Large Transport; Low Wing; 2 Turbojet Eng
Flight Crew Last 90 Days 180.82; Flight Crew Type 4971.32
Human-Machine Interface; Situational Awareness
Deviation - Altitude Crossing Restriction Not Met; Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Published Material / Policy
Flight Crew Became Reoriented
Aircraft; Human Factors
I am not sure what I/we did wrong.  I only know something did not 'go right'.  We briefed for a Stadium Visual Runway 29.  We discussed early configuration and speed control; we discussed when we could/would set FAF altitude ('Cleared For Approach'); and when we could/would set TDZE (Published Segment of Approach).  We confirmed proper waypoint sequencing; and altitudes/airspeeds in Legs Page of FMC.  I/we have done this approach several times; with no issues.  It is actually 'good practice' for our Non-Precision Approach Procedures.  We were 'Cleared the Stadium Visual Approach; proceed direct to GIMEE; remain at/above 2;500 until Stadium Complex'.  We confirmed 2;500 on the Legs Page for GIMEE (Roughly the 'Stadium Complex'); we set 1;500 in the Altitude Select Window (FAF altitude at SLIMR); and double checked.  I DO NOT RECALL IF I CHECKED MY FMA.  Either way; I followed my Flight Director Pitch Command Bar; assuming all was well.  I was right on the Command Bar.  As we approached GIMEE from the NorthWest; I visually sighted a departure from Teterboro Airport; departing to the North; passing beneath us; left to right.  There was no TCAS RA. There was no radio call from Approach Control or from Tower.  The traffic just 'looked closer' than I thought normal.  That 'looked closer' prompted me to check my altitude.  We were at 2;300 MSL; roughly 200 feet below the 2;500 MSL restriction for the 'Stadium Complex' at our twelve o-clock position. I immediately disconnect autoflight systems; leveled off at 2;300 MSL until clear of traffic; and resumed the Stadium Visual Approach without incident.  Upon block-in; I debriefed with the First Officer and I called the Newark Tower Controller to discuss the incident.  The Tower Supervisor had no comments; there had been no 'Alert'; there was 'no issue'.  Nonetheless; I have been TRYING to determine what went wrong; and why.  I CAN ONLY CONCLUDE that I DID NOT SELECT VNAV; OR VNAV DID NOT ENGAGE.  Hence; the Flight Director took me to the 1;500 MSL in the Altitude Select Window; not 'respecting' the 2500 MSL restriction on the Legs Page for GIMEE (the 'Stadium Complex').  That is the only possible explanation.  LESSON LEARNED: We have all learned this lesson many times; and perhaps it is simply a lesson we have to keep learning every now and then; but I NEED to ALWAYS confirm my FMA.  It is as simple as that.
An airline Captain reports descending early on the Stadium Visual to Runway 29 at EWR. The reporter believes that either VNAV was not selected or did not engage. The error was not detected until 200 feet low.