An A320 FMS computed a higher recommended maximum cruise altitude during OPEN CLIMB than was computed when the FMS transitioned to ALTITUDE CAPTURE at cruise. The crew descended down to the REC MAX computed for cruise.

Narrative

First Officer; pilot not flying; on A320. Flight plan had initial altitude of FL280; but Center cleared us directly to FL360. I thought the altitude was unusually high for our high gross weight; but the (new) FMS display a REC MAX of FL363; so we accepted the clearance and maintained OPEN CLIMB to FL360. The instant we entered 'ALT CAP' at FL360; the managed speed index moved from approx .78M to approx .72M (Green Dot/VLS). When I saw this; I selected the PROG PAGE and noticed that the REC MAX altitude changed from FL363 to 334. Since the Captain was new on the airplane and was unfamiliar with the idiosyncrasies and consequences of low mach at the upper limit of altitude capability; I manually selected a speed of .77M and suggested we request a lower altitude. We requested a lower altitude from Center and received a clearance down to FL340; at which time the REC MAX display changed from FL334 to FL343. We captured FL340 and the PERF page at that new altitude showed a new optimum speed of .77M; whereupon we re-selected managed speed. It appears that; in the new FMS; the performance algorithm for calculating REC MAX altitude in the climb is different than that used for cruise. Thus; the displayed REC MAX altitude changed from FL363 to FL334 the instant we changed from OPEN CLIMB to ALT CAP. This may be an item for the fleet technical group to look at. On our flight; the displayed REC MAX altitude was unrealistically high during the climb and could have resulted in a high-altitude unacceptably low-speed condition had we not intervened in overriding the automatic features of the FMS/AutoPilot.