Tail Tiedown Repair
From Bondline
Introduction
The tail tiedown can become damaged two different ways. First, a hard landing with the nose high can cause the tail tiedown to strike the runway. Second, a common method to pull an airplane back into a hangar involves using a winch connected to the tiedown ring. See the towing page for an idea on how to safely winch a Grumman. In both cases, loads are being placed on the tiedown ring that it was not designed to handle. The ring and its supporting structure are designed to take a mostly downward, axial load (it's good not to exceed 45 degrees from the vertical) from a tiedown rope. A hard landing may shove the ring up in the fuselage, or it may bend it to either side or front to back twisting the attached structure. Put enough force on the tiedown with a winch, and the ring will start to pull aft twisting the attached structure.
Tiedown Structure
The attached structure mentioned in the introduction is the aft fuselage bulkhead, which is primary structure. The aft spar of the horizontal stabilizer, which takes most of the stabilizer and elevator loads, attaches to this bulkhead. The bulkhead is bent on the sides and bottom to form a lip that fits tightly over the fuselage skin, and the two are bonded with rivets in a few spots for good measure. Someone else will have to confirm or fix this, but it appears the factory installation of the ring is through the bottom lip into a nut plate on the interior. The aft bulkhead is not very thick, and the lip is not that wide. Any non-vertical load on the ring has the potential to very easily start deforming the lip and debond the bulkhead from the fuselage skin.
Suspected Winch Damage
... And how not to repair it.
The images to the right illustrate a repair to the tiedown ring and surrounding structure and what it looks like from the outside. Notice that the channel section does not appear to lie flat against the fuselage bottom. You can possibly see that the washer is deformed. Right now, it is hard because of the oil and dirt.
After removing the tiedown ring and the existing repair, there was obvious damage. The middle third of the bondline along the bottom is gone, and the aft bulkhead has started cracking towards the radius that separates the bottom from the back face of the bulkhead. The lateral cracks towards the old nut plate rivet holes are not nearly as concerning. But to install the channel section, the mechanic ground down the rivets on either side of the lower lip.
One of two things happened at this point. First, whoever installed the tiedown ring after the repair torqued it down so tight that it severely deformed the large area washer, the lower aft bulkhead, and initiated cracks. Another possibility is that someone winched the airplane into their hangar using the tiedown ring. The way the structure and washer was deformed is consistent with an off-axis load on the tiedown ring from a winch.
