AN ACR MLG MD-88 DSNDED THROUGH ITS ASSIGNED ALT.

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WE WERE INSTRUCTED TO CROSS THE TIROE FIX AT 14000 FT MSL. NO SPD RESTR WAS ISSUED. WE WERE DSNDING AT THE NORMAL ECON SPD OR APPROX 320 KTS. WE WERE EXPERIENCING A SIGNIFICANT TAILWIND; WHICH THE FMS (VNAV DSCNT) WAS HAVING ITS TYPICAL PROB DEALING WITH. (MD-88) OUR DSCNT RATE WAS CONTINUALLY INCREASING ALONG WITH AIRSPD-NOW NEAR 335 KTS. AT OUR VERY HIGH RATE OF DSCNT. THE ALT FUNCTION CAPTURED WELL ABOVE THE NORMAL POINT. APCH CTL ISSUED INSTRUCTIONS TO 'SLOW TO 250 KTS. IN ATTEMPT TO COMPLY WITH THE SPD AND ALT REQUIREMENTS. THE FO (PF) MADE INPUTS INTO THE DFGC USING BOTH IAS; THEN VERT SPD MODES WHICH INADVERTENTLY CANCELED THE LEVEL OFF MODE ON THE AUTOPLT. THIS ANOMALY WAS NOTED AT ABOUT 14500 MSL. APPROPRIATE INPUTS WERE MADE TO LEVEL OFF BUT AT SUCH A HIGH RATE OF DSCNT (APPROX 4500 FPM) THE ACFT BOTTOMED OUT AT ABOUT 13600 MSL. WHILE ALL THIS WAS GOING ON; APCH SWITCHED OUR FREQ AND SIMULTANEOUSLY CHANGED OUR RWY TO 26R. NEEDLESS TO SAY; CONFUSION WAS IN ABUNDANCE. AFTER CHKING IN WITH THE NEW CTLR; WE WERE GIVEN A HDG. BY NOW WE HAD CONNECTED BACK TO ALT. NOW SHOWING 13940 MSL. THE CTLR ASKED IF WE WERE AT 14000 MSL. I RESPONDED IN THE AFFIRMATIVE AND STATED 13940 FT. NO OTHER COMMENTS WERE MADE. I CAN ONLY SURMISE THAT HE SHOWED A BRIEF ALTDEV TO 13.7 OR SO. THIS SORT OF DEV SEEMS TO BE A CONTINUING PROB WITH THE MD-88 AND THE POOR DESIGN OF ITS DIGITAL FLT GUIDANCE COMPUTER. THERE ARE JUST TOO MANY DIFFERENT FUNCTIONS THAT CAN CTL AIRSPD AND DSCNT RATES ALL OF WHICH CAN CTL THE ALT CAPTURE FEATURE. IN ADDITION; WE NOW HAVE 2 DIFFERENT 'VERT SPD' DFGC'S. ONE ALLOWS YOU TO CONTINUE TO ROTATE THE VERT SPD WHEEL IN THE ALT CAPTURE MODE. THE OTHER RESTRICTS THE WHEEL SO THAT YOU KNOW YOU'VE CAPTURED OR STOP MOVING IT TO PREVENT DISABLING THE CAPTURE. WE DON'T KNOW WHICH IS WHICH AND PERHAPS HAD A MALFUNCTION OF THIS FEATURE.