When to change the donuts

[NotarPilot]

My landing gear donuts were last replaced in March of 2006.  They still feel like they have some cushion and sense no need to replace them.  I know there's a gap test that can be done by jacking up the plane but so far they appear to feel fine.  The plane is hangared almost all the time.  What's the opinion?  Should I change them regardless or hold out til the ride starts to feel still?  They're going on 13 years.  How old are your donuts?

[N201MKTurbo]

Mine still say 1977 on them. They still work fine.

[carusoam]

They have a procedure to measure...

The heavier the plane, the shorter the interval... generally.

PP thoughts, not a mechanic...

Best regards,

-a-

[Hank]

I thought mine said "06/96" [the writing was upside down] but when I replaced them at the end of 2012, it was actually "09/69." They worked well, but are oh-so-much-nicer now. Mymkandings improved immediately, and there was no comparison in taxiing.

On the other hand, they're $105 each, and there's 11 of them. If they pass the test, keep 'em.

[StevenL757]

56 minutes ago, NotarPilot said:

My landing gear donuts were last replaced in March of 2006.  They still feel like they have some cushion and sense no need to replace them.  I know there's a gap test that can be done by jacking up the plane but so far they appear to feel fine.  The plane is hangared almost all the time.  What's the opinion?  Should I change them regardless or hold out til the ride starts to feel still?  They're going on 13 years.  How old are your donuts?

Several thread on here about shock discs, but a couple of ways to tell if they need replacing that the factory uses are...

1.  Jack the aircraft.  Get on your butt in front of each main tire and use one of your feet to press backward and upward on each tire.  If there’s any slack, or “give” to the motion, they need changing

2.  Look for cracks in the rubber...mainly vertical cracks.  Doesn’t matter how old they are.  Once they crack, their useful life is rapidly approaching...whether they pass the above test or not

The age of everyone’s respective discs shouldn’t be a barometer to determine whether yours are at-risk.  Don’t be penny-wise and pound-foolish if they are close to needing replacement.  Fuel tank leaks have been known to be caused, in part, by defective discs.  Also, the fuller you leave your fuel tanks with the airplane sitting over regular periods of time without flying it dramatically reduces disc life.

Steve

[salty]

21 minutes ago, StevenL757 said:

Several thread on here about shock discs, but a couple of ways to tell if they need replacing that the factory uses are...

1.  Jack the aircraft.  Get on your butt in front of each main tire and use one of your feet to press backward and upward on each tire.  If there’s any slack, or “give” to the motion, they need changing

2.  Look for cracks in the rubber...mainly vertical cracks.  Doesn’t matter how old they are.  Once they crack, their useful life is rapidly approaching...whether they pass the above test or not

The age of everyone’s respective discs shouldn’t be a barometer to determine whether yours are at-risk.  Don’t be penny-wise and pound-foolish if they are close to needing replacement.  Fuel tank leaks have been known to be caused, in part, by defective discs.  Also, the fuller you leave your fuel tanks with the airplane sitting over regular periods of time without flying it dramatically reduces disc life.

Steve

I can’t imagine how you could prove that a fuel tank leak was caused by defective discs, let alone how you could prove full tanks could reduce disc life. Sounds like a case of turning wild speculations into “facts” because they’ve been repeated a lot. 

Edited November 22, 2018 by salty

[MIm20c]

From my extensive experience (sample size of 2) the long body aircraft (more weight on mains/front) will use up a set of pucks in 10-12 years. This is observed on a Midwest plane stored in a hanger since birth.  They still look like new but are dramatically shorter. 

They were changed on my C three years ago and I expect them to last twice as long. 

[StevenL757]

Salty, adding several hundred pounds of gas to your tanks and letting the aircraft sit repeatedly for days, weeks, or longer causes the disks to compress and stay compressed more than they would with a lighter fuel load.  Discs that are compressed excessively beyond their design limits exhibit a harder ride.  The harder ride puts undue stress on the airframe.  For aircraft with wet wings, this has been known to weaken fuel sealant and cause leaks.  I’m not sure how you interpret repeated events due to some or all of the causal factors I mentioned as a wild interpretations.  That logic doesn’t check.

