 |
37000 Feet | Browse and search NASA's Aviation Safety Reporting System |
|
B737 flight crew reported excursion for level assigned altitude with autopilot engaged for no apparent reason.
| Attributes | |
| ACN | 1705090 |
| Time | |
| Date | 201911 |
| Local Time Of Day | 1201-1800 |
| Place | |
| Locale Reference | ZTL.ARTCC |
| State Reference | GA |
| Environment | |
| Flight Conditions | VMC |
| Aircraft 1 | |
| Make Model Name | B737 Undifferentiated or Other Model |
| Operating Under FAR Part | Part 121 |
| Flight Phase | Cruise |
| Flight Plan | IFR |
| Component | |
| Aircraft Component | Autoflight System |
| Person 1 | |
| Function | Captain Pilot Flying |
| Qualification | Flight Crew Air Transport Pilot (ATP) Flight Crew Multiengine Flight Crew Instrument |
| Experience | Flight Crew Last 90 Days 132 Flight Crew Total 10908 Flight Crew Type 7020 |
| Person 2 | |
| Function | Captain Pilot Not Flying |
| Qualification | Flight Crew Air Transport Pilot (ATP) Flight Crew Instrument Flight Crew Multiengine |
| Events | |
| Anomaly | Aircraft Equipment Problem Less Severe Deviation - Altitude Excursion From Assigned Altitude Deviation - Procedural Clearance |
Narrative:
In cruise flight at FL380 near the beginning of the ozzzi 1 RNAV arrival into atl; told to descend to FL350. Pm (pilot monitoring) set altitude selector to 35000 ft. And I pressed altitude intervene button. We descended to FL350 and were at that altitude for several minutes when the aircraft began to descend near wllsn waypoint. There was a moment of disbelief and why is it descending and how is it descending when 35000 ft. Is in the altitude window. I disengaged the autopilot and auto throttle to take control of the aircraft and climb back to FL350. Lowest altitude descended to was 34;600 ft. After reestablishing autopilot and auto throttles back on at FL350 we tried to assess what had happened. Our top of descent point wasn't for another 80 to 100 nm; 35000 was in the altitude window and we were in VNAV path. We could not determine why the autopilot descended the aircraft at that time.
Original NASA ASRS Text
Title: B737 flight crew reported excursion for level assigned altitude with autopilot engaged for no apparent reason.
Narrative: In cruise flight at FL380 near the beginning of the OZZZI 1 RNAV arrival into ATL; told to descend to FL350. PM (Pilot Monitoring) set altitude selector to 35000 ft. and I pressed altitude intervene button. We descended to FL350 and were at that altitude for several minutes when the aircraft began to descend near WLLSN waypoint. There was a moment of disbelief and why is it descending and how is it descending when 35000 ft. is in the altitude window. I disengaged the autopilot and auto throttle to take control of the aircraft and climb back to FL350. Lowest altitude descended to was 34;600 ft. After reestablishing autopilot and auto throttles back on at FL350 we tried to assess what had happened. Our top of descent point wasn't for another 80 to 100 nm; 35000 was in the altitude window and we were in VNAV path. We could not determine why the autopilot descended the aircraft at that time.
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.