B737 Undifferentiated or Other Model
Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 114; Flight Crew Type 1237
Situational Awareness; Distraction
Deviation - Altitude Crossing Restriction Not Met; Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Weather / Turbulence
Air Traffic Control Issued Advisory / Alert; Flight Crew Became Reoriented
Human Factors
We were given 'descend via the PHLBO arrival' from Washington Center which meant to descend to 8000 ft while crossing certain fixes at or above specific altitudes. On the descent outside of STEFE intersection on the PHLBO arrival; we were encountering moderate turbulence. At some point when given 'descend via' on the arrival to 8000 ft; I didn't notice we left VNAV mode on the MCP in the descent while in the midst of asking the flight attendants to take their jump seats. The non-flying pilot was also occupied with Washington Center trying to get clearance to deviate around weather. Approaching STEFE; the restriction was to cross STEFE at or above 13;000 ft. Since the MCP was in LVL CHG mode due to my oversight; we were unfortunately at 9500 ft. Washington Center noted that we were 3500 ft lower than we were supposed to be.
B737 Captain reported descending below a 'descend via' crossing restriction when the crew failed to note they were in Level Change rather than VNAV mode.