Narrative:

Cruising at FL410, given clearance to cross 15 mi southeast of ykm FL350 or below, discretion to FL240. This crossing restr was entered as a waypoint and executed. Descent was begun and progressed nicely, indicating we would make the restr. I began reviewing the arrival and approach plates. ZSE called asking if we would make the restr. At this point, we both noticed the FMS had dropped the ykm 15 fix and had reverted to the descent necessary for the next crossing restr. As a result, we missed the restr and crossed that point descending through FL35.8 with a rate of descent in excess of 4000 FPM. Contributing factors: significant: 1) reliance upon a normally very reliable function of the FMS. 2) distraction during a workload increase period. 3) fix was pretty close when restriction was given. 4) getting late and late departure (1 hour late). Why did the FMS drop the fix/restriction? I don't really know. But, very shortly thereafter, the aircraft slewed right, then left. EICAS displayed rudder ratio and we lost 1 yaw damper. Could this have been part of the reason? I really have no clue. Callback conversation with reporter revealed the following information: analyst called captain to counsel and inquire what he understood of why this incident happened. Captain had been unable to find out why the FMC dropped the crossing fix. Analyst informed him that FMC will drop anything that aircraft cannot comply with in its performance envelope. Also, when a descent or climb restr is given, the altitude limiter can be rotated to crossing restr altitude to see if the performance of the aircraft will accept the restr. Reporter was not aware of these points of operation with the FMC.

Related reports:

Google
 

Original NASA ASRS Text

Title: ACFT MISSES PROPER XING ALT DURING DSCNT.

Narrative: CRUISING AT FL410, GIVEN CLRNC TO CROSS 15 MI SE OF YKM FL350 OR BELOW, DISCRETION TO FL240. THIS XING RESTR WAS ENTERED AS A WAYPOINT AND EXECUTED. DSCNT WAS BEGUN AND PROGRESSED NICELY, INDICATING WE WOULD MAKE THE RESTR. I BEGAN REVIEWING THE ARR AND APCH PLATES. ZSE CALLED ASKING IF WE WOULD MAKE THE RESTR. AT THIS POINT, WE BOTH NOTICED THE FMS HAD DROPPED THE YKM 15 FIX AND HAD REVERTED TO THE DSCNT NECESSARY FOR THE NEXT XING RESTR. AS A RESULT, WE MISSED THE RESTR AND CROSSED THAT POINT DSNDING THROUGH FL35.8 WITH A RATE OF DSCNT IN EXCESS OF 4000 FPM. CONTRIBUTING FACTORS: SIGNIFICANT: 1) RELIANCE UPON A NORMALLY VERY RELIABLE FUNCTION OF THE FMS. 2) DISTR DURING A WORKLOAD INCREASE PERIOD. 3) FIX WAS PRETTY CLOSE WHEN RESTRICTION WAS GIVEN. 4) GETTING LATE AND LATE DEP (1 HR LATE). WHY DID THE FMS DROP THE FIX/RESTRICTION? I DON'T REALLY KNOW. BUT, VERY SHORTLY THEREAFTER, THE ACFT SLEWED R, THEN L. EICAS DISPLAYED RUDDER RATIO AND WE LOST 1 YAW DAMPER. COULD THIS HAVE BEEN PART OF THE REASON? I REALLY HAVE NO CLUE. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: ANALYST CALLED CAPT TO COUNSEL AND INQUIRE WHAT HE UNDERSTOOD OF WHY THIS INCIDENT HAPPENED. CAPT HAD BEEN UNABLE TO FIND OUT WHY THE FMC DROPPED THE XING FIX. ANALYST INFORMED HIM THAT FMC WILL DROP ANYTHING THAT ACFT CANNOT COMPLY WITH IN ITS PERFORMANCE ENVELOPE. ALSO, WHEN A DSCNT OR CLB RESTR IS GIVEN, THE ALT LIMITER CAN BE ROTATED TO XING RESTR ALT TO SEE IF THE PERFORMANCE OF THE ACFT WILL ACCEPT THE RESTR. RPTR WAS NOT AWARE OF THESE POINTS OF OP WITH THE FMC.

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.