Narrative:

The approach was thoroughly briefed prior to let down. The first officer was to fly the non precision approach on autoplt using LNAV with raw data as a backup. Approach gave us a right turn to final and cleared us to descend from 3000 ft to 2000 ft, cleared to intercept the localizer, cleared for the approach, maintain 2000 until established on the localizer. The first officer requested I enter a direct intercept leg on the FMC to cohop (final approach fix). He then dialed in 2000 ft on the on the mode control panel and I acknowledged the altitude change. After I 'executed' the intercept on the FMC the first officer selected LNAV (autoplt was in heading select). The autoplt immediately began to roll out of its right hand turn and began a left turn. I told the first officer to take it out of LNAV and fly using raw data. He did so. I crosschecked the raw data display and altimeter and observed us rolling out exactly on the localizer and passing through 2200 ft. I then looked out in an attempt to visually acquire the airport. I could not see it and told the first officer. He requested the gear down and final landing checklist. As I lowered the gear and began performing the checklist items, approach control switched us over to lga tower. I accomplished all of this and again looked out for the airport and could not see it. I told the first officer that 'something was not right, we should have visual contact by now.' it was then that I looked at the altitude window on the mode control panel and saw 700 ft dialed in. The last altitude that I acknowledged was 2000 ft and if anything it should have been 1600 ft. We were descending through 1000 ft on the altimeter. I told the first officer to stop the descent and I checked the DME. It read 7 NM. I looked at his FMC legs display expecting to see a few mi to cohop but it was no longer on the page. We should have been at 1600 ft. The FMC had somehow dropped the approach from the legs display. Tower called at this time and asked if we had a visual on the airport. I replied 'negative' and told the first officer to began a climb. The tower said to climb to 2000 ft and report the airport in sight. Once we climbed we acquired the airport visually and made an uneventful landing. This incident could have been prevented by following the briefed procedures. 1) when it was apparent the LNAV function was not working correctly a full transition to raw data should have been performed. By using a combination of raw data localizer and FMC leg page for fix information and insidious error was permitted to grow worse by the first officer. 2) by not calling out and receiving a positive verification from me the altitude changes in the mode control panel altitude window, the first officer completely removed me from the loop. Since I heard no altitude callout from the first officer I assumed we were at 2000 ft 'dirtying up' in preparation for descent to 1600 ft to cohop. I was visually trying to acquire the airport perform the landing checklist. Switching frequency and monitoring the localizer capture. Supplemental information from acn 290735: arriving at 1600 ft I checked the FMS and the cohop fix had disappeared, indicating (I thought) that we had passed it. I put 700 ft in the altitude window and began to descend to minimums. I have never felt comfortable with the setup of flying an approach coupled to LNAV while I am looking at raw data. I believe it is confusing and compounds the chance for error, and will personally not do it again. I think the PF should be monitoring the information he is controling, and xchking against another source of information should be the function of the PNF. I feel we were distracted and confused by rather insidious FMS malfunction which had appeared normal to both of us.

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Original NASA ASRS Text

Title: MLG DSNDS BELOW MINIMUMS AS AUTOPLT MALFUNCTIONS AND FINAL APCH FIX DISAPPEARS FROM SCREEN.

