After an arrival fix crossing altitude was changed to 'at or below FL240'; both pilots verified the ...

Attributes
ACN	974547
Time
Date	201110
Local Time Of Day	0601-1200
Place
Locale Reference	ZZZ.ARTCC
State Reference	US
Environment
Flight Conditions	VMC
Light	Night
Aircraft 1
Make Model Name	B737-700
Operating Under FAR Part	Part 121
Flight Phase	Descent
Flight Plan	IFR
Component
Aircraft Component	FCC (Flight Control Computer)
Person 1
Function	Pilot Not Flying Captain
Qualification	Flight Crew Air Transport Pilot (ATP)
Experience	Flight Crew Last 90 Days 163
Person 2
Function	Pilot Not Flying First Officer
Experience	Flight Crew Last 90 Days 213 Flight Crew Type 5649
Events
Anomaly	Aircraft Equipment Problem Less Severe Deviation - Altitude Crossing Restriction Not Met Deviation - Procedural Clearance

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.

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.

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.