Narrative:

We were cleared to cross an arrival intersection at FL240. The aircraft was descending via VNAV/LNAV. Approaching that intersection the controller changed the clearance to cross it at or below FL240 and descend to FL200. I changed the intersection altitude on the legs page to 240B and the cruise page to FL200. The first officer verified and I initiated. I got distracted with other functions and did not notice that the VNAV light went out and the aircraft reverted to speed function and shallowed the descent to 1;000 FPM. By the time we caught it and the controller alerted us; it was too late. The first officer and I tried to figure out why it would go out of VNAV but [could not]. When you are fresh and alert; the new automation can work great. But when you are tired and worn out after a long day; automation can be your worst enemy! Put the clearance that you will receive on the chart so that it is programmed in the computer. Any time a pilot has to alter the computer descent altitudes; you are asking for trouble.

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Original NASA ASRS Text

Title: After an arrival fix crossing altitude was changed to 'at or below FL240'; both pilots verified the B737-700 MCP and FMC entries but somehow the FMC transitioned to Vertical Speed without the pilots seeing; and the crossing restriction was missed.

Narrative: We were cleared to cross an arrival intersection at FL240. The aircraft was descending via VNAV/LNAV. Approaching that intersection the Controller changed the clearance to cross it at or below FL240 and descend to FL200. I changed the intersection altitude on the LEGS page to 240B and the Cruise page to FL200. The First Officer verified and I initiated. I got distracted with other functions and did not notice that the VNAV light went out and the aircraft reverted to speed function and shallowed the descent to 1;000 FPM. By the time we caught it and the Controller alerted us; it was too late. The First Officer and I tried to figure out why it would go out of VNAV but [could not]. When you are fresh and alert; the new automation can work great. But when you are tired and worn out after a long day; automation can be your worst enemy! Put the clearance that you will receive on the chart so that it is programmed in the computer. Any time a pilot has to alter the computer descent altitudes; you are asking for trouble.

Data retrieved from NASA's ASRS site as of April 2012 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.