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 altitude intv. Less than a minute later; ATC instructed us to maintain FL360. Aircraft had descended approximately 200 ft. Fp selected vertical speed to stop the descent and to climb aircraft back to FL360. Aircraft subsequently climbed to FL360 and altitude 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.

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Original NASA ASRS Text

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

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.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.