Commercial Fixed Wing
Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP)
Human-Machine Interface; Situational Awareness
Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Weather / Turbulence
Flight Crew Overcame Equipment Problem
Human Factors; Environment - Non Weather Related
Prior to entering the ZZZ Center airspace; during cruise at FL330; I had just returned back to my seat as 'as pilot monitoring' after using the rest room.  As a matter of practice; I read aloud the FMA parameters (which all appeared normal); upon taking my seat and assuming the role as 'pilot monitoring.'  Immediately after that; the Captain had also returned back to his seat as 'pilot flying' from his rest break; and exchanged his position with the IRO who had been occupying the captain's seat as the 'pilot flying' during the captain's break. At that time the Captain noticed that the aircraft had momentarily climbed to and leveled off at an indicated altitude of 33;500. The Captain immediately commenced a descent and returned the aircraft back to its original altitude of FL330; without any further consequence.We had become aware of the altitude when the Captain brought it to our attention In the cockpit. However; it was on the following day in ZZZ1 during my deadhead trip; that the captain recollected the event details and informed me that we had a deviation that may warrant the filing of a report. I believe that the aircraft may have climbed 500 feet and leveled off by the inadvertent touching of the altitude selector knob; either by the handling of the clipboard or by using disinfectant wipes in close proximity to the FCP; by one of the crew members; while getting in and out of the seats.  None of the crew members is sure of what caused the movement of the altitude selector knob and the gain in altitude.  The Captain immediately commenced a descent and returned the aircraft back to its original altitude of FL330; without any further consequence. As crew members; we were all equally surprised by the inadvertent deviation in altitude. Exercising greater level of situational awareness by all crew members; increased monitoring of the FMA; especially during the exchange of pilot seats and exchange of pilot roles while commencing and ending of crew rest periods.  Exercising extreme care during handling of clipboard and using disinfectant wipes in vicinity of FCP during flight.
Aircraft wandered off of assigned altitude in excess of 300 feet in cruise flight. [It was identified] when the Captain pointed it out. Mountain waves and or FO accidentally bumping the VSI or altitude pre-select while cleaning/sanitizing the FCP [may have caused it].I confirmed that the altitude was off the assigned; and needed to be corrected immediately. Advise all flight crews to use extreme caution when moving things around or cleaning the FCP inflight.
Air carrier flight crew reported an altitude deviation after the altitude selector knob was inadvertently moved. The flight crew stated the use of disinfectant wipes on the control panel may have been a contributing factor.