Visual flight rules
pilot is responsible for separation from other traffic and from terrain, he is not allowed to enter clouds and must keep visual refference to surface. flying above the sea or above cloud coverage of more than 4/8 is not VFR (in Europe)
Instrument flight rules
pilot is maintaining position and attitude with refference to instruments, and most of the responsibility for separations is up to the ATC /with exception of VFR traffic, in case the IFR flight enters the airspace class that allows simultaneous traffic of both IFR and VFR /class D airspace for instance
if flying IFR, and performing the precise radar assisted landing, the ATC is also responsible for the separation from the surface.
in all other situations, regardless wheter UFR or VFR, the captain is the finally responsible person onboard for maintaining the separation from the surface.
the air law [in my country] states that IFR is anything that includes flying
above clouds of more than 4/8
flying in between two cloud layers
flying over sea
hoodie flights (IFR for the hooded pilot)
the weather which is alreadz considered IR is usually the 5kilometers horizontal visibility, though there's special VFR allowing to reduce the visibility down to 2.5kilometers for airplanes and 0.8 km for helicopters. the special VFR is ONLY performed within the CTR and is much treated like an IFR flight, including separations. only one aircraft at a time may perfoerm a special VFR within the CTR.
flying in clouds obviously requires IFR, since there is yero visibility :)
moreover helicopters may fly VFR flights outside CTR down to 0.8kilometers, providing there is low probabilit of encountering other traffic. that's what I call flying VFR in IMC conditions.
· 1 decade ago