High Quality Content by WIKIPEDIA articles! San Marcos Municipal Airport (ICAO: KHYI, FAA LID: HYI) is a public use airport located in Caldwell County, Texas, United States. It is four nautical miles (7 km) east of the central business district of San Marcos, a city in Hays County. The airport is owned and operated by the City of San Marcos. It is located east of the border of Caldwell County and Hays County. Although most U.S. airports use the same three-letter location identifier for the FAA and IATA, this airport is assigned HYI by the FAA but has no designation from the IATA.