Disappearing Fuel M20M

[DavidP]

Hi All,

I am seeking some advice from my learned US Mooney friends.

I have a 1996 M20M fitted with Monroy tanks. Lately I have been noticing my fuel quantity indications reducing over time while the aircraft is sitting idle on the ramp. In particular, the left main tank, which a few years ago I had fully resealed.

"Obvious fuel leak" you may say - only problem is that there is absolutely no indication of leakage - nothing dripping onto the pavement, only a very tiny smear around one inspection port which I doubt is the indication considering the reduction in quantity on the gauges, and zero fuel smell in the cabin.

I have had my mechanic look at it several times and he cannot find any visible indication of fuel leak.

Any ideas on where else to look? - I am talking about a large quantity of fuel apparently "disappearing", so if it were leaking there would be an unbearable smell associated it if it was pooling somewhere, and other clearly obvious visible indications.

I am certain the fuel isn't being stolen - could it be a gauge issue? Do the wing gauge magnets become weaker over time?

I have read everything I could find regarding Monroy tanks, but cannot find any detailed diagrams on how they are fitted and how the exactly operate. How much fuel must be in the mains before the Monroys start filling? Could it be fuel draining back from the mains to the Monroys after a flight?

Any ideas would be well appreciated.

Safe flying!

David

VH-TLS

 

Edited March 3, 2018 by DavidP

[carusoam]

David,

You have two independent gauges, per wing...

If the fuel is dissapearing, it would be evidenced by the two different gauges.

If the fuel is always being stolen from the same tank and not the other, that would take a criminal psychologist to figure out...

One thing for sure... fuel that leaks can go pretty far before actually escaping.  When this happens there is going to be a very dark blue stain to go with that.

So, If you only have one tiny smear, post a picture... Inspection panels are pretty easy to reseal.  There might be another that is better hidden....

Sounds like there are four sides to a fuel tank. Inboard, aromas enter the cabin pretty easily.  Front side the fuel has a long way to go before exiting... Backside, there are a couple of places fuel will drain out leaving a stain... outer side, has a second fuel level sensor with a seal that can easily be the culprit...

Temperature makes a difference.  In winter, the fuel evaporates slower and travels further to escape.

MS has a guy that is really knowledgeable on the Monroy LR tanks.  He might have some good ideas...  @Piloto (invitation)

PP ideas only, not a mechanic...

Best regards,

-a-

[DavidP]

Thanks for your reply, carusoam

[kortopates]

Are you new to the plane and noticing this or are you describing a recent change?

It is entirely normal for the main tanks to settle out into extended tanks. Since the fuel senders don't measure fuel in the extended tanks you only see the panel and wing gages drop in quantity while this happening. If you think this explains what you're experiencing the simple solution is to measure the fuel in your extended tanks and add that to what your wing guage is showing for a total. Your POH AFMS for the extended tanks will give you guidance on how to mark a dipstick for measuring your extended tanks. Then you can at least monitor the total volume of fuel in your tanks and know if you are really losing any. But after you fuel the mains only, gas will settle in the extended tanks and your fuel gauges will no longer be accurate until your tanks are about half full again, reading lower than actual. Additionally you won't be able to top the mains off until they are about half full either (if your intent is to only fill the mains) since there will be still be fuel that is in the extended portion of the tank. Thus you could get yourself into an overgross situation with more fuel than planned.

 

 

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[DavidP]

Hi kortopates, thanks for your thoughts.

I have had the aircraft for about 5 years, and have been aware of the draining, quantity and other issues related to the extended tanks. I had the tank resealed about 3 years ago. But I have noticed that this issue has become much more noticeable of late and is not reflected in the behaviour of the right tank. The right side may drop a little, but the wing gauge on the left indicates almost empty if the aircraft has sat idle for a few weeks. It is also reflected by a drop in the cockpit gauge.

If it is a leakage issue, then I am really stumped as to where it is draining to, as I have said there is absolutely no indication of leakage by sight or smell. I would expect that if it was draining to a hidden location, by the quantity involved, it would have overflowed or otherwise become noticeable to smell. At my last annual we looked at this, and my engineer applied some more sealant as a precaution. At this point it is hard to accept that the tank is leaking.

However, I am a little frustrated at the lack of information regarding the extended tanks. Aside from a very low resolution schematic on the web site, I have been unable to find any information on how the tanks are installed, where they lie specifically within the wing structure, and when filling, at what point do the extended tanks begin to fill. How many drain holes are there between the tanks? I do understand that the principle is a gravity feed into the mains because of the low nose attitude in the cruise. This gives rise to my thoughts that perhaps on landing when the aircraft resume it's nose up attitude, fuel then drains back into the extended tanks.

