On an airliner, the pilot in command is called the captain. The captain, who generally sits on the left side of the cockpit, is ultimately responsible for everything that happens on the flight. This includes making major command decisions, leading the crew team, managing emergencies and handling particularly troublesome passengers. The captain also flies the plane for much of the trip, but generally trades off with the first officer at some point.
The first officer, the second in command, sits on the right side of the cockpit. He or she has all of the same controls as the captain, and has had the same level of training. The primary reason for having two pilots on every flight is safety. Obviously, if something happens to the captain, a plane must have another pilot who can step in. Additionally, the first officer provides a second opinion on piloting decisions, keeping pilot error to a minimum.
Most airliners built before 1980 have a cockpit position for a flight engineer, also called the second officer. Typically, flight engineers are fully trained pilots, but on an ordinary trip, they don't fly the plane. Instead, they monitor the airplane's instruments and calculate figures such as ideal takeoff and landing speed, power settings and fuel management. In newer airliners, most of this work is done by computerized systems, eliminating the need for the flight-engineer position. In the future, it will be phased out entirely.
All three pilots in the flight crew have equal levels of training, but they usually have varying degrees of seniority. At most airlines, the career track is based almost completely on length of service. To become a captain, you have to rise through the ranks and wait until it's your turn and a position opens up. Seniority also dictates the sorts of planes a pilot flies, as well as his or her schedule. Pilots who are relatively new to the airline will fly reserve, meaning they do not have a set flying schedule. A reserve pilot may have "on call" duty for 12 hours or longer at a stretch. In this time, the pilot has to be packed and ready to fly, because the flight scheduler might page them at any moment. If a pilot is called in, he or she reports to the airport immediately for a flight assignment (for many airlines, the pilot must be ready to go within an hour of being paged). Reserve pilots are called up when the scheduled pilot becomes ill or can't make the flight for some other reason. The life of the reserve pilot is largely unpredictable: Pilots might spend several days on reserve and never get paged, or they might get paged every day. And when they report for duty, they could be flying over to the next state or they might be putting in a three-day trip to another part of the world. With this hectic schedule, it's no wonder flights are occasionally delayed while waiting around for crew members to arrive.
Pilots with more seniority pick out a regular flight schedule, called a line. Pilots holding a line live a more "ordinary" sort of life, in the sense that they know ahead of time when they'll be working. But even these pilots spend a lot of time away from their families, and they never know what delays they'll encounter. In the United States, a pilot's scheduled flight time should not exceed 8 hours in a row for domestic flights or 12 hours for international flights. In actuality, however, pilots may work for more than 16 hours straight, since flights are often delayed or extended.