for differences between the actual aircraft and the flight performance data). In case of an in-flight emergency it may be necessary to determine whether it is quicker to divert to the alternate airfield or continue to the destination. This can be calculated according to the formula (known as the Vir Narain formula) as follows: where C is the distance from the Critical Point (equitime point) to the destination, D the distance between the destination and the alternate airfield, O is the groundspeed, A is the airspeed, θ = Φ +/- d (where Φ is the angle between the track to the destination and the track from the destination to the alternate airfield), and d is the drift (plus when the drift and the alternate airfield are on the opposite sides of the track, and minus when they are on the same side). Flight plan timeline Flight plans may be submitted before departure or even after the aircraft is in the air. However flight plans may be submitted up to 120 hours in advance either by voice or by data link; though they are usually filled out or submitted just several hours before departure. The minimum recommended time is 1 hour before departure for domestic flights, and up to three hours before international flights. This time depends on the country the aircraft is flying out of. Other flight planning considerations Holding over the destination or alternate airports is a required part of some flight plans. Holding (circling in a pattern designated by the airport control tower) may be necessary if unexpected weather or congestion occurs at the airport. If the flight plan calls for hold planning, the additional fuel and hold time should appear on the flight plan. Organized Tracks are a series of paths similar to airways which cross ocean areas. Some organized track systems are fixed and appear on navigational charts (e.g., the NOPAC tracks over the Northern Pacific Ocean). Others change on a daily basis depending on weather, west or eastbound and other factors and therefore cannot appear on printed charts (e.g., the North Atlantic Tracks (NAT) over the Atlantic Ocean). Description of flight plan blocks (FAA) Domestic Flight Plan Form 7233-1, Type: Type of flight plan. Flights may be VFR, IFR, DVFR, or a combination of types, termed composite. Aircraft Identification: The registration of the aircraft, usually the flight or tail number. Aircraft Type/Special Equipment: The type