Flight Crew Air Transport Pilot (ATP)
Confusion
Aircraft Equipment Problem Less Severe; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Published Material / Policy
Air Traffic Control Issued New Clearance; Aircraft Equipment Problem Dissipated; Flight Crew Became Reoriented; Flight Crew Returned To Clearance
Human Factors; Aircraft
At FL260; we were assigned an arrival change. As I was setting up my charts on iPad (looking right); I became aware; due to 'bank angle' 'bank angle' and sound of Autopilot being turned off. At that time I noticed we were in excess of 60 degrees of bank; nose low (but not extreme). The Captain had taken the controls; and was hand flying recovery. It all happened fast; but I do not think I had any sensory cues that we were in bank; until the aforementioned aircraft sounds. I was a little confused on what exactly happened and was happening; but Captain executed excellent recovery. I notified ATC that we had lost 500 feet due to flight control issue; and they told us to descend to FL240; which we did. I asked Captain what had happened; and he told me he was also looking at iPad; and when he looked up; we were well past 45 degrees of bank; still on autopilot. He told me he disconnected autopilot; and executed recovery. We ascertained if there were any injuries in back of plane; and were told none.Rest of flight was uneventful; except for one of us always keeping a hand on yolk; in case anomaly reoccurred. We discussed episode; could not figure out what had occurred. Best we came up with was autopilot directed bank and did not stop; until disengaged. I had quick connection to dead head after flight; discussed with Captain who said he would write up and fill out reports. Really have no idea on why it occurred. We were both seated; no chance of accidentally bumping yoke. Cannot explain.
B737 First Officer experienced excessive bank angle at FL260 with the Captain flying and autopilot engaged. The Captain disconnected the autopilot and regained the 500 feet of altitude lost. Both pilots were focused on their iPads and did not see what initiated the event.