Visual Flight rule (VFR)
Visual Flight rule are a set of regulations under which a pilot operates an aircraft in weather conditions generally clear enough to allow the pilot see where the aircraft is going.

In order for this to happen the weather must be:

  •  Able to operate the aircraft with visual reference to the ground.
  • Better than basic Visual Flight Rule (VFR) weather minima i.e. in visual meteorological condition (VMC), as specified in the rules of the relevant aviation authority. The pilot must be able to operate the aircraft with visual reference to the ground and by visually avoiding obstructions and other aircraft.

One should note that, if the weather is below Visual Meteorological Conditions, pilots are required to use instrument flight rules and operation of the aircraft will primarily be through referencing the instruments rather than visual reference.
Visual flight rule requires a pilot to see outside the cockpit in order to control the aircraft’s attitude, navigate and avoid obstacles and other aircraft.

Instrument Flight Rules (IFR)
Instrument flight rules is one of two sets of regulations governing all aspects of civil aviation aircraft operations. According to Federal Aviation Administration, IFR are rules and regulations established by the FAA to govern flight under conditions in which flight by outside visual reference is not safe. Instrument flight rules flight is dependent upon flying reference to instruments in the flight deck while navigation is acquired by reference to electronic signals.

Differences.
Visual flight rules are far much simpler than instrument flight rules hence need less training and practice. VFR provides a great degree of freedom which allows pilots to go where they want, whenever they want thus allowing them a much wider latitude in determining how to get there.
On the other hand, when operation of an aircraft under VFR is not safe because the visual cues outside the aircraft are concealed by weather or darkness, instrument flight rule is used instead. IFR allows an aircraft to operate in instrument meteorological conditions which is essentially any weather condition less than VMC but in which aircraft can still operate safely.

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