MD88 OVERSHOT 10000 FT IN DSCNT.

Narrative

WE WERE CLRED TO DSND FROM FL240 TO 10000 FT ON THE EASTSIDE 1 ARR INTO SNA. DSCNT WAS INITIATED IN THE 'VNAV' MODE OF AUTOFLT AND CHANGED TO 'IAS' DURING THE DSCNT. AS I RECALL; THIS WAS DONE AROUND 15000 FT MSL. COCKPIT WORKLOAD AT THIS TIME INCLUDED DSCNT AND APCH CHKLISTS. AS THE ACFT WAS DSNDING THROUGH APPROX 11000 FT; I NOTICED THAT 1) THE ALT 'ARM' WINDOW IN THE FLT MODE ANNUNCIATOR WAS BLANK (IT SHOULD HAVE ANNUNCIATED 'ALT') AND THAT 2) THE DSCNT RATE REMAINED HIGH. I DISCONNECTED THE AUTOPLT AND LEVELED OFF THE ACFT. THE ACFT DSNDED TO APPROX 9600 FT MSL BEFORE CLBING BACK TO 10000 FT (ASSIGNED). NO TFC CONFLICTS WERE RPTED BY ATC OR TCASII. FLT LANDED AT SNA WITH NO FURTHER INCIDENT. I TAKE FULL RESPONSIBILITY FOR THIS ALTDEV; BUT I BELIEVE THAT A CONTRIBUTING FACTOR WAS THE FACT THAT THE 'ARM' WINDOW OF THE FLT MODE ANNUNCIATOR (FMA) BLANKED OUT MOST PROBABLY WHEN CHANGING FROM 'VNAV' TO 'IAS' ON THE FLT GUIDANCE PANEL. THIS CANCELED THE AUTO LEVEL OFF AT 10000 FT. THIS ANOMALY IS PECULIAR TO THE MD88/MD90 AUTOFLT SYS.