Adding shoulder harnesses in at least the front seats should be a must for any aircraft that does not have them. A&P/IA Kristin Winter reports on the recent installation of a three-point system on an aircraft that previously had only lap belts.
Before the early 1970s, Piper did not provide shoulder harnesses for its aircraft. In fairness, that was only slightly after the auto manufacturers did the same thing.
Lack of shoulder harnesses have resulted in life-altering brain injuries from accidents in which the front seat occupants could have walked away virtually unharmed. Fortunately, a number of companies have STCs for the retrofitting of shoulder harnesses in Piper aircraft.
Several retrofit options
The largest manufacturer of restraint systems is AmSafe. It supplies much of the OEM market, be it a GA manufacturer or a commercial aircraft manufacturer like Boeing.
The only product AmSafe currently offers as a GA retrofit is its seatbelt airbags. (Winter has a set of seatbelt airbags she plans to install in her Twin Comanche, and will report on that project in a future issue of Piper Flyer. —Ed.)
The largest holder of shoulder harness STCs for Piper aircraft is Alpha Aviation in Minnesota. Alpha Aviation has STCs covering early PA-23, PA-24, PA-28, PA-30, PA-32 and PA-39 aircraft.
B.A.S. in Washington has an STC for the PA-28/32/34. The kit offered by B.A.S. is a four-point shoulder harness/lap belt system.
Aero Fabricators, a company affiliated with Wag-Aero, has several STCs as well. Wisconsin-based Wag-Aero offers kits for J-3, PA-11, PA-18 and PA-20/22 aircraft, as well as PA-28s.
Univair in Aurora, Colo. markets AmSafe shoulder harness restraint systems for J-3s, PA-11s, PA-12s and PA-18s.
A brief look at the kits
Most of the STC kits have two sets of components. One is the belts and fittings.
Some shoulder harnesses have inertia reels, and some include a fixed belt. The latter type generally costs less, but can be a bit less convenient for the pilot when he or she has to loosen the belt in order to reach something in the cockpit.
The other set of components in a kit is whatever is required to provide the necessary structure in order to mount the shoulder harness with enough strength to provide the necessary protection to the user.
This typically involves some reinforcements that need to be attached to the fuselage, usually involving riveting. For that reason, this is not a project for an owner alone unless he or she possesses an A&P license and the tools necessary for the job.
In addition, a shoulder harness installation constitutes a major alteration, and requires that an A&P/IA inspect and sign an FAA Form 337.
Required tools and supplies
The tooling necessary to complete this project will vary a bit depending on what structure is required. Some airplanes may already have the structure installed because shoulder harnesses were an optional item for that particular model year; others may need quite a bit of reinforcing pieces installed in order to provide the necessary support.
Regardless of the kit, the tools necessary for installing solid rivets—and possibly blind rivets as well—will be a necessity.
Any practicing A&P is likely to have the necessary tooling, but for an owner that is interested in participating and wants his or her own tools, the major item is a 2X rivet gun. U.S. Industrial Tool, Sioux Tools and Chicago Pneumatic are just a few of several rivet gun manufacturers.
The most important part of the rivet gun is a good “teasing” trigger that lets the operator control the force and frequency of the blows. The gun also needs a rivet set, which is the part that actually touches the rivet, and a spring retainer to hold it to the rivet gun.
In addition, a selection of Cleco temporary fasteners and pliers will also be necessary.
It goes without saying that the ability to drill holes will be key. A #30 drill bit is used for a 1/8-inch rivet and #40 is for a 3/32-inch rivet. There are some excellent YouTube videos done by EAA on the basics of sheet metal work. (See Resources for additional information. —Ed.)
This is a project that any owner with mechanical aptitude can tackle with supervision by an A&P. Doing so will be a great learning experience for those interested in understanding more about what is involved in aircraft maintenance.
A Comanche 250 project
Recently I participated in, inspected and signed the Form 337 on the installation for a 1959 PA-24-250 Comanche which had never had shoulder harnesses installed. The new owner was keen to have the safety advantage of shoulder harnesses.
