Narrative:

During descent to bna, 7000 ft, the autoplt did not capture said altitude. My technique for descent is, use IAS hold but at 1000 ft and not less than 500 ft prior to desired altitude, to select vertical speed, reduce the rate by 1/2 and then ensure the altitude is still armed. I feel the S80 autoplt in IAS hold/autothrottles on, gives excessively abrupt leveloffs and power advances. Reducing the rate lessens this. On this flight during descent into bna, I used this technique, to include verifying that the altitude was still armed. Between 500 ft above and the desired altitude of 7000 ft, the captain was discussing my method of keeping the icing lights out. Consequently, I missed observing the capture and leveloff at 7000 ft. Why the aircraft continued below 7000 ft I do not know. The altitude was armed at 7500 ft. However, when I noticed that the aircraft had descended below the assigned 7000 ft altitude. I immediately disengaged the autoplt and autothrottles, added power and pulled on the yoke. Shortly after beginning this maneuver the altitude warning went off. The aircraft did not descend below 6700 ft on the center altimeter, at any time during this momentary deviation. The subsequent leveloff and rest of the approach were visually hand flown by me. ATC did not mention this altitude deviation. The captain and I did not discuss this event until he handed me his version in written form.

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Original NASA ASRS Text

Title: ALT OVERSHOOT ON DSCNT.

Narrative: DURING DSCNT TO BNA, 7000 FT, THE AUTOPLT DID NOT CAPTURE SAID ALT. MY TECHNIQUE FOR DSCNT IS, USE IAS HOLD BUT AT 1000 FT AND NOT LESS THAN 500 FT PRIOR TO DESIRED ALT, TO SELECT VERT SPD, REDUCE THE RATE BY 1/2 AND THEN ENSURE THE ALT IS STILL ARMED. I FEEL THE S80 AUTOPLT IN IAS HOLD/AUTOTHROTTLES ON, GIVES EXCESSIVELY ABRUPT LEVELOFFS AND PWR ADVANCES. REDUCING THE RATE LESSENS THIS. ON THIS FLT DURING DSCNT INTO BNA, I USED THIS TECHNIQUE, TO INCLUDE VERIFYING THAT THE ALT WAS STILL ARMED. BTWN 500 FT ABOVE AND THE DESIRED ALT OF 7000 FT, THE CAPT WAS DISCUSSING MY METHOD OF KEEPING THE ICING LIGHTS OUT. CONSEQUENTLY, I MISSED OBSERVING THE CAPTURE AND LEVELOFF AT 7000 FT. WHY THE ACFT CONTINUED BELOW 7000 FT I DO NOT KNOW. THE ALT WAS ARMED AT 7500 FT. HOWEVER, WHEN I NOTICED THAT THE ACFT HAD DSNDED BELOW THE ASSIGNED 7000 FT ALT. I IMMEDIATELY DISENGAGED THE AUTOPLT AND AUTOTHROTTLES, ADDED PWR AND PULLED ON THE YOKE. SHORTLY AFTER BEGINNING THIS MANEUVER THE ALT WARNING WENT OFF. THE ACFT DID NOT DSND BELOW 6700 FT ON THE CTR ALTIMETER, AT ANY TIME DURING THIS MOMENTARY DEV. THE SUBSEQUENT LEVELOFF AND REST OF THE APCH WERE VISUALLY HAND FLOWN BY ME. ATC DID NOT MENTION THIS ALTDEV. THE CAPT AND I DID NOT DISCUSS THIS EVENT UNTIL HE HANDED ME HIS VERSION IN WRITTEN FORM.

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.