AN ALT OVERSHOOT DURING DSCNT WHEN THE ALT HOLD FUNCTION OF THE FMS BECOMES DISARMED AND IS NOT NOTICED BY THE CREW.

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CLRED FROM FL290 TO FL260. ALT PRESELECTOR SET FL260. SELECTED 'DSND' ON FLT DIRECTOR MCP; WHICH SELECTS VERT SPD MODE AT 2000 FPM AND ARMS ALT HOLD. AT FL270; 1000 FT ALERT SOUNDED. PF CALLED 'FL270 FOR FL260.' PF DID NOT CONFIRM ALT HOLD ARMED. FL260 ALERT SOUNDED; BUT BOTH PLTS WERE FOCUSED ON NEW WX GRAPHICS NOW DISPLAYED ON FMS IN THE PEDESTAL. AT SOME POINT; THE ALT HOLD WAS INADVERTENTLY DISARMED. AT FL254; ATC CALLED 'SAY ALT.' DSCNT ARRESTED AT FL252; CLBED BACK TO FL260; AND RPTED ALT TO ATC CLBING THROUGH FL255. NO OTHER ACFT ON TCASII AND NO TFC CONFLICT RPTED BY ATC. PF DECIDED TO FLY WITH LITTLE SLEEP 2 NIGHTS PRIOR TO FLT DUE HEAD COLD. NEITHER PLT NOTICED WHETHER ALT ARM WAS ENGAGED. PLTS WERE LOOKING INTO A LOW SUN. WHITE 'ALT' HOLD PROMPT IS ON A CYAN EFIS BACKGND AND HARD TO SEE IN THIS SUNLIGHT. THE 'ALT' ARM CAN BE DISENGAGED BY TOUCHING THE AUTOPLT PITCH WHEEL WHEN 'ALT' CAPTURE IS ACTIVE. BOTH PLTS WERE ACTIVE IN THE PEDESTAL AREA WHERE THE PITCH WHEEL IS LOCATED. 35 YRS OF ATTN TO ALT YIELDED A FEW MINOR ALTDEVS (200 FT). PERHAPS THE ADAGE 'THERE'S THOSE WHO HAVE AND THOSE WHO ARE GOING TO' FINALLY CAUGHT UP WITH THIS PLT. FORTUNATELY; THIS LESSON LEARNED ENDED WELL; ESPECIALLY SINCE NO CONFLICTING TFC OR TERRAIN WERE FACTORS.