Carl Link – Pivot joint and hinge point wear and play.
I have some fore and aft joint play or wear at the top attachment point on my rudder. I intend to get it fixed as soon as possible. The only thing my parts book shows me is a bushing there. It sounds like I need a new bushing, or have bigger problems with elongated holes. If anyone has dealt with this, or has some insight into this, I’d appreciate hearing from you. Incidentally, the bushing (169-640000-99) is listed at Tulsair for $4.11, but they don’t have any. Rapid wants $65 and they have 10.
This is a point worth emphasizing, as it applies to both fastener hardware in pivoting joints, and to hinge pins. While it is counter-intuitive, in many cases it is the steel “pivot pin” (usually a bolt or clevis pin), or a steel bushing/sleeve, that accumulates the most wear in a moving joint. This is especially true in locations that get oiled (as during the annual inspection). What happens is that the oil attracts fine grit. Since the airframe portions of the joint or hinge are softer aluminum, the grit “embeds” in the surface and doesn’t move relative to the softer aluminum. The surface then becomes a fine sandpaper, that slowly erodes the surface of the harder steel pin or hinge. You can easily see this effect when you remove an old fastener from this type of usage; the wear surfaces on the fastener will be very obvious, often with a wear edge that can be felt with a fingernail.
In many cases, at least 75% of the play can be removed just by changing the steel fastener in a connection; especially if the play is caught early, before the “hammer effect” can damage the mounting and bushing holes. This doesn’t mean that I advocate leaving in old bushings while just changing the bolt, or that excess airframe play should be left unaddressed. It does mean that it is worth changing fasteners and pins first, when play is first noticed, before it gets really excessive.
If anyone untrained hops on this without supervision, you’d better make darned sure that you are making replacements with the correct hardware. The side effects of incorrect hardware can go beyond the obvious risk of an in-flight failure. For example, a Grade Two or Grade Five hardware store bolt, substituted for the proper AN-grade bolt, may not literally fail unless you get a momentary flutter, or get close to VNE and pull some G. But the softer steel will embed grit just like the aluminum in the airframe, and will start shifting the wear to the substantially more expensive (to fix) airframe parts. The different plating on non-aviation steel fasteners also won’t provide the needed sacrificial protection from galvanic corrosion; the fastener will rapidly rust due to direct contact with aluminum if moisture is available.