Bolt-on steps - Part 1

The goal here is to make the steps boltable, using AN3 bolts. Why bolts? The steps have been updated several times. Will these changes eliminate the corrosion problems encountered by some Sling 2 operators? Time will tell. I have also had a step fail under me, after applying an excess dynamic load. I stepped forward heavily (facing to the rear of the aircraft) onto the step when disembarking. The top support failed, where it attaches to the fuselage. The bottom support - which remained attached - applied unknown flexural stress on the underside structure. I jumped clear as soon as I felt the step give way, limiting the extent of the flexure. The precise nature of the failure is unknown to myself as it was hidden under the fairing, it could be one or more of: rivet shear failure; the rivets pulling out of the holes after sustained fretting; the quite narrow bridge of metal around the rivet attachments fatiguing, fretting or tearing.

I decided that it was worth strengthening the connections and making the steps readily removable/replaceable.

I have tested AN3 bolt-nutplate combinations for tensile strength. The bolt fails before the nutplate does, by thread stripping. Combining with some data obtained by builder Pascal Latten, I have concluded the bolt-nutplate combination is at least twice as strong - in tension at least - as the specified 4mm Avinox rivets.

https://www.latten.net/sling2/2018/06/30/torque-vs-tension/

There are three main parts to this scheme.

First-off the steps have been shifted forward one rivet pitch (30mm) in recent manufactured Sling 2 aircraft. I am doing the same. While moving the step forward by this amount does not greatly increase the dynamic step load required to tip over the aircraft onto its tail (only about 3-4kgs from calculation) I believe that by moving the step closer to the wing step-on-skin (about 10cm down from 13cm) this change will discourage disembarking the aircraft facing backwards. I have found that if one is forced to disembark facing forwards, like descending a ladder, the dynamic excess load on the step will be minimal. From my own experience, disembarking facing backwards and lurching forwards onto the step is far more likely to apply a significant excess dynamic load on the step.

As ever, two people standing on the step simultaneously will inevitably tip the aircraft onto its tail - this cannot be avoided. The use of the tail skid and strengthening the rear rib structure will greatly mitigate the damage likelyhood.

1) Upsize the holes in the steps and skins to 3/16" - not 4.9mm! - for a snug fit for the AN3 bolts, as these will be substantially under shear as well as tensile load

2) Strengthen the step support. Particularly after upsizing to 3/16", there is not a lot of metal between the hole and the edge. The solution is to create a doubler, strongly rivetted to the support

3) Create a mounting plate for holding AN3 nutplates which will fit inside the upper and lower step support channels RF-CHL-008 and RF-CHL-009. My original thought was to attach the nutplates directly to the channels. There are issue with this however. The nutplates would need to be chamfered slightly to provide clearance with the curved fold in the channel. This chamfering might affect the load bearing properties of the nutplate. There is also the problem of removing the nutplates if the bolting scheme proves problematic. Assuming the nutplates can be removed, the holes would be suitable for use with 4.8mm Avinox stainless steel OBE61 00614 rivets, after slightly upsizing to 4.9mm

The idea of a separate plate (holding the rivnuts) was proposed (in discussions) by Pascal. The advantage of the plate are threefold: it eliminates to need to apply a slight chamfer to the nutplates themselves; it allows this upgrade to be applied retro-actively; it allows the whole bolting scheme to be dropped if it proves problematic, as this plate can be easily removed.

An important detail is that the plate must be carefully chamfered to provide clearance with the fold in the channel which is close to the holes.

Another important detail is that the 3/16" holes are recentred inward from the edge of the aircraft as much as possible when starting from the original 3.3mm holes punched so as to shift them as far away from the fold in the step support channels. By doing this it is possible to fabricate the load bearing plate such that its flat load bearing surface is separate from the needed edge chamfer.

Another detail is that the load bearing plate is held at its ends with 4mm rivets, also reusing 3.3mm holes present in the original skins and channels. However these holes are also recentred such that when upsized to 4.1mm they fall within the outline of a 3/16" hole if that hole position were otherwise to be adapted for use with a bolt. The reason for this is that if there is any need to shift the step backwards or forwards this hole can be repurposed to take a bolt, using the same custom plate.

The thicknesses of the mounting plates, and the doubler plates (both 2.4mm) have been chosen with the following bolting scheme in mind

Upper step support: AN6-7A bolt with AN960-10 (1/16") washer under head

Lower Step attachment: AN6-6A bolt with AN960-10 (1/16") washer under head

Success depends on very precise alignment of the nutplates with respect to the holes in the plate. I used an AN3 bolt with the head cut off to centre the nutplate accurately while rivetting.