The airspace of the North Atlantic which links Europe and North America is the busiest oceanic airspace in the world.
Aircraft separation assurance and hence safety are nevertheless ensured by demanding the highest standards of horizontal and vertical navigation performance/accuracy and of operating discipline; this airspace requires Minimum Navigation Performance
The vast majority of North Atlantic flights are performed by commercial jet transport aircraft in the band of altitudes FL290 -- FL410. To ensure adequate airspace capacity and provide for safe vertical separations, Reduced vertical separation minimum - RVSM is applied throughout the ICAO NAT region.
For the greater part in the North Atlantic, ATS Radar Surveillance is unavailable.
A large portion of the airspace of the NAT, which, incidentally, contains the majority of these NAT crossing routes, is designated as the NAT High Level Airspace (NAT HLA) between FL 285 and FL 420 inclusive.
The lateral dimensions of the NAT HLA include the following Control Areas (CTAs): REYKJAVIK, SHANWICK (excluding SOTA & BOTA), GANDER, SANTA MARIA OCEANIC, BODO OCEANIC and the portion of NEW YORK OCEANIC EAST which is north of 27°N.
The North Atlantic Region is a High Level Airspace. The HLA extends in the horizontal limits of the oceanic airspaces and oceanic transition airspaces, between FL285 and FL420 included. Aircraft shall follow MNPS and Performance Based Navigation (PBN) minimas of the airspace. RVSM are also in force in HLA airspace.
In order to ensure aircraft and crew are competent to fly in HLA airspace, it is the responsibility of the State of Registry or the State of Operator of the aircraft to perform the required crew trainings and aircraft maintenance.
Aircraft or crew not meeting all the requirements shall not fly inside the HLA airspace, above FL420 or below FL285.
Below are the Oceanic FIRs as represented on IVAO Webeye.
These FIRs are domestic FIRs but affected by oceanic procedures because of their proximities to oceanic FIRs.
Scottish FIR (EGPX): IVAO XU division
Shannon FIR (EISN): IVAO XU division
Brest FIR (LFRR): IVAO FR division