RVSM and Semi-Circular Cruising Rules Simplified
Ask a sim pilot why they cruise at FL350 instead of FL340 and you might get a shrug. The answer is a pair of real-world rules — semi-circular cruising and RVSM — that keep opposite-direction jets separated by at least 1,000 feet. NG ROUTE bakes these conventions into the CRUISE suggestion on your summary card. Knowing the logic helps you pick realistic levels on VATSIM and understand odd ETAs when heading changes.
Semi-circular rule in plain language
In most IFR cruise airspace, magnetic track determines whether you fly odd or even thousands of feet:
- Magnetic track 000°–179° (generally eastbound) → odd flight levels: FL330, FL350, FL370, FL390…
- Magnetic track 180°–359° (generally westbound) → even flight levels: FL320, FL340, FL360, FL380…
Track is not the same as heading — it is the path over the ground. A strong crosswind can make heading differ from track, but flight planning uses track for level selection.
NG ROUTE estimates great-circle track between departure and destination when suggesting cruise. A London-to-Athens flight trends southeast (eastbound-ish) and tends toward odd levels; the return leg flips to even.
What RVSM adds
RVSM (Reduced Vertical Separation Minima) allows aircraft certified for it to separate by 1,000 feet vertically between FL290 and FL410 instead of the older 2,000-foot spacing. That is why odd/even alternation at every thousand feet matters instead of every two thousand.
Below FL290, regional rules vary; above RVSM airspace, separation returns to wider criteria. For typical airline sim flights in the low thirties and high thirties, RVSM thinking applies.
How NG ROUTE combines distance and direction
Cruise suggestions use both semi-circular direction and route length:
| Distance band | Typical eastbound (odd) | Typical westbound (even) |
|---|---|---|
| Under 200 NM | FL250 | FL240 |
| 200–500 NM | FL330 | FL320 |
| 500–1500 NM | FL370 | FL360 |
| Over 1500 NM | FL390 | FL380 |
Your aircraft ceiling caps the suggestion — a regional jet profile will not receive FL390. Distance drives how high you need to fly for efficiency; direction picks odd versus even within that band.
Examples sim pilots recognize
EHAM to LFPG — roughly east-southeast magnetic track → odd level such as FL330 on a medium hop.
LFPG to EHAM — reverse track → even level such as FL320 for the same distance.
Transatlantic eastbound — often FL370–FL390 odd as distance and jet streams dictate.
Compare ETA when rebuilding the same city pair in both directions on a windy day — westbound headwinds plus a slightly lower even level can add noticeable time.
VATSIM and online ATC
Controllers may assign a different level for traffic flow. File the NG ROUTE suggestion as your requested cruise, but comply with ATC clearances even when they break odd/even for sequencing. In solo sim, flying the semi-circular level trains the habit controllers expect in voice briefings.
Transition altitude reminder
Semi-circular rules apply in cruise airspace. Below transition altitude you set local QNH; above transition level you set standard 1013.25 hPa (29.92 inHg). Climb through transition with the correct altimeter setting before leveling at your RVSM cruise — NG ROUTE METAR panels help with departure and arrival QNH.
Exceptions you should know exist
Real-world operations have numerous local exceptions: directional TACAN routes, organized track systems over oceans, military airspace, and flow programs. NG ROUTE models general ICAO conventions, not daily NAT track notices. For simulation, the semi-circular suggestion is the right default unless you are deliberately mimicking a specific dispatch package.
Practice exercise
- Build the same route both ways between two cities you know.
- Compare CRUISE for each direction.
- Fly both legs in the sim at the suggested levels.
- Note fuel burn and time differences in the FMS progress page.
After two or three pairs you will predict odd versus even before looking at the summary card — a small skill that makes online IFR sessions sound professional on the radio.
RVSM and semi-circular rules are not bureaucracy for its own sake. They double usable flight levels in busy airspace. NG ROUTE applies them automatically so your simulated airline flights start at the same altitudes real traffic expects.
Cruise level suggestions are for flight simulation. Actual level assignments come from ATC and dispatch in real operations.