Narrative:

We were instructed to cross the tiroe fix at 14000 ft MSL. No speed restr was issued. We were descending at the normal econ speed or approximately 320 KTS. We were experiencing a significant tailwind, which the FMS (VNAV descent) was having its typical problem dealing with. (Md-88) our descent rate was continually increasing along with airspeed-now near 335 KTS. At our very high rate of descent. The altitude function captured well above the normal point. Approach control issued instructions to 'slow to 250 KTS. In attempt to comply with the speed and altitude requirements. The first officer (PF) made inputs into the dfgc using both IAS, then vertical speed 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 descent (approximately 4500 FPM) the aircraft bottomed out at about 13600 MSL. While all this was going on, approach switched our frequency and simultaneously changed our runway to 26R. Needless to say, confusion was in abundance. After checking in with the new controller, we were given a heading. By now we had connected back to altitude. Now showing 13940 MSL. The controller 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 altitude deviation to 13.7 or so. This sort of deviation seems to be a continuing problem with the md-88 and the poor design of its digital flight guidance computer. There are just too many different functions that can control airspeed and descent rates all of which can control the altitude capture feature. In addition, we now have 2 different 'vertical speed' dfgc's. One allows you to continue to rotate the vertical speed wheel in the altitude 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.

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Original NASA ASRS Text

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

Narrative: 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.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.