Ok, this is simple: get highest speed possible while staying in one piece, then land the plane (anyhow damaged is ok, but destroyed on landing is not). No modded maps, non-vanilla planes and any other fancy stuff, of course - just vanilla game.
Got the whole run recorded. 2653 km/h visible by 8:10 mark in the video: https://www.youtube.com/watch?v=Du1XlFbi2Ec
Anyone dare / able to go faster than this? Bring it on!