Several times up to FL 180 to test and to use special surveillance and observation equipment inside and outside (hooked pod) the helicopter. During the flights the ship was sometimes heavy overloaded and needed more than 45 min to climb up to the end level (take off in 1000 ft). Climbing only at Vy and level flight 20 percent around Vy. Never used oxygen, but we (the pilots) were all pressure chamber tested. But as a helicopter driver you feels so alone at such an altitude and good ol earth is so far away...
Remember me on the up to 10 minutes autorotations with the very cold cabine due to the idling engines.