Vans aircraft announced a Service Bulletin a while back that applies to my plane. The SB requires recurring inspection of a section of the main spar of the horizontal stabilizer that has been shown to develop cracks on some aircraft over time . This is not a particularly worrisome development -- GA airplanes get these types of service bulletins or ADs (Air worthiness Directives) all the time.
What is a Service Bulletin and how does it differ from an AD? SB are maintenance recommendations from a manufacturer and do not carry the force of law. Air worthiness Directives are mandatory compliance issues that come from the FAA. An AD may reference an SB in which case the SB does become mandatory. Even though an SB may not be mandatory, it is probably not a good idea to just ignore them.
That Vans Aircraft has responded with the SB and provided a retrofit kit is a good thing and shows that the company is safety conscious and is proactive in maintaining their good reputation. The SB states that the retrofit kit is not required until cracks have developed. As a preventative measure, I have decided to implement the fix anyway since the horizontal stabilizer is not yet on the plane and has not been painted. There will never be a more convenient time to do the work so I might as well do it now even though it will delay my completion date a few days.
The first step is to disassemble the center section. This means removing the inside ribs and the attachment flanges. There are a lot of rivets in here. The last time I worked on this piece was in early 2012. I always hoped that I would be get to fly the plane a little before I started tearing it apart.
The rivets attaching the inside ribs are removed.
The horizontal stabilizer main spar is that thin piece of sheet metal in the foreground. The area of concern are the four rounded notches right at the edge of the skins, This is, evidently, a high stress area, which is why the notches are placed there in the first place. The theory behind the notches is that the slow curvature of the inside edge of the notch would allow the stress to flow around this area rather than build up at a single point, resulting in crack formation.
The theory has worked on most planes, but a few have developed cracks which led to the service bulletin.
The retro fit kit further beefs up this intersection with doubler plates on each side.
The doublers are clecoed in place prior to riveting
This is the front mostly riveted.
And the rear with the doublers visible.
Now the ribs are re-installed.
The final step is to complete the painting.
Horizontal stabilizer service bulletin (and painting) complete!
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