A lot of certified planes seem to have an extra ground strap/braid from the engine to the motor mount.
I guess this is a redundant path, if the main direct engine to airframe bond fails. In our case, the engine mount is well grounded to the basic airframe via the 6 mounting screws, so it like a good idea for a "back-up" ground...
Has anyone done this on a RV?
gil A
Gil, I have seen this done a number of times. The secondary path is terminated to a hole drilled in a gusset on the -6, -7, or -8 mount and held in place with an 8-32 bolt & nut. Of course, you need to remove any powder coating. (BTW, the -9 doesn't have these gussets on its engine mount.)
The ground strap/braid from the engine to the airframe or engine mount is the primary load path. This returns the current to the storage unit aka battery during startup and operations.
A secondary path is a "nice to have" in the event the primary path fails such as if a tab breaks off.
The engine mount can be used to carry that primary and secondary path, if you chose. Typically I see the primary ground go directly to the fuselage and the secondary go to the engine mount. BTW, make sure you don't terminate either the primary or secondary path at the same location just incase a failure at that connection doesn't compromise both paths.
What are peoples thoughts on WHERE to attach to the engine?
I picked up one of the vacuum pump bolts for my primary.
PS Someone mentioned the steel brade on the oil & fuel lines. Surely the logic of two grounds is to protect these from ever carrying the starter current? If they do, is that not a good way to start a fire since there is surely not enough 'meat' on them?
The idea of the secondary path is to keep you running, should the primary path fail. It is not intended to carry a starting load and I suspect most secondary paths would burn up under a starting load, transferring the current to oil and fuel pressure stainless steel lines. BTW, most secondary paths use a smaller wire, say 12 AWG. Again, the idea is to keep the engine running and all the engine gauges working (EGT, CHT, etc.).
It is unlikely that using the oil and fuel pressure lines as a secondary ground will cause a fire as the electricity goes around the oil and fuel and not through it. Just my opinion. (Kind of like the guy who repairs high tension lines by climbing from a helicopter doesn't get knocked out by the static charge.)