[salty]

No, the logic that doesn’t check is thinking that correlation equals causation. In fact, you haven’t even shown correlation.

I know what you are saying is common belief, but that doesn’t make it fact. You may very well be right, I’m only arguing about the way you are presenting it as fact. I’ve seen nothing but anecdotal evidence about it.

 

Edited November 22, 2018 by salty

[Guest]

31 minutes ago, salty said:

I can’t imagine how you could prove that a fuel tank leak was caused by defective discs, let alone how you could prove full tanks could reduce disc life. Sounds like a case of turning wild speculations into “facts” because they’ve been repeated a lot. 

Sounds like someone else whose name escapes me.  Just say it often enough and people will believe it.

Clarence

[carusoam]

1 hour ago, salty said:

I can’t imagine how you could prove.... let alone how you could prove full tanks could reduce disc life. Sounds like a case of turning wild speculations into “facts” because they’ve been repeated a lot. 

Salty,

Some people share their Mooney experience, and copy it around often... because it is helpful when you don’t have the experience or knowledge yourself...

 

Some people actually have some skills that measure this weird stuff for a living...

Some of these people might be called polymer scientists or polymer engineers...

They may have only studied rubber manufacturing and design for a few days...

Some actual rubber test equipment can be found here...  

https://www.testresources.net/standards/astm/astm-d1229-rubber-compression-set-test-equipment/

 

It is true, rubber creeps (spreads out) over time... under pressure, in warm environments... (glass does too, even slower...)

The hotter the environment, the faster the creep...

The heavier the load compressing the rubber, the faster the creep...

These physical characteristics can be modified by chemical design...

 

It is also true that some people write in a way that it sounds like they are stating facts...

 

If we have to prove everything we say, and leave a reference each time... It won’t be enjoyable enough for anyone to hang around...

Consider it all a casual conversation amongst friends... you can learn enough to find the info you are looking for...

if you ask for a reference, often people will supply that too...

 

If you want to point out some wild ideas... find José when he discusses some of his ice prevention coatings... that may work for an unknown brief period of time...

Everyone has some knowledge to offer, it isn’t going to be perfect...

José is really good at fuel tank facts. Probably nobody around knows as much.  So don’t block him out because he states something factually weird about his pee tube...

You get a lot of good, with all the quirkiness... sometimes somebody will use humor without letting you know it is intended to be humor...  we have a sign for that as well... 

 

And sometimes you get a phrase like this one...

PP thoughts only, from a polymer engineer, not a mechanic...  

Best regards,

-a-

[carusoam]

On another similar topic...

 

Salty,

You wrote an interesting piece regarding rebuilding and installing your engine, and working a full time job at the same time...

You may not qualify as an expert mechanic, but I can assure you there are probably a few dozen people on MS that would be interested in your less than expert presentation of how you went about doing what you did...

See what I mean, everyone has great information to be shared...not everyone writes perfectly well...

Best regards,

-a-

[LANCECASPER]

2 hours ago, salty said:

I can’t imagine how you could prove that a fuel tank leak was caused by defective discs, let alone how you could prove full tanks could reduce disc life. Sounds like a case of turning wild speculations into “facts” because they’ve been repeated a lot. 

http://www.knr-inc.com/knr_inc_joomla/shoptalk-articles/25-shoptalk/23-200904-does-your-mooney-need-a-chiropractor

The fourth paragraph in this article is just one of many places where this has been mentioned. There are many threads on Mooneyspace that discuss this. Also has been presented many times at MAPA seminars by people that work on Mooneys every day and see a direct correlation between bad shock discs and wear on other landing gear components and more frequent fuel sealant failure.

[LANCECASPER]

1 hour ago, salty said:

No, the logic that doesn’t check is thinking that correlation equals causation. In fact, you haven’t even shown correlation.

