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Losing VNAV data on descents (long hauls)

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Hi all


I don't often fly long hauls but on the two recent occasions that I have, my VNAV data seemed to drop out in the latter part of the flight.

Planning software being used



The setup for the flight is all pretty standard using the typical setup/route loading convensions - no particular issues of note.

However - I was on my way to EGLL from OMDB yesterday. (Total flight time was 7h15). On commencing the descent into London, my FMC display began to flicker and I noticed that the speed/alt data had disappeared - except for the mandatory speed/ALT restrictions (in magenta) at certain waypoints.  The MCP dropped out of VNAV mode and set an estimated descent VS of -1800fpm.  Decent forecast winds were loaded prior to descent as per normal.


On reaching 7000' at LAM VNAV seemed to recapture the data and I could re-select VNAV mode. ;While maintaining 7000' in a hold it dropped out again and the FMC display started to flicker - with non-mandatory speed/alt waypoint data disappearing.  It recaptured again and I was able to transition from VNAV/LNAV onto the LOC and APP intercepts.

It only seems to happen on long hauls - experienced the same between KJFK and EGLL 2 weeks ago.  I tend to do alot of < 4 hour 777 trips and I don't seem to experience the problem then.

More than likely me overlooking something but stumped as to what is causing the issue.  Nothing should be corrupting purely because of the length of the flight

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There is a known bug in the 777 that you see when selecting an ILS27 approach and the OCK transition.  Do not select the OCK transition.  The STAR is okay and the ILS27 is okay, only the transition causes the problem,

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Very interesting, I didn't know that.  I believe last night I did select the OCK transition for ILS27R.   I cant remember what I did on the flight from NY


Anyway good to know that there isn't a bigger problem with corruption in the data system.





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