Replacing shock discs

[A64Pilot]

The Thrush crop duster has a very similar rubber disk landing gear system, they are differently shaped, they aren’t flat but are thinner at the edge so they can compress more, but very similar

In the Thrush people don’t change them until they crack, interestingly for some that’s decades, others every couple of years

[EricJ]

10 hours ago, DCarlton said:

Getting my tanks sealed in Sep and wondered if I should have the disks checked.  I’m not feeling a strong argument to do it in this thread.  ??? 

Unless they crack or are otherwise disintegrating or don't pass inspection for compression, there may not be a reason to change them.

[Shadrach]

13 hours ago, DCarlton said:

Getting my tanks sealed in Sep and wondered if I should have the disks checked.  I’m not feeling a strong argument to do it in this thread.  ??? 

What's the date of mfg on you shock discs?  I am dubious on the theory that leaks are induced by rough landings or old discs.  The discs are either flexible or they are hard.  Either way, any impact that would generate sufficient wing flex to delaminate the sealant would likely result in a condemned wing.

[A64Pilot]

IF and I’m not saying they are, but if they are like the crop duster ones they get hard when they get old, and when they get hard they crack.

Most likely they will do that or go soft and bulge out like an old man’s vertebra disks.

I think if they look fine, it’s likely they are fine.

My guess is for those that don’t read this Forum it’s likely they never give them a thought, like the no back spring.

Flip side to that is I guess if you replace them you should be good for 20 years or so?

 

Edited July 25, 2023 by A64Pilot

[RobertGary1]

 

On 7/25/2023 at 8:25 AM, EricJ said:

Unless they crack or are otherwise disintegrating or don't pass inspection for compression, there may not be a reason to change them.

I disagree a bit. The service manual spec allows for a lot of wear of the mains. Basically on jacks the gear can rise and fall nearly an inch and be in spec. That means only gravity is dropping the gear for the squat switch. In the winter they may hang up and you get scary alarms. Since they’re not difficult to change I’ll probably change mine again once I get that slop. 

[EricJ]

6 minutes ago, RobertGary1 said:

I disagree a bit. The service manual spec allows for a lot of wear of the mains. Basically on jacks the gear can rise and fall nearly an inch and be in spec. That means only gravity is dropping the gear for the squat switch. In the winter they may hang up and you get scary alarms. Since they’re not difficult to change I’ll probably change mine again once I get that slop. 

That's certainly a consideration for airplanes with a squat switch.   It's obviously less of an issue for airplanes with an airspeed switch. 

[kortopates]

What's the date of mfg on you shock discs?  I am dubious on the theory that leaks are induced by rough landings or old discs.  The discs are either flexible or they are hard.  Either way, any impact that would generate sufficient wing flex to delaminate the sealant would likely result in a condemned wing.

Sort of agree with you here.
I think the main contributor to wing tank leaks is just the gear supporting the wing directly. Hard disk, have to contribute in some way to the amount of shock loads transferred to wings but not very quantifiable.
But what convinced me is the Cessna Cardinal has exactly the same sealed tanks as do our Mooney’s but it’s very rare for a Cardinal to need a reseal unlike our Mooneys. The big difference of course is the gears.


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[A64Pilot]

I don’t think it’s the gear so much as it’s 

1. different wing design

2. Cardinal gear attach to the fuselage, whereas Money gear attach directly to the wing. 

I think a better comparison would be other low wing aircraft with wet wings.

It’s common for many other wet winged aircraft to need re-sealing at around 20 years or so, nothing lasts forever.

I think there are many other considerations like a hangar, does it stay full of fuel or often left at a lower fuel state etc. 

 

Edited July 27, 2023 by A64Pilot

[DCarlton]

On 7/25/2023 at 11:09 AM, Shadrach said:

What's the date of mfg on you shock discs?  I am dubious on the theory that leaks are induced by rough landings or old discs.  The discs are either flexible or they are hard.  Either way, any impact that would generate sufficient wing flex to delaminate the sealant would likely result in a condemned wing.

I’ll take a look.  Need to dredge thru logs too and see when they were replaced.  It was before my ownership.  Mine seem fine. Don’t seem hard or cracked.  Tire pressure seems to make as much of a difference as anything.  I try to keep them inflated correctly.  