I know what you are saying is common belief, but that doesn’t make it fact. You may very well be right, I’m only arguing about the way you are presenting it as fact. I’ve seen nothing but anecdotal evidence about it.

 

How can you prove the cigarette smoking causes lung cancer without first showing correlation? More people that smoked end up with lung cancer. There are still people that say that doesn't prove that was the cause  . . .  Fine the correlation is good enough for me.

 

Fuel tank failure is much more common in newer heavier Mooneys (especially from 1989-2006 M20M, R & S before they started sandwiching the sealant between pieces before putting everything together in the tanks) than in the older lighter Mooneys, yet the sealant chemistry is better now than it was years ago.

The only thing that has remained constant is the suspension on Mooneys - the shock discs. The newer planes fail the test in very few years (sometimes as little as 2-3 years on heavily optioned airplanes - long range tanks, TKS, A/C) compared to years ago when shocks discs on lighter Mooneys would pass the test for 12-15 years. Sealant failure and shock disc failure happening on the same airplanes sure sounds like a correlation. If all of our landings were perfect there may be no problems, but if there's a hard landing and hardly any give on "bad" shock discs something has to absorb the shock. It doesn't take much flex to break the sealant. After replacing the shock discs the ride on the ground is so much more firm - it feels like you are taxiing a new airplane. Here we go with correlation again . . but I venture to say that some prop strikes while taxiing may have been avoided with better shock discs on the nose gear.

To get back to the original poster's question . . . if they pass the test keep them. If they don't pass, you aren't really saving any money in the long run by trying to keep worn out parts on the airplane. 

[ArtVandelay]

If bad donuts cause fuel sealant failure, then so does chop and turbulence which also causes wing flexing.
Unless the sealant becomes completely inflexible I don’t see how wing flexing for whatever reason can cause fuel leaks.
I’ve been told heat/cold will cause sealant to deteriorate, planes that sit outside will have more problems then hangared, those stored with full tanks will have less leaks.
What causes old (non teflon) hoses to have limited lifespans even if left on the shelf?
Unless someone can talk about the actual chemistry involved, I’ll take it all with a healthy grain of salt.

[c177tx]

Just a lurker here, "I have a Cessna Cardinal, drooling over a Mooney" One thing in common between the Cardinal and the Mooney's are wet wings.  My 68 Cardinal has only leaked from the fuel sender's gasket and the rest of the Cardinal fleet seldom report leaking fuel tank's.  Now that is not to say that some in the fleet have not had wing tank leaks but they are the exception not the rule.  Both Mooney's and Cardinal's wing flex in flight, but the Cardinal's landing gear are not attached to wing's where the Mooney's are.  " just trying to find a correlation between leaks and weak landing pucks"

[Mooneymite]

I've operated off of turf for 20 years and my tanks don't leak.

Oh, wait.  I have bladders.

[carusoam]

TJ,

See How this logic works...

1) People that really control their landings probably don’t get leaks as much as people that collide with the ground...

2) Polymers and high temps generally don’t go well together... high temp polymers aren’t very flexible... for chemistry...something about double bonds improves the heat resistance, but ruins the flexibility...?

3) Wing top surfaces get really hot sitting outside... filling the tanks helps transfer heat away from the surface... not allowing them to get so hot...

4) hitting big cracks in the pavement must be different than big bumps in the air... I’ve seen the iPad bounce off the ceiling in turbulence we had to slow down for... no leaks...

5) One thing about bumping on the ground... is the speed of the energy transferred to the wings... there are a lot of terms to go with impact dynamics... and time.  The distribution of the forces is delivered by the gear in a small area...compared to the whole surface of the wing. Bending moments will be different

6) spreading forces out over time and surface area is going to be beneficial to lowering peak impact forces...  old compressed donuts are less effective at spreading the compression out time wise. They have less give to give... 

7) Teflon is pretty much impervious to all solvents... other rubber hoses aren’t that Impervious over time...  realistically the Teflon inside hoses is more of a chemical variation of the white rigid material we are mostly familiar with...

