Narrative:

Flight was assigned a crossing restriction of 35 southwest of bld VORTAC at 15000'. I programmed the FMC (flight management computer) for this crossing restriction and verified this information on the FMC cdv. The copilot also confirmed the correct data entry. At approximately 35.5 DME southwest of bld VORTAC, lax center asked us our DME from bld. (Note: the FMC indicated 42 mi southwest of bld, ie, 6 mi from the fix.) we told the controller to standby so we could verify the DME distance with raw data directly from the navigation receiver. Upon accomplishing this, the DME indicated was actually 35.5 mi, a discrepancy of 6.5 mi from what the FMC was indicating. We told the controller we were 35.5 DME from bld. The controller then asked us our altitude. We responded, 'we are passing through 18000' (the controller then handed us off to las vegas approach). The copilot and I believe the problem was caused by the computer malfunctioning. The trip had originated in ontario, ca. It took the copilot and I 1 hour to align the IRS system. We believe that this difficulty could have had an effect on the erroneous DME information the computer was generating. In summary, the copilot and I believe that computer malfunction was responsible for the missed crossing restriction. However, we have learned that backing up the computer with raw data from the navigation receiver could prevent an occurrence of this sort in the future. Incidentally, flight did not pose any conflict to other aircraft as a result of this error and we were not asked to contact anyone with regard to this incident.

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Original NASA ASRS Text

Title: ACR MLG ALT DEVIATION UNDERSHOT ALT CROSSING RESTRICTION DURING DESCENT.

Narrative: FLT WAS ASSIGNED A XING RESTRICTION OF 35 SW OF BLD VORTAC AT 15000'. I PROGRAMMED THE FMC (FLT MGMNT COMPUTER) FOR THIS XING RESTRICTION AND VERIFIED THIS INFO ON THE FMC CDV. THE COPLT ALSO CONFIRMED THE CORRECT DATA ENTRY. AT APPROX 35.5 DME SW OF BLD VORTAC, LAX CENTER ASKED US OUR DME FROM BLD. (NOTE: THE FMC INDICATED 42 MI SW OF BLD, IE, 6 MI FROM THE FIX.) WE TOLD THE CTLR TO STANDBY SO WE COULD VERIFY THE DME DISTANCE WITH RAW DATA DIRECTLY FROM THE NAVIGATION RECEIVER. UPON ACCOMPLISHING THIS, THE DME INDICATED WAS ACTUALLY 35.5 MI, A DISCREPANCY OF 6.5 MI FROM WHAT THE FMC WAS INDICATING. WE TOLD THE CTLR WE WERE 35.5 DME FROM BLD. THE CTLR THEN ASKED US OUR ALT. WE RESPONDED, 'WE ARE PASSING THROUGH 18000' (THE CTLR THEN HANDED US OFF TO LAS VEGAS APCH). THE COPLT AND I BELIEVE THE PROB WAS CAUSED BY THE COMPUTER MALFUNCTIONING. THE TRIP HAD ORIGINATED IN ONTARIO, CA. IT TOOK THE COPLT AND I 1 HR TO ALIGN THE IRS SYS. WE BELIEVE THAT THIS DIFFICULTY COULD HAVE HAD AN EFFECT ON THE ERRONEOUS DME INFO THE COMPUTER WAS GENERATING. IN SUMMARY, THE COPLT AND I BELIEVE THAT COMPUTER MALFUNCTION WAS RESPONSIBLE FOR THE MISSED XING RESTRICTION. HOWEVER, WE HAVE LEARNED THAT BACKING UP THE COMPUTER WITH RAW DATA FROM THE NAV RECEIVER COULD PREVENT AN OCCURRENCE OF THIS SORT IN THE FUTURE. INCIDENTALLY, FLT DID NOT POSE ANY CONFLICT TO OTHER ACFT AS A RESULT OF THIS ERROR AND WE WERE NOT ASKED TO CONTACT ANYONE WITH REGARD TO THIS INCIDENT.

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