B737-800
Flight Crew Air Transport Pilot (ATP)
Situational Awareness
Deviation - Altitude Excursion From Assigned Altitude; Inflight Event / Encounter Loss Of Aircraft Control
Flight Crew Returned To Clearance; Flight Crew Became Reoriented; Flight Crew Regained Aircraft Control; General Maintenance Action
Human Factors; Aircraft
At FL260 and 310 knots assigned airspeed; the aircraft executed an uncommanded roll. The FO and I had just been discussing the possibility of an arrival change; due to the landing configuration at destination and were referencing our iPads in anticipation of the change. When I returned my attention to the instruments the aircraft was rolling through 60 degrees of bank. It took a few moments to process what the aircraft was doing. At approximately 70 degrees of bank I disconnected the #1 autopilot and initiated a recovery. Approximately 500 feet of altitude was lost during the recovery. With control of the aircraft reestablished; we notified ATC of the deviation. ATC then directed us to descend to FL240. Although noticeable to the passengers and cabin crew; the seatbelt sign was on and no injuries were reported. The remainder of the flight was unremarkable. After landing; I entered the anomaly in the logbook and spoke with Maintenance before leaving the aircraft. I am still puzzled as to the cause of roll; but I am grateful for the unusual attitude recovery training we practice at recurrent.
B737-800 Captain reported recovering from an autopilot malfunction that put the aircraft into an uncommanded 70 degree bank and resulted in an altitude loss of 500 ft.