Dorsal Fin Replacement

From Bondline

This page is still under construction, the installation of the dorsal fin on the channel has not been documented completely.

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Dorsal Fin Replacement on 74 and later 4 place

Maintenance Difficulty:

Introduction

The Travelers from 72-73 had a smaller aluminum dorsal fin that was riveted to the turtledeck. The 74 up models and all the Cheetahs and Tigers, including the AG series planes have a removable dorsal fin assembly, with the visible fin pull riveted to a stamped aluminum channel. The AG series had fiberglass fins from the factory. The 70's production planes had fins made from ABS plastic, like the elevator tips, HS inboard fillets, tail cone, sump fairings, main wheel pants, etc. As with all the other plastic parts, the 70's plastic dorsal fin ages, and then it cracks and warps. The repair for this is to replace the plastic with a fiberglass replica. There have been many sources for them over the years. Some PMA approved, some commercially produced but not approved, and more than a few have been made as "owner produced parts", using the OEM plastic as a form to build a replacement on.

This article covers the work required to remove the existing dorsal from the plane, disassemble the aerodynamic fin from the mounting channel, the fitting and attachment of the replacement fin to the mounting channel.

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Taking the Fin Assembly Off

At the aft edge of the dorsal there are a series of #6 flush sheet metal screws and dimpled washers that attach the dorsal to the vertical stabilizer. Remove all 12 screws and washers. The fin is now held by friction and 3 clips on the turtledeck that the aluminum mounting channel slides aft into. To release the dorsal, the fin must slide forward about 1.5". Due to the tight fit of the clips, age related distortion of the plastic fin causing the aluminum channel to bind, and in some cases, non-standard sealant or filler that owners or paint shops have applied to the edges where the fin and the turtledeck meet, the fin may not move easily. If there is any sort of sealant around the base, slide a thin putty knife under the fin to break the sealant loose, and work around the entire periphery to free the fin. Slide the fin forward, using caution to not SLAM it into the ELT antenna base on those equipped with Narco ELT 10 and similar antennas that protrude up out of the dorsal. The fin will become very loose as it begins to slide, and using excess force to begin its movement can cause it to damage the aluminum mounting channel when the fin suddenly frees up and slides forward, impacting the ELT antenna base. After moving the fin assembly forward to free it from the clips, carefully extract the ELT antenna from the hole in the top of the dorsal.

Inspection

With the dorsal assembly off the turtledeck, inspect the mounting hardware on the vertical stabilizer and on the turtledeck. The Tinnerman sheet metal clip nuts on the vertical fin are likely to be rusted and broken after decades of service. It is also possible that someone has run #8 (and even #10) sheet metal screws into the clip nuts when the #6 screws began to strip out. It would be a good idea to replace ALL the clip nuts and the mounting screws. Note that many would-be mechanics do not know the difference between a type "A" sheet metal screw, and a type "B" sheet metal screw. Type A screws have sharp points and are intended to be inserted into sheet metal where the threads will bear on the actual skin. Type B screws are blunt and have a different thread pitch, and are intended for Tinnerman clip nuts. Installing Type A screws into Tinnermans will cause the tabs on the Tinnermans to be deformed as they are bent to the different thread pitch of the pointy type A screws. Don't make this beginner's mistake. Obtain and install the correct hardware.

The clips on the turtledeck are held on by 2 Avex 1601-0410 pull rivets, each. After decades of service it is not uncommon to see that the zinc chromate protection on the steel clips has failed, and some rust has begun. It is also not all that uncommon to see the Avex rivets failing due to age, stress, and dis-similar metal corrosion. Now would be a good time to remove the clips, glass bead them to remove any corrosion and old primer, and then etch and prime them with a fresh coat. To remove the rivets, drill through the head with a 1/8" bit and then snap the head off with a 1/8" pin punch. The body of the rivet can then be tapped through the turtledeck without risk of over-sizing the hole. An automatic center punch is also a handy tool to help remove the Avex rivets. Drive the center mandrel back into the rivet with the automatic center punch, and the rivet head will be easier to drill off, since the bit will only be cutting the aluminum alloy body of the rivet, and not trying to cut the steel mandrel in the rivet.

With the hardware inspected and replaced as needed, the aluminum channel should be similarly inspected for condition. The 70's production planes had little if any corrosion protection on the channel. Also it could have had previous repair attempts made, and the holes in the mounting flanges might be over-sized or new holes may have been drilled, leaving the flange with the appearance of swiss cheese, due to all the extra holes.

Removing the Plastic Fin

Removing the plastic dorsal without distorting the aluminum channel or doing damage to the mounting holes is the next challenge. As with the clips on the turtledeck, the plastic fin is held to the aluminum channel with Avex rivets. The difference is that they are flush rivets instead of domed head. (Avex 1604-0412 rivets, which are 1/8" nominal diameter with a 120 degree flush head are the correct call-out. Some parts manuals refer to the older "Chobert" rivets, but the Avex is the current replacement.)

It is a little more difficult to remove the flush rivets than the domed head ones, if only because they tend to have the center hole plugged with the steel mandrel, right to the surface, making the pre-drilled centering hole on the domed rivets not so easy to use on the flush ones. Using the automatic center punch mentioned before, drive the mandrel back into the rivet, and then drill through the head with a 1/8" bit, and snap the head off with a 1/8" pin punch. This prevents drilling into the aluminum channel and damaging the hole(s). Once the plastic has been freed from the aluminum channel, the remainder of the rivet bodies can be pulled from the back side with a pair of diagonal cutters. Correctly used, the cutters will squeeze the rivet right at the joint of the body and the mandrel's bulb, pulling the mandrel back further, and allowing the body of the rivet to collapse and easily come out of the channel.

Gil Alexander suggests an alternative method to remove the rivet heads - "if you have an original plastic part that is already junk - is to just sand off the rivet heads (and the surrounding plastic) with a 2 inch disk grinder."

Removing the Rivets from the Mounting Channel

Surprise! There may be extra holes in the aluminum mounting flange. In this case, there were 3/32" pilot holes that the factory made, but that were missed during manufacture. Note that you may have additional holes from previous replacements, or the holes may be over-sized or not round. Standard sheet metal methods for repair are applicable. It may be necessary to respace your rivet holes, or flush doublers can be installed on the inside of the flange, such that the rivet holes can be re-drilled. By dimpling the channel flange and using 3/32" flush rivets, it is possible to make an invisible repair, allowing for new rivet holes. Hopefully you are doing the first repair, and won't have to deal with "swiss cheese" from many extraneous holes. The rivet bodies can be removed from inside the channel by gripping them with a pair of diagonal cutters. Placing the cutters just below the bulb at the end of the mandrel, one can squeeze the rivet and pull the mandrel out. This leaves a nice pilot hole for a 1/8" drill bit to remove the rest of the rivet without oversizing the hole or distorting the channel from twisting or pulling on the rivet.

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Fitting the New Dorsal Fin

The new part, regardless of source, will require some trimming to match your airframe. The part is probably too long, too tall, and very likely not straight on the bottom edge. Each of these must be dealt with to achieve a smooth fit without gaps.