A321
Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Flight Crew Total 17000
Situational Awareness; Troubleshooting
Aircraft Equipment Problem Less Severe; Deviation - Altitude Excursion From Assigned Altitude; Deviation - Speed All Types; Inflight Event / Encounter Weather / Turbulence
Air Traffic Control Provided Assistance; Flight Crew Returned To Clearance
Weather; Aircraft
While in cruise flight at FL350 in IMC with only green precipitation depicted on radar display; we encountered moderate turbulence causing an increase in airspeed followed by a sudden decrease in airspeed. The autopilot was unable to maintain altitude. We lost approximately 400 feet of altitude before the descent could be halted and a climb initiated to get back to our assigned altitude. ATC noticed our altitude deviation and queried us about it. We made a PIREP [Pilot Report] regarding the turbulence and inability to maintain altitude. ATC advised us that there had not been any separation conflict due to our altitude deviation. Unknown weather created moderate turbulence and loss of energy to maintain altitude.
A321 pilot reported a loss of 400 feet of altitude due to autopilot failure; weather; and turbulence.