B737 Undifferentiated or Other Model
Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 220; Flight Crew Total 10000; Flight Crew Type 7000
Aircraft Equipment Problem Less Severe; Deviation - Altitude Crossing Restriction Not Met; Deviation / Discrepancy - Procedural Clearance
Flight Crew FLC Overrode Automation; Flight Crew Became Reoriented
Procedure; Human Factors; Aircraft
Descending on the DRLLR 1 RNAV arrival to IAH. Given a hard altitude to cross MPORT at; then 'descend via the DRLLR 1 arrival.' The FMC was correctly calculating the altitudes and speeds until the hard altitude crossing was given at the fix. Unknown as to the reason; but once the hard altitude is substituted for the between altitude; the FMC shows the aircraft significantly above the path. In my case yesterday; I was 2;000 FT above the FMC generated PDI path. Had to use full speedbrakes to make the hard altitude crossing restriction; then the rest of the RNAV descent was uneventful. You could probably easily re-create this in a simulator to the FAA controllers.
B737 Captain reports being issued a hard altitude to cross MPORT then descend via the DRLLR 1 RNAV to IAH. When the hard altitude is entered in the FMC the aircraft is instantly 2;000 FT high on the descent path; requiring speedbrakes and high speed to make the crossing.