On one flight, with only a crew of three (no passengers) onboard the Global Express and light on fuel, I requested a climb to FL510 (max certified altitude) as none of us had ever been that high. We usually cruise at FL450 or lower.
With masks on, of course, we inched on up slowly as climb performance diminished.
It felt good to see what the airplane would actually do.
After ten minutes of so, I said to my co‑pilot, “Don’t ask ATC for lower, ask them for re‑entry.”
The controller was happy to comply and remarked that he didn’t know our type could go so high.
Grand Prairie, TX