Narrative:

Prior to descent into ZZZ; we briefed a CAT ii as the temp/dew point was 3/3; and on the descent weather dropped below our mins (1/8 mile). We began to plan on an alternate; and declined a descent to save fuel in anticipation of holding. Laterally; they continued us on the STAR. We got a little distracted after putting in the alternate airport; the flight plan had us shoot the approach in ZZZ; fly direct to [the] alternate airport; then fly back to ZZZ; so our landing fuel made no sense until we reconciled this discrepancy. Even after we fixed this; we only had 400 or so pounds of extra fuel to play with.ATC gave us a further descent; so we asked if the visibility had increased. Yes; and during the descent it rose quite a bit. ATC then gave us vectors for our descent since we were high; and I configured flaps 1 and spoilers to get down. Eventually we were cleared to 2000; vectored onto; and cleared for the approach. Approaching the penultimate fix the magenta glide path (gp) began to descend below us. My first officer (first officer) remarked on this. I responded that we hadn't past intermediate fix (if) yet with a crossing altitude of 2000 feet. Passing if; I hit appr mode and nothing happened except for a message on the FMS that said vertical glide path (vgp) not available. At the time; I didn't register the meaning of this message properly; instead thinking it pertained to the approach plate note; 'ILS gp and vgsi not coincident' that I assumed must be part of the new ACARS functions and messages. I said appr mode didn't work; to my first officer; and hit appr again. Nothing happened. Meanwhile the gp was getting away from us; so I touch control steered (tcs) it down to catch the gp. We were in VMC above a layer of IMC. As we got close to the clouds; I zoomed in I'm my mfd and realized we hadn't passed the FAF yet with its restriction of 1800 feet. I remarked on this and started climbing. Seconds later ATC issued a low altitude alert and I called for and executed a missed approach. My first officer believes our lowest altitude was 1200 feet. ATC reported visibility of 6000 feet at that point; so we flew a second approach to a landing; without further incident.detection was part mfd situational awareness; part getting close to descending into the clouds; and part that nagging feeling that this isn't right. There were several contributing factors: a very early wake-up call; concerns about fuel and an unexpected alternate; ATC clearances that we either didn't accept or had to question. Additionally an FMS message neither of us had seen before; and a runaway gp that neither of us had seen or heard of before. I think the fuel-related pressure to make this approach work played a role.suggestions: if something doesn't look right; take the time to investigate it; especially at night or in or approaching IMC. In our case; once we remarked that we had a runaway gp; we should have just both gone green needles or asked for vectors off the approach to reconcile the problematic FMS.

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Original NASA ASRS Text

Title: EMB-175 flight crew reported that they descended below the FAF crossing altitude while approach; and subsequently received a low Altitude alert from ATC.

Narrative: Prior to descent into ZZZ; we briefed a CAT II as the Temp/dew point was 3/3; and on the descent weather dropped below our mins (1/8 mile). We began to plan on an alternate; and declined a descent to save fuel in anticipation of holding. Laterally; they continued us on the STAR. We got a little distracted after putting in the Alternate Airport; the flight plan had us shoot the approach in ZZZ; fly direct to [the] Alternate Airport; then fly back to ZZZ; so our landing fuel made no sense until we reconciled this discrepancy. Even after we fixed this; we only had 400 or so pounds of extra fuel to play with.ATC gave us a further descent; so we asked if the visibility had increased. Yes; and during the descent it rose quite a bit. ATC then gave us vectors for our descent since we were high; and I configured Flaps 1 and spoilers to get down. Eventually we were cleared to 2000; vectored onto; and cleared for the approach. Approaching the penultimate fix the magenta Glide Path (GP) began to descend below us. My First Officer (FO) remarked on this. I responded that we hadn't past Intermediate Fix (IF) yet with a crossing altitude of 2000 feet. Passing IF; I hit APPR mode and nothing happened except for a message on the FMS that said Vertical Glide Path (VGP) not available. At the time; I didn't register the meaning of this message properly; instead thinking it pertained to the Approach plate note; 'ILS GP and VGSI not coincident' that I assumed must be part of the new ACARS functions and messages. I said APPR mode didn't work; to my FO; and hit APPR again. Nothing happened. Meanwhile the GP was getting away from us; so I Touch Control Steered (TCS) it down to catch the GP. We were in VMC above a layer of IMC. As we got close to the clouds; I zoomed in I'm my MFD and realized we hadn't passed the FAF yet with its restriction of 1800 feet. I remarked on this and started climbing. Seconds later ATC issued a low altitude alert and I called for and executed a missed approach. My FO believes our lowest altitude was 1200 feet. ATC reported visibility of 6000 feet at that point; so we flew a second approach to a landing; without further incident.Detection was part MFD situational awareness; part getting close to descending into the clouds; and part that nagging feeling that this isn't right. There were several contributing factors: a very early wake-up call; concerns about fuel and an unexpected alternate; ATC clearances that we either didn't accept or had to question. Additionally an FMS message neither of us had seen before; and a runaway GP that neither of us had seen or heard of before. I think the fuel-related pressure to make this approach work played a role.Suggestions: If something doesn't look right; take the time to investigate it; especially at night or in or approaching IMC. In our case; once we remarked that we had a runaway GP; we should have just both gone green needles or asked for vectors off the approach to reconcile the problematic FMS.

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.