Frontier Airlines announced an expansion of its network Tuesday — including 21 new destinations from Denver International Airport — and a fare sale to fill the seats. The move comes as the Denver-based airline prepares for an initial public offering, and it signals a renewed focus on Frontier’s primary hub after several years of reductions here. Frontier said the expansion, including non-Denver routes, increases the number of destinations served by the carrier by 30 percent and double the number of total routes. Airline executives joined airport officials in making the announcement at DIA Tuesday, hailing the expansion as an important step for both. Read more on Colorado Springs Gazette