ALTDEV ON DSCNT.

Narrative

HAVING RECEIVED CLRNC TO FL280; THE CAPT PROGRAMMED THE FMS TO DSND TO FL280 BY A POINT 35 MI W OF MEI. THE ACFT STARTED THE DSCNT EARLY; SO THE CAPT CHANGED FROM AN FMS DSCNT TO A VERT SPD DSCNT. AT THIS TIME 'ALT ARM' LIGHT WAS ILLUMINATED FOR ALT CAPTURE AT FL280. AT APPROX 250 FT BELOW FL280; THE ALT WARNING HORN SOUNDED AT WHICH TIME THE DSCNT WAS STOPPED AND THE ACFT WAS RETURNED TO FL280. NO CONFLICT WAS NOTED. STANDARD CALLOUTS WERE ACCOMPLISHED. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR FLIES THE MD-88 FOR A MAJOR UNITED STATES ACR. HE LIKES THE ACFT AND CALLS IT 'AN OLD ACFT WITH NEW SOFTWARE.' HE THINKS THIS EPISODE WAS PRIMARILY A 'SOFTWARE PROB.' HE AGREES THAT HE SHOULD HAVE BEEN WATCHING THE ACFT MORE CLOSELY. THE ACFT HAS MADE HIM MORE AWARE AND WATCHFUL. THERE WAS NO WORD FROM ARTCC. IT IS HIGHLY PROBABLE THAT THE CTLR DID NOT NOTICE. THE RPTR'S ACR'S POLICY IS TO AVOID CHANGING THE ALT MODE WITHIN THE LAST 1000 FT OF CAPTURE SO AS TO AVOID LOSING THE ALT CAPTURE FEATURE. THE CREW DID NOT CHANGE THE ALT MODE.