A step-by-step guide for making and installing cable seals on a PA-18.
When my 1952 Piper PA-18 Super Cub Special was restored in 2014, no detail was too small—even the black faux-leather cable seals where the control cables pass through the fabric skin were new and perfectly applied. However, after three years of heat, cold, exhaust gases and cleaning solutions, plus the buffeting of the slipstream, the edges of those seals started to dry, curl and peel away.
After pulling off a few of the seals and inspecting the material, it was obvious that their lack of flexibility and the old, dried adhesive would make them difficult to glue back in place. The seals seemed to be made of a plasticized fabric. The fabric had a woven backing coated with a black vinyl embossed in a faux leather pattern and was about 1 millimeter thick.
I wondered if the original 1950s seals were leather, or if by that time they had been replaced by synthetic materials. Artificial leather, such as the brand-name material Naugahyde, was available as early as the 1930s. It’s likely that in the very earliest days of aviation this sort of item would have been genuine leather glued to the fabric, but I have no idea when Piper made the changeover to plastics. (Maybe a former factory worker will know?)
What are these things?
These cable seals might more precisely be called grommets, which is anything inserted around the edges of a hole through a thin material, which could be wing fabric, sheet metal, fiberglass, wood, paper or almost anything at all.
Grommets perform several functions. They may be applied to prevent tearing of the pierced thin material, or to discourage abrasion by whatever passes through the hole. Or, it might be the other way around: to protect the wire, cable or whatever is passing through the hole from abrasion. Grommets can help keep dirt and water out of an opening, especially if the grommet is made of a flexible material, essentially making a big hole into a smaller hole. Since it doesn’t completely seal the opening from the elements, I wouldn’t call it a seal, exactly.
Grommets are also used to cover the sharp edges of a hole so you don’t cut yourself.
Often a grommet performs all these functions, which certainly seems to be the case for the fabric grommets used where control cables pass through the fabric on vintage airplanes.
When most people think of a grommet, they envision a metal or plastic ring that’s pressed, like a rivet, into a fragile surface. But a grommet could also be something as lightweight as those self-sticking reinforcements students use for three-ring notebook paper. The stitching around a buttonhole could be considered a grommet in that it keeps the buttonhole from fraying.
In the electrical business, rubber grommets are often known as “insulating bushings” and are commonly made of molded rubber. There are certainly a few of these on most aircraft. For large and irregular openings, long strips of cushioning materials may be applied to sharp edges. This is known as grommet edging.
Fabric airplane control cable grommets
On my Super Cub, the control cables pass through the fabric in four pairs of places—where the lower aileron cable passes from the floor of the cockpit and runs upward along the wing strut; where that cable enters the bottom of the wing (it emerges from the top of the wing under a metal shield); where the upper aileron cable running inside the wing passes out through the bottom surface of the wing to the aileron; and where the rudder cables exit the fuselage in front of the empennage to connect to the rudder arms. Two of these pairs of grommets are circular; two are elongated shapes. They are meant to prevent chafing along the fabric.
If you prefer premade grommets, Wag-Aero sells a Piper Naugahyde Cable Seal Kit (part No. M-423-003) for $7.50. The kit consists of “two slotted fuselage and two aileron punched cable seals. Set of 4.” Aircraft Spruce has a similar item on its website (part No. 09-00335) for $9.55 per set of four. The seals from Aircraft Spruce and Wag-Aero are all teardrop-shaped.
To replace all the seals (i.e., grommets) on my Super Cub, I would need two kits. Depending on which company I order from, pricing would run $15 to $19, plus tax and shipping.
However, I enjoy working on my airplane, so I decided to take a shot at making the grommets myself.
Can you legally DIY?
My local mechanic said making these homemade grommets falls under FAR Part 43, Appendix A, which allows “preventive maintenance to be performed by a certificated pilot, holding at least a Private certificate, on an aircraft owned or operated by that pilot, provided the aircraft is not used in commercial service.” FAR Part 43, Appendix A, subpart (c), item No. 7 specifically mentions “making simple fabric patches not requiring rib stitching or the removal of structural parts or control surfaces.”
My mechanic said he would consider these grommets no different than a pilot performing a small fabric repair job.
I visited a nearby fabric store to look for replacement material. I found a few rolls of black vinyl in the upholstery section. One was called “marine vinyl” and the tag revealed it had a -10 F cold-crack rating. (“Marine” also led me to think that if it’s a good material for boat upholstery, it might be good for airplanes.)
Since I was making just a few small grommets, I needed a minimal amount of material. The vinyl comes on a yard-wide roll. You must buy the material by length off the roll and the smallest amount I could buy was 1/8th of a yard, or 4.5 inches. At $19.99 per yard, it cost $2.50 plus tax.
After getting the material home, I checked the fabric store’s website and discovered the fabric’s brand name was Spradling. I researched a bit and learned that Spradling is a company that specializes in upholstery materials for automotive, marine and general upholstery. According to Spradling, marine vinyl is essentially the same as regular vinyl, with the addition of UV inhibitors and mildew or antifungal additives, all of which seem like good attributes for airplanes sitting inside damp hangars or out in the weather.
Making and installing grommets: step by step
1. Carefully peel the old grommets from the fabric surfaces. I suppose if they didn’t come off reasonably easily, you wouldn’t be replacing them. Mine peeled right off.
2. Use the old grommet as a template and trace the shape onto the back side of the new vinyl. I used a fine point Sharpie marker, which made a 1/16-inch thick line.
3. Cut out the new vinyl with scissors or a sharp knife. I cut around the outer edge of the black line, making the new grommet about 1/8th of an inch larger than the original. This helps to cover any old glue that may be on the fabric and will give a clean look to the new installation.
4. Glue the new grommets in place. I chose original Gorilla Glue to adhere my newly-cut grommets. Make sure you keep this glue warm; put the bottle in your pocket for a while, otherwise it’ll be very thick and hard to spread. To use it, you must first wet the surfaces and then apply the glue. I used a small brush to spread the glue over the grommet’s surface to its edges. Gorilla Glue also comes in a “clear” version that I would select if I was buying glue again. It might also be good to use a spray adhesive, which would provide an even and thin application that wouldn’t squeeze out when the grommet is pressed on, keeping things neat.
5. Wipe off any excess glue that gets squeezed from under the grommet before taping it in place (I used masking tape) until the glue sets. Be careful or you’ll glue the tape to the fabric.
6. After the glue dries, remove the tape and ensure that none of your control wires were glued to anything and they move freely. It’s also worth paying special attention to the “controls free and correct” item on your checklist in your next few flights.
7. Admire your handiwork.
From start to finish—including my travel time to the stores for the fabric and the glue—this project took around two hours.
Know your FAR/AIM and check with your mechanic before starting any work.
Dennis K. Johnson is a writer and a New York City-based travel photographer, shooting primarily for Getty Images and select clients. He spends months each year traveling, flies sailplanes whenever possible and is the owner of N105T, a newly-restored Piper Super Cub Special. Send questions or comments to .
Spradling International, Inc.
NAUGAHYDE CABLE SEAL KITS
– PFA SUPPORTERS
Aircraft Spruce & Specialty Co.