I guess I am a little fussy on this details as I am a former Captain on oil tankers, so I have something in my DNA that needs to know where fluids run to!

 

[kortopates]

You're implying an indication issue on your left tank. If so, you need an independent method of measuring tank volume from both fuel caps to verify. But you haven't given an estimate on the amount of missing fuel over the course of the couple weeks you mention. For example, is it large enough to be noticeably different visually looking into both the main and extended tanks? Or are we discussing more like small number of gallons that you won't be able to tell the difference visually but still plenty to be more than a seep?

First, there is no secret place fuel can drain too. Any leak is going to be very obvious, fuel is going to seep or drip directly out from a leak, or if inside the wing, such as up higher, it will run downhill towards the center of the aircraft inside the wing till it finds a place to drip out which could be anywhere downstream including inside the cabin. It will be very evident and leave a unmistakable blue stain where ever it flowed, dripped or seeped from. Its certainly not always obvious exactly where a leak originates from since it can flow down the side of a tank and in the wing for quite a ways before it comes out but it will become visible externally on the aircraft or in the cabin soaking the carpet if its a significant leak i.e. more than a seep.

With no evidence of a leakage, it suggest an issue with one of your senders - there are two in each tank wired in series. The wing gauge is not an independent gauge but magnetically senses the arm of the fuel sender below it - so it would make sense for them both to be off. Which brings us back to the point of an alternative method of measuring fuel volume in your left tank.  I suggest you stick your Left main and extended tanks. Recording height of fuel level is adequate since you just need to know if the volume is really changing while the aircraft is sitting in the hangar.  

If there is no leak, its got to be an indication issue if someone isn't borrowing fuel from your tank. 

But so far, this doesn't sound related to having extended tanks since you are familiar with the how the tank gauges indicate differently as the fuel added to the main tank settles out into the extended tanks (since the extended tanks are a mere extension of the main tanks uphill and outboard of the main tanks in front of your speed brakes). 

[Piloto]

David, if there is no stains or smell and the fuel indication on the wing gauge is lower I think your fuel is getting borrowed. This can happen by syphoning through the filler hole or by draining through the drain valve. Even if your plane is hangared someone in maintenance can have a copy of your padlock key. Change your hangar padlock and let us know your finding. You are not the first.

José

[DavidP]

4 hours ago, kortopates said:

You're implying an indication issue on your left tank. If so, you need an independent method of measuring tank volume from both fuel caps to verify. But you haven't given an estimate on the amount of missing fuel over the course of the couple weeks you mention. For example, is it large enough to be noticeably different visually looking into both the main and extended tanks? Or are we discussing more like small number of gallons that you won't be able to tell the difference visually but still plenty to be more than a seep?

I recently left the aircraft for about 4 weeks (too busy to fly) and the wing tank gauge went from 16 to 0. At these levels no fuel is visible from the filler point.

4 hours ago, kortopates said:

First, there is no secret place fuel can drain too. Any leak is going to be very obvious, fuel is going to seep or drip directly out from a leak, or if inside the wing, such as up higher, it will run downhill towards the center of the aircraft inside the wing till it finds a place to drip out which could be anywhere downstream including inside the cabin. It will be very evident and leave a unmistakable blue stain where ever it flowed, dripped or seeped from. Its certainly not always obvious exactly where a leak originates from since it can flow down the side of a tank and in the wing for quite a ways before it comes out but it will become visible externally on the aircraft or in the cabin soaking the carpet if its a significant leak i.e. more than a seep.

These were exactly my thoughts - if it was a leak eventually it would manifest itself somewhere and become evident. As I have mentioned, no such signs can be found.

4 hours ago, kortopates said:

With no evidence of a leakage, it suggest an issue with one of your senders - there are two in each tank wired in series. The wing gauge is not an independent gauge but magnetically senses the arm of the fuel sender below it - so it would make sense for them both to be off. Which brings us back to the point of an alternative method of measuring fuel volume in your left tank.  I suggest you stick your Left main and extended tanks. Recording height of fuel level is adequate since you just need to know if the volume is really changing while the aircraft is sitting in the hangar.  

This is what I would like to think as the cause. Bearing in mind the Bravo has one filler point with extended tanks. When the tank gets down to less than 20 on the wing gauge I will no longer see any fuel, so this is where a certain reliance on the gauges comes in to give some idea of what is in the tank and a dip stick therefore is of no use. The only thing that bugs me is that the loss is also reflected on the cockpit gauges which makes me think that there is no such thing as a coincidence and it surely could not be issues with both the wing gauge AND the fuel sender(s).

However after having flown it today, on my return I left enough fuel to be visible at the filler point, so I will monitor this and see what happens.