Interesting input 7TX.

PP thoughts only, not a mechanic...

Best regards,

-a-

[mike_elliott]

13 hours ago, NotarPilot said:

My landing gear donuts were last replaced in March of 2006.  They still feel like they have some cushion and sense no need to replace them.  I know there's a gap test that can be done by jacking up the plane but so far they appear to feel fine.  The plane is hangared almost all the time.  What's the opinion?  Should I change them regardless or hold out til the ride starts to feel still?  They're going on 13 years.  How old are your donuts?

Its false economics to keep putting it off if it is needed. The Maintenance manual calls out the recommended tolerances (0-.6") between the retaining collar and the retaining plate with full fuel and weight on landing gear. If it is not within spec, it is time to replace. It doesnt matter what is printed on the old disks or how old they are or what our opinions are. See 32-60-04 of the Service manual

[Raptor05121]

Just my $0.02, but when we resealed my tanks I ran the "grass strips ruin tank sealant" topic past my IA and he showed me some tools that had the dried sealant on them. You could twist, bend, pull, and the sealant flexed just fine. I'll let you draw your own conclusions.

[N201MKTurbo]

2 hours ago, Raptor05121 said:

Just my $0.02, but when we resealed my tanks I ran the "grass strips ruin tank sealant" topic past my IA and he showed me some tools that had the dried sealant on them. You could twist, bend, pull, and the sealant flexed just fine. I'll let you draw your own conclusions.

Properly mixed and properly applied sealant is very robust. Most leaks I find are caused by poorly mixed or applied sealant. 

Flexing caused by rough landings can cause poor sealant to fail. It can cause shear stresses that can cause weak sealant to debond.

Sealing a tank takes many batches of sealant and a lot of surface preparation. One bad batch or one missed spot on surface prep can cause a weak spot in the tank.

[ArtVandelay]

Just my $0.02, but when we resealed my tanks I ran the "grass strips ruin tank sealant" topic past my IA and he showed me some tools that had the dried sealant on them. You could twist, bend, pull, and the sealant flexed just fine. I'll let you draw your own conclusions.

Put the tools in the freezer overnight and try your experiment again.

[carusoam]

Put the donuts in the freezer as well....

Physical properties of polymers, including rubber, do change with temperature... they can have a freezing point, and a secondary transition temperature (like freezing, but becoming even more stiff than before)...

Keep in mind that fuel tank sealants don’t absorb moisture, so a temperature around 32°F probably won’t have any strange behavior in particular...   so no frozen sponge effect...

+1 on hard landings exacerbating existing problems...

PP thoughts only, not a mechanic...

Best regards,

-a-

[Oldguy]

You can change on condition, time, or whatever you want to use as a guideline.

Change on condition would be when the actual gap measurement does not meet the service manual measurement.

Change on time would be every (pick a number) years.

The final option would be to change them when ever you feel like you want to do it or have it done.

I am getting mine done this annual as they are old enough to warrant it (12-14 years) and barely passed the gap check last year. Plus, as @Hank mentioned, my landings can always use a little help.

[Shadrach]

On 11/24/2018 at 11:26 AM, teejayevans said:

Put the tools in the freezer overnight and try your experiment again.

Have you ever done this with PRC or Flamemaster sealants? It’s amazing how well this stuff fares on the pressure vessels of high altitude turbine aircraft as well their fuel tanks. You seem to think the sealant loses some significant amount of elasticity with temp. You are mistaken. These sealants remain flexible we’ll into sub zero temperatures. Have you ever seen the wing on a commercial or business jet flex on landing? How cold soaked do you think that fuel (and sealant)  is after five hours at 35,000 to 45,000 feet? 

The “worn donut fuel leak” theory is an old wives tail in my opinion. I’m not saying you couldn’t have a hard enough landing to damage your fuel tanks. I am saying is that if you had a hard enough landing to damage the tanks the plane would likely be totaled. 

 

 

Edited November 26, 2018 by Shadrach