Vertical fin spar crack

[8475]

My mechanic found a couple of cracks in the vertical fin spar. These are at the corners of the rectilinear hole at the base of the spar. The cracks are in the doubler (PN 0431129). We first drilled out the doubler and found that only the doubler was cracked. However, given the age of the parts and the fact that we have the Univair spar replacement, we opted to replace the spar as well.

N76415 vert fin spar crack.jpg (277.21 KiB) Viewed 5075 times

N76415 vert fin spar crack closeup.jpg (278.46 KiB) Viewed 5075 times

[8475]

Here is a pic of the removed spar. Note that the upper rudder hinge is mounted on a doubler. I cannot find this doubler in the Cessna parts catalog. Is this part of a service bulletin or AD? Or just a discretionary field addition?

N76415 vert fin spar removed.jpg (181.77 KiB) Viewed 5074 times

[8424]

Hi Dave.

The doubler is part of the C140A rudder.

In the C140A Parts Manual, it's on page 21, Figure 13, Fin Assembly, number 3. The part number is 0431124. Channel - Fin Spar Reinforcement.

The Fin Assembly is the same part number for the 120, 140 and 140A, even tho the 140A assembly has the top hinge reinforcement channel.

I can't find an AD directing the addition of the doubler under the top hinge.

It would be good to have John Cooper/6643 confirm all this!

Hope this helps.

Steve A H

[6643]

Don't know...

To the best of my knowledge, there is on AD or service bulletin relating to the top hinge.

[8475]

Steve, thank you for that C140A advice! Super helpful. Searched the Univair catalog with that number and it's available for about 10 bux.

John Cooper, thanks for the advice about the AD. Appreciate the insight.

I think I'll put this back on the new spar. Can't hurt.

[a64pilot]

First annual I did after buying my 140, after spinning it pretty regularly, I found that the main spar was rotted through under the doubler that I believe an AD had required be added.
When I replaced everything, I put fuel tank sealer under the doubler before we shot it on, that sealed it really well and hopefully the spar won’t rot out ever again.

[8475]

First annual I did after buying my 140, after spinning it pretty regularly, I found that the main spar was rotted through under the doubler that I believe an AD had required be added.
When I replaced everything, I put fuel tank sealer under the doubler before we shot it on, that sealed it really well and hopefully the spar won’t rot out ever again.

Your doubler at issue, what was its location? At the base of the spar? Or was it the one I pointed out, under the top-most hinge?

[a64pilot]

Base of the spar. I replaced my spar 10 years or so ago.
I just went out and looked, my upper hinge point also has a doubler exactly like yours. I don’t remember if the new spar came that way or what.
However you can see where we sealed it also with fuel tank sealer.

We built crop dusters and had to shim a mounting point to achieve perfect alignment, but we had three hinge points and our 140’s only have two? I was thinking the doubler may be a shim, but with only two hinge points I don’t think alignment is as much of an issue?

Best pic I could get with rudder installed.

If this video works, forward about 1 min into it, you can see the vertical fin had no flex at all, even with the rotted out spar
<iframe title="vimeo-player" src="https://player.vimeo.com/video/9791700" width="640" height="428" frameborder="0" allowfullscreen></iframe>

[8475]

Thanks, Jody. Appreciate the info. Also, love the video.