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Boeing first officer reported a possible false glide slope capture and incorrect descent using aircraft automation.
| Attributes | |
| ACN | 1651419 |
| Time | |
| Date | 201906 |
| Local Time Of Day | 0001-0600 |
| Place | |
| Locale Reference | SDF.Airport |
| State Reference | KY |
| Environment | |
| Flight Conditions | VMC |
| Light | Night |
| Aircraft 1 | |
| Make Model Name | Boeing Company Undifferentiated or Other Model |
| Operating Under FAR Part | Part 121 |
| Flight Phase | Final Approach |
| Route In Use | Visual Approach |
| Flight Plan | IFR |
| Component | |
| Aircraft Component | ILS/VOR |
| Person 1 | |
| Function | Pilot Flying |
| Qualification | Flight Crew Commercial Flight Crew Instrument Flight Crew Private Flight Crew Multiengine |
| Experience | Flight Crew Last 90 Days 180 Flight Crew Total 3500 Flight Crew Type 250 |
| Events | |
| Anomaly | Aircraft Equipment Problem Less Severe Deviation - Altitude Excursion From Assigned Altitude |
Narrative:
Visual approach to runway 35L. Descended to 2;500 feet MSL approaching crdnl (FAF) and armed the GS and localizer. 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 altitude 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.
Original NASA ASRS Text
Title: Boeing First Officer reported a possible false glide slope capture and incorrect descent using aircraft automation.
Narrative: 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.
Data retrieved from NASA's ASRS site 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.