Bruce:
I bought 292WC from Tom in July and it now lives here in Ottawa, Canada. I will likely get Tom to send me the material he has anyway, and will plan to overhaul the nose gear at the next Annual in July of this year. That said, can you offer any advice on what is reasonable in terms of play on this. My expectations is that the worst that can happen is that the nose gear will begin to shimmy and feel loose. I have not noticed this at all and if Tom already replaced the side bushings then hopefully all that is needed is the shims I read about in the MM Archives.
Not sure what to make of this but on Friday my mechanic jacked the plane to verify that they were ordering all of the right bushings for the nose gear. He said most of the play seemed to be in the two upper bolts that attach the gear assembly to the Airframe. He said that the bolts did not seem very tight, so he checked the specs, then torqued them to spec. He said that at this point it had cleared up 70% of the play he was seeing before, and concluded that a rebuild was not required. I am happy not to waste money on bushings that do not need to be replaced, but I am curious if this is something that has been reported before in the Sierra line?
Bob Steward, A&P-IA:
The play described (1/4″) seems a lot. There is a LOT of linkage and many bushings in the Sierra nose gear. If Tom replaced those in the nose gear support, then I’d be looking at all the others in the pivot mechanism to see if maybe the play wasn’t a combination of other small amounts of looseness adding up.
The important thing is to be sure the gear is set up properly. I see way too many folded nose gears from incorrect assembly and rigging of the gear actuators.
Locate the play and isolate it. You may find wear from 30+ years of operation is giving you that looseness. There are MUCH worse things that can happen with a retract than a little shimmy on landing. I’d say this falls under the standard maintenance and inspection protocol, and that with all the problems in properly maintaining and rigging of the nose and main gear on retracts in general, that you are fortunate to have found the problem before it contributed to a gear collapse.
I’ve included Mike Rellihan in the reply list, as he has a Sierra and might want to comment further.
Editor’s Comments:
I could be wrong in what I am about to describe; if so, please accept my apologies. It is very difficult to address this kind of problem hands-off, and from a distance.
I suspect that the two bolts mentioned are the upper pivot bolts. They cannot be too tight, or the nose gear will not free fall properly during emergency extension. Please make sure that the mechanic tested not only power retraction/extension, but also tested emergency extension (with the plane on jacks), when he completed this work. If memory serves, these bolts have special Teflon-coated thrust washers, just to make sure that they can be snug (not tight), while still allowing free motion. If the mechanic did not do an emergency extension test after this work, you are at risk. My personal practice is to do this testing after any gear work on a retract, including tire changes. You never know what else will turn up, and if you don’t find it, and something goes wrong, it will be blamed on the work that was last done.
Just tightening the pivot bolts should not have removed a lot of lateral play. If it did, it is hiding a wear problem that will resurface very quickly. It is highly likely that the gear needs new grip bushings, thrust washers, and bolts at the pivots.
The wear points that result in Sierra nose gear lateral play, as measured at the bottom of the tire, are as follows:
1. The wheel bearings being left too loose.
2. The knee pivot bushings and pin have too much play. This is a high wear point that does not get greased often enough.
3. The main support bearings in the top of the housing. They don’t get greased properly (weight off the nose gear), and their installation design cause rocking wear. This is especially true if the shim fit isn’t kept snug. It has to be just loose enough to allow emergency extension (the gear rotates on the main bearing as it falls).
4. The pivot bolts and bushings.
Unless everything is new, it is hard to get the play below 1/16-1/8″ (at the bottom of the tire). This is partly due to the rocking action of the main support bearing.
The parts that are critical to avoid a collapse are the downlock cylinder, downlock hook engagement, downlock (overcenter) spring, and the adjustment of the gear switch. On some planes a misadjusted switch causes the gear pump to cut off before the gear is fully locked down. The owner stays lucky for a long time, as in effect the gear makes the last 1/8″ or so and locks in place through momentum and pump spool-down. Until the day it doesn’t quite make it before touch-down. The nose gear is always the last to lock, because of wind load. The mains will virtually always lock, because of the reduced wind load and the heavy over-center springs on the mains. That’s why the nose gear collapses before anything else, on a Sierra. The gear itself is far more robust than that on other brands. But like any other retract, the gear needs proper attention to prevent problems.