The MS-1 on a Space shuttle flight sat behind the pilot on the flight deck, and monitored displays and checklists. The MS-2 was the flight engineer, and sat behind the commander. The Flight Engineer assisted the commander and pilot, and acted as the third member of the flight deck crew, and an additional set of eyes during the critical phases of a mission. The hopes of NASA management that a CDR-PLT-MS2 team would be able to fly three or four missions a year were never realized. After a single mission as pilot, a pilot astronaut became eligible to be commander on their next mission. Although some of the mission specialists were fully qualified pilots, none ever flew as pilot on a mission, and therefore never served as a mission commander. As more than one mission specialist flew on each flight, they began flying at a faster rate than their pilot classmates. Two pilot astronauts, David Griggs and Steven Nagel, flew their first missions as mission specialists. Nagel later flew as the pilot on STS-61-A and as the commander on STS-37 and STS-55. Griggs was assigned as the STS-33 pilot, but he was killed in an air crash prior to the mission.
