Following completion of his first operational tour, Herrington returned to VP-31 as a Fleet Replacement Squadron Instructor Pilot. While assigned to VP-31, he was selected to attend the United States Naval Test Pilot School at Naval Air Station Patuxent River, Maryland in January 1990. After graduation in December 1990, he reported to the Force Warfare Aircraft Test Directorate as a project test pilot for the Joint Primary Aircraft Training System. Herrington conducted additional flight test assignments flying numerous variants of the P-3 Orion as well as the T-34C and the de Havilland Canada Dash 7. Following his selection as an Aeronautical Engineering Duty Officer (AEDO), Herrington reported to the U.S. Naval Postgraduate School, where he attained a Master of Science degree in aeronautical engineering in June 1995. Herrington was assigned as a special projects officer to the Bureau of Naval Personnel Sea Duty Component when selected for the astronaut program.