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A B737-800 autopilot twice transitioned from VNAV path to cws pitch without command and the during the first event at FL360 began a dive loosing 300 ft before recovery was affected.
| Attributes | |
| ACN | 1055630 |
| Time | |
| Date | 201212 |
| Environment | |
| Light | Night |
| Aircraft 1 | |
| Make Model Name | B737-800 |
| Operating Under FAR Part | Part 121 |
| Flight Phase | Cruise |
| Flight Plan | IFR |
| Component | |
| Aircraft Component | Altitude Hold/Capture |
| Person 1 | |
| Function | Pilot Flying |
| Events | |
| Anomaly | Aircraft Equipment Problem Critical Deviation - Altitude Excursion From Assigned Altitude Inflight Event / Encounter Loss Of Aircraft Control Inflight Event / Encounter Weather / Turbulence |
Narrative:
In turbulence; the pitch mode of the autopilot dropped out of VNAV path and reverted to cws pitch; and the plane immediately started diving at what must have been about 4;000 FPM. I grabbed the yoke; disconnected the autopilot and stopped the descent at a loss of about 300 ft. Later in the flight; the same thing happened on descent but there was no altitude deviation.suggestions for avoiding recurrence: keep the autopilots up.
Original NASA ASRS Text
Title: A B737-800 autopilot twice transitioned from VNAV Path to CWS Pitch without command and the during the first event at FL360 began a dive loosing 300 FT before recovery was affected.
Narrative: In turbulence; the PITCH mode of the autopilot dropped out of VNAV path and reverted to CWS Pitch; and the plane immediately started diving at what must have been about 4;000 FPM. I grabbed the yoke; disconnected the autopilot and stopped the descent at a loss of about 300 FT. Later in the flight; the same thing happened on descent but there was no altitude deviation.Suggestions for avoiding recurrence: Keep the autopilots up.
Data retrieved from NASA's ASRS site as of July 2013 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.