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In ICAO Annex 11 it is recommended:

When the lower limit of a Control Area is above 900 m (3000 ft) MSL it should coincide with a VFR cruising level

Why should it coincide with a VFR cruising level?

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    $\begingroup$ Welcome to aviation.SE! $\endgroup$ – Pondlife Jan 19 at 4:22
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VFR cruising levels are always located between two IFR cruising levels. With half of the required radar separation minimum to each the IFR level above and below it.

For example: required separation minimum: 1000ft IFR lvl: 200 VFR lvl: 195 IFR lvl: 190

The sectors are split at VFR cruising levels to ensure, that both the upper and lower sector can use ALL of their IFR cruising levels while maintaining separation to aircraft that may fly at the highest/lowest level of the sector below/above.

Example: Sector A is working from FL145-FL245 Sector B is working from FL245-FL355

Sector A may use every IFR cruising level within the sector (FL150, FL160, ..., FL240). Sector B as well (FL250, FL260, ..., FL350).

As the difference between the highest and lowest level is always 1000ft, separation is ensured also between aircraft flying on top of each other in different sectors!

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  • $\begingroup$ I agree this is the most obvious answer, but why would the ICAO not just define it as "the lower limit of a control area should be x500 ft". Why specifically mention VFR? I feel like there could be some less obvious reason behind it $\endgroup$ – expeditedescent Jan 23 at 8:22
  • $\begingroup$ @expeditedescent because in some cases it might be that the VFR levels are NOT x500ft (non-RVSM airspace). Before EU-RVSM was implemented all the division levels above FL290 had to be in implements of 1000, but still in between 2 IFR cruising level e.g.: FL310 and FL330 are highest and lowest usable levels in two sectors, they would then split at FL320 $\endgroup$ – pcfreakxx Jan 23 at 9:38
  • $\begingroup$ Again, I agree completely with your logic. I just think it's a bit weird that they are bringing flight rules into the mix, when the rule seems to have nothing at all to do with flight rules $\endgroup$ – expeditedescent Jan 23 at 19:23
  • $\begingroup$ @expeditedescent it for the sake of easiness. Check ICAO annex 2 Apendix 3, it‘ll be clearer then :) $\endgroup$ – pcfreakxx Jan 23 at 19:37

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