Date: 201908
Local time of day: 1201-1800
Location: ZZZ.ARTCC, US
Flight conditions: Marginal
Light: Daylight
Aircraft: B737 Next Generation Undifferentiated
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Cruise
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Reporter organization: Air Carrier
Reporter function: First Officer; Pilot Not Flying
Reporter qualification: Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument
Human factors: Workload; Distraction; Human-Machine Interface
Second reporter function: Pilot Flying; Captain
Anomaly: Conflict Airborne Conflict; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural Clearance
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Returned To Clearance; Flight Crew Overcame Equipment Problem; Flight Crew Became Reoriented
Contributing factors: Human Factors
Primary problem: Human Factors

Narrative: A/P CMD A ENGAGED. At FL360; we requested lower altitude of FL320 for ride. ATC cleared aircraft to descend to FL350. FP (Flying Pilot) initiated descent with VNAV ALT INTV. Less than a minute later; ATC instructed us to maintain FL360. Aircraft had descended approximately 200 ft. FP selected VERT SPEED to stop the descent and to climb aircraft back to FL360. Aircraft subsequently climbed to FL360 and ALT HLD was selected. Both pilots observed the aircraft level off at FL360; however; both pilots failed to confirm that the ALTITUDE window was not selected and changed from 35000 ft back to the original reassigned altitude of 36000 ft. The FP selected VNAV with the pretense that FL360 would be maintained. Both pilots became distracted while preparing a plan to navigate around the building cumulonimbus on the arrival route and failed to diligently monitor the aircraft altitude. An audible Traffic Advisory (TA) was heard and the aircraft was observed to be in a shallow descent while another aircraft was converging at FL350. We quickly re-climbed to FL360; the ALTITUDE window was selected to 36000 ft; VNAV was then selected; the aircraft FMA indicated that VNAV PTH was the engaged mode; and the aircraft was now maintaining level flight at FL360. After passing traffic; the aircraft was cleared to descend to FL320; vectored around observed weather; and flew the remaining portion of the flight as any other routine day. The TA event was discussed among the pilots and it was agreed that this could have been prevented by being more diligent in closely monitoring what is selected in the Mode Control Panel (MCP) and confirming the outcome with the Flight Mode Annunciations (FMAs). And finally; both pilots cannot be heads down while the aircraft goes unmonitored. We both learned a lesson today.


Second narrative: At FL360; we requested lower altitude of FL320 for ride. ATC cleared aircraft to descend to FL350. FP (Flying Pilot) initiated descent with VNAV ALT INTV. Less than a minute later; ATC instructed us to maintain FL360. Aircraft had descended approximately 200 ft. FP selected VERT SPEED to stop the descent and to climb aircraft back to FL360. Aircraft subsequently climbed to FL360 and ALT HLD was selected. Both pilots observed the aircraft level off at FL360; however; both pilots failed to confirm that the ALTITUDE window was not selected and changed from 35000 ft back to the original reassigned altitude of 36000 ft. The FP selected VNAV with the pretense that FL360 would be maintained. Both pilots became distracted while preparing a plan to navigate around the building cumulonimbus on the arrival route and failed to diligently monitor the aircraft altitude. A 400 ft deviation occurred. An audible Traffic Advisory (TA) was heard and the aircraft was observed to be in a shallow descent while another aircraft was converging at FL350. We quickly reclimbed to FL360; the ALTITUDE window was selected to 36000 ft; VNAV was then selected; the aircraft FMA indicated that VNAV PTH was the engaged mode; and the aircraft was now maintaining level flight at FL360. After passing traffic; the aircraft was cleared to descend to FL320; vectored around observed weather; and flew the remaining portion of the flight as any other routine day.This event occurred because I leveled off using altitude hold at FL360 and failed to recognize that I had neglected to reset FL360 from FL350 in the Altitude Alerter. I desired to reengage VNAV at that time. When I did I noted VNAV Path annunciated but failed to recognize that the aircraft was shortly to begin a descent to FL350 because FL350 was set in the Altitude Alerter.


Synopsis: B737 flight crew reported altitude excursion due to lack of automation management.