to be stabilize).  I’ve never had fuel odors or seen stains, and I wasn’t worried about a leak until now.

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49 minutes ago, DXB said:

This doesn’t happen to everyone?? Last thing I need is another maintenance worry. I also generally see the fuel level down about a half gallon to a gallon when looking in the tank a week after topoff. Plane lives in a non- temp controlled hangar.  Honestly, I never thought anything of it until now. I assumed  it was some combination of evaporation, temp- volume changes,  venting overboard by expansion, and/or redistributing among the bladders (though I top off slowly at the end and watch for level to be stabilize).  I’ve never had fuel odors or seen stains, and I wasn’t worried about a leak until now.

If you have bladders and you see it as well, then I am almost certain that it is redistribution.

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We don't see it because we don't top off.  We park with 30 - 35 gallons on board to leave more 'useless' load capacity for whoever flies next.

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12 hours ago, Shadrach said:

I'll also say that if you have the payload available on a long XC there’s really no reason not to do a max fill up. Planning minimizes risk, it doesn't eliminate it.

I can think of two good reasons.

1.  Lower weight means better takeoff and climb capability.  This can be especially important at high density altitudes like Rock Springs, WY in the summer.

2.  I use extra capacity to tanker cheap gas to avoid buying expensive gas.  If I fill up every time, I lose some of that capability.  I will have filled up by buying expensive gas at my current location which means I can't buy as much gas at my next stop which has much cheaper gas.  For example, if I fill up at home (S50) I'll be buying gas at $5.76/gallon.  If I'm heading toward SLC I can stop at U76 (Mountain Home, ID) $4.55 or JER (Jerome, ID) $4.39.  Leaving home with 36 or 42 (VFR) gallons and landing with an hour of fuel will allow me to save about $35.  To be honest, even then I'd probably only fill to the 50 gallon tabs outbound and around 60 on the way home so I can arrive at home with the required 30 - 35 gallons we've agreed to.

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3 hours ago, Bob - S50 said:

I can think of two good reasons.

1.  Lower weight means better takeoff and climb capability.  This can be especially important at high density altitudes like Rock Springs, WY in the summer.

2.  I use extra capacity to tanker cheap gas to avoid buying expensive gas.  If I fill up every time, I lose some of that capability.  I will have filled up by buying expensive gas at my current location which means I can't buy as much gas at my next stop which has much cheaper gas.  For example, if I fill up at home (S50) I'll be buying gas at $5.76/gallon.  If I'm heading toward SLC I can stop at U76 (Mountain Home, ID) $4.55 or JER (Jerome, ID) $4.39.  Leaving home with 36 or 42 (VFR) gallons and landing with an hour of fuel will allow me to save about $35.  To be honest, even then I'd probably only fill to the 50 gallon tabs outbound and around 60 on the way home so I can arrive at home with the required 30 - 35 gallons we've agreed to.

both of those calculations are built into my payload analysis. If I am embarking on a 700-800NM XC, It would be highly unusual for me to chose a short, high altitude strip as a starting point. If you know your numbers, you know your numbers...  I tanker gas out of an 1800" strip year round.  I could do it at gross, but I wouldn't.

Edited by Shadrach

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18 minutes ago, Shadrach said:

both of those calculations are built into my payload analysis. If I am embarking on a 7-800NM XC, It would be highly unusual for me to chose a short high altitude strip as a starting point. If you know your numbers, you know your numbers...  I tanker gas out of an 1800" strip year round.  I could do it at gross, but I wouldn't.

Sorry, by payload I thought you meant available useful load.  Didn't realize you were adjusting that downward based on conditions.

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3 hours ago, Bob - S50 said:

Sorry, by payload I thought you meant available useful load.  Didn't realize you were adjusting that downward based on conditions.

With me, bags and tanks filled to the brim (even with a wing shake). I’m still an 450lbs under max gross.

I limit trips with non flying passengers to 450NM for comfort. It also coincides with the nearly 800lbs in available payload I have left with 45gals aboard.

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Maybe a little thievery?  Siphoning a few gallons from different planes so it’s not noticed ?  Probably not...seems like a lot of work for a little gas but maybe?

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I had a 1963 C for 25 years.  The last year I had it it was losing a lot of fuel.  If i parked it in a closed locked hanger with both tanks full for a month almost all of the fuel would be gone.  No stains under the wings, no blue marks on the ground, no fuel smell in the cabin.  Mechanic told me it was due to evaporation but never did believe that and never did find out the problem.  I now have an F and it does not lose any fuel. 

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From what the OP is describing I'd say the fuel is just changing volume due to changing temperatures, or even settling in the tanks a bit.  A fair amount of air usually goes when I fuel the aircraft, and the gas gets all churned up,incorporating more.  When all that air separates it'll look like you've a little less fuel.

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I just had another thought.  What if a tank had a slow leak that drained into a part of the wing that was sealed enough on the bottom to prevent dripping, yet had openings elsewhere that would allow evaporation?  Say for example out the front of the fuel tank into the inspection panel area.  If it evaporated nearly as fast as it leaked, the nose up attitude of the plane might keep the liquid from reaching the inspection panel to leak out, yet evaporation could allow it to escape through the weep holes.

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2 hours ago, wburger1 said:

I had a 1963 C for 25 years.  The last year I had it it was losing a lot of fuel.  If i parked it in a closed locked hanger with both tanks full for a month almost all of the fuel would be gone.  No stains under the wings, no blue marks on the ground, no fuel smell in the cabin.  Mechanic told me it was due to evaporation but never did believe that and never did find out the problem.  I now have an F and it does not lose any fuel. 

Is the F in the same hangar?

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