Boeing Company Undifferentiated or Other Model
Flight Crew Commercial; Flight Crew Instrument; Flight Crew Private; Flight Crew Multiengine
Flight Crew Last 90 Days 180; Flight Crew Total 3500; Flight Crew Type 250
Confusion; Human-Machine Interface; Situational Awareness; Time Pressure
Aircraft Equipment Problem Less Severe; Deviation - Altitude Excursion From Assigned Altitude
Flight Crew FLC Overrode Automation; Flight Crew Became Reoriented; Flight Crew Overcame Equipment Problem
Aircraft
Visual approach to RWY 35L. Descended to 2;500 feet MSL approaching CRDNL (FAF) and armed the GS and LOC. Level at 2;500 feet; and approaching the GS from beneath; I noted the GS CAP when my instruments still showed us a dot and a half below GS. The plane began a descent; and I announced to the Captain that I thought we had a false GS capture. He stated that his instruments showed us right on GS. I looked back to mine; and saw that we were now below 2;400 feet (the GS intercept altitude at CRDNL); but still some distance from CRDNL. I disengaged the AP and began a level off as the altitude alert sounded. We leveled at about 2;000 feet MSL; still well short of CRDNL. We turned the FDs off; then back on; but the approach mode was still engaged; and the GS capture was still shown; so we repeated turning off the FDs; and cycling them back on. This time it cleared the approach mode. We selected ALT HOLD at some point in here; and then when we saw we were on the actual GS (confirmed with the PAPIs) we reengaged approach mode and continued the approach. We were stable at 1;000 feet (the second 1;000 feet; not the false 1;000 feet call that you get flying over the dump); and continued the approach to landing. We were visual with the field and the terrain during the entirety of this approach. In retrospect; I would have disengaged the AP sooner; rather than trying to sort out the difference between my indications and the Captain's as we descended 500 feet on a false GS capture.
Boeing First Officer reported a possible false glide slope capture and incorrect descent using aircraft automation.