2 hours ago, Piloto said:

David, if there is no stains or smell and the fuel indication on the wing gauge is lower I think your fuel is getting borrowed. This can happen by syphoning through the filler hole or by draining through the drain valve. Even if your plane is hangared someone in maintenance can have a copy of your padlock key. Change your hangar padlock and let us know your finding. You are not the first.

José

Thanks Jose', I know what you are saying. Unfortunately I am not hangared (too expensive). Having said that, I am pretty confident lifting the fuel is not the issue (relatively speaking we are a pretty honest mob here), although one can't rule anything out, but if this was so, the right tank remains untouched. On the subject of the extended tanks, do you have anything available that gives me precise details of the extended tank installation?

Thanks for your suggestions and thoughts guys.

 

[kortopates]

Extended tanks without the second filler port? What is your total fuel capacity?


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[DavidP]

8 minutes ago, kortopates said:

Extended tanks without the second filler port? What is your total fuel capacity?


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118 USG. The Bravo Monroy fit did not come with a separate tank. This is the reason why I'd like more details of its installation.I also have speed brakes which alters the standard extended capacity.

Edited March 4, 2018 by DavidP

[kortopates]

118 USG. The Bravo Monroy fit did not come with a separate tank. This is the reason why I'd like more details of its installation.I also have speed brakes which alters the standard extended capacity.

I asked because there was another mod to modify the original filler neck to hold 100 gal by just filling the tank higher. As far as I know all of the Monroy mods wet an additional area behind the main tank area as shown in the diagram at this website: https://www.emapa.aero/Monroy-Mooney-Fuel-Increase-Upgrade-p/mooney-fuel-increase-upgrade.htm

 

[mention=6932]Piloto[/mention] as the owner of the STC will be able to answer that best.

 

 

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[DavidP]

3 minutes ago, kortopates said:

I asked because there was another mod to modify the original filler neck to hold 100 gal by just filling the tank higher. As far as I know all of the Monroy mods wet an additional area behind the main tank area as shown in the diagram at this website: https://www.emapa.aero/Monroy-Mooney-Fuel-Increase-Upgrade-p/mooney-fuel-increase-upgrade.htm

Yes, thanks, that's the low-res schematic I referred to in an earlier post. Hopefully Jose' might have some more detailed information that might be available for release.

[LANCECASPER]

I've owned three M20M's, all with Monroy long rang tanks and there's never been an issue of fuel quantity indications moving around as you are suggesting. It has to be going somewhere, and if it was leaking you'd have a noticeable amount on the ground and blue stains on your airplane somewhere.

You have a fuel totalizer on your M20M - either a Hoskins or a Shadin. These measure only what goes through the transducer. How does that compare when you fill up?  If there's a discrepancy and it's not leaking - no blue stains - the only other explanation I can think of is that someone is stealing your fuel.

I can't imagine owning a plane and not hangaring it. Your argument is that it's too expensive. What do you consider too expensive? (Unfortunately your profile doesn't mention where you are located.) I feel that hangaring is just one of the necessary costs of owning an expensive airplane like yours. People justifiably hangar airplanes that are worth much less. (Rain, snow, wind, hail, thieves, airframe corrosion, paint damage, UV light damaging your windows, interior, avionics, other airplanes taxiing into your airplane, wind constantly moving your control surfaces, people stealing your fuel, water getting in your fuel, fuel tank sealant not lasting as long, birds another animals nesting in your airplane, etc, etc, etc.)

Regardless of what the current market value is of your airplane, if you look at parts prices, you are maintaining a half million dollar machine. At least from my point of view, leaving it outside doesn't make any sense. I bet your fuel thief appreciates it though,. 

 

[mike_elliott]

David, if you are losing fuel out of your left tank only as it seems you are saying, and this has happened repetitively, as it seems you are saying, then I suspect a leak. Pull the inspection panel behind the landing light and take a flashlight and mirror to look at the back side of the spar if you havent already. This simple 15 minute check might give you some clues.

[carusoam]

1) Somebody once posted internal wing drawings indicating the drain holes at the top and bottom of the ribs...

This can explain some slowness in fuel and air moving between segments of the fuel tank.

2) Standard Long Body tanks can hold a total of fuel near 100gallons.  Some Eagles got the fuel necks that made it difficult to put in the full 100gallons.  More of a challenge moving air out and fuel in...  Monroy extended tanks can add another 15gallons per side... for a total of 130 gallons...

3) MS has shown three independent ways of measuring fuel.   They should all be able to give the same answer.  Wing mounted gauges. Instrument panel gauges and the fuel totalizer on the instrument panel...

4) it sounds like, one day, 16? gallons disappeared... that is easily measured by all three systems... even the wing’s simple float gauge is marked in 5 gallon increments.

