B767 flight crew reported while on the ILS approach the aircraft descended below the glide slope resulting in flight towards terrain. Flight crew recovered and continued.
Synopsis
B767 flight crew reported while on the ILS approach the aircraft descended below the glide slope resulting in flight towards terrain. Flight crew recovered and continued.
Narrative
FO was PF getting vectors to final from right base for XXL. Joined loc and g/s at 2000 feet on the right autopilot while also selecting flaps 20 and doing the before landing checklist at the same time. The aircraft pitched down with the selection of flaps 20 and continued a downward pitch trajectory despite having captured G/S on the pitch flight mode annunciator. I was heads-down doing the flow and checklist items. After about a two to three hundred foot descent; I heard the master caution alert and I noticed an amber line through the g/s pitch FMA (Flight Mode Annunciator); indicating invalid guidance. I directed the FO to disengage the autopilot; which he did while pitching up to arrest the sink rate; level out and recapture a good glide path to the runway. The flight director took a couple of hundred feet on proper descent path to give good guidance for the remainder of the approach. The rest of the approach and landing were uneventful.Too many things happening at once to provide a proper capture of the glide slope or a temporarily invalid glide slope signal due to traffic being on the runway ahead. Also; being heads down and not monitoring a proper capture of the glide path by the auto flight system. Suggestions: Being aware that improper capture might occur or spacing out required tasks to properly monitor the proper capture of the glide path.
Second reporter narrative
I was Pilot Flying getting vectors to ILS XXR into ZZZ. Given vectors to final at 2600' and cleared to maintain 2000 until established; cleared ILS XXR". We were at approximately 170 knots at flaps 15. I selected FLCH (Flight Level Change) and descended to 2000" and as we approached the LOC and the glide path from: below; I directed the APP tile and called for "Gear down; flaps 20; Before Landing Checklist". As the flaps rolled to 20; we simultaneously saw the FMA (Flight Mode Annunciator) capture both the LOC and GS along with a pitch change that seemed more abrupt than normal. We continued to configure the aircraft and conducted the before landing checklist. Shortly thereafter; at perhaps 1500' MSL; we heard an alert and saw that the aircraft had commanded a flightpath well below the glide path.I disconnected the autopilot and leveled the aircraft at approximately 1200' to reintercept the glide path from below. As we approached the visual glide path on the PAPIs; I continued the descent to an uneventful landing. Of note; even at the correct visual glide path; the Flight Director was still commanding a wildly incorrect pitch; directing the aircraft well above the glide path. At approximately 500'; it seemed to have corrected to the proper glide path.ATC never said anything.I am not 100% certain if it was a false glide slope or equipment malfunction; but we did intercept the glide slope where we expected to.We intercepted the glide path at the appropriate point and verified glide slope and localizer captured with the FMAs; so actually preventing the event doesn't seem possible without more data from the aircraft. What is always worth discussing; however; is the speed at which we; as a crew; recognize and react. I think I could have recognized it slightly quicker when we first began the pitch down for the glide path by not trying to do before landing checks right away after recognizing a slightly more aggressive pitch change than normal. We still had 1000' to the required stable gate and it was probably worth extra attention. I might have been able to recognize the growing divergence slightly quicker in that case."
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.