B737-800
Flight Crew Air Transport Pilot (ATP)
Distraction; Situational Awareness
Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural Clearance
Flight Crew Became Reoriented; Flight Crew Returned To Clearance
Human Factors
Arrival into JFK; weather CAVU. Captain was pilot flying; First Officer was pilot monitoring. Planned and briefed the visual 13L with the RNV RNP RWY 13L approach as backup. Approach cleared us direct to ASALT; cross ASALT 3;000; cleared approach. During the descent we received several calls for a VFR target at our 10-12 o'clock position. We never acquired the traffic visually; but had him on TCAS. Eventually Approach advised traffic no factor; contact Tower. On contact with Tower we were cleared to land. Approaching ASALT I noticed we were approximately 500 feet below the 3;000 foot crossing altitude. Somewhere during the descent while our attention was on the VFR traffic the plane dropped out of VNAV path and I didn't catch it. I disconnected the autopilot and returned to 3;000 feet. Once level I reengaged VNAV and completed the approach with no further problems.
FMA mode changes are insidious. In clear weather; with your head out of the cockpit clearing for traffic in a high density environment; especially at your home field on a familiar approach; it is easy to miss a mode change. This is a good reminder to keep instruments in your cross check on those relatively few great weather days.
B737-800 flight crew reported an altitude deviation on approach to JFK that resulted when they lost autoflight mode awareness.