Rules of the Air — Page 126, Lesson 200

I want to walk you through the semi-circular flight level rules, RVSM, and special VFR. This is where the Rules of the Air chapter gets into the practical business of vertical separation — how we keep aircraft apart vertically when they're cruising. Let's start with the semi-circular rules. The whole point is vertical separation inside CAS — that's Controlled Airspace. Inside CAS, ATC will allocate specific flight levels to individual aircraft, and those allocations may or may not comply with the semi-circular rule. But for planning purposes, and outside of CAS, pilots should select the required cruising level — either IFR or VFR — from the ICAO table of flight levels. From those tables, the semi-circular rules can be derived, and they permit the selection of flight levels in accordance with the aircraft's magnetic track. Now, tracks are specified as either 'eastbound' or 'westbound'. Eastbound tracks are 000°M to 179°M inclusive — that's magnetic, so from north through east to just before south. Westbound tracks are 180°M to 359°M inclusive — from south through west and back to just before north. Specific flight levels are allocated to VFR and IFR traffic. Here's the key distinction: IFR flight levels are whole thousands of feet, whereas VFR levels are whole thousands plus 500 feet, up to FL285. So an IFR level might be FL250, while a VFR level would be FL255. Eastbound levels are defined as 'odd', and westbound as 'even', based on the first two digits of the flight level number. So FL250 — the first two digits are 25, which is odd — that's an eastbound level. FL260 — first two digits 26, even — that's westbound. Above 30,000 feet, generally, the vertical separation is doubled, and the pattern changes: IFR levels are all odd, and VFR levels are even. So above FL300, the separation between levels becomes 2000 feet instead of 1000. Now, a special system of flight level allocation is applied where reduced vertical separation minima — RVSM — is applied. Let me show you the tables for this. shows the IFR and VFR flight levels up to and including FL290, and shows the levels above FL290. Now let's get into RVSM itself. The purpose is to make more flight levels available to turbojet traffic in the cruise. The system prohibits VFR flight — that makes the VFR levels available to IFR traffic. To achieve this, the separation between IFR levels between FL290 and FL410 inclusive is reduced from 2000 feet down to 1000 feet. So instead of the doubled separation above FL300, we squeeze those levels closer together to create more cruising levels for the jets. There's a requirement for aircraft using the RVSM system: they must be fitted with A/TCAS — that's Airborne Traffic Alert and Collision Avoidance System — and they must be approved by the airspace authority. Above FL410, the altimeter errors are considered too great to continue the 1000-foot separation, so the reduced separation stops there. shows the reduced vertical separation minima. Now let's move to Special VFR — SVFR. This has a history worth understanding. With the introduction of airspace restrictions in the late 1960s, military aerodromes close to large international aerodromes — specifically Northolt, in proximity to the rapidly expanding Heathrow — found that IFR procedures were mandatory in the new control zones, when previously VFR procedures were generally accepted. To allow aeroplanes to fly into and out of Northolt, in what was then the Heathrow Special Rules Zone, a procedure was set up based on a corridor in which visual navigation was required. The logic was simple: providing the pilot could see the ground, he could navigate, and provided he remained clear of cloud, he could avoid collisions. So a system of 'not quite' IMC — Instrument Meteorological Conditions — or special VFR was invented. Until the late 1970s, this was applied in what was known as the Northolt special VFR corridor. It was then expanded to include the general aviation aerodrome at Denham, and its advantages for aeroplanes and pilots unable to comply with IFR were obvious. When the classes of airspace — A through G — were introduced, ICAO also adopted special VFR as a procedure with appropriate international amendments. So SVFR is defined by ICAO as a VFR — and that's where the excerpt cuts off. The definition continues, but the key point is that special VFR is a VFR procedure that allows flight in conditions that would otherwise require IFR — typically within controlled airspace when the weather is below normal VFR minima, but the pilot can still maintain visual reference to the ground and clear of cloud. Let me make sure you've got the core contrasts clear. First, IFR levels are whole thousands; VFR levels are whole thousands plus 500 up to FL285. Second, eastbound is odd, westbound is even, based on the first two digits. Third, above 30,000 feet the separation doubles and IFR becomes all odd, VFR even. Fourth, RVSM squeezes IFR separation from 2000 down to 1000 feet between FL290 and FL410, requires A/TCAS and airspace authority approval, and stops above FL410 because of altimeter error. And fifth, special VFR grew out of the Northolt corridor as a way to fly visually when IFR was otherwise mandatory.