There are two different rules under which aircraft can operate, both based on the instruments on board and the qualification of the crew:
- Visual Flight Rules (VFR).
- Instrumental Flight Rules (IFR).
In order operations to be carried out under VFR, the meteorological conditions must be good enough to allow pilots identify the visual references in the terrain and other aircraft. Such meteorological conditions are referred to as Visual Meteorological Conditions (VMC). VFR require a pilot to be able to see outside the cockpit, to control the aircraft’s altitude, navigate, to maintain distance to surrounding clouds, and avoid obstacles and other aircraft. Essentially, pilots in VFR are required to see and avoid.
If the meteorological conditions are below the VMC threshold, the flight must be performed under IFR. Such meteorological condition are referred to as Instrumental Meteorological Conditions (IMC). Notice that IFR flights are under control by ATC services.
Figure 10.11: Classes of Airspace in the USA (altitudes AGL in feet).
Since sometimes VFR and IFR flights must share the same airspace, it was necessary to regulate the operations. With that aim, ICAO has defined seven different classes of airspaces: A, B, C, D, E, F, and G. The most restrictive one is Class A, where only IFR flights are permitted. The least restrictive is Class G, where both IFR and VFR flights are permitted. In any of the other airspace classes, sovereign authorities derive additional rules (based on the ICAO definitions) for VFR cloud clearance, visibility, and equipment requirements. Classes A-E are referred to as controlled airspace. Classes F and G are uncontrolled airspace.Figure 10.11 sketches the classes of airspace in the US (notice that class F in not use in the US). Table 10.1 include some of the main features by ICAO.
Table 10.1: Airspace classification.