5) When leaks start, they don’t usually fix themselves.... it is possible to have dirt in a fuel drain that clears, but 16 gallons is going to leave a lasting stain.... a couple of gallons coming out an errant fuel cap leaves a big mess that requires a purposeful clean-up...

6) One can suspect some borrowed fuel, but proof is going to be required to know there isn’t a leak somewhere.

7) Moving forwards... exacting records of fuel data is a good idea... how many gallons put in each tank, how many gallons on the two level systems, and fuel used from both tanks using the totalizer and a note pad...

8) video camera mounted at the airport? Reporting and discussing with the airport management is a pretty open idea...

9) Locking mechanism for the tank caps?  (Doesn’t prevent theft through the drains)

10) Briefly, prove to yourself the three methods of measuring fuel use, all tell the same story. If they don’t, you know exactly how much is missing, from where, over a known time frame.

11) Most age and heat related leaks occur near the top of the tanks.  Four level sensor leaks can occur near the mid level of the tanks. Fuel drain leaks occur near the bottom of the tanks...

12) Fill the tanks to the top. Verify the level on the four gauges. Reset the fuel totalizer to zero.... hang a sign on the plane indicating the plane is being monitored by CCTV... wait a few days to see what happens.... or use it normally, but keep good notes on fuel in, and fuel used.  The results will speak for themselves....

13) if it was 16 or so gallons missing, it has left a tell tale somewhere. Or, they wouldn’t bother to use so much blue fuel dye...

14) Use those instruments and report back...  

PP thoughts only, not a mechanic...

Best regards,

-a-

[DavidP]

6 hours ago, LANCECASPER said:

I've owned three M20M's, all with Monroy long rang tanks and there's never been an issue of fuel quantity indications moving around as you are suggesting.

I'm very happy for you

6 hours ago, LANCECASPER said:

It has to be going somewhere, and if it was leaking you'd have a noticeable amount on the ground and blue stains on your airplane somewhere.

I've said this from my original post

6 hours ago, LANCECASPER said:

I can't imagine owning a plane and not hangaring it. Your argument is that it's too expensive. What do you consider too expensive?

I didn't come here to be lectured on the pro's and con's of hangarage, but to seek advice from the largest and most experienced Mooney population on the planet on how best to troubleshoot this issue I'm having.

I am well aware of these issues and I made my decision as such. I live in Australia (YMMB). We do not live in a friendly GA environment here. As an aircraft owner you are automatically put into the "rich boys toys" category, and we are price gouged as such. I don't mind paying good money if I believe I am getting value for money.

6 hours ago, LANCECASPER said:

I bet your fuel thief appreciates it though

I wish he would stop following me as I make my way around the country.

Otherwise thanks for your thoughts.

[DavidP]

1 hour ago, carusoam said:

PP thoughts only, not a mechanic...

Thanks for your help, I'll go through that list and hopefully it may shed some light.

[carusoam]

Feel free to post pics of all your findings...  from a known starting position to the finished results.

Logic and reasoning will provide solid answers.  If not, a different approach is going to be needed.

Best regards,

-a-

[DavidP]

6 hours ago, mike_elliott said:

Pull the inspection panel behind the landing light and take a flashlight and mirror to look at the back side of the spar if you havent already. This simple 15 minute check might give you some clues.

Thanks Mike, that's a good tip, thank you for that I'll give it a try.

Edited March 4, 2018 by DavidP

[DavidP]

1 minute ago, carusoam said:

Feel free to post pics of all your findings...  from a known starting position to the finished results.

Will do, really appreciate your advice and thoughts.

[carusoam]

The Power Of MS and the MS community! 

All stuff I learned here.

Best regards,

-a-

[DavidP]

5 minutes ago, carusoam said:

The Power Of MS and the MS community! 

All stuff I learned here.

Best regards,

-a-

Oh yes. As I responded in the "hangarage" post - you guys are the guru of Mooneys, that's why I posted here. I would love to spend time in the USA Mooney'ing around, your GA environment is so much more user-friendly than out here.

My M20M is 1 of only 2 in the country!

Edited March 4, 2018 by DavidP

[carusoam]

Go ahead and update your avatar info as much as practical.  This helps us help you even better...

Best regards,

-a-

[N201MKTurbo]

Are you by any chance leaving the fuel selector on the left tank every time? The leak could be down stream of the fuel selector. It may just be dripping on the tarmac from the engine compartment.

[DavidP]

8 minutes ago, N201MKTurbo said:

Are you by any chance leaving the fuel selector on the left tank every time? The leak could be down stream of the fuel selector. It may just be dripping on the tarmac from the engine compartment.

Not sure about every time, but it would be left on the last tank used. My engineer did suggest I put it to OFF, which I tried but didn't seem to change anything.