The rescue fire services are based on a critical aircraft based on a statistical analysis of movements (take-offs and landings) on the airport. The aerodrome category is based on the size of the largest aircraft which will operate at the aerodrome. In addition, the number of movements of the critical aircraft is calculated, and the category can be decreased by one if the number of movements is lower than the standard describes. For example, at an airport regularly handling Boeing 737 and Airbus A320 aircraft with a single Boeing 777 service per week, the airport fire service has to cater up to the ICAO category 7 of the 737 and A320; a single 777 movement per week does not justify a full ICAO category 9 fire service. There are also minimum category levels based on e.g. the number of seats in the critical aircraft.