A DSNDING B737-400 FLC OVERSHOOTS THEIR ASSIGNED ALT WHEN THE AUTOPLT DISCONNECTS PRIOR TO LEVELOFF ALT AT JAYBO; MD.

Narrative

ENRTE PHL; ZDC (132.05) CLRED FLT TO CROSS JAYBO AT 13000 FT (FROM 23000 FT 23 NM FROM JAYBO). SUBSEQUENTLY TOLD TO LEVEL AT 14000 FT. ACFT REACHED 13600 FT BEFORE IT WAS RECOGNIZED THAT AUTOPLT HAD ABNORMALLY DISCONNECTED WITH NONE OF THE NORMAL VISUAL OR AUDIO CUES. CAPT RECOVERED ACFT TO ASSIGNED 14000 FT WITHOUT INCIDENT. SUPPLEMENTAL INFO FROM ACN 478707: VNAV WAS ENGAGED AND FMC CORRECTLY COMMANDED A HIGH AIRSPD STEEP DSCNT SINCE 23 MI TO LOSE 10000 FT IS GREATER THAN A NORMAL DSCNT. FMC HOWEVER DID NOT RESPECT STICK SHAKER AND I HAD TO INTERVENE BY USING LEVEL CHANGE (DISENGAGE VNAV). AUTOPLT ONCE AGAIN CONTINUED TO ALLOW ACFT TO FLY AT STICK SHAKER AND PROCEEDED TO DSND BELOW 14000 FT AT WHICH TIME I TOOK OVER MANUALLY AND ARRESTED THE DSCNT AT 13600 FT; CLBING BACK TO 14000 FT. WE WERE MONITORING THE ACFT AND FULLY EXPECTED A LEVELOFF AT 14000 FT NOT KNOWING THE AUTOPLT WAS DISENGAGED. FMC ALLOWED ACFT TO FLY AT STICK SHAKER. AUTOPLT WAS SOMEHOW DISCONNECTED WITHOUT WARNINGS THAT ARE SUPPOSED TO BE SEEN AND HEARD; WARNING SOUND AND FLASHING RED LIGHT ON FMA (FLT MODE ANNUNCIATOR PANEL) WILL PAY MORE ATTN TO ENTIRE MODE INDICATIONS IN FUTURE: 'PADDLE' SWITCH WAS DOWN (AUTOPLT DISENGAGED POS) WHICH WOULD HAVE IMMEDIATELY TOLD ME WHY ACFT WAS FLYING AT CLACKER SPD. ALSO; THERE IS A SMALL 'AUTOPLT OFF' INDICATOR ON FMA.