Narrative:

I was the PF on a night arrival to sna on the KAYOH4 arrival; hector transition. We were on a profile descent when the controllers were stepping us down in increments as we passed hector VOR toward dawna intersection. We were cleared to cross dawna at 13000 ft requiring a high rate of descent via 'step holding.' everything was routine as we descended out of 19000 ft for 17000 ft (assigned) direct to dawna; at just below vne; I experienced an autoplt 'hard-over/upset;' rolling wings abruptly in steep left turn; (past 35 degrees) in a nose down condition. I immediately disengaged the autoplt as we were still in a steep descent; correcting to recover altitude without overstressing G's on a pull-up. A controller cautioned us asking for altitude; the sic responded 16500 ft returning to 17000 ft assigned. (Neither of us heard the 1000 ft above chime) because of hi-blower heating and the suddenness of this violent turn and upset. After this event; the autoplt seemed to work normally; however; I hand flew most of the arrival; just momentarily checking the autoplt. We squawk the FMS-autoplt to ascertain the possible cause for the abrupt l-nose down turn. As a slow turbojet in the high altitude system; we are often side-stepped; or require high rate of dscnts to meet fixed-pt altitudes; to coordinate with the fast traffic; factors contributing to this event were: high rate of descent; near vne; noisy environment and a disorienting lateral maneuver violent enough to throw the crew sideways. It might have helped if I had requested a slower speed to lessen the upset shock? Callback conversation with reporter revealed the following information: the problem has been traced to a faulty connection between the aircraft's attitude heading reference system and the autopilot. This aircraft is over 20 years old and has most of the original electronic equipment.

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Original NASA ASRS Text

Title: CE500 PILOT REPORTS AUTOPILOT HARD OVER DURING DESCENT TO 17000 FT. ACFT DESCENDS TO 16500 FEET BEFORE CONTROL CAN BE REGAINED.

Narrative: I WAS THE PF ON A NIGHT ARR TO SNA ON THE KAYOH4 ARR; HECTOR TRANSITION. WE WERE ON A PROFILE DSCNT WHEN THE CTLRS WERE STEPPING US DOWN IN INCREMENTS AS WE PASSED HECTOR VOR TOWARD DAWNA INTXN. WE WERE CLRED TO CROSS DAWNA AT 13000 FT REQUIRING A HIGH RATE OF DSCNT VIA 'STEP HOLDING.' EVERYTHING WAS ROUTINE AS WE DSNDED OUT OF 19000 FT FOR 17000 FT (ASSIGNED) DIRECT TO DAWNA; AT JUST BELOW VNE; I EXPERIENCED AN AUTOPLT 'HARD-OVER/UPSET;' ROLLING WINGS ABRUPTLY IN STEEP L TURN; (PAST 35 DEGS) IN A NOSE DOWN CONDITION. I IMMEDIATELY DISENGAGED THE AUTOPLT AS WE WERE STILL IN A STEEP DSCNT; CORRECTING TO RECOVER ALT WITHOUT OVERSTRESSING G'S ON A PULL-UP. A CTLR CAUTIONED US ASKING FOR ALT; THE SIC RESPONDED 16500 FT RETURNING TO 17000 FT ASSIGNED. (NEITHER OF US HEARD THE 1000 FT ABOVE CHIME) BECAUSE OF HI-BLOWER HEATING AND THE SUDDENNESS OF THIS VIOLENT TURN AND UPSET. AFTER THIS EVENT; THE AUTOPLT SEEMED TO WORK NORMALLY; HOWEVER; I HAND FLEW MOST OF THE ARR; JUST MOMENTARILY CHKING THE AUTOPLT. WE SQUAWK THE FMS-AUTOPLT TO ASCERTAIN THE POSSIBLE CAUSE FOR THE ABRUPT L-NOSE DOWN TURN. AS A SLOW TURBOJET IN THE HIGH ALT SYS; WE ARE OFTEN SIDE-STEPPED; OR REQUIRE HIGH RATE OF DSCNTS TO MEET FIXED-PT ALTS; TO COORDINATE WITH THE FAST TFC; FACTORS CONTRIBUTING TO THIS EVENT WERE: HIGH RATE OF DSCNT; NEAR VNE; NOISY ENVIRONMENT AND A DISORIENTING LATERAL MANEUVER VIOLENT ENOUGH TO THROW THE CREW SIDEWAYS. IT MIGHT HAVE HELPED IF I HAD REQUESTED A SLOWER SPD TO LESSEN THE UPSET SHOCK? CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE PROBLEM HAS BEEN TRACED TO A FAULTY CONNECTION BETWEEN THE ACFT'S ATTITUDE HEADING REFERENCE SYSTEM AND THE AUTOPILOT. THIS ACFT IS OVER 20 YEARS OLD AND HAS MOST OF THE ORIGINAL ELECTRONIC EQUIPMENT.

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.