Nose gear pucks

[Bartman]

The biscuits on my J were about a year old when I purchased in 2007. The mains still look new sitting in the hangar. However, one of the nose gear biscuits has a split and needs to be addressed. My bird leaks a little oil, just enough to keep a film of oil on the nose gear biscuits. We have eliminated cracks and it's just a nuisance, so let's not get diverted talking about a few drops of oil. 

It's time for annual and although the IA is competent and did a great job last year, he has not changed out these biscuits. He can rent the tools, I have the maintenance manual, and says we can tackle this project. Questions:

1. Is it reasonable to expect that we can do a good job having never done it before ?

2. The mains still look new, would you replace them too ? 

3. I have never had the problem with darting one way or the other, but the brake pedals do vibrate or chatter occasionall on landing and it did not change when we replaced the brake pads. Is this a symptom of an issue with the the nose gear ?

4. Any words of wisdom or link to a website will be greatly appreciated !

[Mooneymite]

I would check for warped brake rotors first if there is chattering in the pedals.

[N201MKTurbo]

If you have the tool, it is easy to change the doughnuts. Wrap some cardboard like the back of a legal pad around the contact points to keep from scratching the paint.

If they were new in '07, it is hard to imagine that they need replacing. If the nose wheel is shimmying, I would look at steering bushings (horn and truss) first.  

[Bartman]

No nose shimmy at all, just brake pedals chattering and warped brake rotor sounds along the lines of what I was thinking. When they replace the pads they also replaced the brake fluid and the old stuff was very thick like consistency of cold syrup.  

One of the doughnuts on the nose is cracked or split and we are going to change those three.  We will look at the truss for dents and damage and address worn bushings and linkages as needed  

I do not have any darting or shimmy, just feel chatter in the pedals even before I touch them to apply the brakes. However, the brake pedal chatter goes away if I just tap left, right, left kinda like I do in turbulence to improve the ride and the rollout is always straight and smooth.  It doesn't even happen every landing, maybe one in 5 or less.  Warped rotor sounds plausible. 

[cliffy]

How deep is the split in the biscuit?  2007 ain't really too old for a J.  Have you checked the tolerance at the top of the strut pack per the MM?  Yes changing the nose pack is doable with care . Not really a big issue   Skygeek tends to have the best pricing on the donuts.

There is a wear measurement for the main's donuts when on the ground or just jack the plane up and if you can't turn the donuts by hand they're OK. They should be OK.

Brake rotors are a possibility for brake pedal vibrations I would however check the nose gear while you have the airplane  on jacks.  With the nose wheel off the ground grab it and turn it back and forth in the motion as it would go while making turns on the taxiway.  5 degrees either side of center is OK,   20 degrees is way too much. This indicates that the steering linkage at the top of the nose strut is worn in the bushings and bolts. 

If the brake pedals vibrate more as the they disc heats up with long brake applications I'd say look for the problem in the discs and calipers.

[Vance Harral]

Not unreasonable for a competent mechanic to replace shock discs as a first time job, IMO.  The structure isn't complicated, the procedure in the service manual is pretty clear, and there are numerous references on the web (here's one with pictures).  The tools are not an absolutely necessity, but are the best way to get the job done.  Great if your mechanic can rent them.

As for replacing the mains, the official guidance is in your service manual.  For the M20J, it reads:

1. Aircraft with full fuel load and weight on landing gear.

   A. Main gear shock discs, (See Fig 32-21).

      (1) Check gap between retaining collar (A) (Figure 32-21) and top retaining plate (B).  Allowable gap is 0.00 to 0.60 inches

      (2) Replace discs when gap exceeds the tolerance.

I've read that on short and mid-bodies, Mooney recommends replacing the shock discs every 8-10 years, but that calendar limit is not discussed in the service manual.  My mechanic is a firm believer in replacing them on condition, rather than on a calendar limit.  He looks for any gap between the collar and the top plate, and also looks at how the gear expand when the airplane is jacked at the annual.  Many people go beyond 10 years on a set of shock discs, at least on the short/mid-body models.  My guess is those are all hangared airplanes, as sun and wind exposure will deteriorate the rubber more rapidly.

At the super-risky end of the spectrum, some people don't know they need new shock disks until they can't raise the gear after takeoff, because the shock disks don't expand enough to trigger the squat switch.  My understanding is you can raise the gear if you wait a minute or so, for the crusty old discs to expand.  Seems like a relatively benign failure mode if you know what's happening, but obviously doesn't work with the manual gear, and certainly not a strategy I'd recommend.

[LANCECASPER]

 

The biscuits on my J were about a year old when I purchased in 2007.

I would check the date stamp on the Lord mounts themselves (biscuits). It should show the month and date of manufacture. Sometimes they look OK but are hard as a rock and don't offer any suspension after so many years.

[Oldguy]

I would only replace mains if they did not meet service limits as stated in the MM.

Like others, I would look to the rotors for the source of the shimmy and then the geometry of the nose truss. If you are around a long, wide runway, you might even try taxiing and alternately applying individual brakes to see if the shimmy is affected by single side application. Just be careful not to rodeo off into the grass!

