LASAR Truss and SB 202

[Mooneymite]

Nose gear truss repair and SB 202.

My nose gear truss had dents in it when I bought my '74 C twenty years ago.  They were close to limits, but within limits.  Recently I rechecked the dents in the truss and much to my surprise, one side was dented well beyond limits.  I have no idea when, or where it happened; my last annual was a year ago.
I ordered a replacement re-built truss from LASAR and got three shock biscuits from Aircraft spruce.  Working under the direction of an IA, I got the rebuilt truss installed with the new biscuits.  Definitely not a fun job unless you like old, dirty grease and working in impossible spaces.

The replacement involved not just a logbook entry, but a 337 as well because the replacement truss was not just rebuilt, but modified (STC) with the "over-steer" protection tabs.

Optimistically, I taxied out for a check flight, but that's when the project went south.  My Mooney which has always taxied and tracked straight and true was like a wandering lover...all over the place.  Having read about this phenomenon on Mooneyspace, I knew to look up SB M20-202 (https://cdn2.hubspot.net/hubfs/4147179/technical_documents/service_bulletins/sbm20-202.pdf) and Don Maxwell's piece on this issue: http://donmaxwell.com/the-eight-second-ride-sb-m20-202/

I called Dan at LASAR parts and told him about my troubles with the LASAR modified truss and after some back and forth, he agreed to send me the spacer detailed in SB 202 (M20-202-3).  Dan admitted that all the rebuilt trusses were rebuilt on the same jig regardless of the year of the plane it was destined for.  I presume that their jig is based on an earlier model than my '74 which has never needed the spacer.

After jacking up and leveling the plane, the trouble was obvious, the plumb line was well aft of the nose geat axle.  The castering geometry was way off.

We re-compressed the biscuits and installed the spacer.  The change was amazing considering the spacer is only 0.12" thick.  The axle was now well aft of the plumb line. 

 The subsequent check flight was fine with no evidence of nose wheel wandering.

As a side note, I did not have, or use the Mooney tool to compress the biscuits.  I used a combination of normal aircraft weight on the nose wheel and a  turnbuckle where the shock strut mounted.  The nose biscuits we removed were dated 3/83 and were within limits, but I replaced them because I didn't want to have to redo this job when they did, in fact fail to check out.  36 years on a set of biscuits is pretty impressive.  The attached picture shows the difference between three 36 year old biscuits and new ones.

Edited December 29, 2019 by Mooneymite

[David Lloyd]

Mooney changed a bolt size in 1968 or 1969, otherwise the dimensions are the same.  My understanding was the only tubes replaced were the ones that had damage.  They do add the stops requiring the 337 unless you already had stops.  I had the truss on my '75 replaced a few months back.  You may have gotten mine after it was repaired.  Mine traced straight before and after replacement with no shim. 

Years ago I had a 1965 C that was dented on one side.  A few days after buying it, there was a dent on the other side.  Check every time so you know who did it. Then kill 'em. Back before Al invented the internet, I had almost every part in the nose gear and steering replace to cure it's darting and self steering.  Then a friend told me about SB202.  Different airplane after that. I was almost scared to fly it because sooner or later it had to slow down on the ground.  

[carusoam]

See if this helps to plumb the story...

Great details, Mooneymite!

Thanks for sharing them.

Best regards,

-a-

 

[Hank]

Glad you got it figured out, Gus!