B737 Undifferentiated or Other Model
Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 75; Flight Crew Total 32000; Flight Crew Type 11000
Workload; Human-Machine Interface; Distraction; Confusion; Situational Awareness
Deviation - Altitude Crossing Restriction Not Met; Deviation - Altitude Excursion From Assigned Altitude; Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Clearance
Flight Crew Became Reoriented; Flight Crew Took Evasive Action; Flight Crew Returned To Clearance
Human Factors; Procedure
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.
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.
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.