Revisit seat base mechanism

I was not happy with several aspects of the seat base mechanism. There was slight binding of the pins in the side channels. The use of AN3 nylon washers (diameter approx 10mm) instead of the original 4mm solid rivet meant that the seat locating pin cable was displaced from straight in the resting position more than was originally the case. This greater displacement has two effects. Firstly, the pins did not extend beyond the seat channels quite as much as originally. The straight end-to-end length of the seat pins is 449.5mm tip to tip. With the original solid rivet, there is a slight deflection of about 3mm or so, the tip-to-tip length is 448.8mm. With the AN3 nylon washer this is reduced to 448.0mm, a total reduction of 1.5mm or 0.75mm each side. This reduction is not so bad, but there is a second effect from the use of the washer, and the greater deflection. There is a kind of memory effect where if the pins are pushed from balanced state, then the pressure released, the pins will not quite return to the original position. This memory effect is caused by a slight resistance to motion of the cable when pushed against the nylon washer to the greater extent. It is also caused by the cable pressing and rubbing against the stop plates, a situation that is not relieved in the resting position. There is also a contribution from any binding of the seat pin with the seat channel.

My solution is to refabricate seat adjustment plates ST-PLT-002 with a greater length, changing from the original 40mm between the hole centres up to 46mm. 3mm of this increase compensates for the greater diameter of the nylon washer compared to the solid rivet. Another 3mm of this increase also allows the cable to relax to a almost fully straight-line configuration. The result of this change is that the seat pin tip-tip distance becomes 449.5mm, the same as the full length of the cable.

The aim and result of these changes means the pins extend an extra 0.75mm each side. This ensures that the seat will help to ensure reliable registration with the fuselage seat channels. I have observed with club aircraft that the seats can be easily detach from the seat channels with some minor forcing, owing to a combination of wear due to extremely frequent usage and possible memory effect and/or binding, even with the original design - combined with the fact that the 433mm wide seat base can move laterally a bit within the 436mm fuselage seat channels, and also that the seat pins have a 2.5mm taper at each tip.

In detail, the original design 448.8mm tip-tip distance means that each the flat of ech tip will register in the fuselage seat channel over (448.8-433)/2 (average seat base channel to pin tip) - 1.5 (seat shifted hard to other side) - 2.5 (taper) = 3.9mm. This can be a lot less due to any memory effect or binding, leading to a loss of registration. This figure is improved to 4.7mm, with a lot less variation due to elimination of binding and reduction of memory effect to a small fraction of a mm.