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When ATC deleted their speed restriction at mnstr on the geela RNAV STAR to phx the flight crew of a B737-700 was uncertain of how to do so in the FMC while retaining the crossing window above 15;000 and below 16;000. They opted to exit VNAV path and continue their descent in speed intervene mode in anticipation the FMC predicted crossing would comply with the restriction. It didn't and they crossed the waypoint 800 ft low.
| Attributes | |
| ACN | 1056549 |
| Time | |
| Date | 201212 |
| Local Time Of Day | 1201-1800 |
| Place | |
| Locale Reference | PHX.Airport |
| State Reference | AZ |
| Environment | |
| Flight Conditions | VMC |
| Light | Night |
| Aircraft 1 | |
| Make Model Name | B737-700 |
| Operating Under FAR Part | Part 121 |
| Flight Phase | Descent |
| Flight Plan | IFR |
| Component | |
| Aircraft Component | FMS/FMC |
| Person 1 | |
| Function | First Officer Pilot Flying |
| Experience | Flight Crew Last 90 Days 115 Flight Crew Type 7000 |
| Person 2 | |
| Function | Captain Pilot Not Flying |
| Qualification | Flight Crew Air Transport Pilot (ATP) |
| Events | |
| Anomaly | Aircraft Equipment Problem Critical Deviation - Altitude Crossing Restriction Not Met Deviation - Altitude Overshoot Deviation - Procedural Clearance Deviation - Procedural Published Material / Policy |
Narrative:
While descending toward mnstr on GEELA6 into phx; we were told to disregard the speed restrictions (i.e. Not necessary to slow from 280 to 250). I deleted the speed restriction from mnstr; which also removed the altitude restriction to cross between 15;000 and 16;000. I then selected speed intervention 280 KIAS. The CDU showed that we would cross mnstr at about 15;500; so I didn't bother reentering the restriction. The increased speed; however; caused us to descend faster and we actually crossed mnstr at about 14;200. There was no traffic conflict; nor did ATC mention the deviation.I should have entered an altitude at mnstr after deleting the speed. Also; I think returning to VNAV path would have had us crossing mnstr at a correct altitude. A limitation of the FMC is that one cannot remove a speed without also deleting the associated altitude.
Original NASA ASRS Text
Title: When ATC deleted their speed restriction at MNSTR on the GEELA RNAV STAR to PHX the flight crew of a B737-700 was uncertain of how to do so in the FMC while retaining the crossing window above 15;000 and below 16;000. They opted to exit VNAV PATH and continue their descent in speed intervene mode in anticipation the FMC predicted crossing would comply with the restriction. It didn't and they crossed the waypoint 800 FT low.
Narrative: While descending toward MNSTR on GEELA6 into PHX; we were told to disregard the speed restrictions (i.e. not necessary to slow from 280 to 250). I deleted the speed restriction from MNSTR; which also removed the altitude restriction to cross between 15;000 and 16;000. I then selected Speed Intervention 280 KIAS. The CDU showed that we would cross MNSTR at about 15;500; so I didn't bother reentering the restriction. The increased speed; however; caused us to descend faster and we actually crossed MNSTR at about 14;200. There was no traffic conflict; nor did ATC mention the deviation.I should have entered an altitude at MNSTR after deleting the speed. Also; I think returning to VNAV PATH would have had us crossing MNSTR at a correct altitude. A limitation of the FMC is that one cannot remove a speed without also deleting the associated altitude.
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.