[RobertGary1]

23 hours ago, DCarlton said:

I’ll take a look.  Need to dredge thru logs too and see when they were replaced.  It was before my ownership.  Mine seem fine. Don’t seem hard or cracked.  Tire pressure seems to make as much of a difference as anything.  I try to keep them inflated correctly.  

When on jacks is there slop in the mains? 

[N201MKTurbo]

23 hours ago, DCarlton said:

I’ll take a look.  Need to dredge thru logs too and see when they were replaced.  It was before my ownership.  Mine seem fine. Don’t seem hard or cracked.  Tire pressure seems to make as much of a difference as anything.  I try to keep them inflated correctly.  

If they are half flat, it lands smoother. Not good on the tires….

You don’t need to check the pressure, when it gets hard to drag it out of the hangar, you need more air.

[Kelpro999]

Got 61yrs. out of my firestones  but upgrading because I heard it’s better 

[N201MKTurbo]

5 minutes ago, Kelpro999 said:

Got 61yrs. out of my firestones  but upgrading because I heard it’s better 

They just don’t make them like they used to.

[DCarlton]

1 hour ago, RobertGary1 said:

When on jacks is there slop in the mains? 

Is there a spec for slop in the manuals ?  

[RobertGary1]

14 hours ago, DCarlton said:

Is there a spec for slop in the manuals ?  

Yes but it’s overly generous. I was well within the spec but started getting gear alarm in the winter. Talked to Don Maxwell who said  this. 

[DCarlton]

On 7/25/2023 at 11:09 AM, Shadrach said:

What's the date of mfg on you shock discs?  I am dubious on the theory that leaks are induced by rough landings or old discs.  The discs are either flexible or they are hard.  Either way, any impact that would generate sufficient wing flex to delaminate the sealant would likely result in a condemned wing.

Here's a few pics of my discs.  My F is a '67.  If the number (11968) that looks like a date is a date, then my '67 didn't get completed until '68 and my discs are 55 years old.  Haven't looked through logs to see if they were ever replaced.  They don't seem to ride harsh and I don't have any obvious signs of them being a problem.  

[RoundTwo]

3 hours ago, DCarlton said:

Here's a few pics of my discs.  My F is a '67.  If the number (11968) that looks like a date is a date, then my '67 didn't get completed until '68 and my discs are 55 years old.  Haven't looked through logs to see if they were ever replaced.  They don't seem to ride harsh and I don't have any obvious signs of them being a problem.  

I believe that to be the part number. The date code is normally on the raised circles, like on these Lord engine mounts. 

[Shadrach]

On 7/26/2023 at 5:22 PM, kortopates said:

Sort of agree with you here.
I think the main contributor to wing tank leaks is just the gear supporting the wing directly. Hard disk, have to contribute in some way to the amount of shock loads transferred to wings but not very quantifiable.
But what convinced me is the Cessna Cardinal has exactly the same sealed tanks as do our Mooney’s but it’s very rare for a Cardinal to need a reseal unlike our Mooneys. The big difference of course is the gears.


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Agree about the shock being delivered direct to the wing. Perhaps it does contribute in some way. That being said, I have done a number of tank repairs over the years. I don’t see how minor wing flex compromises the sealant bond.  It would seem to me that if the leaks were gear induced, the failures would be in a specific area of a specific station but that has not been my experience. Additionally, my wings suffer far more motion and flex in sumner time turbulence than they ever see on landing.

[Shadrach]

On 7/28/2023 at 4:31 PM, DCarlton said:

Here's a few pics of my discs.  My F is a '67.  If the number (11968) that looks like a date is a date, then my '67 didn't get completed until '68 and my discs are 55 years old.  Haven't looked through logs to see if they were ever replaced.  They don't seem to ride harsh and I don't have any obvious signs of them being a problem.  

@RoundTwo is correct. J-11968-5 is the part number. Date code is just month and year I believe.

[outermarker]

A few comments on what I learned when the disks were changed on my 65’E. 
1. age of the disks(20+ years old) and assumed they were age hardened.

2. distance from prop tip to ground clearance assuming motor mounts are good.

3. make sure you have a drive pin (bolt) to align the nose gear assembly. This is a must.