Fuel tank

[Indy]

Hello

I am looking at few Mooneys as my first airplane. I am concerned about fuel tanks leaking on Mooneys. I was told that all Mooneys have fuel leak problems, or are there only certain make and model mooneys thank you

[Bob - S50]

It mostly depends on how you treat them.  We have a '78 J, tanks are original, no leaks.

Just keep some fuel in them, don't keep them parked them in the sun all the time, and they'll last a long time.

[carusoam]

Indy,

My 65C lived outdoors, the last decade in NJ... 45years on original sealant...

There are dozens of things to be concerned about when buying old machines...

when you put your priority list together the fuel tank status will be on the list further down than engine OH...

It is important, fixable, and for known costs... and has options like bladders...

Similar to Paint status, prop status, window status, donut status, and anything else that costs more than an AMU...

Don’t have fear... Mooneys aren’t much different than other planes when it comes to the cost of maintenance...

Plan on doing your homework, start reading log books to get to know how to determine some status issues yourself... a skill you will always benefit from having...

PPIs are important...

PP thoughts only...

Best regards,

-a-

[Gagarin]

11 hours ago, Bob - S50 said:

It mostly depends on how you treat them.  We have a '78 J, tanks are original, no leaks.

Just keep some fuel in them, don't keep them parked them in the sun all the time, and they'll last a long time.

100% Agree. I have a 1982 M20J with original sealant and no leaks. I keep the plane hangared to preserve interiors, paint and tanks.

[Greg Ellis]

So I will answer the other way.  I have a 63 C model and had to have the tanks re-sealed.  That was 8 or 9 years ago and all is well.  However, when removed my sealant was in bad shape and the tanks had been patched so much it was a mess inside.  These photos are the before, after cleaning, and then after the re-seal.

[RobertGary1]

Also, if useful load is important to you be careful with bladders. Especially the 64 gallon ones reduce useful load.

 

-Robert

[Vance Harral]

FYI, for the OP, there is a judgement component to fixing wet wing fuel leaks.  One which doesn't seem to get discussed much here in the midst of the black-and-white "fuel leaks are a disaster" vs. "I've never had any trouble" vs. "get bladders" debates.

If you read either the Mooney maintenance manual or AC 43-13, you'll find that many wet wing fuel leaks are actually airworthy.  Both documents provide guidance on how to determine airworthiness, and the standard is not zero leaks.  I quote from the maintenance manual on our airplane: "Slow-to-heavy seeps occurring in open areas, such as wing surfaces exposed to the airstream, are leaks which do not constitute a flight hazard and need not be repaired prior to flight, providing the condition causing the leak cannot result in a leak of greater intensity during flight."

So that Mooney you see on the ramp with a blue trickle emanating from a couple of inspection panel screws and/or an outboard lap joint isn't necessarily owned by a cheapskate rule breaker.  It might be, but it's about as likely it's been formally judged as both airworthy and safe, by an A&P, and that the owner is continuing to fly it while contemplating long-term options.  Not all leaks are airworthy, of course; and any leak of any kind legitimately decreases the value of the airplane.  But among people who have never dealt with wet wing airplanes, there is often a perception that the first time a small stain shows up anywhere, it permanently grounds the airplane until a very expensive complete strip and re-seal is accomplished.  That's just not the case.  Spot patches and watchful waiting are both appropriate ways to deal with fuel tank seeps.  In most cases you'll have months/years to notice a leak, decide what to do about it, and still fly the airplane and eventually ferry it to a shop to have the work done as desired.  I speak from personal experience here: we bought an airplane with original sealant plus an amateur patch job, in 2004.  We knew it would leak soon enough, just based on history.  In the 16 years of ownership since, we've had the fuel tanks patched three times.  Average cost for the patch work was $1-2K each time.  Maybe around $5000 total, or about $300/year, or about $3/hour of flight.  So for about half the cost of a full strip and reseal, we've been limping along for twice as long as the warranty a full-strip-and-reseal shop will give you.  I'm not saying that's the right answer for everyone, though.  If you have a show-quality airplane with a really nice paint job, seeing it seep fuel is painful if for no other reason than cosmetics.  Our airplane is a workhorse, which influences our thinking about leaks and patch jobs.

To be clear, I'm not suggesting you buy an airplane with a bunch of heavy seeps at face value and consider it no big deal.  The problem never gets better on its own, and it will require maintenance in the range of $1-10AMU, depending on how bad the problem is and how you plan to fix it.  But to the extent you're concerned about the basic design of wet wings, it might make you feel better to know that they rarely go from leak-free to unairworthy in a short time.    By the way, our favorite A&P had a long career with a major US air carrier, and he said many airliners have wet wings and leak a little fuel essentially all the time.  The joke was that if you could jump over the puddle it was making on the ramp, it was still airworthy.  That's only a slight exaggeration from the actual guidance.

[steingar]

If you're buying an older Mooney and it hasn't had its tanks resealed or bladdered, it is either leaking or will.  All airplanes have expensive systems, usually engine and paint.  Some have an extra, like covering a rag airplane.  Mooneys have an extra, the tanks can get old and leak.  It costs a crap load once, and then it's done.

[Gagarin]

5 hours ago, Greg Ellis said:

So I will answer the other way.  I have a 63 C model and had to have the tanks re-sealed.  That was 8 or 9 years ago and all is well.  However, when removed my sealant was in bad shape and the tanks had been patched so much it was a mess inside.  These photos are the before, after cleaning, and then after the re-seal.

