Assembling the Horizontal Stabilizer – Rear Spar

Now that some of the skeleton parts for the horizontal stabilizer have been primed and the primer has fully dried, it’s finally time to start riveting the assembly together. This starts with the rear spar, consisting of a left and right spar channel, the top and bottom reinforcement bars that overlap the center of the spar, and the hinge brackets for the elevator control surfaces.

In the above picture, you can see the aft side of the rear spar, with the reinforcement bars, the brackets, and the factory heads of the rivets holding it all together. The factory head of the rivet is the part that is pre-shaped in the manufacture of the rivet. In this case, the rivets are all standard round-head rivets.

In the following picture, you can see the shop heads on the fore side of the spar, formed during the setting process by whatever tool is being used. All of these rivets were set using a pneumatic squeezer.

Here’s another close-up view of the aft side. The holes that have been left empty will be used later to attach ribs on the interior of the stabilizer.

The center bearing for the elevator is attached using bolts, washers, and nuts. These are aircraft-grade nylon lock nuts that must be set to a specific torque. The torque spec for these bolts is 20-25 inch-pounds, but the drag torque (that needed to overcome the drag of the nylon insert) needs to be measured and then added to the total torque. Using a beam-style torque wrench, I measured the drag torque at about 6 inch-pounds, bringing the total torque to 26-31 inch-pounds. I set each of the nuts at 30 inch-pounds.

The orange/red goop is torque-seal. It provides no structural strength, but is designed to shatter if the underlying base moves. Over the years, when the airplane gets its annual condition inspection, the mechanic (that would be me) will check the torque seal to make sure that both the nuts and bolts remain in the exact position they were in when first tightened down. If not, they will need to be removed and re-torqued. When applying the torque seal, I was very careful to keep it off the bolt threads. I understand that torque seal on threads makes it almost impossible to remove the nut without cutting the bolt.

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