December 12, 2004

This is your tailwheel. (Sound of sizzle.) This is your tailwheel on drugs. Yeah, I know, looks pretty ugly. Trust me, I'm planning on having it powder coated one of these days when I can stand not to fly it for a few days. The tailwheel spring and swivel take a real beating back there, getting pelted with whatever loose crap it can find, not to mention every little bit of stray oil and grease and gunk that makes its way along the belly. During the course of more than 300 flight hours, a fair amount of sludge has managed to find its way back there. Anyway, the other day I disassembled it and cleaned and regreased everything. The rain we had recently really served to wash out the grease and dry it out (not good). As I was working with it I noticed that there was a "clicking" if I pushed up on it and rotated the tailwheel. At first I thought something was catching inside the swivel, but I quickly realized that it was the whole tailwheel assembly that was rotating and clicking as it hit the limits of the range of play.

Here's a movie (1mb WMV) that illustrates how much play there was. At first I thought the play was in the area of those two AN3 bolts. I replaced those bolts and didn't notice any elongation of the holes or marring/notching/flattening on the old bolts. After replacing them I realized that the play was in the front, where the tailwheel spring attaches to the weldment in the tailcone, not at the rear. There's a single AN4 (1/4" diameter) bolt up there. When I went to loosen the nut and remove that bolt, I found that it definitely wasn't fully torqued anymore. I had originally torqued it to 75 inch-pounds back in March, but over the course of a few hundred hours, I guess it had the opportunity to wear in a little and loosen up a bit. Definitely something to keep an eye on.

So here's that AN4 bolt removed and cleaned. Sorry for the fuzzy photo, but you can still see the marks on the bolt where it was getting clanked around in there.

The hole in the spring looked fine -- I mean, this is hardened steel, so it's the strongest piece of the puzzle. The weldment tube is what would give out first, most likely, in form of elongated holes. Still, I didn't notice any elongation there either (naked eye inspection).

At first I just replaced the AN4 bolt and AN365-428 nut with new ones and reinstalled and retorqued everything. That took out most of the play, but there was still a tiny bit. So I bought a few AN174-14A bolts, which are "close tolerance" bolts designed to have a slightly bigger diameter than regular AN4 bolts. When I installed the AN174 and torqued it, the play is almost completely gone. There's still a minute trace of play, although I think most of it is in the swivel area now. I can definitely live with it, as long as I keep an eye on it.

I spoke to Ken Krueger (Van's engineer) about this, and I asked him what he recommended doing if the play got worse even with the AN174 bolt in there. I was wondering if that front hole could be enlarged to 5/16" for an AN5 bolt. His response was:

Dan,
I do not know how much you can "oversize" the hole attaching the tailspring to the tailspring mount.

You might be okay with a 5/16 bolt. I would strongly recommend that if you elect to drill a larger hole that you use a 1/64 oversize hole with a 1/64 oversize bolt. These bolts have a shank that is 1/64 larger than that of an otherwise identical NAS type bolt. These are called "repair" bolts, or more correctly "dash X" bolts. (There are also 1/32 oversize repair bolts, a.k.a. "dash Y" bolts.)

I have zero concern if you elect to go 1/64 over, a tiny bit of concern if you elect to go 1/32 over, and some concern if you elect to go to 5/16.

Repair bolts are sometimes hard to come by and are always expen$ive but they can be really nice when you are in a pinch.

Anyway, this is useful information. I wasn't aware of the "dash X" or "dash Y" hardware. I learn something new about aircraft hardware every day, it seems.

Absolute worst case, I could replace the spring mount weldment and the entire tailwheel assembly if I had to. But fortunately it sounds like there are several tolerable steps before it gets to that point. My plan is to keep an eye on this and watch for any play developing. It's on my "stuff to check at every oil change" list now.

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Dan Checkoway ()