A flight plan is a document which provides detailed information about a planned flight. The document is filed with aviation officials, and forwarded to officials at the plane's destinations or waypoints to ensure that they have the data in hand. Filing a flight plan is required by law in many cases, and it is also a good idea from a safety perspective, as it ensures that if a flight goes missing, someone will start looking for it.
Several pieces of information are included in a flight plan. The names of the captain, crew, and passengers are included, along with descriptions of any cargo which may be carried. The type of aircraft is also discussed, as is the type of flight, indicating whether the pilot will be flying with instruments, or under visual flight rules. The flight plan also details the departure and arrival points of the aircraft, the estimated route the plane will take, and the expected duration of the flight.
In addition to providing this basic data, a flight plan also usually details alternate airports which it will use in the event of an emergency. It may also specifically address concerns about controlled or restricted airspace, and other issues which may come up during the flight. The idea is to create a complete picture of what is going to occur on the flight, and to demonstrate that the crew have prepared for unexpected events.
From an air traffic control perspective, flight plans are very important, as they alert officials to the presence of planes in the sky. Using filed flight plan data, controllers can time the arrival and departure of planes, and send out specific information about the altitude and heading which various planes should follow to avoid collisions. Without flight plan information, air traffic controllers would find their jobs would be much more complicated than they already are.
Another concern is fuel consumption, because planes burn a lot of fuel, and running out of gas in the middle of the sky is not a desirable occurrence. Using information provided by the manufacturer of the aircraft, the person who files the flight plan can estimate how much fuel will be used, and whether or not it will be necessary to stop and refuel. Fuel allowances must also account for bad weather, which could increase fuel consumption, and in some regions, pilots are mandated to carry extra fuel so that they are prepared in the event of an unexpected event, which could vary from needing to hover over an airport to wait to land to losing fuel due to damage.