ATC asking if you have oxygen

Is UNL higher than FL500 ?

I think UNL basically means “This (VFR) chart only shows features up to a certain FL (which is listed at the front) and this particular feature exceeds that FL).”

I can’t imagine the ISS having to ask some LARS for a DACS every time their orbit happens to be over that particular DA.

UNL is used in the AIP and is defined simply as “unlimited”.
So that would mean that the restriction applies at any (atmospheric) altitude.

So, unless you can make it into low earth orbit, it might be easier just to go round it.

They were active up to FL160 that day, hence the amusing response. I didn’t mind at all – thought it was funny.

Incidentally, that is the only time I’ve ever not been able to cross that area, yet I know loads of people automatically go round it. There’s no harm in asking… if in fact you even have to ask – a lot of the time, there’s just a recorded message.

It’s quite interesting flying over Salisbury plain. You feel like you’re doing something a little bit naughty and that you shouldn’t really be there. That said, there’s bugger all to see though!

every single Corvalis/Columbia (the turbo one) I’ve ever seen was flying at FL100.

…ergo the ones not seen could be at a different level. We normally flight plan the Columbia 400 at +FL180. Never been asked about O2, but ATC (UK, France, Italy, etc.) sometimes query aircraft type.