B737-700
Flight Crew Last 90 Days 115; Flight Crew Type 7000
Training / Qualification; Situational Awareness; Human-Machine Interface; Confusion
Aircraft Equipment Problem Critical; Deviation - Altitude Overshoot; Deviation - Altitude Crossing Restriction Not Met; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural Clearance
Flight Crew Returned To Clearance; Flight Crew Became Reoriented; Flight Crew FLC Overrode Automation
Aircraft; Chart Or Publication; Human Factors; Procedure
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.
We discussed the best way to delete the speed [at MNSTR] while keeping the altitude restriction (I don't think there is one!). We selected delete without executing and it showed us crossing fix at 15;600 (small font [which identifies a predicted altitude; not a hard one]) if we were on profile for subsequent fixes.I'm still not sure of the best way to delete a speed restriction on a fix that also has a crossing altitude 'window' (below and above altitudes). But we know that we shouldn't have selected Speed Intervention and we should have stayed in [VNAV] PATH. At the very least; if we had entered the 'floor' altitude of the fix in the page; we wouldn't have descended below it. Obviously better coordination and monitoring also would've prevented this error.
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.