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B737 flight crew reported the aircraft overshot the altitude restriction because the autopilot disconnected.
| Attributes | |
| ACN | 1566345 |
| Time | |
| Date | 201808 |
| Local Time Of Day | 1201-1800 |
| Place | |
| Locale Reference | ZZZ.Airport |
| State Reference | US |
| Environment | |
| Light | Daylight |
| Aircraft 1 | |
| Make Model Name | B737-800 |
| Operating Under FAR Part | Part 121 |
| Flight Phase | Descent |
| Flight Plan | IFR |
| Person 1 | |
| Function | Captain Pilot Flying |
| Qualification | Flight Crew Multiengine Flight Crew Instrument Flight Crew Air Transport Pilot (ATP) |
| Experience | Flight Crew Last 90 Days 439 Flight Crew Type 12000 |
| Person 2 | |
| Function | Pilot Not Flying First Officer |
| Qualification | Flight Crew Air Transport Pilot (ATP) Flight Crew Instrument Flight Crew Multiengine |
| Experience | Flight Crew Last 90 Days 418 |
| Events | |
| Anomaly | Aircraft Equipment Problem Less Severe Deviation - Altitude Crossing Restriction Not Met Deviation - Altitude Overshoot Deviation - Procedural Clearance |
Narrative:
We were cleared to descend via the RNAV arrival into ZZZ. While at FL270; ATC stated 'do me a favor and cross zzzzz intersection at FL190.' the FMC wouldn't initially take FL190 at zzzzz intersection because the following waypoint (ZZZZZ1) had a restriction of FL200A. While working with the FMC to correct this problem; we failed to clarify if ATC wanted us to 'descend via; except cross zzzzz at FL190'; so with the ZZZZZ2 crossing restriction of FL240A (which is only 7.5 miles west of zzzzz) it left an extremely steep descent after the ZZZZZ2 intersection. Passing ZZZZZ2 pilot flying (PF) intervened with vertical speed and increased the rate of descent to make the crossing restriction. PF failed to immediately reset the MCP altitude to FL190 for the degrade in automation. Passing FL197; at a high rate of descent; the altitude window was reset to FL190; but further intervention and automation degradation was required to keep from busting through FL190. During this intervention; neither the PF nor the pilot monitoring (pm) noticed that the autopilot had disengaged. As a result the aircraft leveled initially at FL190 and then continued to descend to FL183. The next waypoint [after ZZZZZ1] on the arrival is ZZZZZ3; which has an altitude restriction of FL190. We descended to FL183 ft before correcting back up to FL190. The rest of the arrival and landing were uneventful. There were a number of things we could have done to prevent this event from occurring: clarify the clearance with ATC. The removal of the FL240A restriction just prior to the zzzzz intersection would have allowed us to remain in the highest level of automation and allowed the FMC to correctly program a descent formula. If we had immediately reset the MCP altitude to FL190; the autopilot could have assisted in leveling off at the proper altitude. Proper phraseology by ATC could have mitigated the problem. Both pilots should have been more vigilant in monitoring the aircraft to detect that the autopilot had disengaged.
Original NASA ASRS Text
Title: B737 Flight Crew reported the aircraft overshot the altitude restriction because the autopilot disconnected.
Narrative: We were cleared to descend via the RNAV Arrival into ZZZ. While at FL270; ATC stated 'Do me a favor and cross ZZZZZ intersection at FL190.' The FMC wouldn't initially take FL190 at ZZZZZ intersection because the following waypoint (ZZZZZ1) had a restriction of FL200A. While working with the FMC to correct this problem; we failed to clarify if ATC wanted us to 'descend via; except cross ZZZZZ at FL190'; so with the ZZZZZ2 crossing restriction of FL240A (which is only 7.5 miles west of ZZZZZ) it left an extremely steep descent after the ZZZZZ2 intersection. Passing ZZZZZ2 Pilot Flying (PF) intervened with Vertical Speed and increased the rate of descent to make the crossing restriction. PF failed to immediately reset the MCP altitude to FL190 for the degrade in automation. Passing FL197; at a high rate of descent; the altitude window was reset to FL190; but further intervention and automation degradation was required to keep from busting through FL190. During this intervention; neither the PF nor the Pilot Monitoring (PM) noticed that the autopilot had disengaged. As a result the aircraft leveled initially at FL190 and then continued to descend to FL183. The next waypoint [after ZZZZZ1] on the arrival is ZZZZZ3; which has an altitude restriction of FL190. We descended to FL183 ft before correcting back up to FL190. The rest of the arrival and landing were uneventful. There were a number of things we could have done to prevent this event from occurring: Clarify the clearance with ATC. The removal of the FL240A restriction just prior to the ZZZZZ intersection would have allowed us to remain in the highest level of automation and allowed the FMC to correctly program a descent formula. If we had immediately reset the MCP altitude to FL190; the autopilot could have assisted in leveling off at the proper altitude. Proper phraseology by ATC could have mitigated the problem. Both Pilots should have been more vigilant in monitoring the aircraft to detect that the autopilot had disengaged.
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.