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.