The aircraft was given a requirement to reach FL310 by a specified time (43) in order to gain separation from opposite direction traffic at FL290. After the specified time (at 44) the pilot reported that he was unable to reach FL310 and was unable to maintain FL300. The aircraft was given immediate descent to FL270. Shortly afterwards the pilot advised that he was able to climb to FL310. A procedural breakdown in separation with the opposite direction traffic occurred.
The crew reported that they had maintained the climb rate using the vertical speed of the aircraft autopilot. This had caused the airspeed to decay because the climb performance of the aircraft was unable to sustain the climb rate demanded. It then became necessary for the aircraft to descend in order to regain its speed and performance.
