737 Strange DEU/FD problem
The other day, we had a strange problem. On arriving on stand, the ground power died just as we put it on, leaving the usual requirement to reset the Yaw Damper &c. All other flight deck indications were normal.
FDs were switched on, but LNAV would not arm prior to departure. Upon rotation, the flight directors disappeared completely. This was solved by cycling the FD switches and re-selecting the desired modes.
The suggestions was that occasionally intermittent power failures cause CDS failures/faults that do not annunciate and only present themselves in this fashion. Despite many GPU failures, this is the first time I've experienced this before and would welcome any suggestions as to the root cause.