Arc’teryx Aerios 18 Backpack Black Grid. New No Tags Condition, no signs of wear. The Aerios 18 Backpack has a patent-pending interchangeable harness system that allows you to finetune your fit before you start your hike, so you won’t spend every step thinking about your pack. The ultralight, ultra-strong grid pack fabric sheds light rain and stands up to all the drops, drags, and snags a pack encounters on the trail.