Wet Wing Longevity

[Marc_B]

1 minute ago, MikeOH said:

Honestly, I think having the plane hangared is much more critical as I believe the temperature cycling and direct sun exposure to be far more damaging to the sealant, especially on the top of the wings.

agreed 100%.  same with bladders or fuel cells I'm sure as well!

[Pinecone]

18 hours ago, Marc_B said:

I suspect it's a solution looking for a problem.  Tons of Mooneys have been firmly planted on the ground compressing the shock discs that last 12-15 years.  Long bodies shock discs don't last as long due to heavier engine and higher empty weight on the discs.

None of the Mooneys need this.  But perhaps it eeks out a few years on shock discs for long bodies with full fuel tanks?

My hangar neighbor had their M20J covered in a series of Goodwill sheets tip to tail, wing to wing, and this kept their aircraft paint less covered in dust, less need to clean and scratch with dirt, and their original paint looked pretty factory fresh after 30 years.  Sometimes it seems that an ounce of prevention keeps the "pounds" of time at bay! But @Vance Harral I'm with you, I can't imagine leaving my Mooney up on jacks routinely.

I agree.

So, I have to change the shock discs a few years earlier.  Less of a hassle than jacking and lowering the plane every time.

[Marc_B]

48 years on original sealant and finally some leaks prompting maintenance.  Outstanding!  Hope you took the poll above so your Mooney could be counted @Doug G!

[Marc_B]

I didn’t realize there was "disdain" for the way Mooney made their wet wings.  Been trying to learn more about various fuel storage solutions, and specifically how the major manufacturers constructed their fuel cells and wet wings.  Pardon my ignorance and please correct my errors/fill in the gaps...

One way constructing "wet wings" by using faying sealant between the ribs and skin, using clecos to secure everything in place (basically clamping the pieces together to let the “glue” dry), then once sealant is cured shoot rivets to hold together, then go back and apply sealant to the inside of the tank seams.

Vs. Mooney just rivets skin dry and then seals the tank seams on the inside.  Sounds like at one point someone at Mooney tried to place sealant between the rib and the skin but maybe were shooting the rivets before the sealant cured?? and this just squirted all the sealant out of the seam…lead to issues in a couple year models (did I see something about 2006-2007 maybe??) and so they went back to the original way.

BUT, I’m not entirely sure of how the Mooney technique compares regarding longevity and leaking vs. other techniques.  I assume that ALL sealant has a useful service life and I’d assume that every wet wing will wind up needing a reseal at some point in the typical lifetime of the airframe (with aircraft lasting in some cases over 60 years).  However given that the primary goal of sealant is to hold fuel...this is almost always performed "on condition" when leaks become airworthy issue (sometimes a prophylactic reseal is performed prior to repaint if the sealant is old to prevent having to open up access panels or harm paint with reseal).  Minor seeps may not be an airworthy issue, but rather evidence that maintenance (in the form of patch or reseal) may be required in the near future.

However, with the wet wings with the faying sealant between the rib and skin, does this mean that to reseal you have to unrivet the skins, clean off old stuff and reapply??  Seems that if you were just using chemicals to strip then you’d likely strip the sealant in the seam as well.  How are fuel tanks resealed for this type of construction and how long do they last before leaks/need for reseal?

Have there been any studies or information put out comparing the typical lifespan of various types of fuel tanks (bladders, wet wings, fuel cells, etc.)?

I gather that the "disdain" is born of the idea that if an airframe lasts only 20-30 or so years in the fleet, that the sealant was expected to last that duration?  So I'm gathering that the issue is one of the idea of should fuel tank reseals even exist??  However given the GA fleet has such a long lifespan in some cases, what are the chances that a wet wing (of any construction type) wouldn't need patching or reseal at some point in it's life span??

 

One interesting thing is that the Cirrus fiberglass fuel cell is made in halves that are epoxied together and then sealant is used at the seam as added protection.  It appears Cirrus used Polythioether (PTE) sealant and possibly changed to Polysulfide sealant?, but I'm not sure the rationale behind this choice/details.  The use of PTE has been questioned by some, however it appears that PTE has a 10-fold increase in thermal resistance compared with Polysulfide...not sure if that's what Cirrus was targeting??  But polysulfide has more fuel/chemical resistance compared to PTE.  The other thing that’s interesting is that given the fiberglass construction, if opening and resealing a cirrus fuel tank you don’t use chemicals to remove the sealant as it would degrade the composite.  So sealant has to be removed mechanically.  In general, it was felt that Cirrus tanks are much more leak resistant.  So something that makes a Cirrus fuel cell leak seems that it would raise a lot of red flags that either the fuel is detrimental, or the manufacturing process was bad.  

Regarding bladders, Griggs Mooney bladders are made with vulcanized neoprene and nylon and come with a 5-year warranty.  Many bladders were constructed with synthetic rubbers such as Nitrile, although some manufacturers used urethane polymers.  They're suggested to last from 5-20 years.

Fuel cells consists of a fuel resistant polymer (typically liquefied nylon, Neoprene, Buna-N / Nitrile or Urethane) laminated to a fabric substrate (typically polyester or nylon) which provides structural support and puncture resistance.  They're suggested to last from 5-25 years.

 

In general most of the fuel solutions have a similar life span and they all have pros and cons between the choice from one to another.  Seems to me that Mooney's goal of maximizing efficiency was seen in the choice of a wet wing.  Curious if any A&P's or engineers out there have info to add and if this all is accurate?  Thanks!

 

[EricJ]

36 minutes ago, Marc_B said:

I didn’t realize there was "disdain" for the way Mooney made their wet wings. 

There is for people trying to make a legal argument that serves their own purposes.   I don't think it goes beyond that.

A job description for maintainers of large airplanes (e.g., airliners, transport aircraft) is "tank diver", because repairs are constantly needed and they're all wet wings.  There are many, many small companies that make a living as mobile tank repairers servicing all manner and sizes of turbine aircraft, because there is a constant need for repairs to those aircraft.

Houston Tank Specialists do a lot of Mooneys, but their website also says they do Pipers and Twin Cessnas.   Fuel tank repair is a common enough thing that we had a whole section on it in A&P school.   It wasn't a casual mention, we spent some time on it, and Mooneys weren't even remotely mentioned.

These guys are local, and iirc I called them once and they mostly do turbines, and because they mostly do turbines they're pretty expensive;

https://aircraftfueltankrepairarizona.com/

It seems evident to me that George is singling out Mooneys because the first two publicized tank failures due to G100UL were Mooneys and some folks here held his feet to the fire a bit when he wasn't very forthcoming with answers.    His filing says little to nothing about the numerous other failures to date.

[MikeOH]

31 minutes ago, EricJ said:

There is for people trying to make a legal argument that serves their own purposes.   I don't think it goes beyond that.

BINGO!

As @Marc_B related, the Mooney tank 'system' is just an engineering trade-off between various fuel tank solutions.  They ALL require periodic maintenance.

This wasn't an issue until GAMI needed to defend G100UL in the marketplace.