I'm starting this thread to contain some content that came up in one of the Icon A5 threads that is of more general interest and doesn't really belong there.
Actually, they did not remove the bushings at all. I don't remember the nature of the bushings, but based on past experience with Grob's machinery-centric designs they are probably something you can remove with snap ring pliers.
Getting slightly off-topic, but covering some material that might be of general interst:
The things that they did remove, the "spigots," are the spanwise pins at the ends of each spar stub that plug into the bushings on the root rib of the opposite wing. My understanding of the situation is that the design was subjected to analysis and tests by the RAF as part of their due diligence prior to or while operating G103s in their cadet program. That program does a lot of winch launches, which pile on a lot of fatigue cycles pretty fast. You don't necessarily feel it in the cockpit, but a winch launch subjects the wings to the kind of bending moment you see at around 4G load factors. That's well within the flight envelope, of course, but when you do a lot of it as in cadet training, the fatigue accumulates a lot faster than it might otherwise.
Anyhow, the RAF evaluated the design, and their evaluation showed that the spigots might fatigue and crack within the operational lifespan of the aircraft. The bad thing there is that it is hard to check for incipient fatigue cracks, and the failure of a wing spar spigot has some pretty drastic consequences. So they took their concerns to Grob, and regardless of their internal considerations Grob released a technical bulletin, and later an AD, that addressed the concerns of one of their largest customers. As I understand it, at least.
The original Grob spigots were steel pins welded to steel plates. This assembly was then riveted to a piece of plywood that serves as a spar stub shear web stiffener, and the whole thing was wrapped in about 8 plies of 9oz fiberglass, never more to be seen. Until the technical bulletin, at least.
The upgrade process that I recall was basically to slot the spar between the spar cap and the top and bottom of the steel spigot plate, then cut vertically along the plate's inboard edge to liberate the spigot assembly and the plywood it's riveted to from the spar. This left the unidirectional spar cap fibers unmolested all the way to the ends, with a cutout between them where the shear web and spigot assembly used to be. I did tap such a spar lightly with a hammer to see if it would resonate like a tuning fork, but it did not.
The process reminded me of the old adage: "The way to a man's heart is through the stomach. Unless you're a cardiologist; then you just saw straight through the sternum."
Once the old spigot was removed, the new spigot assembly was fitted into the opening and positioned with a steel fixture located according to the lift pins and perhaps other features on the face of the root rib. The replacement spigots were machined and/or forged from billets, and were overall heavier and bulkier than the welded originals. I think that the new spigot was fitted to a new piece of plywood. I don't recall if any effort was expended to scarf the new plywood into the old, but I don't think so. Anyhow, the new spigot was floxed into place in the fixture and let cure. Then it was wrapped in about a dozen plies of something like 92125, post-cured, and that was it.
Thanks, Bob K.
Actually, they did not remove the bushings at all. I don't remember the nature of the bushings, but based on past experience with Grob's machinery-centric designs they are probably something you can remove with snap ring pliers.
Getting slightly off-topic, but covering some material that might be of general interst:
The things that they did remove, the "spigots," are the spanwise pins at the ends of each spar stub that plug into the bushings on the root rib of the opposite wing. My understanding of the situation is that the design was subjected to analysis and tests by the RAF as part of their due diligence prior to or while operating G103s in their cadet program. That program does a lot of winch launches, which pile on a lot of fatigue cycles pretty fast. You don't necessarily feel it in the cockpit, but a winch launch subjects the wings to the kind of bending moment you see at around 4G load factors. That's well within the flight envelope, of course, but when you do a lot of it as in cadet training, the fatigue accumulates a lot faster than it might otherwise.
Anyhow, the RAF evaluated the design, and their evaluation showed that the spigots might fatigue and crack within the operational lifespan of the aircraft. The bad thing there is that it is hard to check for incipient fatigue cracks, and the failure of a wing spar spigot has some pretty drastic consequences. So they took their concerns to Grob, and regardless of their internal considerations Grob released a technical bulletin, and later an AD, that addressed the concerns of one of their largest customers. As I understand it, at least.
The original Grob spigots were steel pins welded to steel plates. This assembly was then riveted to a piece of plywood that serves as a spar stub shear web stiffener, and the whole thing was wrapped in about 8 plies of 9oz fiberglass, never more to be seen. Until the technical bulletin, at least.
The upgrade process that I recall was basically to slot the spar between the spar cap and the top and bottom of the steel spigot plate, then cut vertically along the plate's inboard edge to liberate the spigot assembly and the plywood it's riveted to from the spar. This left the unidirectional spar cap fibers unmolested all the way to the ends, with a cutout between them where the shear web and spigot assembly used to be. I did tap such a spar lightly with a hammer to see if it would resonate like a tuning fork, but it did not.
The process reminded me of the old adage: "The way to a man's heart is through the stomach. Unless you're a cardiologist; then you just saw straight through the sternum."
Once the old spigot was removed, the new spigot assembly was fitted into the opening and positioned with a steel fixture located according to the lift pins and perhaps other features on the face of the root rib. The replacement spigots were machined and/or forged from billets, and were overall heavier and bulkier than the welded originals. I think that the new spigot was fitted to a new piece of plywood. I don't recall if any effort was expended to scarf the new plywood into the old, but I don't think so. Anyhow, the new spigot was floxed into place in the fixture and let cure. Then it was wrapped in about a dozen plies of something like 92125, post-cured, and that was it.
Thanks, Bob K.
