Date: 201310
Local time of day: 1201-1800
Location: ZZZ.TRACON, US
Flight conditions: VMC
Light: Night
Aircraft: B737 Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Final Approach
Airspace: Class D ZZZ
ATC / advisory: TRACON ZZZ
Aircraft component: Autoflight System
Component problem: Improperly Operated
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Workload; Human-Machine Interface; Distraction; Confusion; Situational Awareness
Second reporter function: Pilot Not Flying; First Officer
Anomaly: Deviation - Altitude Crossing Restriction Not Met; Deviation - Altitude Excursion From Assigned Altitude; Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Clearance
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Became Reoriented; Flight Crew Took Evasive Action; Flight Crew Returned To Clearance
Contributing factors: Human Factors; Procedure
Primary problem: Human Factors

Narrative: Cleared direct IAF for the RNAV XXL. Approach told us to turn left 40 degrees for traffic and to descend to 3;000 FT. With the off course heading I deselected VNAV; engaged SPD; and slowed to 220 KTS. Once the traffic passed Approach Control cleared us direct the Initial Fix (IF) which is on the final approach course. I initially used HDG SEL and once heading toward the IF engaged LNAV. The VNAV page of the FMC was commanding 250 KTS so I decided to delay engaging VNAV until it commanded VM0 speed. Prior to the IF I asked the First Officer to set 2;000 FTabout the same time Approach handed us off to the Tower. The First Officer called the Tower just prior to the IF. At that time we both noticed the plane was descending out of 3;000 FT. I arrested the descent with the yoke and pitch was now in CWS. Once the path came back in I engaged VNAV. The remainder of the approach was uneventful.


Second narrative: We were in a descent; on a vector; cleared to proceed direct to the Initial Fix (IF) intersection; maintain 3;000 until the IF and simultaneously cleared for the RNAV Z XXL approach. I was instructed to set 2;000 feet into the altitude window (next altitude after the IF) by the pilot flying; which I did. As we approached the IF; Center instructed us to contact Tower. I made the frequency change; checked in and as I returned my attention to the approach; noticed that we were descending past 3;000 FT. At first I thought we had passed the IF but a cross check showed that we had not yet passed it. I estimate that we were within 1 mile of the IF at the time. I announced that we were below 3;000 FT at which point the pilot flying leveled the aircraft (approximately 300 FT low) and we passed the IF a few moments later.Subsequent discovery showed that VNAV was not engaged so the altitude restriction was not honored by the autoflight system. VNAV was re-engaged and the remainder of the flight continued uneventfully.


Synopsis: A B737 Captain flying on a RNAV Approach at 3;000 FT for the IF constraint; instructed the First Officer to select 2;000 FT on the MCP for the FAF with the SPD engaged and VNAV disengaged. The aircraft transitioned to CWS and began descended 300 FT before the crew caught the error.