Narrative:

Descended from cruise altitude to FL270 on account of light chop using FMC cruise descent. Cleared to den via powdr 2 arrival, expect to cross powdr at FL190 and 250 KIAS. Expected crossing restrs were entered into FMC to provide top of descent calculation for a VNAV descent. PNF confirmed data entry. Top of descent point displayed seemed to be too close to powdr for altitude to be lost and existing tailwind component of 80-90 KTS. ATC cleared us to descend to FL230 then cross powdr at FL190 and 250 KTS. FMC cruise mode descent was initiated from FL270 to FL230 to allow recomputation of top of descent point. New top of descent point again seemed too close to powdr. I attempted to force the start of descent using the descent/capture mode. This was not available (ie, not displayed on FMC descent page). Started to descend using vertical speed mode of autoplt. At this point ATC issued 'descend now to FL190' clearance. At this point I recognized that the FMC and autoplt inputs would not make the restr and disconnected the autoplt to hand fly the aircraft. Powdr was crossed slightly high and fast (first officer said altitude was FL193 and exact speed not noted). No traffic conflict was noted or reported by ATC. No comment was made by ATC. Discovery of the late/high profile condition was discovered by observation of instruments combined with unexpected ATC clearance to descend now to FL190. Contributing factors: older, slow FMC system (B737-500). Lack of information on parameters used by FMC to calculate descent and formula used, keeping me from doing a good xchk on FMC calculations. It always seems to descend later than I would when hand flying, but has, in my experience, always made good its calculations -- until now. I have been flying FMC equipment since 1991. Possible use of cruise mode descent masking or delaying top of descent calculation and display. High tailwind component 80-90 KTS. Fatigue -- rough ride due to turbulence. Fatigue -- second day of an afternoon schedule after a month of flying early mornings. Communication -- I did not voice my suspicions of the FMC display error to the PNF or solicit his opinion. Corrective actions: rely more directly on basic plting skills -- especially in adverse conditions. Discuss all observations (ie, think out loud) about flight progress with the other pilot.

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Original NASA ASRS Text

Title: B737 CREW DID NOT CROSS THE CLRED FIX AT THE CLRNC ALT OR SPD.

Narrative: DSNDED FROM CRUISE ALT TO FL270 ON ACCOUNT OF LIGHT CHOP USING FMC CRUISE DSCNT. CLRED TO DEN VIA POWDR 2 ARR, EXPECT TO CROSS POWDR AT FL190 AND 250 KIAS. EXPECTED XING RESTRS WERE ENTERED INTO FMC TO PROVIDE TOP OF DSCNT CALCULATION FOR A VNAV DSCNT. PNF CONFIRMED DATA ENTRY. TOP OF DSCNT POINT DISPLAYED SEEMED TO BE TOO CLOSE TO POWDR FOR ALT TO BE LOST AND EXISTING TAILWIND COMPONENT OF 80-90 KTS. ATC CLRED US TO DSND TO FL230 THEN CROSS POWDR AT FL190 AND 250 KTS. FMC CRUISE MODE DSCNT WAS INITIATED FROM FL270 TO FL230 TO ALLOW RECOMPUTATION OF TOP OF DSCNT POINT. NEW TOP OF DSCNT POINT AGAIN SEEMED TOO CLOSE TO POWDR. I ATTEMPTED TO FORCE THE START OF DSCNT USING THE DSCNT/CAPTURE MODE. THIS WAS NOT AVAILABLE (IE, NOT DISPLAYED ON FMC DSCNT PAGE). STARTED TO DSND USING VERT SPD MODE OF AUTOPLT. AT THIS POINT ATC ISSUED 'DSND NOW TO FL190' CLRNC. AT THIS POINT I RECOGNIZED THAT THE FMC AND AUTOPLT INPUTS WOULD NOT MAKE THE RESTR AND DISCONNECTED THE AUTOPLT TO HAND FLY THE ACFT. POWDR WAS CROSSED SLIGHTLY HIGH AND FAST (FO SAID ALT WAS FL193 AND EXACT SPD NOT NOTED). NO TFC CONFLICT WAS NOTED OR RPTED BY ATC. NO COMMENT WAS MADE BY ATC. DISCOVERY OF THE LATE/HIGH PROFILE CONDITION WAS DISCOVERED BY OBSERVATION OF INSTS COMBINED WITH UNEXPECTED ATC CLRNC TO DSND NOW TO FL190. CONTRIBUTING FACTORS: OLDER, SLOW FMC SYSTEM (B737-500). LACK OF INFO ON PARAMETERS USED BY FMC TO CALCULATE DSCNT AND FORMULA USED, KEEPING ME FROM DOING A GOOD XCHK ON FMC CALCULATIONS. IT ALWAYS SEEMS TO DSND LATER THAN I WOULD WHEN HAND FLYING, BUT HAS, IN MY EXPERIENCE, ALWAYS MADE GOOD ITS CALCULATIONS -- UNTIL NOW. I HAVE BEEN FLYING FMC EQUIP SINCE 1991. POSSIBLE USE OF CRUISE MODE DSCNT MASKING OR DELAYING TOP OF DSCNT CALCULATION AND DISPLAY. HIGH TAILWIND COMPONENT 80-90 KTS. FATIGUE -- ROUGH RIDE DUE TO TURB. FATIGUE -- SECOND DAY OF AN AFTERNOON SCHEDULE AFTER A MONTH OF FLYING EARLY MORNINGS. COM -- I DID NOT VOICE MY SUSPICIONS OF THE FMC DISPLAY ERROR TO THE PNF OR SOLICIT HIS OPINION. CORRECTIVE ACTIONS: RELY MORE DIRECTLY ON BASIC PLTING SKILLS -- ESPECIALLY IN ADVERSE CONDITIONS. DISCUSS ALL OBSERVATIONS (IE, THINK OUT LOUD) ABOUT FLT PROGRESS WITH THE OTHER PLT.

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.