Sunday, November 11, 2018

Doing Service Bulletin 18-02-02 Part 1 of 3

Back in February Van’s Aircraft issued Service Bulletin 18-02-02 involving possible cracks forming on the horizontal stabilator’s front spar. It appears a very tiny number of RV-12’s have developed stress cracks where the bearing bracket mounts onto the stabilator’s front spar. (Some believe the cracks are forming from the common practice of using the seatbelt to lock the control stick full aft which places the stabilator on the stop so it does not move and bang into the stops on windy days. As a result, some believe the energy from strong winds (or passing airplanes) is transferred directly into the spar at the nutplates for the stabilator mounting bolts causing the stress fractures over time).The service bulletin calls for removal of the stabilator and associated hinge brackets for an inspection of the front spar for cracking … if no cracks are found, the inspection is to be performed every 100 hours or annual inspection. If cracks are found, the service bulletin is to be installed immediately.

Just to be clear, NO cracks were developing on the DOG Aviation RV-12 … however, decided while the RV-12 is down for a condition inspection may just as well go ahead and install Service Bulletin 18-02-02 now and be done with it. I’m here to tell you getting the stabilator off and reinstalled just to inspect every 100 hours is not a piece of cake. As such, decided may just as well go ahead and install the service bulletin now as a preemptive strike and have one less thing to worry about. Plus, doing the work now means all the new parts will be on the stabilator when the RV-12 gets painted this coming spring.

Installing the stabilator service bulletin involves removing the inboard and outboard hinge brackets and throwing them away. The old hinge brackets are replaced with a new hinge brackets that sit on a new doubler plate … in addition, supporting parts are added that create a box around the hinge brackets which tie into the stabilator’s skins and spar. There are also additional doubler plates riveted under the stabilator horns for the control cables. This is not a 10 minute job … plan on at least a full day’s work to install this service bulletin.

Prior to attempting this service bulletin, you will absolutely want to procure a 12” long #30 drill bit to easily access some of the rivets that need to be removed and for drilling some of the new rivet holes. (As a side note, builders/owners may want to also consider replacing all the bolts that screw into nutplates with new bolts. In this case that involves replacing 16 AN3-4A bolts and 2 AN4-12A bolts. Is this truly necessary? Probably not. But I have it on very good authority that “ideally” bolts that screw into nutplates should be replaced after each use. Small bolts like these are really inexpensive, so during final assembly, the DOG Aviation RV-12 will receive new hardware at those locations on the stabilator where bolts screw into nutplates).

 Some of the parts included with the service bulletin need to be separated on the band saw … after doing that all the parts were dressed on the Scotch-Brite wheel to remove burrs and smooth edges then prepped for priming. Because my Akzo epoxy primer is now way out of date and changing color, decided to use SEM self-etching primer and place the parts in a hot box for drying.
The parts for the Stabilator service bulletin primed and ready for instillation.

For ease of removal and instillation of the stabilator assembly the RV-12’s tail was lowered by placing the nose gear on blocks so the stabilator was about the same level as the workbench. Before the stabilator can be unbolted from the fuselage a few items need to be removed … starting with the tail cone’s upper and lower faring’s, the pitch trim servo pushrod and of course the upper and lower stabilator cables. Others have reported that the stabilator cables can be removed without unscrewing the adjusting barrels ….. so that was the tact taken. The cables were easily removed by pulling aft on the cables gently and sliding the bolt out. Prior to removing the cables, a piece of safety wire was wrapped around the cable and secured to the tail cone …. Doing this will insure the cable won’t fall into the tail cone if it slips out of the hand. Mike K. gave me a hand removing the stabilator from the tail cone … thanks Mike.
After removing the stabilator cables from the upper and lower stabilator horns, tie wraps were used to help prevent the cables from falling back into the tail cone …. note the safety wire is the insurance policy.

Note: Before discussing the actual instillation of the new parts, fellow RV-12 builders/owners and readers of this Blog should be aware I deviated from the instillation guide in three areas …. I did not drill the control horns per the plans (more on that a couple of paragraphs below), three solid rivets were used on the inboard flanges of the HS-01231B instead of two solid rivets and a pop rivet (more work but done for a consistent look) and lastly because flush rivets were installed on all the DOG Aviation’s RV-12 exterior skins, the rivet holes on the HS-01231C stabilator hinge gussets common to the stabilator’s skins were machine countersunk for flush rivets (99.9 % of builders won’t need to consider this unless, like myself, you used flush rivets on the stabilator’s skins).

