The Government requires the TCICAA to secure the most efficient use of airspace consistent with the safe operation of aircraft and the expeditious flow of air traffic whilst taking into consideration the requirements of operations and owners of all classes of aircraft. Additionally environmental implications and national security issues must be considered. Flights in TCI airspace are operated in accordance with Visual Flight Rules (VFR) or Instrument Flight Rules (IFR). VFR flight is not permitted in any TCI airspace at night; night is defined as the time from half an hour after sunset until half an hour before sunrise, sunset and sunrise being determined at surface level. In general, separation standards are not applied by ATC to or between VFR flights and therefore separation from other aircraft remains the responsibility of the pilot in command of a VFR flight. The exception to this applies in Class C Airspace – where ATC will separate VFR from IFR but not VFR from VFR. Minimum values of visibility and separation from cloud are defined, in excess of which a flight can be operated under VFR. Visual Meteorological Conditions (VMC) details the actual weather conditions permitting flight under VFR. Should weather conditions deteriorate below the proscribed minima for cloud and visibility, then Instrument Meteorological Conditions (IMC) exist and the flight must operate under more stringent IFR. For an aircraft to be flown within IMC, it must be: fitted with the necessary instrumentation and certified by the TCICAA the pilot must hold an instrument rating. IFR is, for the most part, mandatory within controlled airspace and an IFR flightplan must be filed. This enables the controllers to apply the required levels of separation to aircraft operating within controlled airspace and for traffic information to be provided to IFR traffic operating outside of controlled airspace so that the pilot can separate themselves sufficiently.