Date: 201810
Local time of day: 1201-1800
Location: LGA.Airport, NY
Flight conditions: IMC
Light: Daylight
Aircraft: EMB ERJ 170/175 ER/LR
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Initial Approach
Airspace: Class B LGA
ATC / advisory: Tower LGA
Aircraft component: FMS/FMC
Component problem: Improperly Operated
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Human factors: Situational Awareness; Time Pressure; Workload
Anomaly: ATC Issue All Types; Conflict Airborne Conflict; Deviation - Altitude Crossing Restriction Not Met; Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Automation Aircraft RA; Person Flight Crew
When detected: In-flight
Result: Flight Crew Became Reoriented; Flight Crew Returned To Clearance
Contributing factors: Human Factors; Procedure
Primary problem: Human Factors

Narrative: As a first time to LGA; both CA (Captain) and FO (First Officer) briefed extensively the STAR and approach RNAV 13. We planned possible vectors and how ATC would connect us to the approach. We were told to maintain 250 KTS until advised. We were given direct to LGA VOR and cross at 4;000 feet. After crossing turn left to 040 degrees; then descend to 3;000 feet. The Captain asked if we could slow down and ATC said to maintain 250 KTS. Still not knowing what was going to happen for vectors we saw previous aircraft loop around and direct to CHNZO. We were then given a left turn to 020 descend to 2;000 feet direct to CHNZO; speed your discretion and cleared approach. At this point we were behind the aircraft and when the Captain selected 1;300 feet for MEATZ FAF on the RNAV approach he forgot to arm the VNAV. The aircraft descended below 2;000 feet altitude required at CHNZO to RABBY intersections. The CA noticed the wrong altitude and then got the RA. It was only momentary as the Captain climbed back up and continued the approach without incident. ATC made no calls and the approach was completed without any other issues.Ultimately; the one problem was not arming the VNAV. But the factors of [being] unfamiliar [with] arrival procedures; an unfamiliar arrival and approach vectors; told to maintain speed of 250 KTS until just before CHNZO IAF in a turn; task saturation in a very short time; and given an arrival contrary to what we thought our arrival briefing would be to connect to the RNAV approach; all contributed to being behind the aircraft.I feel this report would help any pilot on a first time into LGA. Having a better idea of traffic flow and how they connect to the IAF of the LGA RNAV 13. Speed management and when to expect to be able to slow to approach speeds. Understanding the task saturation after crossing LGA with vectors; lower altitude assignments; and speed control all in a rushed moment requires planning. Again; as Captain I could [have] briefed the NFP to make sure the proper FMA readings are active. I might add that it was a little nerve wracking to see the runway off center from the approach as we broke out into visual conditions.




Synopsis: EMB-175 Captain reported an altitude deviation with an RA occurred during the arrival into LGA.