EXCESS DATA
**** Hidden Message ***** QUESTIONSometimes, the amber message "EXCESS DATA" appears in the Navigation Display. The only meaning given by
Boeing is : "the amount of data sent to the navigation display exceeds the display capability".
Can we lose any information displayed on the instrument?
ANSWER
The Navigation Display (ND) gets most of its data from the FMC, and this data comes in as a stream of 32-bit
digital words to ARINC 429 spec. The data is divided between dynamic data (airspeed, winds, active waypoint,
distance to go,etc.) and map background data which includes the track line, route waypoints, plus overlay data
such as waypoints, navaids, airports, etc. in the area, depending on which you have selected on the Display
Control Panel (DCP). The FMC also puts out map data covering a slightly larger area than the ND displays, so
that you cannot fly off into blank space before the map has a chance to update.
The map background data is organized into 64-word blocks, and up to 8 blocks can be sent. The first and last
word of each block are start and end words, and so you can have up to 62 x 8, or 496 words of map background
data displayed at any one time. After that, the data is repeated in (up to) 8 new blocks. Each data word can carry
either a latitude, a longitude, an arc radius, arc subtended angle, three characters of an ident., etc. (so a waypoint
could take up to 3 or 4 words - lat., long, ident.).
The FMC sends these 8 data blocks at a minimum refresh rate of every 5 seconds, OR immediately when a DCP
selection has been made.
Now, if the FMC cannot send more data than can be fitted into 496 words, so algorithms in the FMC determine
which data to drop, and "EXCESS DATA" is displayed on the ND (set by a logic "1" bit in one of the control
words). Usually, it is off-route map overlay data (waypoints, navaids, airports, etc.) that causes the overflow. The
FMC drops off those points most distant from the aircraft first (and gives priority to your track data). Since the
FMC is putting out data covering a larger area than the ND can display, it may be that NO data is lost to the crew
at all.
If you get the EXCESS DATA message, you are probably out at 320 or 640 range, and have map overlay data
selected - most likely waypoints, which can be numerous in terminal areas. The best thing to do would be to
either deselect the waypoints on the DCP or reduce the range one click. This will trigger the FMC to initiate a
fresh set of background data blocks - hopefully totaling less than 496 words of data.
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