ALTDEV ALT OVERSHOT IN DSCNT IN MD-88 FLC DEV FROM ATC CLRNC.

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OUR ACFT WAS ENRTE FROM BOS TO DCA. WE WERE IN THE INITIAL APCH PHASE OF THE FLT BEING CTLED BY BWI APCH CTL. WX WAS IMC CONDITIONS; EXTREMELY HAZY AND AN OBSCURED CEILING. AT THE TIME OF THE ALTDEV INCIDENT; THE FO WAS FLYING THE ACFT ON AUTOPLT AND AUTOTHROTTLES. WE WERE GIVEN SUCCESSIVE LEVEL OFF ALTS BY ATC AS THE ACFT CONTINUED A DSCNT IN THE APCH ENVIRONMENT. FROM A STABILIZED ALT OF 12000 MSL WE WERE CLRED TO 10000 MSL AND THEN DOWN TO 8000 MSL BEFORE STOPPING AT 10000 MSL. THE ACFT SPD WAS 250 KIAS WITH SLATS DEPLOYED AS WE DSNDED THROUGH 10000 FT MSL. AT APPROX 8500 MSL; ATC REQUESTED WE SLOW TO 210 KIAS. THE FO HAD BEEN CTLING THE DSCNT RATE AND SPD USING THE 'IAS' FUNCTION ON THE DIGITAL FLT GUIDANCE PANEL AND IDLE PWR SELECTED. BOTH PLTS HAD VERIFIED THAT 8000 FT WAS SELECTED ON THE DIGITAL FLT GUIDANCE PANEL AS THE LEVEL OFF ALT. I TURNED MY ATTN THEN TO LOADING APCH DATA INTO THE FMS TO AID IN FLYING THE RWY 18 RIVER VISUAL APCH INTO DCA. AT THIS POINT; TO COMPLY WITH THE 210 KIAS SPD REQUEST BY ATC; THE FO SELECTED THE 'VERT SPD' FUNCTION ON THE DIGITAL FLT GUIDANCE PANEL; SETTING 600 FPM DSCNT RATE AND CHANGING THE 'SPD SELECT' WINDOW TO SHOW 210 KTS. BECAUSE OF A FLAW IN THE MD-88'S DIGITAL FLT GUIDANCE PROGRAM; LAST SECOND CHANGES IN ANY OF THE VERT CTL FUNCTIONS WILL REMOVE THE ALT LEVEL OFF COMMAND PREVIOUSLY SELECTED. THE ALT WARNING SIGNAL AND A TCASII 'TA' GOING OFF SIMULTANEOUSLY AT 7700 MSL BROUGHT OUR ATTN TO THE DEV. FULL PWR AND PITCH INCREASE TO 20 DEGS NOSE UP STOPPED OUR DSCNT AT 7600 MSL AND PROVIDED A QUICK CLB BACK TO 8000 MSL. APCH CTL QUESTIONED US ABOUT OUR ALT AS WE CLBED BACK THROUGH 7900 MSL. NO FURTHER COMMENTS WERE RECEIVED FROM ATC ABOUT THE INCIDENT. IT IS MY OPINION THAT THIS SCENARIO IS REPEATED AT THE VERY LEAST; DAILY; IN THE MD-88 ACFT. THE WORKLOAD THAT CONFRONTS PLTS IN THE ATC SYS WILL NEVER DECREASE AND WILL ONLY INCREASE WITH SYS EXPANSION AND INCREASED FLYING LEVELS IN THE UNITED STATES. THE MOST EFFECTIVE SOLUTION TO THIS RECURRING PROB IS TO CHANGE THE SOFTWARE PROGRAM FOR THE MD-88 DIGITAL FLT GUIDANCE SYS TO ALLOW LAST SECOND CHANGES TO PITCH COMMANDS WITHOUT REMOVING THE ALT LEVEL OFF FUNCTION.