B737 CREW HAS FMC VNAV FAILURE DSNDING INTO ORD.

Narrative

ON DSCNT INTO ORD ON THE JVL 4 ARR; THE CTR GAVE US A XING RESTR OF 20 NM N OF JVL AT 13000 FT. WE HAD BEEN GIVEN A 280 KT SPD BY THE PREVIOUS CTLR. I ENTERED THE DSCNT ON THE LEGS PAGE OF FMC ALONG WITH THE ASSIGNED SPD ON THE DSCNT PAGE. THE CAPT CONFIRMED AND I EXECUTED. THE WINDS ALOFT AND DSCNT WINDS HAD BEEN PROGRAMMED PREVIOUSLY. UPON REACHING MSN VOR (WHICH IS WHERE THE TOP OF DSCNT WAS POSITIONED) THE FMC DUMPED ALL OF OUR RNAV INFO; THE SPD FELL BACK TO THE DEFAULT 210 KTS AND THE ACFT BEGAN A DIVE TO MAKE THE RESTR. WE ALSO NOTICED THAT THERE WAS NO FLT PATH ANGLE OR VERT SPD INFO ON THE DSCNT PAGE. THE DSCNT INFO ON THE EADI THEN SHOWED US 4000 FT HIGH. I HAD TO HAND FLY PUSHING THE BARBER POLE AT THIS POINT TO MAKE OUR DSCNT. WE TOLD ATC WE NEEDED ALT AND/OR AIRSPD RELIEF TO MAKE THE DSCNT. HE RESPONDED TO MAKE THE ALT AS BEST AS POSSIBLE (WHICH WE TOOK TO MEAN AIRSPD RELIEF). AS A RESULT OF THIS; WE HAD AN AIRSPD DEV OF 40-50 KTS TEMPORARILY (APPROX 1 MIN) IN ORDER TO MAKE OUR RESTR. WE DID MAKE OUR ALT RESTR BUT AT AN INCREASED AIRSPD WHICH PROBABLY CLOSED A SEPARATION GAP FOR ATC SINCE THEY INQUIRED OF OUR AIRSPD AT THE BOTTOM OF THE DSCNT. I FELT IT WAS NECESSARY TO FILE THIS (THOUGH INITIALLY I DID NOT) BECAUSE OF SEVERAL VNAV INCIDENTS I HAVE HAD OVER THE LAST 12 MONTHS INVOLVING MISTAKES MADE TO ENTRIES; LATE ATC INSTRUCTIONS; PROGRAMMING ERRORS AND VNAV FMC SCREW-UPS. I REALIZE THAT VNAV SHOULD ALWAYS BE BACKED UP WITH 3:1 INFO; HOWEVER; VNAV OFTEN COMPUTES TOP OF DSCNTS INSIDE THE 3:1. THIS WAS AN EXAMPLE OF THIS THAT DID NOT RING ANY ALARMS FOR US. ALL OF THE AVAILABLE INFO WAS CORRECTLY ENTERED AND CONFIRMED IN THE FMC; HOWEVER; THIS INCIDENT AND SEVERAL OTHER OF THE SAME HAVE HAPPENED TO BE IN THE LAST YR INVOLVING OUR FMC JUST DUMPING DSCNT INFO THAT THE COMPUTER COULDN'T FIGURE OUT AND LEAVING US TO SCRAMBLE. IT SEEMS THE MORE INFO WE FEED INTO THE MACHINE; THE WORSE IT OPERATES! IF THESE FMC'S CANNOT HANDLE TODAY'S STAR REQUIREMENTS; THEN WE SHOULDN'T RELY ON THEM. IF THE AUTOMATION DOES NOT WORK; IT ADDS ONE MORE LEVEL OF THREAT AT THE BUSIEST TIME OF THE DSCNT PRIOR TO APCH. THIS CAUSES PEOPLE TO NOT USE THE AUTOMATION THUS CREATING INEFFICIENCIES IN OUR SYS. IF THIS HAS HAPPENED TO ME; I KNOW IT IS HAPPENING TO OTHERS THAT ARE NOT BEING RPTED. FIX OUR FMC'S SO THAT WE CAN RELY ON THEM TO EASE OUR WORKLOAD AND HELP US WITH SITUATIONAL AWARENESS INSTEAD OF ADDING ADDITIONAL PROBS.