Narrative:

While on descent with autoplt on; we were being given step-down altitudes during the arrival. At FL190; in descent; we were given a clearance to descend via the ott 6 and; as PF; I put in the next crossing restr of 16000 ft in the altitude window while I verified the bottom crossing altitude of 6000 ft. I then set 6000 ft in altitude window and pressed 'altitude intervention' to continue on the VNAV path. Both pilots verbalized socco altitude; but I failed to catch that the path had been changed in the FMS to cross below the crossing restr altitude. Just prior to fix; I leveled off approximately 1000 ft low with no comments or conflicts from ATC. Arrival continued normally thereafter. I missed the fact that I had inadvertently changed the FMS altitude crossing restr by pressing the 'altitude intv' button while we were established on the VNAV path. Both pilots discussed my error briefly in descent and after the flight on the ground. I will be more conscious of re-pressing 'altitude intervention' and the changes it makes while electing other vertical mode selections. Supplemental information from acn 802362: this occurred due to inadvertent; reflexive use of the altitude intervention button that deleted the 16000 ft altitude constraint from the FMC accompanied with normal workload distraction at just the wrong time. In my experience; this feature of the altitude intervention button to delete altitude constraints from the FMC is rarely called upon in normal operations. This shows a possible human factor issue with the function since there is no confirmation of the deletion prior to execution.

Related reports:

Google
 

Original NASA ASRS Text

Title: A B737 CREW REPORTS MISSING AN ARR CROSSING RESTRICTION AFTER THE FO PRESSED 'ALT INTV' THEREBY DELETING THE NEXT FIX'S ALT CONSTRAINT.

Narrative: WHILE ON DSCNT WITH AUTOPLT ON; WE WERE BEING GIVEN STEP-DOWN ALTS DURING THE ARR. AT FL190; IN DSCNT; WE WERE GIVEN A CLRNC TO DSND VIA THE OTT 6 AND; AS PF; I PUT IN THE NEXT XING RESTR OF 16000 FT IN THE ALT WINDOW WHILE I VERIFIED THE BOTTOM XING ALT OF 6000 FT. I THEN SET 6000 FT IN ALT WINDOW AND PRESSED 'ALT INTERVENTION' TO CONTINUE ON THE VNAV PATH. BOTH PLTS VERBALIZED SOCCO ALT; BUT I FAILED TO CATCH THAT THE PATH HAD BEEN CHANGED IN THE FMS TO CROSS BELOW THE XING RESTR ALT. JUST PRIOR TO FIX; I LEVELED OFF APPROX 1000 FT LOW WITH NO COMMENTS OR CONFLICTS FROM ATC. ARR CONTINUED NORMALLY THEREAFTER. I MISSED THE FACT THAT I HAD INADVERTENTLY CHANGED THE FMS ALT XING RESTR BY PRESSING THE 'ALT INTV' BUTTON WHILE WE WERE ESTABLISHED ON THE VNAV PATH. BOTH PLTS DISCUSSED MY ERROR BRIEFLY IN DSCNT AND AFTER THE FLT ON THE GND. I WILL BE MORE CONSCIOUS OF RE-PRESSING 'ALT INTERVENTION' AND THE CHANGES IT MAKES WHILE ELECTING OTHER VERT MODE SELECTIONS. SUPPLEMENTAL INFO FROM ACN 802362: THIS OCCURRED DUE TO INADVERTENT; REFLEXIVE USE OF THE ALT INTERVENTION BUTTON THAT DELETED THE 16000 FT ALT CONSTRAINT FROM THE FMC ACCOMPANIED WITH NORMAL WORKLOAD DISTR AT JUST THE WRONG TIME. IN MY EXPERIENCE; THIS FEATURE OF THE ALT INTERVENTION BUTTON TO DELETE ALT CONSTRAINTS FROM THE FMC IS RARELY CALLED UPON IN NORMAL OPS. THIS SHOWS A POSSIBLE HUMAN FACTOR ISSUE WITH THE FUNCTION SINCE THERE IS NO CONFIRMATION OF THE DELETION PRIOR TO EXECUTION.

Data retrieved from NASA's ASRS site as of May 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.