Regional Jet 700 ER/LR (CRJ700)
Flight Crew Air Transport Pilot (ATP)
Distraction; Situational Awareness; Workload
Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter CFTT / CFIT
Air Traffic Control Issued Advisory / Alert; Flight Crew Became Reoriented
Human Factors; Weather
Going into LGA we were told to expect the RNAV GPS approach for runway 13. After briefing and setting up for the approach we were given a 030 heading after the LGA VOR and then another vector to join the final before the CHNZO fix. ATC broadcasted that the visibility had dropped to 1.5 miles at about the same time we were joining the final. We were discussing the minimums when we were cleared for the approach. I mistakenly started my descent at the CHNZO fix instead of the RABBY fix to 1300 feet. The FO was putting the gear down and getting us configured so he was busy with the checklist while I descended. By the time I caught my error we were at the MEATZ fix which allowed a crossing at 1300 feet so I continued the approach with no further problems. I discovered that I had descended about 2 miles early. The main threat that distracted me was the broadcasting of the deteriorating visibility while we were joining final along with task saturation of getting configured for the approach. Slow down and if you get behind just go around and set back up for the approach.
Once [I] saw a corrective action by the PF (selected ALT); [I] looked away to make the new configuration/frequency changes and when [I] looked back up; the plane was leveling at 1300. Before getting the first word in with Tower; the Controller made contact and indicated she was getting a low altitude alert on us and to continue. [I] acknowledged. The short remainder of the approach and landing went smoothly and as reported earlier by the Approach Controller; we broke out of the weather right above minimums.We established several threats in this event. The RNAV GPS 13 is an approach that is rarely used on a runway that is also used for landing fairly infrequently. Both my Captain and I have lots of experience operating in and out of LGA and we both believe that this day was the first time flying this particular approach leading to a lack of familiarity even though the details were thoroughly briefed both times we flew it. The information the Approach Controller issued us also lead to a significant threat. We were just outside the CHNZO fix on vectors when the conversation about deteriorating weather began and making us the second aircraft to try the approach with these conditions. This obviously led to an increase in workload as we double checked the approach plate minimums and continued with heading/altitude changes. Then the Controller gave us a strange clearance (twice). By clearing us to cross CHNZO above 2000; it caused a distraction from the fact that 2000 actually needs to be maintained beyond the next fix (RABBY) leading to the undesired aircraft state of starting the descent early and being low at RABBY. Workload for both pilots reached a maximum at the point of altitude correction/configuration and frequency change just prior to Tower noticing the low altitude alert.If I could change any behavior in what I learned from this event; it would be to express a concern as PM more clearly to the PF in the form of a direct observation/correction. What needed to be said at the CHNZO fix was 'we need to maintain 2000 until beyond RABBY' and the deviation could have been corrected quickly or not happened at all. I would argue that the clearance from approach control did not help the situation of flying a complicated; unfamiliar approach but nothing he said was incorrect; just distracting. The most important learning tool from this event is something we already hammer down on in every day operations but needs to be reiterated: a thorough approach briefing is absolutely necessary; especially when flying a full IMC approach; and both pilots need to be clear on every detail of every segment of the approach before it begins.
CRJ-700 flight crew reported receiving a low altitude alert from ATC after descending early on approach to LGA.