B737 flight crew reported a 9 mile map shift as they approached top of descent; causing them to miss a crossing restriction.
Synopsis
B737 flight crew reported a 9 mile map shift as they approached top of descent; causing them to miss a crossing restriction.
Narrative
We were in cruise flight at FL340. We had been previously cleared direct to LHO. Between 80 to 100 NM east of LHO on the NORDK3 arrival into SLC we were given a clearance to cross 60 NM east of LHO at FL300. I entered 300 into the altitude window and then entered the restriction into the CDU; the Captain checked it; and I hit enter. We both monitored the top of descent point as we proceeded. We saw the top of descent near 5 NM. Less than 30 seconds later the top of descent was gone and the CDU showed us between 3 and 4000 FT high. I saw there was less than 1 NM to the crossing point and immediately started descending. Before the Captain had a chance to call ATC; they were calling us to see if we could make the restriction. The Captain informed them that we had a navigation error and wouldn't make the restriction. He requested a vector; but the Controller re-cleared us to descend to FL300. By this time we were past the crossing restriction point and still descending. The Controller never indicated to us that there was a problem with missing the restriction and she never gave us a vector to compensate. As we continued the flight and had time to try to analyze what happened; the only thing we could figure out was the FMS must have been off by miles and when it updated our position; instantly changed to almost on top of our crossing restriction point.
Second reporter narrative
ATC assigned 60 East Brigham City VOR LHO at FL 300. FMS was programmed with fix. At 5.7 DME to top of descent I began thinking it was getting close to time to go down. I literally looked out the window; was beginning to key mic to advise ATC of altitude change; when I noted the descent page went from 0 (high/low) climbed rapidly to 4000' high; then changed to the next fix in the descent profile. Simultaneously ATC queried if we were going to 'make the restriction.' At this point the First Officer (pilot flying) stated that we were showing 57 DME from LHO! At no time did we receive any kind of RNP alert. In a space of seconds the FMS 'moved' 9 miles. I believe that we were in a poor VOR coverage area and the system lost itself. I realize that we don't think these systems can 'get lost' without an alert; but without redundancy in the CDUs; the only backup is the IRS. If it's in agreement with the FMS; where is the doublecheck?
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.