Narrative:

We were given a clearance to cross womac at 13000 ft MSL and 250 KTS, the restr was given to us relatively late and the captain selected IAS mode on the digital flight guidance control panel. He selected 330 KIAS (the aircraft uses pitch to control airspeed in this mode) and idle power. As the aircraft approached 13000 ft MSL it became obvious the aircraft was not going to level at the selected altitude of 13000 ft MSL. The captain attempted to prevent the altitude deviation by descended approximately 500 ft below 13000 MSL before correcting back. I believe the rate of descent way have been too great for the system to capture the altitude (although it should not) or the captain may have adjusted the IAS wheel during the altitude capture mode (this can disable the capture mode). Lessons learned: (1) don't allow yourself to be distracted with other duties during critical phases of flight, such as, approaching level off. (2) be aware of the potential problems that may arise when utilizing the autoplt, such as, increasing the rate of descent to make an altitude restr. Callback conversation with reporter revealed the following information: reporter stated that he was sure that the autoplt limitations had been exceeded since the captain used a very high rate of descent using speed brakes at high speed, as well as adjusting the speed after the automatic descent was started which cancels the 'capture mode'. He further stated that the need to go down faster was a higher than normal tailwind and airspeed at the start of the descent. The aircraft was a model MD88. He believes that there was no problem with the autoplt but pilots have to know its limitations since they are not always 'advertised'.

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Original NASA ASRS Text

Title: CAPT OF AN MD88 ACR ACFT OVERSHOT DSCNT ALT AND AIRSPD DURING DSCNT.

Narrative: WE WERE GIVEN A CLRNC TO CROSS WOMAC AT 13000 FT MSL AND 250 KTS, THE RESTR WAS GIVEN TO US RELATIVELY LATE AND THE CAPT SELECTED IAS MODE ON THE DIGITAL FLT GUIDANCE CTL PANEL. HE SELECTED 330 KIAS (THE ACFT USES PITCH TO CTL AIRSPD IN THIS MODE) AND IDLE PWR. AS THE ACFT APCHED 13000 FT MSL IT BECAME OBVIOUS THE ACFT WAS NOT GOING TO LEVEL AT THE SELECTED ALT OF 13000 FT MSL. THE CAPT ATTEMPTED TO PREVENT THE ALTDEV BY DSNDED APPROX 500 FT BELOW 13000 MSL BEFORE CORRECTING BACK. I BELIEVE THE RATE OF DSCNT WAY HAVE BEEN TOO GREAT FOR THE SYS TO CAPTURE THE ALT (ALTHOUGH IT SHOULD NOT) OR THE CAPT MAY HAVE ADJUSTED THE IAS WHEEL DURING THE ALT CAPTURE MODE (THIS CAN DISABLE THE CAPTURE MODE). LESSONS LEARNED: (1) DON'T ALLOW YOURSELF TO BE DISTRACTED WITH OTHER DUTIES DURING CRITICAL PHASES OF FLT, SUCH AS, APCHING LEVEL OFF. (2) BE AWARE OF THE POTENTIAL PROBS THAT MAY ARISE WHEN UTILIZING THE AUTOPLT, SUCH AS, INCREASING THE RATE OF DSCNT TO MAKE AN ALT RESTR. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATED THAT HE WAS SURE THAT THE AUTOPLT LIMITATIONS HAD BEEN EXCEEDED SINCE THE CAPT USED A VERY HIGH RATE OF DSCNT USING SPD BRAKES AT HIGH SPD, AS WELL AS ADJUSTING THE SPD AFTER THE AUTO DSCNT WAS STARTED WHICH CANCELS THE 'CAPTURE MODE'. HE FURTHER STATED THAT THE NEED TO GO DOWN FASTER WAS A HIGHER THAN NORMAL TAILWIND AND AIRSPD AT THE START OF THE DSCNT. THE ACFT WAS A MODEL MD88. HE BELIEVES THAT THERE WAS NO PROB WITH THE AUTOPLT BUT PLTS HAVE TO KNOW ITS LIMITATIONS SINCE THEY ARE NOT ALWAYS 'ADVERTISED'.

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.