Narrative:

Aircraft at FL370 cruise on flight plan route, kessel 2 arrival. Autoplt in command mode LNAV and VNAV engaged. FMC programmed to cross druzz intersection at 11000' and 250 KTS. This clearance had not been given, but it was expected we would get it. About 70 mi west of kessel ATC issued this clearance: 'cross 25 mi west of kessel at and maintain 25000'.' I mentioned to the captain that I had experienced problems with the FMC's on some other of our aircraft starting descent late. We each agreed 'to see how this one does.' I said there was something screwy--the FMC said 'distance to top of descent,' and 'distance to fix' only 2 mi apart--how could that be? About this time ATC told us to 'start your descent now,' first hint of possible problem. Using the 'old' 3 for 1 descent formula, it looked as though we had our work cut out for us to make the restriction 25 west of kessel. ATC again, 'you did get 25 west of kessel at FL250?' I stated that if we can't make it, we should tell them. We crossed the VOR at 26100' and leveled at FL250 about 15 seconds later. Both captain and I have enough FMC experience and in our discussion agreed that the system does not seem to function acceptably in descent. The FMC requires monitoring at all times, like any other navigation system. Its unique capabilities and performance can lull the crew into a degree of 'monitoring complacency,' which can be insidious. For my part (as I did early in my FMC flying experience), I will be much more vigilant while operating the FMC to ensure the system performs as it is programmed. As for the reason for the problem in this case, I am at a loss to explain it. However, this is likely to occur again and I plan to report it to the company so that the situation can be rectified.

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Original NASA ASRS Text

Title: FLT CREW OF MLG DEPENDED ON AUTO NAVIGATION AND FMC TO ACCOMPLISH CROSSING ALT, BUT EQUIPMENT WAS SLOW TO START DESCENT AND CROSSING RESTRICTION NOT MET.

Narrative: ACFT AT FL370 CRUISE ON FLT PLAN ROUTE, KESSEL 2 ARR. AUTOPLT IN COMMAND MODE LNAV AND VNAV ENGAGED. FMC PROGRAMMED TO CROSS DRUZZ INTXN AT 11000' AND 250 KTS. THIS CLRNC HAD NOT BEEN GIVEN, BUT IT WAS EXPECTED WE WOULD GET IT. ABOUT 70 MI W OF KESSEL ATC ISSUED THIS CLRNC: 'CROSS 25 MI W OF KESSEL AT AND MAINTAIN 25000'.' I MENTIONED TO THE CAPT THAT I HAD EXPERIENCED PROBS WITH THE FMC'S ON SOME OTHER OF OUR ACFT STARTING DSCNT LATE. WE EACH AGREED 'TO SEE HOW THIS ONE DOES.' I SAID THERE WAS SOMETHING SCREWY--THE FMC SAID 'DISTANCE TO TOP OF DSCNT,' AND 'DISTANCE TO FIX' ONLY 2 MI APART--HOW COULD THAT BE? ABOUT THIS TIME ATC TOLD US TO 'START YOUR DSCNT NOW,' FIRST HINT OF POSSIBLE PROB. USING THE 'OLD' 3 FOR 1 DSCNT FORMULA, IT LOOKED AS THOUGH WE HAD OUR WORK CUT OUT FOR US TO MAKE THE RESTRICTION 25 W OF KESSEL. ATC AGAIN, 'YOU DID GET 25 W OF KESSEL AT FL250?' I STATED THAT IF WE CAN'T MAKE IT, WE SHOULD TELL THEM. WE CROSSED THE VOR AT 26100' AND LEVELED AT FL250 ABOUT 15 SECS LATER. BOTH CAPT AND I HAVE ENOUGH FMC EXPERIENCE AND IN OUR DISCUSSION AGREED THAT THE SYS DOES NOT SEEM TO FUNCTION ACCEPTABLY IN DSCNT. THE FMC REQUIRES MONITORING AT ALL TIMES, LIKE ANY OTHER NAV SYS. ITS UNIQUE CAPABILITIES AND PERFORMANCE CAN LULL THE CREW INTO A DEG OF 'MONITORING COMPLACENCY,' WHICH CAN BE INSIDIOUS. FOR MY PART (AS I DID EARLY IN MY FMC FLYING EXPERIENCE), I WILL BE MUCH MORE VIGILANT WHILE OPERATING THE FMC TO ENSURE THE SYS PERFORMS AS IT IS PROGRAMMED. AS FOR THE REASON FOR THE PROB IN THIS CASE, I AM AT A LOSS TO EXPLAIN IT. HOWEVER, THIS IS LIKELY TO OCCUR AGAIN AND I PLAN TO RPT IT TO THE COMPANY SO THAT THE SITUATION CAN BE RECTIFIED.

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.