Citationjet (C525/C526) - CJ I / II / III / IV
Flight Crew Air Transport Pilot (ATP)
Confusion; Situational Awareness
Aircraft Equipment Problem Less Severe; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter Loss Of Aircraft Control
Flight Crew Regained Aircraft Control; Flight Crew Returned To Clearance
Aircraft
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.
C525 Captain reported the autopilot rolled the aircraft into a 30 degree bank and descended.