A B737 captain flying on a RNAV approach at 3;000 ft for the if constraint; instructed the first off...

Attributes
ACN	1124968
Time
Date	201310
Local Time Of Day	1201-1800
Place
Locale Reference	ZZZ.TRACON
State Reference	US
Environment
Flight Conditions	VMC
Light	Night
Aircraft 1
Make Model Name	B737 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Flight Phase	Final Approach
Flight Plan	IFR
Component
Aircraft Component	Autoflight System
Person 1
Function	Captain Pilot Flying
Qualification	Flight Crew Air Transport Pilot (ATP)
Experience	Flight Crew Last 90 Days 75 Flight Crew Total 32000 Flight Crew Type 11000
Person 2
Function	Pilot Not Flying First Officer
Events
Anomaly	Deviation - Altitude Crossing Restriction Not Met Deviation - Altitude Excursion From Assigned Altitude Deviation - Altitude Overshoot Deviation - Procedural Clearance

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 speed; 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 heading 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.

Original NASA ASRS Text

Title: 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.

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.

Data retrieved from NASA's ASRS site as of July 2013 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.

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 speed engaged and VNAV disengaged. The aircraft transitioned to cws and began descended 300 ft before the crew caught the error.