Narrative: THE APCH WAS THOROUGHLY BRIEFED PRIOR TO LET DOWN. THE FO WAS TO FLY THE NON PRECISION APCH ON AUTOPLT USING LNAV WITH RAW DATA AS A BACKUP. APCH GAVE US A R TURN TO FINAL AND CLRED US TO DSND FROM 3000 FT TO 2000 FT, CLRED TO INTERCEPT THE LOC, CLRED FOR THE APCH, MAINTAIN 2000 UNTIL ESTABLISHED ON THE LOC. THE FO REQUESTED I ENTER A DIRECT INTERCEPT LEG ON THE FMC TO COHOP (FINAL APCH FIX). HE THEN DIALED IN 2000 FT ON THE ON THE MODE CTL PANEL AND I ACKNOWLEDGED THE ALT CHANGE. AFTER I 'EXECUTED' THE INTERCEPT ON THE FMC THE FO SELECTED LNAV (AUTOPLT WAS IN HDG SELECT). THE AUTOPLT IMMEDIATELY BEGAN TO ROLL OUT OF ITS R HAND TURN AND BEGAN A L TURN. I TOLD THE FO TO TAKE IT OUT OF LNAV AND FLY USING RAW DATA. HE DID SO. I XCHKED THE RAW DATA DISPLAY AND ALTIMETER AND OBSERVED US ROLLING OUT EXACTLY ON THE LOC AND PASSING THROUGH 2200 FT. I THEN LOOKED OUT IN AN ATTEMPT TO VISUALLY ACQUIRE THE ARPT. I COULD NOT SEE IT AND TOLD THE FO. HE REQUESTED THE GEAR DOWN AND FINAL LNDG CHKLIST. AS I LOWERED THE GEAR AND BEGAN PERFORMING THE CHKLIST ITEMS, APCH CTL SWITCHED US OVER TO LGA TWR. I ACCOMPLISHED ALL OF THIS AND AGAIN LOOKED OUT FOR THE ARPT AND COULD NOT SEE IT. I TOLD THE FO THAT 'SOMETHING WAS NOT R, WE SHOULD HAVE VISUAL CONTACT BY NOW.' IT WAS THEN THAT I LOOKED AT THE ALT WINDOW ON THE MODE CTL PANEL AND SAW 700 FT DIALED IN. THE LAST ALT THAT I ACKNOWLEDGED WAS 2000 FT AND IF ANYTHING IT SHOULD HAVE BEEN 1600 FT. WE WERE DSNDING THROUGH 1000 FT ON THE ALTIMETER. I TOLD THE FO TO STOP THE DSCNT AND I CHKED THE DME. IT READ 7 NM. I LOOKED AT HIS FMC LEGS DISPLAY EXPECTING TO SEE A FEW MI TO COHOP BUT IT WAS NO LONGER ON THE PAGE. WE SHOULD HAVE BEEN AT 1600 FT. THE FMC HAD SOMEHOW DROPPED THE APCH FROM THE LEGS DISPLAY. TWR CALLED AT THIS TIME AND ASKED IF WE HAD A VISUAL ON THE ARPT. I REPLIED 'NEGATIVE' AND TOLD THE FO TO BEGAN A CLB. THE TWR SAID TO CLB TO 2000 FT AND RPT THE ARPT IN SIGHT. ONCE WE CLBED WE ACQUIRED THE ARPT VISUALLY AND MADE AN UNEVENTFUL LNDG. THIS INCIDENT COULD HAVE BEEN PREVENTED BY FOLLOWING THE BRIEFED PROCS. 1) WHEN IT WAS APPARENT THE LNAV FUNCTION WAS NOT WORKING CORRECTLY A FULL TRANSITION TO RAW DATA SHOULD HAVE BEEN PERFORMED. BY USING A COMBINATION OF RAW DATA LOC AND FMC LEG PAGE FOR FIX INFO AND INSIDIOUS ERROR WAS PERMITTED TO GROW WORSE BY THE FO. 2) BY NOT CALLING OUT AND RECEIVING A POSITIVE VERIFICATION FROM ME THE ALT CHANGES IN THE MODE CTL PANEL ALT WINDOW, THE FO COMPLETELY REMOVED ME FROM THE LOOP. SINCE I HEARD NO ALT CALLOUT FROM THE FO I ASSUMED WE WERE AT 2000 FT 'DIRTYING UP' IN PREPARATION FOR DSCNT TO 1600 FT TO COHOP. I WAS VISUALLY TRYING TO ACQUIRE THE ARPT PERFORM THE LNDG CHKLIST. SWITCHING FREQ AND MONITORING THE LOC CAPTURE. SUPPLEMENTAL INFO FROM ACN 290735: ARRIVING AT 1600 FT I CHKED THE FMS AND THE COHOP FIX HAD DISAPPEARED, INDICATING (I THOUGHT) THAT WE HAD PASSED IT. I PUT 700 FT IN THE ALT WINDOW AND BEGAN TO DSND TO MINIMUMS. I HAVE NEVER FELT COMFORTABLE WITH THE SETUP OF FLYING AN APCH COUPLED TO LNAV WHILE I AM LOOKING AT RAW DATA. I BELIEVE IT IS CONFUSING AND COMPOUNDS THE CHANCE FOR ERROR, AND WILL PERSONALLY NOT DO IT AGAIN. I THINK THE PF SHOULD BE MONITORING THE INFO HE IS CTLING, AND XCHKING AGAINST ANOTHER SOURCE OF INFO SHOULD BE THE FUNCTION OF THE PNF. I FEEL WE WERE DISTRACTED AND CONFUSED BY RATHER INSIDIOUS FMS MALFUNCTION WHICH HAD APPEARED NORMAL TO BOTH OF US.

Data retrieved from NASA's ASRS site as of July 2007 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.