Rear Seat Belts Modification

I spent most of my Saturday pondering what to do about seat belts in the rear seat. My problem is that the Sling 4 kit was designed to use lap belts (only) for the rear seat. However, the likelihood of surviving a crash is greatly improved by having 3-point belts, which is why they’ve been mandatory in cars since 1977 and in production small planes since 1988. The FAA even has a dedicated GA page, which talks about safety, shoulder harnesses and survivable accidents.

If your airplane has shoulder harnesses, use them
If your airplane is not equipped with shoulder harnesses, have them installed as soon as possible

FAA General Aviation Safety Page [link]

A small plane like this would be required to have lap and shoulder belts for all seating positions under part 23, the portion of FAA rules that apply to certified airplanes. Since my plane will be certified under experimental rules, it’s not required to comply with part 23, which is likely part of the reason the kit lacks 3-point belts. The front seats have 3-point belts.

The later-designed Sling TSi adds a shoulder harness for the rear seat. My fuselage kit was made in 2018 during the ramp-up to TSi production, so it has some of the provisions needed for the harnesses, but I don’t have any of the parts. I would have to de-skin the rear fuselage to install the factory parts. I’d also have to put a lot of the project on hold while I wait for the parts to arrive, which could be a very long time during the COVID-19 pandemic.

A recent post on Peter V’s blog helps explain the history a bit. At some point, a Sling 4 customer requested shoulder harnesses for the rear seat, so TAF designed something for that customer, and subsequently made it available as an off-the-menu option that I didn’t know about. When the TSi went on sale, this option was made standard. However, since my kit is not a TSi, it was missing the plates and cables shown in the linked blog post.

I don’t have these mounting tabs (image courtesy of Peter V’s blog)

I could probably order / request the parts (as Peter did), but it will likely require a long delay as I already mentioned. Further, I’ve seen a few pictures of completed Sling TSi’s, and I’m not sure I like it anyway. The shoulder harness stretches across the luggage area, as you can see in the next picture from Philip R’s blog (this picture is of N135WT, the first TSi that flew in the US). I don’t particularly like this arrangement of having the belt draped across the luggage area. There’s no retractor, and not much to prevent the belt from rubbing against or getting snagged by something in the luggage compartment.

Routing of the rear seat shoulder belt (image courtesy of Philip R’s blog)

So, I decided to come up with my own design, which will incorporate a 3-point belt with a retractor, much like the front seats. To do this, I have to find a place to mount the retractor (a.k.a. inertia reel) and the shoulder anchor point, both of which must be rigidly attached to withstand the force associated with a 215-pound person accelerating to a 9G crash load, plus a “fitting factor” of 1.33x, for a total load of just over 2500 pounds. That’s a lot of force!

In researching a solution, I discovered that FAA Advisory Circular (AC) 43.13-2b, “Acceptable Methods, Techniques, and Practices―Aircraft Alterations”, has a whole chapter dedicated to shoulder harnesses. In addition to showing common methods for attaching anchors, the AC also is helpful in providing some data needed for calculating the stresses that I need to design to.

Other FAA resources:

The front seat belts have the shoulder anchors attached to the canopy. Unfortunately I can’t do the same for the rear seat — the anchor needs to be behind the occupant, so that means the anchor needs to be somewhere in the luggage compartment. I mocked up a solution in the next few pictures. In the first picture, I taped a wood block representing where I’ll put a bracket for the shoulder anchor. AC 21-34 recommends that mounting to beltframes (aka ribs) is acceptable, assuming they have been reinforced.

Mockup of the shoulder anchor attachment

I developed a CAD model of the modification and performed a stress analysis. The shoulder anchor will attach to a new intercostal, made of a 0.080″ formed hat section that attaches to the beltframe (rear fuselage rib #2), the upper rear fuselage skin, and to longeron 201, to spread the load across the structure. The beltframe and skin are pretty thin (0.025″ and 0.020″, respectively), so I’ll have to add doublers to reinforce them. The model predicts the area will deform under this “ultimate” load, but that is acceptable as long as it doesn’t fail.

CAD model of the anchor attachment point
Stress analysis predicts that deformation will occur, but there is positive margin so the structure should survive

The retractor will be located in the floor beneath the rear seat bench. There is plenty of unused space between the floor ribs. There is a fairly stout beltframe where the center and rear fuselages join – that is where I plan to attach the reel. From there, the belt webbing will have to come up behind the seat, so I’ll need to cut some slots in the floor to route the belt.

The retractor reel will get mounted in unused space below the rear bench seat, and will attach to the beltframe or floor rib, not sure yet
Candidates for slots in luggage floor channel, to pass the belt from the retractor reels up to the shoulder anchors

I will have to drill out rivets to access the area below the rear seat bench. I am not happy about having to do this design modification. I did not plan to do this work, which is diverting my time away from other tasks. But, for the safety of anyone who travels in this plane with me, I think I owe it to them to provide this essential protection.

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