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Optional Service Bulletin Kit for RV-12 Main Gear Channel Update

greghughespdx

Well Known Member
Advertiser
We've released Service Bulletin SB-00040, which can optionally be installed on legacy/original model RV-12 aircraft. The SB-00040 kit includes the parts and instructions needed to proactively add additional reinforcement to the landing gear beam and was developed based on field experience where flight school aircraft subject to repeated hard landings had rivet failure and/or fatigue issues as described in the bulletin.

We have produced a handful of kits that can be used to make the changes described in this service bulletin. Note that this is not a mandatory service bulletin. Van's recommends compliance for operating environments where repeated heavy and unusual landing loads might occur, such as in flight training or on unusually rough landing surfaces.

This service bulletin applies only to the original model RV-12 and is not applicable to the RV-12iS, which utilizes a different design/structure.
 
That looks like quite a bit of work to install. Fortunately, it's not an upgrade that my 12 needs (not yet anyway), but all credit to Vans for putting in the hard work to develop the kit. Listed as $360 on Vans site.
 
If someone had upgraded, or has plans to upgrade, to the gun drilled gear legs, are there any interference or install compatibility issues with this SB kit?
 
My aluminum carry-thru gear channel developed cracks at the rear “mouseholes” at ~500TT. I had previously installed SB 12-11-09 per Van’s recommendation. This Service Bulletin added support plates to the bottom side of the u-channel but did not address stress concentrated at the AN5 washer/nut on the top side of the channel.

I’m sure rough landings and/or operating on rough fields adds to this problem, but I have a theory that in-flight loads are also a major contributor.

The spring landing gear leg, with heavy cantilevered wheel assembly, causes an up/down flexing motion – not necessarily fore/aft. Might even happen when airplane leaves the ground, gear drops suddenly, with springing motion up/down before settling. The springing action of the gear leg would be seen by the outboard u-bracket and would be vertical forces at that location. Repetitive up/down force on the u-bracket will cause the thin bottom web of the channel to develop stress cracks that emanates from the mounting holes.

As stated above, I think the root cause of the problem is flight loads and not necessarily ground operations. When the airplane is on the ground all the force loaded onto the gear is in one direction - up. I don’t think heavy braking is to blame because this would impart a vertical force in the “up” direction at the rear mounting bolts. If braking was a contributor, we would see tearing at the front mounting holes. Also, the wheels have an “instant center” when they contact the ground on landing. Even a smooth landing has violent forces when the wheel spins-up at the instant center point of contact on the ground.

In turbulent air, the gear leg flexes up/down with the heavy cantilevered wheel assembly at the far end of the spring gear leg. Constant up/down repetitive forces caused by flight turbulence could easily cause fatigue cracking that we see. This would explain why there is damage even when plane is landed correctly most of time.

Upward motion of the gear leg (i.e. – landing) puts force into vertical web of channel which is good. Downward force of the gear tries to “tear” the u-bracket bolts through the channel floor. The upward force is spread somewhat with the face of the u-bracket that straddles and supports the outboard gear mounting. The downward force is isolated to the AN5 washer and lock nut. It is this small concentrated area of force in the downward direction that is trying to rip and tear the AN5 bolt thru the thin base of the aluminum u-channel. The landing gear is not visible from inside the airplane – but there is a lot of motion going on below the wing during flight. My theory is that the downward force of the landing gear in turbulent flight is trying to rip the AN5 main gear bolts down thru the 3/16” thick extruded aluminum channel. Force is localized at the face of the washer and needs to be spread over a much larger area on top of the channel to eliminate stress concentration. It looks like this new SB finally addressed the root cause of the problem.

Several years ago, a small group of local RV-12 owners developed a set of steel “sandwich plates” that are designed to spread all the forces from the u-bracket into a large area of the carry-thru channel in order to mitigate stress concentration at the AN5 bolt holes. I installed a set of these plates at ~500TT and consider the problem solved...
 
Interesting theory Jim, but I don't see myself doing that extensive SB. I guess everyone has opinions and I fall on the side of believing enough in Van's engineering of the RV12 to be confident that my landing gear is strong enough to handle aerodynamic loads while flying. I guess time will tell for me..
 
The spring landing gear leg, with heavy cantilevered wheel assembly, causes an up/down flexing motion – not necessarily fore/aft. Might even happen when airplane leaves the ground, gear drops suddenly, with springing motion up/down before settling. The springing action of the gear leg would be seen by the outboard u-bracket and would be vertical forces at that location. Repetitive up/down force on the u-bracket will cause the thin bottom web of the channel to develop stress cracks that emanates from the mounting holes.

As stated above, I think the root cause of the problem is flight loads and not necessarily ground operations.

I agree that there are vertical loads applied at this point in flight but disagree that it is a root cause of crack development in the channel.

The loads induced by ground operations are orders of magnitude higher than what would be induced by the gear leg mass in flight, but instead of debating with theories we can look at actual data.
If your theory was correct, every high time RV-12 would be developing cracks in the channel just from all of the flight hours, but they aren't. Otherwise it would not have been offered as an optional change.
 
Cudos to Van's for making the sig. investment of time to develop and test this optional SB. After reading through 19 pages, I won't be taking it on, but I'll definitely remind myself on short final to avoid slamming the aircraft down on the runway. Those pictures were disturbing - should be shown to every new RV-12 pilot.
 
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