Nose Wheel Strut Issue

[RMarkSampson]

Looking for anyone with experience of a DA-20 NLG strut and its wheel pant. The wheel pant and the fairing around the strut has been rubbing. The wheel pant is held in place by two bolts near the rear of the fork assembly and the tow bar pins that insert into forward part of the fork assembly. The angle between the strut and wheel fork is a fixed dimension. When I removed the fairing around the strut I could see very little clearance between the wheel pant and the NLG strut. My worry is that the strut is becoming bent in such a way that that clearance has diminished. I've included a picture of the NLG with the strut fairing removed and the wheel pant still in place.

The NLG likes to shimmy - frequently it will start osculating a few wags upon NLG touchdown, on occasion violently until you either get the NLG unloaded up in the air again or slow down while being shaken like a dog with its toy, so I am struggling with that issue also. I'm aware of the castellated nut and the spring scale to set the proper torque - but it remains an issue. Not sure if the two issues are connected.

Appreciate any feedback. Regards Mark

[Steve]

Your symptoms are what I would expect with a severely worn bearing on the fork where the strut pivots. I would jack the airplane so that weight is off the nosegear and see if the fork pivots up and down (it should only turn left and right).

Steve

[krellis]

I'm not an expert on the C1, but I believe the nose strut assembly is virtually identical in form and function to my A1. I suspect the rubber dampers in the "shock absorber" are worn out, changing the height and therefore geometry of the nose strut. The easy check is the prop clearance from ground to tip - your AMM probably has a specification for this. The height of the shock absorber itself is shown in section 32-20-00 of the C1 AMM (245mm).

Krea Ellis
DA20-A1 N853DF "Princess Amelia"

[Steve]

The height of the rubber damper stack won't affect the geometry of the fork-strut clearance, as apparently is the case here. Bearing wear could cause it, as well as the bad shimmy...

Steve

[krellis]

Yes, my bad. The pivot bushings are probably worn (20-3220-01-05) which would cause both problems. Good time to check the condition of the welded stud on the end of the NLG strut, as these have been known to crack and fail, allowing the nose wheel assembly to depart the airplane.

Krea Ellis

[RMarkSampson]

Thanks for the feedback - I already have the rubber damper stack on order but did not see that curing the clearance issue. I'll inspect the pivot bushings and replace them too.

Mark

[RMarkSampson]

A bit of feedback after the NLG rehab. Replacement of the NLG Steering Stop Plate (p/n 20-3220-00-23) likely had the most benefit to my shimmy issue. Aluminum Stop Plate vs. steel strut means the aluminum will wear down - there was noticeable sideways freeplay with the old Stop Plate that was eliminated with the replacement of the Stop Plate. I also replaced the pivot bushings (p/n 20-3220-01-05) as recommended but the old ones seemed to be in reasonably good shape - but as Steve mentioned they do wear so it seems to be good practice to replace those when chasing a shimmy issue. The strut and stud are solid.

The rubber dampers were worn out - at least they were significantly "thinner" than the new ones. The uploaded picture shows the height difference between the new and old dampers - it is like adding an additional damper to the stack. So squeezing the new dampers to allow the end nut to be threaded became an issue. Now I know why Diamond sells the entire assembly for $900 vice the $300 to just replace the rubber parts. I only replaced about half of the dampers because of that issue. I did not see anywhere in the AMM where there was a specific "squeeze pressure" that was required for the strut assembly - only the distance. So we achieve that exact distance with a combination of new and old dampers that allowed us to hand-squeeze the dampers to get the end nut threaded. The outcome was satisfactory. I was interested in knowing if anyone has dealt with this issue. I also can envision how to build an inexpensive jig using two threaded rods to squeeze the dampers beyond what can be done by hand - was interested if others had done something similar.

No excessive shimmy noted with a couple of post-maintenance landings. The wheel pant remains off - I don't think I have figured out the clearance issue between the strut and wheel pant. For another day...

[Steve]

Mark:

Yes, replacing the elastomer elements is a b**ch! I have done it twice on my DA-40. I used a large woodworking vise on my workbench to compress the stack enough to get the nut started, but it wasn't easy (or safe, for that matter, since one slip and the rubber donuts may have gone flying!). This recent SB about inspecting the center rod on the elastomer pack has an appendix which shows a spring compressor tool you could construct to make the job easier:

http://support.diamond-air.at/fileadmin ... r-tube.pdf

As an aside, I would either replace all, or none of the elements. I don't think it is a good idea mixing old and new ones.

Steve

PS: Isn't it amazing how much Diamond charges for those standard DIN rubber cushions? I found a source for them in England and the cost for a set (including DHL shipping) was less than $100.

[RMarkSampson]

Steve,
Thanks for the PDF of the SB - my A&P called Diamond Customer Service and got the same info. We will be getting all the new rubber dampers in the stack after I build the jig. So my two threaded rod jig idea will turn into a three rod one per the SB. With three rods there will be no chance of a Darwinian lesson and the jig torqueing itself into a double helix and removing any digits from yours truly.

So I was feeling good about avoiding the $900 "just replace everything" solution - going for the $300 "dampers only" solution - until you mentioned the $100 solution. Aviation math - everything costs three times as much as it should - and if you get another middle man in the mix - then it is another three times as much for whopping 9 times as much. Got to love it...flying that is...
-Mark

[keithPTC]

The NLG likes to shimmy - frequently it will start osculating a few wags upon NLG touchdown, on occasion violently until you either get the NLG unloaded up in the air again or slow down while being shaken like a dog with its toy, so I am struggling with that issue also. I'm aware of the castellated nut and the spring scale to set the proper torque - but it remains an issue. Not sure if the two issues are connected. Appreciate any feedback. Regards Mark

We had the same aggressive shimmy as described by Mark above on NLG touchdown. Confirmed that the NLG fork pivot friction was adjusted within Diamond specs and then per Service Bulletin C1-32-03 replaced Belleville Spring washers with new style. The NLG still shimmied.

The solution was to replace both Wheel Fork Pivot Bushings (p/n: 20-3220-01-05). NLG shimmy is now gone.

Pivot bushing installation tips: remove the old pivot bushings with a hammer and brass drift (not steel). Use an arbor press or hydraulic shop press to install new bushings. Be sure to grease the inside of bushings and the top and bottom bushing flange (Aeroshell 22) when you install the fork to strut.