On the picture that shows the fuel drain valve adapter plate (just below the fuel pick up) I noticed that the base of the adapter plate is fully covered with sealant. This covers the drain holes for when an F391-53S  drain valve is used. This limits the drain capability of the valve. For better drain performance use an F391-72 valve that has the drain holes just above the mounting threads

F391-53S

 

F391-72

 

 

Edited March 9, 2020 by Gagarin

[Guest]

I’ve zoomed in on the picture of the drain valve nutplate. I can’t tell if the holes are open or blocked.  Installing the wrong drain valve is not a fix to blocked holes.

Ill go beat my dead horse now.

Clarence

[Gagarin]

2 hours ago, M20Doc said:

I’ve zoomed in on the picture of the drain valve nutplate. I can’t tell if the holes are open or blocked.  Installing the wrong drain valve is not a fix to blocked holes.

Ill go beat my dead horse now.

Clarence

Trying to clear the holes after sealant has cured will cause a fuel leak around the perimeter of the drain valve that looks like a blue stain around the valve making you believe is an O-ring problem. The drain holes on the nutplate are so small that eventually they get clogged by sealant debris.  

[redcatcher27]

9 hours ago, Vance Harral said:

FYI, for the OP, there is a judgement component to fixing wet wing fuel leaks.  One which doesn't seem to get discussed much here in the midst of the black-and-white "fuel leaks are a disaster" vs. "I've never had any trouble" vs. "get bladders" debates.

If you read either the Mooney maintenance manual or AC 43-13, you'll find that many wet wing fuel leaks are actually airworthy.  Both documents provide guidance on how to determine airworthiness, and the standard is not zero leaks.  I quote from the maintenance manual on our airplane: "Slow-to-heavy seeps occurring in open areas, such as wing surfaces exposed to the airstream, are leaks which do not constitute a flight hazard and need not be repaired prior to flight, providing the condition causing the leak cannot result in a leak of greater intensity during flight."

So that Mooney you see on the ramp with a blue trickle emanating from a couple of inspection panel screws and/or an outboard lap joint isn't necessarily owned by a cheapskate rule breaker.  It might be, but it's about as likely it's been formally judged as both airworthy and safe, by an A&P, and that the owner is continuing to fly it while contemplating long-term options.  Not all leaks are airworthy, of course; and any leak of any kind legitimately decreases the value of the airplane.  But among people who have never dealt with wet wing airplanes, there is often a perception that the first time a small stain shows up anywhere, it permanently grounds the airplane until a very expensive complete strip and re-seal is accomplished.  That's just not the case.  Spot patches and watchful waiting are both appropriate ways to deal with fuel tank seeps.  In most cases you'll have months/years to notice a leak, decide what to do about it, and still fly the airplane and eventually ferry it to a shop to have the work done as desired.  I speak from personal experience here: we bought an airplane with original sealant plus an amateur patch job, in 2004.  We knew it would leak soon enough, just based on history.  In the 16 years of ownership since, we've had the fuel tanks patched three times.  Average cost for the patch work was $1-2K each time.  Maybe around $5000 total, or about $300/year, or about $3/hour of flight.  So for about half the cost of a full strip and reseal, we've been limping along for twice as long as the warranty a full-strip-and-reseal shop will give you.  I'm not saying that's the right answer for everyone, though.  If you have a show-quality airplane with a really nice paint job, seeing it seep fuel is painful if for no other reason than cosmetics.  Our airplane is a workhorse, which influences our thinking about leaks and patch jobs.

To be clear, I'm not suggesting you buy an airplane with a bunch of heavy seeps at face value and consider it no big deal.  The problem never gets better on its own, and it will require maintenance in the range of $1-10AMU, depending on how bad the problem is and how you plan to fix it.  But to the extent you're concerned about the basic design of wet wings, it might make you feel better to know that they rarely go from leak-free to unairworthy in a short time.    By the way, our favorite A&P had a long career with a major US air carrier, and he said many airliners have wet wings and leak a little fuel essentially all the time.  The joke was that if you could jump over the puddle it was making on the ramp, it was still airworthy.  That's only a slight exaggeration from the actual guidance.

This is a great answer! It fits exactly what I have experienced in my limited experience - except to say - sometimes it might get better...

In 2017 before I purchased my 1968 M20F, it had a couple of blue stains under the wing that would be classified as heavy seeps or maybe even running seeps IAW our maintenance manual. Who knows how long they had been there? We cleaned the stains away fairly easily and nothing returned. The experienced (and renowned) mooney shop that I hired to do the pre-buy and fresh annual took a good look and assured me that it was not an issue.  After the annual, I flew her to her forever home.

Now I wash and wax my plane regularly, but have never seen anymore stains.  Sometimes I'm away for a couple months at a time.  I make sure to keep the tanks full most of the time.  And each time I come back after a long hiatus, I dread crawing under there, expecting the worst - to see more blue stains.  But the fact is, since I cleaned it that very first time, 2 and 1/2 years ago, I haven't seen any at all.  The only thing to add to this mystery is, I did have them replace all the pucks during that first annual. Perhaps that's all my baby needed.   

[LANCECASPER]

When looking at a used airplane there are usually 4 or 5 major things that could be expensive if they aren't the way you want them at the purchase pre-buy.

In order of expense they are usually: 

1. Powerplant (engine/prop)

2. Panel

3. Paint

4. Interior

With Mooneys add: 

5. Fuel Tank re-seal (which are usually a little less than a full interior is to re-do)

[tmo]

0. Corrosion