I saw FL400 today on a EXS flight.
Whats the highest flight level anyone has seen?
seeen 400, heard 600 going down to 400
frank
FL560, commercially - Concorde (but not, obviously via Radarbox).
DC
The Astraeus flight from Stansted to Rekyjavik and return regularly flies over at FL400.
Highest Flight Level I've seen is FL485, for N144KK, a Gulfstream V operated by GC Air Llc. Don't know if that's a genuine FL, as I've seen several over 100,000ft which were obviously spurious readings.
Lots of others around FL450.
When there's a U-2 around, it's common to see it reporting FL600.
The Canbarra PR9 could fly well above FL600.
Once saw a picture from a PR9 showing it looking down on a U-2