Mark, Yes, using lockwire to prevent FOD it makes sense to run it down the middle. But the bloke from Rotax also has longevity in mind, the lockwire around the pipe and pulled tight makes the spring last longer, so far anyways, I guess I can only guarantee it after a few thousand hours of use.
(Yes, I do trust him as he is a most amazing/incredible/brilliant/dedicated guy. He puts hundreds of hours into flying his prototypes so it's not just talk, he proves it.)
I have lock wired my springs like Rob does, but only because the springs were closed at one end. They were stainless steel KTM motocross springs. I didn't have any problems except the lock wire does snap sometimes as it gets pinched by the spring, and the lock wire also cuts into the spring, wearing a groove in it. I think with a standard spring, lock wiring through the centre is best. Just my opinion.
I recently found three of my eight springs had broken, with the pieces still there only because they had been safety wired down the middle, thereby preventing them from departing their locations.
I am wondering, though, whether the Rotax Chief Engineer is suggesting (as found in the installation manual, as well) each pair of springs be wired together in the middle in order to somehow dampen vibration. If so, then I would consider adding the additional wires, but I cannot believe that it makes any sense to consider those to be "safety" wires when they clearly will not prevent the pieces of broken springs from coming loose. In fact, I would be extremely concerned about foreign object damage to the prop if I operated an aircraft with a pusher prop that did not have the springs safety wired down the middle.
So, Rob, can you, possibly, corral the Chief Engineer and find out why he suggests the (hopefully, additional) wiring between the springs and whether he meant for that wire to be in addition to one down the middle, particularly since Section 78-00-00 (p. 3) refers to both minimizing vibration and prevention of FOD?
There appears to be a conflict in the 912 Installation Manual. On the one hand it speaks of safety wiring to prevent foreign object damage ("FOD"). As I can attest, the only way that this can be accomplished is to have the safety wire go down the middle of the spring, so that if a spring breaks neither piece can be dislodged and get loose.
On the other hand, the Installation Manual states that an objective of safety wiring is also to assist in damping vibrations, and the ONLY way illustrated in the Installation is a "safety" wire that basically connects the middle of one spring to the middle of the related pair. As I see it if this is the only "safety" wire used, and either spring breaks, it's an invitation to the portion of the spring that is not still attached to the related pair, to come loose. Nevertheless, Rob indicated that this is the "proper" way to safety wire the springs.
I know that if I had a trike, or something else with a pusher prop, there is NO WAY that I would not safety wire the springs through their middle, but I wonder whether the Installation Manual's reference to avoiding FOD, along with its reference to damping vibration, means that BOTH methods should be used, i.e., a first safety wire through each spring, and a second connecting the two springs in each pair together to assist in vibration damping.
Rob, if you read this, can you run it by the Chief Engineer?
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