C525 captain reported the autopilot rolled the aircraft into a 30 degree bank and descended.

Attributes
ACN	1606235
Time
Date	201812
Local Time Of Day	1201-1800
Environment
Flight Conditions	VMC
Aircraft 1
Make Model Name	Citationjet (C525/C526) - CJ I / II / III / IV
Operating Under FAR Part	Part 135
Flight Phase	Cruise
Flight Plan	IFR
Component
Aircraft Component	Autopilot
Person 1
Function	Captain Pilot Not Flying
Qualification	Flight Crew Air Transport Pilot (ATP)
Events
Anomaly	Aircraft Equipment Problem Less Severe Deviation - Altitude Excursion From Assigned Altitude Deviation - Procedural Clearance Inflight Event / Encounter Loss Of Aircraft Control

Narrative:

In cruise flight at FL290. Approaching fix and aircraft started what appeared to be a smart turn; but the rate of roll started increasing beyond 30 degrees and the aircraft started descending. Aircraft lost approximately 400 feet before we could get the autopilot disconnected and hand fly the aircraft back to the correct altitude and back on course. ATC never asked any questions but did give us a crossing restriction for a future fix. Not sure ATC was even aware. Re-engaged autopilot and no other occurrence.not sure anything could have stopped the occurrence; we both grabbed the controls once the roll kept increasing. Everything on the FMA showed all modes were set correctly.

Original NASA ASRS Text

Title: C525 Captain reported the autopilot rolled the aircraft into a 30 degree bank and descended.

Narrative: In cruise flight at FL290. Approaching fix and aircraft started what appeared to be a smart turn; but the rate of roll started increasing beyond 30 degrees and the aircraft started descending. Aircraft lost approximately 400 feet before we could get the autopilot disconnected and hand fly the aircraft back to the correct altitude and back on course. ATC never asked any questions but did give us a crossing restriction for a future fix. Not sure ATC was even aware. Re-engaged autopilot and no other occurrence.Not sure anything could have stopped the occurrence; we both grabbed the controls once the roll kept increasing. Everything on the FMA showed all modes were set correctly.

Data retrieved from NASA's ASRS site 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.