SEATTLE — Air travelers are returning to the skies and U.S. planes are now typically flying full and with many passengers not wearing masks — a worry for those most vulnerable to COVID-19 infection. Pexco Aerospace, based in Union Gap in Central Washington, plans to offer a solution for airlines that could be ready to fly next year. It has partnered with Seattle-based Teague, a leading design firm, to produce a simple snap-on plastic component that creates an “air shield” system it touts as increasing protection of airline passengers from virus transmission. “There are still a lot of people co…