An A320 FMS computed a higher recommended maximum cruise altitude during open climb than was compute...

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 maximum of FL363; so we accepted the clearance and maintained open climb to FL360. The instant we entered 'altitude 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 maximum 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 maximum 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 maximum altitude in the climb is different than that used for cruise. Thus; the displayed rec maximum altitude changed from FL363 to FL334 the instant we changed from open climb to altitude cap. This may be an item for the fleet technical group to look at. On our flight; the displayed rec maximum 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.

Original NASA ASRS Text

Title: 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.

Data retrieved from NASA's ASRS site as of April 2012 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.

Attributes
ACN	896575
Time
Date	201006
Local Time Of Day	1201-1800
Place
Locale Reference	ZZZZ.ARTCC
State Reference	FO
Environment
Light	Daylight
Aircraft 1
Make Model Name	A320
Operating Under FAR Part	Part 121
Flight Phase	Climb Cruise
Route In Use	Direct
Flight Plan	IFR
Component
Aircraft Component	FMS/FMC
Person 1
Function	First Officer Pilot Not Flying
Experience	Flight Crew Last 90 Days 150 Flight Crew Total 11500 Flight Crew Type 4000
Events
Anomaly	Aircraft Equipment Problem Less Severe Deviation - Procedural Other / Unknown

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 maximum computed for cruise.