ATC authorization for an IFR aircraft to operate in VFR conditions at any appropriate VFR altitude. A pilot receiving this authorization must comply with the VFR visibility, distance from cloud criteria, and the minimum IFR altitudes. The use of this term does not relieve controllers of their responsibility to separate aircraft in Class B and Class C airspace or terminal radar service areas.
Source: US FAA
The procedure is used in the USA and is not an ICAO standard.
In short, a VFR on-top clearance is an IFR clearance that allows pilots to fly at VFR altitudes (i.e. to select a level in lieu of the assigned one). This can be helpful if flying slightly above, or in between layers, and it is preferable to stay out of the clouds. Flying for extended periods of time through IMC can be fatiguing and less comfortable (turbulence is ofthen present in clouds). Also, being able to see distant objects helps avoid air sickness.
The procedure is is not intended to restrict pilots so that they must operate only above an obscuring meteorological formation (layer). Instead, it permits operation above, below, between layers, or in areas where there is no meteorological obscuration. It is imperative, however, that pilots understand that clearance to operate "VFR−on−top/VFR conditions" does not imply cancellation of the IFR flight plan.
When an aircraft has been cleared to maintain VFR-on-top, the pilot is responsible to fly at an appropriate VFR altitude, comply with VFR visibility and distance from cloud criteria, and to be vigilant so as to see and avoid other aircraft. The pilot is also responsible to comply with instrument flight rules applicable to the flight (e.g., adherence to ATC clearances).
Although IFR separation is not applied, controllers must continue to provide traffic advisories and safety alerts, and apply merging target procedures to aircraft operating VFR-on-top. The pilot should advise ATC prior to any altitude change to ensure the exchange of accurate traffic information.
The appropriate level for VFR-on-top is derived by adding 500 ft to the corresponding IFR level, i.e.:
- For magnetic courses 0−179 − odd levels plus 500 ft, e.g. 3500, 5500.
- For magnetic courses 180−359 − even levels plus 500 ft, e.g. 4500, 8500.
VFR-on-top clearances are not available in Class A airspace.
- FAA Publications (external link containing a list of documents including the ones below)
- FAA JO 7110.65 - Air Traffic Control
- FAA Aeronautical Information Manual