Boeing An operator reported that after engaging the flight level change ("FL CH") mode during cruise to change altitude, the autothrottle engine pressure ratio (EPR) limit stayed at the cruise limit rather than changing to the climb limit.
This is normal operation for the autothrottle system on Model 757 airplanes. The autothrottle thrust limit will not change from "CRZ" to "CLB" if the flight level change mode is selected during climb to fly to a higher altitude. In the flight level change mode, the autothrottle system attempts to fly the airplane to the selected altitude within two minutes without changing the target airspeed. The thrust management computer determines the thrust setting required for a two minute level change maneuver by first determining the climb rate required. The required climb rate is based on the difference between the current altitude and the selected altitude. The autothrottle system then advances the throttle levers to achieve the calculated climb rate for obtaining the selected altitude in two minutes, or until the cruise EPR limit thrust is reached.
Boeing Flight Operations In Service Activities Report 93-03, 8th February 1993