ALTDEV ALT EXCURSION.

Narrative

WE WERE ENRTE FROM DCA TO MCO APPROX 30 MINS PRIOR TO LNDG WHEN JAX CENTER ASK US TO DSND FROM OUR CRUISE ALT TO FL310. THIS WAS ACCOMPLISHED USING THE FMS AND A VNAV DSCNT. THE ACFT SMOOTHLY DSNDED TO FL310 AND LEVELED AUTOMATICALLY. VNAV LEVEL WAS DISPLAYED ON THE FLT MODE ANNUNCIATOR. FL310 WAS ALSO DISPLAYED IN THE ALT ALERT ADVISORY WINDOW. A FEW MINS LATER THE FLT ATTENDANT KNOCKED AT THE DOOR AND I TURNED TO LET HER IN. AT THAT MOMENT THE FO SAID WE ARE DSNDING AND HE IMMEDIATELY PRESSED THE ALT HOLD BUTTON. WE DSNDED ABOUT 100-150 FT BEFORE WE RETURNED TO FL310. WE DID NOT DEV MORE THAN 150 FT AND THERE WAS NOT A TFC CONFLICT. THIS PROBLEM OCCURRED BECAUSE I HAD ALREADY ENTERED XING RESTRICTIONS INTO THE FMS THAT WERE DISPLAYED ON THE ARR WE WERE USING INTO MCO. THE FMS BEGAN A DSCNT TO MEET THESE XING RESTRICTIONS WITH COMPLETE DISREGARD FOR THE ALT DISPLAYED IN THE ALT ALERT WINDOW. THIS IS A SOFTWARE PROBLEM THAT HAS RESULTED IN MANY ALT VIOLATIONS. I STRONGLY SUGGEST THAT A CHANGE BE MADE TO THE SOFTWARE PACKAGE. THE FMS SHOULD LOOK AT THE ALT IN THE ALT SELECT WINDOW AS IT'S PRIMARY REF AND THEN OTHER RESTRICTIONS AS SECONDARY REFS. THIS WOULD PREVENT THE ACFT FROM AUTOMATICALLY DEPARTING PRESELECTED ALTS.