Elec Bus Isolation 747-400

[QAVION]

Hi, FELlOw PPRuNers.

Can anyone tell me if Elec Bus Isolation happens on the 747-400 when all three A/P's are _armed_ for approach... or does this only happen when all three A/P's are _engaged_ at 1500' (when LAND3 is annunciated)?

I thought it was the latter, but the Boeing Maintenance Manual says otherwise. If you have a copy handy, the reference is 22-11-00, possibly page 5 (depending on your airline).

Presumably, this is co-incident with the Electrical Synoptic page being blanked, so this should be an easy question for 400 pilots ;-)

Thanks.
Regards.
Q.