Changing the biscuits should be something a good MX could do with instructions and the proper tools. There are several options to rent and it makes the entire process easier.

[Guest]

A loose wheel bearing, a warped disc, or worn torque plate holes and caliber pins can cause pedal pulsing.

Clarence

[Andy95W]

 

A loose wheel bearing, a warped disc, or worn torque plate holes and caliber pins can cause pedal pulsing.

Clarence

As can nose wheel shimmy caused by worn bushings and steering horn wear.  

[Bartman]

So it seems the information I was given was incorrect. If I am reading these numbers correctly these discs on the nose gear were made in May of 1990 and need to be replaced. If its been that long since they were replaced then the nose gear truss, bushings, bolts, and linkages will need attention too.

I wonder if upgrading to the LASAR rebuilt nose gear components is the way to go. If this is not terribly expensive I will consider it.

Hope this file works. I converted it from a 4.8mb PNG to a 633kb JPG

Edited January 14, 2016 by Bartman

[AllanPedrosa]

[carusoam]

Looking at your photo Bartman, 25 years seams to be too much...

1) Rubber that cracks and chunks off is pretty much done.  

2) the donuts typically start life in a cylinder shape.  They take on the toroidal pastry's shape as they slowly compress over time.

3) measuring the gap as outlined by Vance above is the engineering way to be sure.

4) the date thing can be really misleading.  In the case where the numbers can be misread like 6 69  or 9 96 right side up vs upside down.  Some aren't that easy to read as the numbers chip off with age...

5) typical normal useable life can be in the 5 - 15 year timeframe.  Depending on weight and temperature.

6) leaving the tanks full up is good for the tank sealant.  Leaving them empty is good for the donuts.

7) changing donuts every now and then is good for the plane.

How does that sound?

Best regards,

-a-

[N201MKTurbo]

That looks easier then the Mooney tool!

[Bartman]

Wow guys thanks so much for all of the great information!  And especially to Allen for all of the step by step pictures. Having only posted a pic once before, and it was too large, it took me a little while to figure out how to post one, much less doing 10 on one post. 

I looked it up in the logs and the nose gear discs were changed in 2005, and the mains were changed in 2002.

We discussed it last night and have decided to go with the the LASAR options and rebuild the nose gear. My bird was hatched in 1977 and I'm sure it could use some more attention than just the bare minimum shock discs. The mains look great sitting in the hangar, but we will jack her up and measure this weekend, and replace if needed.

[LANCECASPER]

35 minutes ago, Bartman said:

Wow guys thanks so much for all of the great information!  And especially to Allen for all of the step by step pictures. Having only posted a pic once before, and it was too large, it took me a little while to figure out how to post one, much less doing 10 on one post. 

I looked it up in the logs and the nose gear discs were changed in 2005, and the mains were changed in 2002.

We discussed it last night and have decided to go with the the LASAR options and rebuild the nose gear. My bird was hatched in 1977 and I'm sure it could use some more attention than just the bare minimum shock discs. The mains look great sitting in the hangar, but we will jack her up and measure this weekend, and replace if needed.

If they installed shock discs in 2005 that were manufactured it 5/1990 they were used at the time. I would be very suspect of ever using the person that did that maintenance.

[PMcClure]

1 hour ago, LANCECASPER said:

If they installed shock discs in 2005 that were manufactured it 5/1990 they were used at the time. I would be very suspect of ever using the person that did that maintenance.

Looks like pencil whipping to me. I would have my mechanic check anything else that was signed off by that mechanic in the logs. Also, LASAR rents the tool to install. You have to deposit $500 but I think they return $450 when returned. Just did my last week. 

[helitim]

If the discs are 25 years old, I would consider doing exactly what Allen shows and remove the truss, disassemble and inspect the bushings at all the joints.  I did this just a few weeks ago and found excessive wear on two of the bushings. While it was apart, I media blasted and repainted the whole thing. Replaced the steering horn and biscuits. Even added a new tire while it was apart.    Have a new nose wheel assembly now.  Can't wait to get the engine back on and see how it all flies now.

[cnoe]

And don't dismiss what Clarence said above about torque plates and caliper pins. After my last disc change I developed a brake chatter that I first identified while pulling the plane with a tug. Wheel/brake vibration and noise would intermittently start without even applying the brakes. New torque plates and pins solved the problem immediately.

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[Bartman]

We finally got her up on jacks today and we were surprised to find out how much play there was in the nose gear. It had excessive play in all 3 direction ps including front to back, sided to side, and also slop in the hemi linkages for turning.  We have decided to go ahead and replace everything with whatever it needs from LASAR. we have not disassembled anything, but I think we are better off with one of the pre-assembled subassembly packages offered on their website.  I figure we need everything and will go with either package C with rebuilt steering horn, or package D with the new steering horn.  

Any PIREPs or recommendations on this approach would be greatly appreciated.

 

http://www.lasar.com/docs/lasar%20nose%20gear%20packages-3.pdf

[N201MKTurbo]

The work they are doing is easy to do. The hardest part is stripping and reprinting. I would also do the oversized truss bushing.

once you remove the parts, you are mostly done anyway.