can be associated with more than one arrival airport, which can occur when two or more airports are in proximity (e.g., San Francisco and San Jose). Special use airspace In general, flight planners are expected to avoid areas called Special Use Airspace (SUA) when planning a flight. In the United States, there are several types of SUA, including Restricted, Warning, Prohibited, Alert, and Military Operations Area (MOA). Examples of Special Use Airspace include a region around the White House in Washington, D.C., and the country of Cuba. Government and military aircraft may have different requirements for particular SUA areas, or may be able to acquire special clearances to traverse through these areas. Flight levels Flight levels (FL) are used by air traffic controllers to simplify the vertical separation of aircraft and one exists every 100 feet relative to an agreed pressure level. Above a transitional altitude, which can vary from country to country and even within a country, the worldwide agreed upon pressure datum of 1013.25 millibars (corresponding to the pressure at sea level for the ICAO Standard Atmosphere, 101.325 kPa) or the equivalent setting of 29.92 inches of mercury is entered into the altimeter and altitude is then referred to as a flight level. The altimeter reading is converted to a flight level by removing the trailing two zeros: for example, 29000 feet becomes FL290. When the pressure at sea level is by chance the international standard then the flight level is also the altitude. To avoid confusion, below the transition altitude, height is referred to as a numeric altitude, for example 'descend 5000 feet' and above the transition altitude, 'climb flight level 250'. Airways have a set of associated standardized flight levels (sometimes called the "flight model") which must be used when on the airway. On a bi-directional airway, each direction has its own set of flight levels. A valid flight plan must include a legal flight level at which the aircraft will travel the airway. A change in airway may require a change in flight level. In the US, Canada and Europe for eastbound (heading 0–179 degrees) IFR flights, the flight plan must list an "odd" flight level in 2000 foot increments starting at FL190 (i.e., FL190, FL210, FL230, etc.); Westbound (heading 180–359 degrees) IFR flights must list an "even" flight level in 2000 foot increments starting at FL180 (i.e., FL180, FL200, FL220, etc.). However, Air Traffic