Within the US, the recipe would be something along these lines:
Since FL 410 is an eastbound altitude
Moderately short, 1-2 hours
- not so short that climbing high isn't worth it, nor so long that the aircraft takes off so heavy with fuel that 410 can only happen as a step-climb
An aircraft that can reach 410
For example, a 737 NG or Max or a ...
In general, the minimum vertical distance two IFR aircraft can be (if they are not separated laterally) is 1,000 feet. This allows for inaccuracies, for example due to:
aircraft height (compare a Skyhawk, nine feet tall, with an A380, 79 feet tall)
while still making sure aircraft do ...
This answer is specifically for the US--
Above 3000' AGL and below 18000' MSL (which is not the same thing as FL 180)1, cruising VFR traffic flies at MSL altitudes which are "round" numbers plus 500' (e.g. 3500' MSL, 4500' MSL, 5500' MSL, etc) while cruising IFR traffic flies at MSL altitudes which are "round" numbers (e.g. 4000' MSL, ...
The short answer is: yes
Personal Licensing regulation outlined in Annex 1, 2.1.4 "Circumstances in which class and type ratings are required" states that:
126.96.36.199 A Contracting State having issued a pilot licence shall not permit the holder of such licence to act either as pilot-in-command or as co-pilot of an aeroplane, an airship, a helicopter ...