Narrative:

During descent on jawbn nine arrival to runway 16C; the first officer who had limited time on the aircraft; approximately 75 hours; set lower limit of altitude restrictions 12000 ft vs the individual step-down altitudes. Captain verified the altitude; and since aircraft was in VNAV profile with FMC altitude protection decided not to 'instruct' the first officer. Both crewmembers were tired; but had worked well for the flight both up and back. Fairly close to jawbn intersection; sea approach advised they were switching runways and captain went heads down to reprogram FMC for the new ILS and arrival. When I came up from heads down; the aircraft was already considerably below the 16000 ft restriction for jawbn intersection. Notified ATC and he said no conflict and that he should have waited till passing jawbn to give the amended clearance; he then cleared us to 10000 ft. At that time I noticed the aircraft was in cws P instead of VNAV. I have no idea how it got there or when; since I had been heads down; but since the alerter was set to 12000 ft. The aircraft simply continued descend. In the future; simply not allow myself to accept an 'unusual' usage of the automation. There is a strong desire not to continually 'instruct' the copilot; but with new personnel not familiar with normal techniques here; it may be necessary more than usual. There is no 'formal' procedure I am aware of; but the technique of setting each crossing altitude unless using VNAV/rnp approach procedures will guarantee protection for just this kind of event.

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Original NASA ASRS Text

Title: B737-800 DESCENDS BELOW CORRECT ALTITUDE FOR STAR WHEN GIVEN A RWY CHANGE FROM A STRAIGHT-IN SOUTH TO A NORTH RWY.

Narrative: DURING DESCENT ON JAWBN NINE ARRIVAL TO RWY 16C; THE FO WHO HAD LIMITED TIME ON THE AIRCRAFT; APPROXIMATELY 75 HOURS; SET LOWER LIMIT OF ALTITUDE RESTRICTIONS 12000 FT VS THE INDIVIDUAL STEP-DOWN ALTITUDES. CAPT VERIFIED THE ALTITUDE; AND SINCE AIRCRAFT WAS IN VNAV PROFILE WITH FMC ALTITUDE PROTECTION DECIDED NOT TO 'INSTRUCT' THE FO. BOTH CREWMEMBERS WERE TIRED; BUT HAD WORKED WELL FOR THE FLIGHT BOTH UP AND BACK. FAIRLY CLOSE TO JAWBN INTERSECTION; SEA APPROACH ADVISED THEY WERE SWITCHING RUNWAYS AND CAPT WENT HEADS DOWN TO REPROGRAM FMC FOR THE NEW ILS AND ARRIVAL. WHEN I CAME UP FROM HEADS DOWN; THE AIRCRAFT WAS ALREADY CONSIDERABLY BELOW THE 16000 FT RESTRICTION FOR JAWBN INTERSECTION. NOTIFIED ATC AND HE SAID NO CONFLICT AND THAT HE SHOULD HAVE WAITED TILL PASSING JAWBN TO GIVE THE AMENDED CLEARANCE; HE THEN CLEARED US TO 10000 FT. AT THAT TIME I NOTICED THE AIRCRAFT WAS IN CWS P INSTEAD OF VNAV. I HAVE NO IDEA HOW IT GOT THERE OR WHEN; SINCE I HAD BEEN HEADS DOWN; BUT SINCE THE ALERTER WAS SET TO 12000 FT. THE AIRCRAFT SIMPLY CONTINUED DESCEND. IN THE FUTURE; SIMPLY NOT ALLOW MYSELF TO ACCEPT AN 'UNUSUAL' USAGE OF THE AUTOMATION. THERE IS A STRONG DESIRE NOT TO CONTINUALLY 'INSTRUCT' THE COPILOT; BUT WITH NEW PERSONNEL NOT FAMILIAR WITH NORMAL TECHNIQUES HERE; IT MAY BE NECESSARY MORE THAN USUAL. THERE IS NO 'FORMAL' PROCEDURE I AM AWARE OF; BUT THE TECHNIQUE OF SETTING EACH CROSSING ALTITUDE UNLESS USING VNAV/RNP APPROACH PROCEDURES WILL GUARANTEE PROTECTION FOR JUST THIS KIND OF EVENT.

Data retrieved from NASA's ASRS site as of January 2009 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.