The Alpha Aviation kit for the Comanche 250 was very complete and of excellent quality. (See photo 01, page 22.) The kit included all required parts and hardware including restraints for two front seats, an 8130-3 Airworthiness Certificate, an installation manual and a copy of the STC and signed STC authorization.
The first step is to gently remove the headliner from the area to provide access to the structure above the rear window. (See photo 02 on page 24, top.) Removing the headliner can be a challenging project and needs to be undertaken carefully to avoid damaging the headliner.
The structural portion of the kit consisted of a stringer and two doubler plates. The two doubler plates are riveted together with a carefully-laid-out pattern, and to this doubler plate assembly is mounted the attachment point for the inertia reel for the shoulder strap. Then a longitudinal stringer and the assembled doubler plate must be fitted and riveted to the airframe above the rear window.
Measuring and positioning
Careful measurement is key to making sure that the stringer and doubler are properly positioned. This is done by riveting the bottom of the assembled doubler to the existing stringer that runs above the window, as shown in photo 03 (page 24, bottom). Note that blind fasteners were used here.
The installation of the assembled doubler sets the position for the new stringer, which runs from the door frame back to the frame at the back bulkhead.
The photo below shows the assembled doubler and the stringer fitted to the aircraft and held in with spring sheet clamps, referred to colloquially as Clecos. Clecos have been used since before World War II and are indispensable for aircraft sheet metal work. The most common type require a special set of pliers to install and remove them.
Clecos are color-coded based on the size of the hole they are designed to fill, and are used to pull tight the two sheets of metal. As mentioned earlier, there are several good videos available from EAA that cover Clecos and other basic sheet metal techniques and tools.
The temporary fastening process
As is good practice, the initial holes were drilled to a smaller size, in this case 3/32 inch, which later were enlarged to 1/8 inch as called for in the instructions. This technique works to clean up any shifting that takes place so that each hole is reasonably precise.
Once the structure is fitted and all the holes are drilled, the parts are removed in order to clean off the burrs and excess material from around the rivet holes. The parts are then reassembled and held in place with the Clecos.
Anchoring with rivets, and final steps
Solid rivets are generally preferred and more economical, but are not always practical, and the kit from Alpha Aviation provides both solid rivets and blind CherryMax rivets.
CherryMax rivets are souped-up pop rivets made to an aerospace standard and are designed with a locking collar to fasten the stem into the rivet as the stem forms much of the strength of the fastener.
CherryMax rivets are used when you can’t get a bucking bar to the back of a solid rivet. They can be seen in photo 04 (below) as they are used to attach the lower side of the assembled doubler to the aircraft’s existing stringer. CherryMax rivets have a number of special pullers that can be used to install them; some are hand-operated and some are pneumatic.
Once all the rivets are installed, a strong support base has been created to anchor the inertia reel and shoulder harness, as shown in photo 05 (below).
Reinstallation of the headliner, carefully cutting a hole for the bolt, and subsequently bolting on the inertia reel completes the installation—save for the log entry and completing the Form 337.
Labor may vary; added safety will not
The labor necessary to install the shoulder harness kit varies with the amount of structure that must be added. It can take anywhere from a handful of hours to a couple of days’ worth of labor, but shoulder harnesses are most needed in the kind of accident that can happen to even the best pilots.
No restraint will help if you hit a mountain at cruise speed—but landing mishaps, loss of runway control, or even a controlled glide into favorable terrain after an engine failure are more common. These scenarios are where a shoulder harness might make all the difference.
Know your FAR/AIM and check with your mechanic before starting any work.
Kristin Winter has been an airport rat for almost four decades. She holds an ATP-SE/ME rating and is a CFIAIM, AGI, IGI. In addition, Winter is an A&P/IA. She has over 8,000 hours, of which about 1,000 are in the Twin Comanche and another 1,000 in the Navajo series. She owns and operates a 1969 C model Twinkie affectionately known as Maggie. She uses Maggie in furtherance of her aviation legal and consulting practice; she also assists would-be Comanche, Twin Comanche, and other Piper owners with training and pre-purchase consulting. Send questions or comments to .