According to the Federal Aviation Administration, the federal government will close down 173 small airport towers next month. The closures will affect small airports, many of them rural, where towers are manned by unionized contractors rather than FAA employees. The FAA oversees 250 contract towers. The FAA notified operators in a conference call on Tuesday of the closures. The contract program has been around for the last 30 years and during that time only three airports have been shut down. Most of the employees are former FAA and military air-traffic controllers.