2009-09 · NASA ASRS report 853390
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'.
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.
Upon arrival we called Maintenance and they called Maintenance Control. They stated that they knew of the problem. It was a software glitch and we didn't need to write it up ... First and foremost; when we got the VTK flag; we did not know where to look to see what the flag meant and what to expect. Nothing to cover this was in the QRH. The warnings for automation malfunctions need to be in the QRH; readily available in any flying conditions. We need to be trained on how to handle problems and where to look. This could have ended in a violation for us.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.
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