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B737-700 captain reports his aircraft beginning to slow automatically during an RNAV approach while level at 3000 feet; with 2400 feet set in the altitude window for the next step down altitude. ATC asks for 210 knots to be maintained and speed intervention is selected on the MCP which results in an early descent that is quickly corrected.
| Attributes | |
| ACN | 1238146 |
| Time | |
| Date | 201502 |
| Local Time Of Day | 1801-2400 |
| Place | |
| Locale Reference | ZZZ.Airport |
| State Reference | US |
| Environment | |
| Flight Conditions | VMC |
| Light | Night |
| Aircraft 1 | |
| Make Model Name | B737-700 |
| Operating Under FAR Part | Part 121 |
| Flight Phase | Initial Approach |
| Flight Plan | IFR |
| Component | |
| Aircraft Component | MCP |
| Person 1 | |
| Function | Captain Pilot Flying |
| Qualification | Flight Crew Air Transport Pilot (ATP) |
| Experience | Flight Crew Last 90 Days 38 |
| Events | |
| Anomaly | Deviation - Altitude Excursion From Assigned Altitude Deviation - Procedural Clearance Deviation - Procedural Published Material / Policy Deviation - Speed All Types |
Narrative:
On RNAV approach; in LNAV and VNAV; the aircraft started to slow. We were level at 3000 ft with 2400 ft set for the next descent. ATC asked us to maintain 210 knots. I used speed intervention to maintain speed and did not notice the airplane began to drift down to 2400 ft. Caught it about 2600 ft and started back up. By then; it was time to descend to 2400 ft. It was an automation error and totally my fault. I must not understand the system as well as I thought I did.
Original NASA ASRS Text
Title: B737-700 Captain reports his aircraft beginning to slow automatically during an RNAV approach while level at 3000 feet; with 2400 feet set in the altitude window for the next step down altitude. ATC asks for 210 knots to be maintained and speed intervention is selected on the MCP which results in an early descent that is quickly corrected.
Narrative: On RNAV approach; in LNAV and VNAV; the aircraft started to slow. We were level at 3000 FT with 2400 FT set for the next descent. ATC asked us to maintain 210 knots. I used Speed Intervention to maintain speed and did not notice the airplane began to drift down to 2400 FT. Caught it about 2600 FT and started back up. By then; it was time to descend to 2400 FT. It was an automation error and totally my fault. I must not understand the system as well as I thought I did.
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.