Narrative:

At delmo and 11000 ft level and slowing through approximately 230 KIAS, slats out, idle power, and autoplt on, I made the 'prepare for landing' PA and looked down to reselect the audio panel to #1 transmitter. As I looked up, I saw the altitude descending through 10800 ft, did double take and immediately intervened manually, kicking off the autoplt and aggressively returning to 11000 ft (bottomed out at 10700 ft or maybe 10650 ft). During this pull up, I heard 'traffic, traffic,' but didn't look at the TCASII display (no RA) as I was totally focused on precise aircraft control. Don't know if the TA was the normal one for climbing traffic or caused by our deviation, or where or how close the other traffic was. Autoplt didn't disconnect until I clicked it off. I don't know what caused it or how the autoplt started to descend. I know I didn't push the yoke. Autoplt worked fine after the event. I didn't write up the autoplt to maintenance. Controller came up on radio and his words and speech timing made me think he was calling us about the altitude and then mid-sentence saw us climbing and cut off his comment. My only explanation is the S80 autoplt has a long history of failing to capture or maintain capture during rapidly changing conditions, especially large rates. This was my first trip (6TH leg) in 10 ks. Supplemental information from acn 617006: we were making the left turn at delmo intersection on the jen 8 arrival. The autoplt was on. As the aircraft started the turn, it was still decelerating, and the captain asked for the slats. As the slats were extending, the aircraft began to descend out of 11000 ft. The captain intervened at 10700 ft, and was correcting back to 11000 ft, when approach instructed us to descend to a lower altitude. The controller didn't mention anything about the unintentional descent.

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Original NASA ASRS Text

Title: POSSIBLE TFC CONFLICT WITH A TCASII TA PRODUCED BY AN ALT EXCURSION BY A MANEUVERING M82 WHEN MAKING THE TURN, REDUCING SPD AND CONFIGN AT DELMO INTXN, TX.

Narrative: AT DELMO AND 11000 FT LEVEL AND SLOWING THROUGH APPROX 230 KIAS, SLATS OUT, IDLE PWR, AND AUTOPLT ON, I MADE THE 'PREPARE FOR LNDG' PA AND LOOKED DOWN TO RESELECT THE AUDIO PANEL TO #1 XMITTER. AS I LOOKED UP, I SAW THE ALT DSNDING THROUGH 10800 FT, DID DOUBLE TAKE AND IMMEDIATELY INTERVENED MANUALLY, KICKING OFF THE AUTOPLT AND AGGRESSIVELY RETURNING TO 11000 FT (BOTTOMED OUT AT 10700 FT OR MAYBE 10650 FT). DURING THIS PULL UP, I HEARD 'TFC, TFC,' BUT DIDN'T LOOK AT THE TCASII DISPLAY (NO RA) AS I WAS TOTALLY FOCUSED ON PRECISE ACFT CTL. DON'T KNOW IF THE TA WAS THE NORMAL ONE FOR CLBING TFC OR CAUSED BY OUR DEV, OR WHERE OR HOW CLOSE THE OTHER TFC WAS. AUTOPLT DIDN'T DISCONNECT UNTIL I CLICKED IT OFF. I DON'T KNOW WHAT CAUSED IT OR HOW THE AUTOPLT STARTED TO DSND. I KNOW I DIDN'T PUSH THE YOKE. AUTOPLT WORKED FINE AFTER THE EVENT. I DIDN'T WRITE UP THE AUTOPLT TO MAINT. CTLR CAME UP ON RADIO AND HIS WORDS AND SPEECH TIMING MADE ME THINK HE WAS CALLING US ABOUT THE ALT AND THEN MID-SENTENCE SAW US CLBING AND CUT OFF HIS COMMENT. MY ONLY EXPLANATION IS THE S80 AUTOPLT HAS A LONG HISTORY OF FAILING TO CAPTURE OR MAINTAIN CAPTURE DURING RAPIDLY CHANGING CONDITIONS, ESPECIALLY LARGE RATES. THIS WAS MY FIRST TRIP (6TH LEG) IN 10 KS. SUPPLEMENTAL INFO FROM ACN 617006: WE WERE MAKING THE L TURN AT DELMO INTXN ON THE JEN 8 ARR. THE AUTOPLT WAS ON. AS THE ACFT STARTED THE TURN, IT WAS STILL DECELERATING, AND THE CAPT ASKED FOR THE SLATS. AS THE SLATS WERE EXTENDING, THE ACFT BEGAN TO DSND OUT OF 11000 FT. THE CAPT INTERVENED AT 10700 FT, AND WAS CORRECTING BACK TO 11000 FT, WHEN APCH INSTRUCTED US TO DSND TO A LOWER ALT. THE CTLR DIDN'T MENTION ANYTHING ABOUT THE UNINTENTIONAL DSCNT.

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.