Narrative:

For departure from sna to iah, company route was selected and executed on the route page of the control display unit (CDU). The route page was then verified against the predep clearance (pre departure clearance), both showing the BCKBY2 departure to trm. The problem was that the trm transition was not selected on the control display unit and arrival page, and thus the noise abatement procedure ground track was not correctly entered. On departure the FMC wanted to head directly to the point F093T on the BCKBY2 departure instead of executing the noise abatement procedure. We knew something was wrong, but instead of abandoning the FMC commands and executing the departure without the FMC, we were trying to figure out the problem. Then departure told us to turn right to heading 170 degrees and later cleared us to bhose on the SID. The lesson learned are that when the FMC commands something unexpected, just abandon it and use raw data, and that incomplete input into the FMC results in faulty output.

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Original NASA ASRS Text

Title: A B737 FLC EXPERIENCED A FAILURE OF THE FMC DURING THE NOISE ABATEMENT DEP PROC. THE ACFT WOULD NOT FLY THE PROGRAMMED PROC BECAUSE THE FLC HAD FAILED TO SELECT THE TRANSITION ON THE DEP AND ARR PAGE OF THE DISPLAY.

Narrative: FOR DEP FROM SNA TO IAH, COMPANY RTE WAS SELECTED AND EXECUTED ON THE RTE PAGE OF THE CTL DISPLAY UNIT (CDU). THE RTE PAGE WAS THEN VERIFIED AGAINST THE PREDEP CLRNC (PDC), BOTH SHOWING THE BCKBY2 DEP TO TRM. THE PROB WAS THAT THE TRM TRANSITION WAS NOT SELECTED ON THE CTL DISPLAY UNIT AND ARR PAGE, AND THUS THE NOISE ABATEMENT PROC GND TRACK WAS NOT CORRECTLY ENTERED. ON DEP THE FMC WANTED TO HEAD DIRECTLY TO THE POINT F093T ON THE BCKBY2 DEP INSTEAD OF EXECUTING THE NOISE ABATEMENT PROC. WE KNEW SOMETHING WAS WRONG, BUT INSTEAD OF ABANDONING THE FMC COMMANDS AND EXECUTING THE DEP WITHOUT THE FMC, WE WERE TRYING TO FIGURE OUT THE PROB. THEN DEP TOLD US TO TURN R TO HDG 170 DEGS AND LATER CLRED US TO BHOSE ON THE SID. THE LESSON LEARNED ARE THAT WHEN THE FMC COMMANDS SOMETHING UNEXPECTED, JUST ABANDON IT AND USE RAW DATA, AND THAT INCOMPLETE INPUT INTO THE FMC RESULTS IN FAULTY OUTPUT.

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.