Date: 201812
Local time of day: 1801-2400
Location: ZNY.ARTCC, NY
Flight conditions: VMC
Aircraft: B737-700
Operator: Air Carrier
Flight phase: Descent
ATC / advisory: Center ZNY
Aircraft component: FMS/FMC
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Not Flying
Reporter qualification: Flight Crew Instrument; Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP)
Human factors: Situational Awareness
Anomaly: Aircraft Equipment Problem Less Severe; Deviation - Altitude Crossing Restriction Not Met; Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Loss Of Aircraft Control
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Overcame Equipment Problem
Contributing factors: Human Factors; Aircraft
Primary problem: Aircraft

Narrative: This was our second leg in this aircraft and had no issues prior to this event.On descent into EWR on the PHLBO3 arrival; the aircraft just prior to a crossing restriction pitched down. Even with a relatively quick reaction; the aircraft went 800 feet below the altitude restriction.We began our descent from cruise to FUBAR intersection normally; meeting the restriction with no issue. There was a tailwind in the descent but the aircraft was not having any problems maintaining profile.  The only modification the First Officer made to the arrival was the addition of speed restrictions.  They were added to avoid receiving 'STEEP DESCENT AFTER DQO' and 'STEEP DESCENT AFTER SOMTO' scratch pad messages. The first speed restriction added was at DQO. The FO put 300 KTS next to the preprogrammed ALT restriction of 200A.  The next speed restriction added was 270 at SOMTO. Prior to DQO VNAV was not having any problem with the descent. I was watching closely as sometimes with good tailwinds on this arrival the aircraft can struggle maintaining speed.  This was not the case this evening.  There was a DECEL circle just prior to DQO which makes sense as the descent page had 310 KTS programmed and a 300/FL200A restriction. During this event; the aircraft was in VNAV PTH.  As the aircraft reached the DECEL point approximately 2 miles from the DQO VOR at an altitude just slightly above the crossing restriction; the aircraft did the opposite of what I expected it to do. I was expecting the aircraft to slightly pitch up to slow the 10 KTS and meet the FL200A crossing restriction and continue down the arrival. Instead the aircraft pitched down increasing an already high rate of descent.  I made a comment to the First Officer along the lines of 'what is this thing doing now'.  There was a slight hesitation (I am talking 1 potato 2 potato) on both our parts as the aircraft was in VNAV PTH and was now rapidly passing through the FL200A restriction just prior DQO instead of honoring the altitude.  I intervened turning off the AP and pitching up; but not wanting to be aggressive on the controls ended [up] crossing DQO at FL192.  The AP was turned back on and the remainder of the flight continued without any other issues with the VNAV.  As a side note we did receive a message several minutes after passing DQO VOR 'STEEP DESCENT AFTER DQO'. I have not seen a message like that come up after having passed the fix.




Synopsis: B737NG Captain reported overshooting a crossing altitude restriction when the aircraft aggressively pitched down unexpectedly on the PHLBO3 arrival into EWR.