Narrative:

During descent; ATC had us descend to an altitude which; if I remember correctly; was 20;000 ft. Soon after leveling; ATC told us to cross [a point] at 15;000 ft. I; as the non-flying pilot; attempted to put the new cruise altitude into the FMC on the cruise page using L1 (autothrottles and VNAV engaged). Upon pressing L1; nothing happened; and approximately five seconds later; the FMC went to the start page (FMC/ACARS/maintenance) with FMC selection not there and a yellow vtk flag on the pfd. The legs page had no data on it (all of our flight plan was gone) and the pfd did not have the arrival anymore. Approximately 10 seconds later the data came back; but as a route discontinuity and I had to bring [the point] back up as the active waypoint. Vtk flag was gone as well. The cruise altitude insert was attempted one more time with the same result and the same corrections were applied. No further conflicts occurred and the flight arrived without incident. Maintenance was notified by the flying pilot/captain and he informed me that maintenance said that they were aware of the error and that it was a software glitch; nothing they could do about it; and don't worry about writing it up.the biggest thing I noticed right off was that there is not a checklist in the QRH to handle this malfunction. Neither I nor the captain really knew if we were handling the malfunction correctly and only guessed at the procedures based on FMC experience. Was the vtk flag a by-product of the FMC dump? Did the vtk error cause the FMC to lock up? We did not know; and luckily it corrected itself. I believe there is information referencing the vtk flag in the fom but it's not a quick find if you don't know where to look. Why are these pfd flags not in the QRH? We have a checklist for the yellow configuration notice on the center fuel tanks and we get that error almost every time there is fuel in the center tanks and the pumps are not on; but does anyone pull out the checklist for that? Let's put something in the QRH if it affects our navigation capability.

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Original NASA ASRS Text

Title: A B737-700 flight crew twice loses VNAV CDU data and vertical track guidance during descent when ATC altitude clearances were entered and executed on the 'CRUISE' page of the CDU. Flight crew is advised by maintenance to not write up the discrepancy because they were 'aware of the error and that it was a software glitch'.

Narrative: During descent; ATC had us descend to an altitude which; if I remember correctly; was 20;000 ft. Soon after leveling; ATC told us to cross [a point] at 15;000 ft. I; as the Non-Flying Pilot; attempted to put the new cruise altitude into the FMC on the Cruise page using L1 (autothrottles and VNAV engaged). Upon pressing L1; nothing happened; and approximately five seconds later; the FMC went to the start page (FMC/ACARS/MAINT) with FMC selection not there and a yellow VTK flag on the PFD. The LEGS page had no data on it (all of our flight plan was gone) and the PFD did not have the arrival anymore. Approximately 10 seconds later the data came back; but as a route discontinuity and I had to bring [the point] back up as the active waypoint. VTK flag was gone as well. The cruise altitude insert was attempted one more time with the same result and the same corrections were applied. No further conflicts occurred and the flight arrived without incident. Maintenance was notified by the Flying Pilot/Captain and he informed me that Maintenance said that they were aware of the error and that it was a software glitch; nothing they could do about it; and don't worry about writing it up.The biggest thing I noticed right off was that there is not a checklist in the QRH to handle this malfunction. Neither I nor the Captain really knew if we were handling the malfunction correctly and only guessed at the procedures based on FMC experience. Was the VTK flag a by-product of the FMC dump? Did the VTK error cause the FMC to lock up? We did not know; and luckily it corrected itself. I believe there is information referencing the VTK flag in the FOM but it's not a quick find if you don't know where to look. Why are these PFD flags not in the QRH? We have a checklist for the yellow CONFIG notice on the center fuel tanks and we get that error almost every time there is fuel in the center tanks and the pumps are not on; but does anyone pull out the checklist for that? Let's put something in the QRH if it affects our navigation capability.

Data retrieved from NASA's ASRS site as of April 2012 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.