[sokoloff]

The F-15 can regularly climb over FL600 and, under special conditions, has exceeded 100K above MSL.

[jki275]

That was a specialty pre-production version of the F15 that was modified to do basically just altitude and speed records. It never entered operational service.

https://theaviationgeekclub.com/how-the-f-15-streak-eagle-br...

[Manuel_D]

In level flight or in a zoom climb? It's one thing to build up speed and pitch up in an ballistic arc, it's another thing to fly level at altitude.

[sokoloff]

The service ceiling is published as FL650. That’s defined as being able to sustain a 100fpm climb, so the absolute level flight ceiling is somewhat higher than that. Service ceiling figure is not in a zoom climb.

It would easily suffice to engage a balloon at FL600.

[zabzonk]

the venerable english electric lightning (RAF) intercepted a u2 at 66k feet during a nato exercise under full control. it also made it to 86K feet, but that was a zoom climb, and the pilot seems to have found the experience rather scary.