B757 Undifferentiated or Other Model
Flight Crew Last 90 Days 230; Flight Crew Total 11100; Flight Crew Type 8100
Distraction; Human-Machine Interface; Workload; Situational Awareness
Aircraft Equipment Problem Less Severe; Deviation - Altitude Crossing Restriction Not Met; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Weather / Turbulence
Flight Crew FLC Overrode Automation
Aircraft; Chart Or Publication; Human Factors; Procedure; Weather
We were descending into DCA via the FRDMM RNAV STAR; conducting a VNAV Path descent. We monitored the FMA to ensure we stayed on the VNAV PATH and on the VDI (vertical deviation indicator) during the descent. After passing MORLL; still on the correct VNAV PATH; we reset the altimeters at FL180 from 29.92 to 30.56. The next waypoint; PLDGE; is supposed to be crossed at 280 KTS between 17;000 and 15;000. Due to the large altimeter change (which increased our QNH vice QNE indicated altitude by 640 FT) the very short distance between MORLL and PLDGE and the inability of VNAV to deal with the altimeter change in such a short period of time; we crossed PLDGE higher than 17;000. During all this; we monitored the FMA the entire time. We never left VNAV PATH once; yet we still crossed PLDGE high! Upon changing the altimeter; the VDI briefly went away; FMA stayed in VNAV PATH. The VDI came back after maybe 10 seconds and showed us on the correct vertical track. The speed began to slow towards 250 (no idea why [as we were] still in VNAV PATH). As we neared 250; it became apparent that VNAV didn't know what it was doing. We FLCH'd it down as quick as we could; including speed brake use; but there was no way to catch up at that point.
A B757-200 flight crew was unable to comply with the crossing altitude restriction at PLDGE on the FRDMM RNAV STAR into DCA when the switch from QNE to QNH passing FL180 increased their indicated altitude by 640 FT.