A C441 experienced autoplt failure in descent; leading to an altitude and track deviation.

Attributes
ACN	713577
Time
Date	200610
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : zjx.artcc
State Reference	FL
Altitude	msl bound lower : 26000 msl bound upper : 28000
Environment
Flight Conditions	IMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zjx.artcc
Operator	general aviation : personal
Make Model Name	Conquest I/Conquest II
Operating Under FAR Part	Part 91
Flight Phase	cruise : enroute altitude change
Route In Use	enroute airway : j75.airway
Flight Plan	IFR
Person 1
Affiliation	other
Function	flight crew : captain oversight : pic
Qualification	pilot : multi engine pilot : private pilot : instrument
Experience	flight time last 90 days : 47 flight time total : 5100 flight time type : 3100
ASRS Report	713577
Person 2
Affiliation	other
Function	flight crew : first officer
Events
Anomaly	aircraft equipment problem : less severe altitude deviation : overshoot non adherence : clearance other spatial deviation
Independent Detector	other flight crewa
Resolutory Action	flight crew : returned to assigned altitude flight crew : returned to intended or assigned course
Supplementary
Problem Areas	Aircraft Flight Crew Human Performance
Primary Problem	Aircraft

Narrative:

We were in turbulence in IMC at 28000 ft on autoplt. We were instructed by center to descend to 27000 ft. Started descent; set altitude alert; and a few moments later heard 'altitude alert' chime; which we took to be the approaching 27000 ft warning. More turbulence! Suddenly; a few seconds later; realized airplane was both descending and turning rapidly. Stopped turn/bank (about 30 degrees) and descent (at about 26500 ft?) and quickly got back on course at 27000 ft. Later investigation indicated that the 'altitude alert' chime and the 'autoplt off' chime produce identical sounds (perhaps the same chime). What we thought was a warning that desired altitude was approaching; was in fact an autoplt off alert. After thousands of hours of flying that airplane that had never previously occurred; and I had not realized that it could occur. Will know better hereafter!

Original NASA ASRS Text

Title: A C441 EXPERIENCED AUTOPLT FAILURE IN DSCNT; LEADING TO AN ALT AND TRACK DEV.

Narrative: WE WERE IN TURB IN IMC AT 28000 FT ON AUTOPLT. WE WERE INSTRUCTED BY CTR TO DSND TO 27000 FT. STARTED DSCNT; SET ALT ALERT; AND A FEW MOMENTS LATER HEARD 'ALT ALERT' CHIME; WHICH WE TOOK TO BE THE APCHING 27000 FT WARNING. MORE TURB! SUDDENLY; A FEW SECONDS LATER; REALIZED AIRPLANE WAS BOTH DSNDING AND TURNING RAPIDLY. STOPPED TURN/BANK (ABOUT 30 DEGS) AND DSCNT (AT ABOUT 26500 FT?) AND QUICKLY GOT BACK ON COURSE AT 27000 FT. LATER INVESTIGATION INDICATED THAT THE 'ALT ALERT' CHIME AND THE 'AUTOPLT OFF' CHIME PRODUCE IDENTICAL SOUNDS (PERHAPS THE SAME CHIME). WHAT WE THOUGHT WAS A WARNING THAT DESIRED ALT WAS APCHING; WAS IN FACT AN AUTOPLT OFF ALERT. AFTER THOUSANDS OF HOURS OF FLYING THAT AIRPLANE THAT HAD NEVER PREVIOUSLY OCCURRED; AND I HAD NOT REALIZED THAT IT COULD OCCUR. WILL KNOW BETTER HEREAFTER!

Data retrieved from NASA's ASRS site as of January 2009 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.