Separation — Page 298, Lesson 386

Let's pick up with the vertical separation standards themselves, because that's where the real precision lives. I want to walk you through the ICAO level occupancy standard first, because it's the foundation for everything else. The standard is that an aircraft's actual altitude must stay within plus or minus 300 feet of its assigned flight level. That's the ICAO level occupancy standard — plus or minus 300 feet. The UK applies a tighter standard of plus or minus 200 feet. Now, why does this matter? Because when we talk about separating aircraft vertically by 1000 feet, we need a safety buffer. If one aircraft is 300 feet high and the other is 300 feet low, the actual gap between them shrinks. So the nominal separation of 1000 feet includes this buffer area to keep the real-world gap safe. Look at Figure 16.1 — it shows the ICAO level occupancy standard with FL100 and FL110, and you can see the 300-foot tolerance above and below each level, plus the 400-foot safety buffer between them. That's the visual of what I just described. Now, the actual vertical separation minima depend on the airspace and the flight level. Within designated airspace, subject to RAN agreement for RVSM — that's Reduced Vertical Separation Minima — the rule is a nominal 300 metres, which is 1000 feet, below FL410, or a higher level where so prescribed for use under specified conditions. At or above FL410, the separation increases to a nominal 600 metres, which is 2000 feet. But here's the key contrast: within all other airspace, the break point is lower. Below FL290, it's a nominal 300 metres, or 1000 feet. At or above FL290, it's a nominal 600 metres, or 2000 feet. So the RVSM airspace allows the tighter 1000-foot separation up to FL410, whereas non-RVSM airspace requires the wider 2000-foot separation from FL290 upward. Let me now define the RVSM cruising levels precisely. For domestic airspace, outside the NAT MNPSA — that's the North Atlantic Minimum Navigation Performance Specification Airspace — the RVSM cruising levels are those between FL290 and FL410 inclusive. That means both FL290 and FL410 are themselves RVSM levels. So the band is inclusive at both ends. Next, the minimum cruising level. Except when specifically authorized by the appropriate authority, cruising levels below the minimum flight altitudes established by the State shall not be assigned. Area Control Centres — the ACCs — shall, when circumstances warrant it, determine the lowest usable flight level or levels for the whole or parts of the control area for which they are responsible. They use that when assigning flight levels, and they pass it to pilots on request. Unless otherwise prescribed by the State concerned, the lowest usable flight level is that which corresponds to, or is immediately above, the established minimum flight altitude. The portion of a control area for which a particular lowest usable flight level applies is determined in accordance with air traffic services requirements. Now, assignment of cruising level. An ACC will normally allocate only one cruising level to an aeroplane, for flight in the control area or for flight entering another control area. The one exception is where cruise climb is authorized — in that case, more than one level may be involved. Finally, level change. If a change in cruising level is required, the aircraft is to request a level change en route, after the initial clearance has been received. Aircraft with cruise climb authorized will be cleared to operate between two levels. On ATS routes — that's air traffic services routes, the airways — extending beyond the control area, level changes are to be effected over a radio navigation aid. There's also a specific case: if an aircraft has been cleared into a CTA — a control area — below the minimum cruising level for that airspace, the ACC will issue a clearance to climb even though the pilot has not requested it. And when necessary, an aircraft may be cleared to change cruising level at a specified time, place, or rate. So the whole picture is: the occupancy standard defines how precisely you hold your level, the separation minima define how far apart levels must be, and the cruising level rules define which levels you can be assigned and how you change them.