Control (ATC) may assign any flight level at any time if traffic situations merit a change in altitude. Aircraft efficiency increases with height. Burning fuel decreases the weight of an aircraft which may then choose to increase its flight level to further improve fuel consumption. For example, an aircraft may be able to reach FL290 early in a flight, but step climb to FL370 later in the route after weight has decreased due to fuel burn off. RVSM Alternate airports Part of flight planning often involves the identification of one or more airports which can be flown to in case of unexpected conditions (such as weather) at the destination airport. The planning process must be careful to include only alternate airports which can be reached with the anticipated fuel load and total aircraft weight and that have capabilities necessary to handle the type of aircraft being flown. In Canada, unlike the United States, unless specifically exempted by a company Operating Certificate, IFR flight plans require an alternate airport, regardless of the forecast destination weather. In order to be considered as a legally valid alternate, the airport must be forecast to be at or above certain weather minima at the estimated time of arrival (at the alternate). The minimum weather conditions vary based on the type of approach(es) available at the alternate airport, and may be found in the General section of the Canada Air Pilot (CAP). Fuel Aircraft manufacturers are responsible for generating flight performance data which flight planners use to estimate fuel needs for a particular flight. The fuel burn rate is based on specific throttle settings for climbing and cruising. The planner uses the projected weather and aircraft weight as inputs to the flight performance data to estimate the necessary fuel to reach the destination. The fuel burn is usually given as the weight of the fuel (usually pounds or kilograms) instead of the volume (such as gallons or litres) because aircraft weight is critical. In addition to standard fuel needs, some organizations require that a flight plan include reserve fuel if certain conditions are met. For example, an over-water flight of longer than a specific duration may require the flight plan to include reserve fuel. The reserve fuel may be planned as extra which is left over on the aircraft at the destination, or it may be assumed to be burned during flight (perhaps due to unaccounted