B737-700 flight crew reported an altitude deviation occurred when both flight directors and the autopilot dropped offline during cruise. Systems came back online eventually and flight continued.
Synopsis
B737-700 flight crew reported an altitude deviation occurred when both flight directors and the autopilot dropped offline during cruise. Systems came back online eventually and flight continued.
Narrative
At FL360 as the FO and Pilot Flying while transitioning from a heading; to avoid weather; to ZZZZZ waypoint to return to our route. Both flight directors dropped offline unexpectedly; resulting in the autopilot disengaging and the aircraft in the right bank started to climb. We gained about 320' in RVSM airspace (36;320') while recovering the aircraft position. We immediately reported the deviation to ATC. ATC queried us if it was result of a wake turbulence or clear air turbulence event. We were operating in occasional moderate to severe turbulence due to convective activity in the area. We reported to ATC the autopilot unexpectedly dropped off and we immediately corrected the altitude back to FL360. During the event; both flight director switches were in the on position and no intentional disengagement of the autopilot had taken place. Upon arrival in ZZZ [airport]; we wrote up the automation failure and also noticed a similar event had been written up earlier in the month.Suggestion: Because this was an unexpected systems failure; I think the only thing we could've done to improve the outcome would have been to monitor the controls a little bit closer in RVSM airspace. By looking back further in the Maintenance log prior to the flight; we may have been able to see the previous writeup and be a little bit more aware of the potential for this to happen in flight. However; as the earlier write up was very cryptic in its description of the events; I'm not sure that we would've been able to gleam the full effects of the previous failure. A quicker response to the autopilot dropping offline might have resulted in preventing the altitude deviation.
Second reporter narrative
While in cruise at FL360 and after re-intercepting course following weather deviation; the autopilot (CMD B) inadvertently disengaged. Assuming it was pilot induced; I looked at reengaging it but noticed both flight directors had disappeared from view (Flight Director switches remained ON). While assessing; I realized the PF had not taken control manually yet; so I mention it (this was within the first five seconds or so of the disengagement and there had been a startle factor). He took immediate control and corrected to proper path; but the aircraft had drifted up by 300' beforehand.After approximately 30 seconds; the flight directors reappeared on the PFD (Primary Flight Display); and we were able to reengage the autopilot. There were no other cockpit or FMS indications associated with the malfunction. ATC queried (before we had a chance to notify) because there had been a lot of widespread turbulence throughout the region. We informed them and I sent a message to Dispatch to inform him of the malfunction and deviation. I reviewed the entire logbook for previous writeups and found one dated (date). Similar to our situation and Maintenance ran tests at that time but found no faults.Suggestion: This was a first for me; but I would say our initial response to the autopilot kicking off was a little slow. The autopilot's aural alert is so unnatural to hear in cruise that it was a distraction. Then the FD disappearing was confusing. All that led to the deviation because no one took immediate control of the aircraft (for five seconds or so). Simulator training could expose Crews this type of startle factor" event."
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.