I was getting refueled by a truck at a US airport a while back, I took the ground cable from the truck and attached it to the exhaust pipe, the refueller guy took it off and reclamped it to the bolt on the brake caliper, I asked him why, he said he was told by his boss to either use the brake caliper or tie down ring on the wing which ever offered a clean metal to metal connection but not the exhaust. No idea what that was all about.