<P> Despite the protection offered by flight in controlled airspace under IFR, the ultimate responsibility for the safety of the aircraft rests with the pilot in command, who can refuse clearances . </P> <P> It is essential to differentiate between flight plan type (VFR or IFR) and weather conditions (VMC or IMC). While current and forecast weather may be a factor in deciding which type of flight plan to file, weather conditions themselves do not affect one's filed flight plan . For example, an IFR flight that encounters visual meteorological conditions (VMC) en route does not automatically change to a VFR flight, and the flight must still follow all IFR procedures regardless of weather conditions . In the US, weather conditions are forecast broadly as VFR, MVFR (Marginal Visual Flight Rules), IFR, or LIFR (Low Instrument Flight Rules). </P> <P> The main purpose of IFR is the safe operation of aircraft in instrument meteorological conditions (IMC). The weather is considered to be MVFR or IMC when it does not meet the minimum requirements for visual meteorological conditions (VMC). To operate safely in IMC ("actual instrument conditions"), a pilot controls the aircraft relying on flight instruments and ATC provides separation . </P> <P> It is important not to confuse IFR with IMC . A significant amount of IFR flying is conducted in Visual Meteorological Conditions (VMC). Anytime a flight is operating in VMC and in a volume of airspace in which VFR traffic can operate, the crew is responsible for seeing and avoiding VFR traffic; however, because the flight is conducted under Instrument Flight Rules, ATC still provides separation services from other IFR traffic, and can in many cases also advise the crew of the location of VFR traffic near the flight path . </P>

To operate an aircraft under ifr a flight plan