PPRuNe Forums - 767 EHSI "MAP" Flag Confusion

I thought I'd got the logic of this flag figured out, but...

With some earlier experimentation with the EHSI displays on a particular 767, the yellow "MAP" flag appeared to appear under the following situation(s):

On the MAP ("TRK UP" display option) page, when the IRS's were not fully aligned or loss of FMC data (e.g. due to, say, a power loss to the FMCS).

On the PLAN page, the flag only appeared when all four FMC circuit breakers were pulled (computers and CDUs L/R).

This led me to believe that the MAP flag appears when the FMC map data cannot be properly drawn on the displays. PLAN, of course, doesn't need IRS data to show the route (only the compass displays at the top of the PLAN page require IRS input).

However, on another 767, I had trouble generating a flag on the MAP page at all. A fellow engineer commented that he'd had to wait up to five minutes for a MAP flag to appear.

Does the logic vary from 767 to 767.... or were my earlier observations muddied by some kind of time delay? (perhaps)

Thanks for any input (on this and other glass-cockpitted aircraft).

Rgds.. .Q.