When must we make contact with ATC?
Is there any global reference describing when flight crews have to make contact with ATC?


On the ground, you need a clearance from ATC when you are on the maneuvering area, which consists of taxiways and runways. The ramp/apron is typically not part of the maneuvering area, and as such not controlled by ATC, although many large airports have some sort of ground control for these area.

In the air, you need a clearance from ATC whenever you are in controlled airspace. Some countries additionally require pilots flying IFR to be in contact with ATC even in uncontrolled airspace.

Note: ‘controlled airspace’ means an airspace of defined dimensions within which air traffic control service is provided in accordance with the airspace classification; (from SERA, EU 923/2012)