I noticed on flight plans with Simbrief it will make me do a descent and then back up to the normal cruise altitude, is there any reason for that and does it happen IRL?

Could be an airspace restriction above idosi but could also just be simbrief bug.

I think it is due to the ONE rule (Tracking 000 to 179 fly ODD level, EVEN level o.w.). Not sure about how the real world route would be filed in this case.

 

Benny

Does happen IRL. At least in Europe, airspace contains quite some complex restrictions, and sometimes such ”yoyos” happen in flight planning, but I believe they are not actually flown all that often, i.e. ATC will actually allow you to maintain the level.

Another reason might be more favorable winds/atmospheric conditions on the lower level, but looking at the graph that doesn’t look like the case here.

.