ATX AMB ALT DEVIATION EXCURSION FROM CLRNC ALT AND HEADING DEVIATION.

Narrative

WHILE CRUISING IN IMC CONDITIONS AT 22000' WITH THE AUTOPLT ON; THE AUTOPLT SHUT OFF AND THE ACFT EVIDENTLY ENTERED A GRADUAL BANK AND DSCNT. DUE TO THE LOW RATE OF CHANGE OF BANK AND PITCH; THE DISCONNECTED AUTOPLT WAS NOT NOTICED UNTIL APPROX 45 DEGS OF LEFT BANK AND 3-4 DEGS OF NOSE DOWN PITCH. WHEN NOTICED; ACFT HAD TURNED APPROX 55 DEGS AND DSNDED APPROX 500'. CORRECTIVE ACTION WAS IMMEDIATELY INITIATED AT THIS POINT AND ACFT WAS RETURNED TO CORRECT HDG AND ALT. DURING RECOVERY MAX HDG DEVIATION WAS APPROX 80 DEGS AND ALT LOSS APPROX 700'. THE ATC CTLR MADE NO COMMENT TO ME ABOUT THE DEVIATION. I MADE NO RPT AS THERE WAS NO OTHER TFC IN THE AREA; AND THUS NO ACTUAL HAZARD TO OTHER ACFT. CONTRIBUTING FACTORS: THE PLTS WERE CHKING CHARTS AT THE TIME WHICH PARTIALLY OBSCURED THE INSTRUMENT PANEL. THE AUTOPLT HAS NO AURAL WARNING WHEN IT DISCONNECTS. THE ONLY WARNING IS ON THE AUTOPLT ANNUNCIATOR PANEL (VISUAL WARNING). FAILURE IS INDICATED BY THE AUTOPLT ANNUNCIATOR LIGHT TURNING OFF AND FLASHING FOR APPROX 3 SECS. THE AUTOPLT MAY BE INADVERTENTLY SHUT OFF BY LIGHT PRESSURE ON EITHER PLT'S CONTROL WHEEL TRIM SWITCH; OR LIGHT PRESSURE ON THE AUTOPLT ON/OFF SWITCH. ALL OF THESE SWITCHES ARE VERY EASY TO ACCIDENTALLY MOVE. ALSO; THE AUTOPLT SOMETIMES WILL SHUT ITSELF OFF FOR NO APPARENT REASON. THIS IS A POORLY DESIGNED SYS THAT CAN LEAD TO THE NEED TO RECOVER FROM UNUSUAL ATTITUDES. A SIMPLE; DISTINCT AURAL WARNING WOULD IMMEDIATELY GET THE PLT'S ATTN W/O REQUIRING SUCH CONSTANT MONITORING. CLEARLY IT IS THE PLT'S RESPONSIBILITY TO MONITOR THE OPERATION OF THE AUTOPLT; BUT LACK OF AN AURAL WARNING IS ALSO A DESIGN DEFICIENCY THAT MAY CONTRIBUTE TO OR LEAD TO ANOTHER PLT ERROR INCIDENT.