OK, let’s get started. The process for the service bulletin begins by making measurements and marking the WD-1207 & 1208 upper and lower stabilator horns to receive six AN426AD3-4 rivets each. The six rivet locations are to be carefully marked and center punched on each stabilator horn. After center punching the horns, the HS-01232 stabilator horn doubler is centered beneath the WD-1207 stabilator horn then match drilled into the HS-1211 spar cap and secured with Clecos.
The HS-01232 doubler is centered under the WD-1207 stabilator horn and between the edges of the spar box and taped in place. Note the center punching for the six holes to be drilled into the WD-1207 horn can be also seen. Also of note, the center punch marks were biased just outboard of the lines to make it a little easier to squeeze the rivets with the pneumatic squeezer (done later).
Prior to drilling, a clamp was used just to make sure the HS-01232 doubler does not move during the drilling process.
Match drilling the #30 holes in the HS-01232 stabilator horn doubler into the HS-1211 spar cap.
Completed match drilling of the HS-01232 stabilator horn doubler into the HS-1211 spar cap.

Here is where I deviated from the plans and why. At this point Van’s would have the builder/owner drill down through the WD-1207 stabilator horn and through the HS-01232 stabilator horn doubler and then into the HS-1211 spar cap. This really bothered me because ultimately the six holes created in the HS-1211 spar cap serve no purpose other than it was easy to do it this way (and admittedly less work). The six holes in the HS-01232 doubler will ultimately receive countersinking for AN426AD3-4 rivets which will be used to attach the HS-01232 doubler to the bottom of the WD-1207 horn leaving the six #40 holes in the HS-1211 spar cap unused  … same applies for the HS-01232 doubler on the other side for the WD-1208 horn.

My solution was to NOT drill the remaining six holes directly into the HS-1211 spar cap … this was accomplished by first removing the WD-1207 and moving it to a drill press, drilling a #50 lead hole at the six center punched rivet locations, then stepping up the six holes to the final size of #40.
Completed drilling of three rivet holes to #50 on one side of the WD-1207 horn … three down three to go.

The newly drilled WD-1207 horn was bolted back in place over the HS-01232 stabilator horn doubler. Next, the six #40 holes in the WD-1207 horn were used as guides to partially drill into the underlying HS-01232 stabilator horn doubler just enough to leave a nice deep dimple but not enough to drill completely through the piece. Tape was placed on the HS-01232 to note correct orientation and the WD-1207 horn was removed so the HS-01232 doubler could be taken over to the drill press and final drilled using the deep dimples as a guide for the #40 drill bit.
Newly drilled WD-1207 horn bolted back in position so the newly drilled #40 holes can be used to match drill the underlying HS-01232 doubler.
Using the #40 holes in the WD-1207 as guides, one can see the HS-01232 doubler was drilled just enough to create nice dimples that can be used to complete the drilling on the drill press.
The HS-01232 doubler with the newly created six holes … note the spar cap in the background came out unscathed without the additional six holes (that would not be used for rivets) had the service bulletin instructions been followed exactly as written.

To complete the work on the WD-1207 the six #40 holes in the HS-01232 doubler need to be machine countersunk from the bottom for AN426AD3-4 rivets. Next the HS-01232 doubler is riveted onto the WD-1207 stabilator horn. To accomplish this task the pneumatic squeezer was outfitted with the little used “no hole yoke” and the WD-1207 stabilator horn was covered with blue tape to prevent scratches.
A “no hole” yoke was used on the pneumatic rivet squeezer to secure the HS-01232 doubler to the bottom of the WD-1207 stabilator horn using AN426AD3-4 rivets.
Completed riveting of the HS-01232 doubler to the WD-1207 stabilator horn.
Test fit of the WD-1207/HS-01232 doubler assembly on the stabilator spar … fits good.

Gee, this was so much fun let’s flip the stabilator over and do it all again for the WD-1208 stabilator horn on the other side using the same process. After riveting the HS-01232 doublers to both the WD-1207 & WD-1208 stabilator horns, both horn assemblies are set aside and will be installed later.

I’m very happy … this portion of the service bulletin turned out great! And with no extra holes in the HS-1211 spar cap. Admittedly, it took more time doing the drilling this way and lots of care needs to be taken to assure proper alignment … just move slowly if considering performing the work the way I did it. Next up is installing the new hinge brackets which will be documented in the next post.