5 times the speed of sound, only with ARMA physics.

So I tested the hight limit in ARMA 2 last night.

At about 10000ft the controls slowed and ACE blackout stopped, at 50000ft the controls stopped responding completly and I could only disable the engine and set autopilot. On re-entry I hit about 5118km/h and the autopilot, controls, and ACE blackout kicked in at about 10000ft.