I have one from the horses mouth, as fully qualified SLF. I went back to the galley on an AA-777 last month westbound across the pond. Powder blue sky outside, so I bent slightly and looked out of the rear access window.
Coming in the opposite direction, right to left at the same altitude was another silver jet, with the most wonderful grey expanding contrail from the back. It wasn't converging so I wasn't alarmed, and watched it for a few minuites..
It was the wingtip...