Narrative:

Widebody transport FMS system has a known problem. In cruise flight, if VNAV (vertical navigation) is selected, the aircraft automatically climbs to the next higher flight level. In this incident, we were cleared pilots discretion from FL350 to FL240. 240 was selected and armed in the altitude window, a descent point was programmed and entered in the FMS. When approximately 40 NM from the descent point, VNAV was selected, we did not verify the descent accepted into the FMS and did not immediately catch the slow climb initiated by the FMS. Shortly into the climb, we noticed it and began to recover. The aircraft topped out 1680 ft above assigned altitude, we immediately advised the center controller and began a descent. The controller seemed unconcerned and stated that he did not catch our climb. No further problems. The widebody transport FMS should not be programmed to begin a climb when VNAV is selected with a lower altitude loaded into the FMS and dialed into the altitude select window. Recommend tie the altitude select window to the FMS such that the aircraft will not climb with a lower altitude selected. Callback conversation with reporter revealed the following information: the reporting so states that the 2 pilots in the aircraft were not at all surprised at what the aircraft did. For some reason unknown to the reporter, the aircraft will climb 4000 ft if the FMS is programmed for a descent and VNAV is armed well before the descent point selected. The aircraft in question is a widebody transport that has been modified by its air carrier owner to have a more modern cockpit. Some problems have been designed into the system. The air carrier is allegedly working on solving this problem.

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Original NASA ASRS Text

Title: AN ACR WDB ACFT CLBED WHEN IT WAS SUPPOSED TO DSND.

Narrative: WDB FMS SYS HAS A KNOWN PROB. IN CRUISE FLT, IF VNAV (VERT NAV) IS SELECTED, THE ACFT AUTOMATICALLY CLBS TO THE NEXT HIGHER FLT LEVEL. IN THIS INCIDENT, WE WERE CLEARED PLTS DISCRETION FROM FL350 TO FL240. 240 WAS SELECTED AND ARMED IN THE ALT WINDOW, A DSCNT POINT WAS PROGRAMMED AND ENTERED IN THE FMS. WHEN APPROX 40 NM FROM THE DSCNT POINT, VNAV WAS SELECTED, WE DID NOT VERIFY THE DSCNT ACCEPTED INTO THE FMS AND DID NOT IMMEDIATELY CATCH THE SLOW CLB INITIATED BY THE FMS. SHORTLY INTO THE CLB, WE NOTICED IT AND BEGAN TO RECOVER. THE ACFT TOPPED OUT 1680 FT ABOVE ASSIGNED ALT, WE IMMEDIATELY ADVISED THE CTR CTLR AND BEGAN A DSCNT. THE CTLR SEEMED UNCONCERNED AND STATED THAT HE DID NOT CATCH OUR CLB. NO FURTHER PROBS. THE WDB FMS SHOULD NOT BE PROGRAMMED TO BEGIN A CLB WHEN VNAV IS SELECTED WITH A LOWER ALT LOADED INTO THE FMS AND DIALED INTO THE ALT SELECT WINDOW. RECOMMEND TIE THE ALT SELECT WINDOW TO THE FMS SUCH THAT THE ACFT WILL NOT CLB WITH A LOWER ALT SELECTED. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTING SO STATES THAT THE 2 PLTS IN THE ACFT WERE NOT AT ALL SURPRISED AT WHAT THE ACFT DID. FOR SOME REASON UNKNOWN TO THE RPTR, THE ACFT WILL CLB 4000 FT IF THE FMS IS PROGRAMMED FOR A DSCNT AND VNAV IS ARMED WELL BEFORE THE DSCNT POINT SELECTED. THE ACFT IN QUESTION IS A WDB THAT HAS BEEN MODIFIED BY ITS ACR OWNER TO HAVE A MORE MODERN COCKPIT. SOME PROBS HAVE BEEN DESIGNED INTO THE SYS. THE ACR IS ALLEGEDLY WORKING ON SOLVING THIS PROB.

Data retrieved from NASA's ASRS site as of July 2007 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.